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Deburr small-intersecting-holes

This document summarizes deburring techniques for small intersecting holes. It discusses how hole size, tolerance, depth of intersection, and other factors impact the deburring process selection. Only 5 of the 37 major deburring processes are normally suitable for small holes. These include thermal energy method, abrasive jet, abrasive flow, electropolish, and manual methods. The document provides tables comparing deburring process capabilities based on hole size change, edge condition, and other criteria.

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MM . TE IQ MS MR ‘CM PE @(All Rights Resewedl SMALLlNTERSECTINGHOLES BY LaRouxK. Gillespie StaffEngineer BendixCorporation ABSTRACT Beburring intersecting holesis oneof the mostdifficult deburring tasksfacedbymanyindustries. Only 14of the 37majordeburring processesare applicableto mostinter- sectingholeapplications. Onlyfive of theseare normally applicableto smallor miniature holes. This paper summarizes sic processcapabilitiesandtechniques used asa function of holesizesandintersection depths.
ABSTRACT Deburring intersecting holes is one of the most difficult deburring tasks faced by many industries. Only 14 of the 37 major deburring processes are applicable to most intersecting hole applications. Only five of these are normally applicable to small or miniature holes. This paper summarizes basic process capabilities and techniques used as a function of hole sizes and intersection depths. INTRODUCTION Effective deburring of intersecting holes is a function of many variables. For example, the following must be considered in any analysis of the 'best' deburring techniques. Hole Sizes Hole Tolerances Hole Surface Finishes Type r,E Holes (Blind or Thru) Initial Edge Condition Material Compatibility (Deburring Side Effects) Part Schedules Depth of Intersection Wall Thicknesses Desired Edge Condition Workpiece Material Quantity of Intersections Accessibility of Intersection Few deburring processes will &burr small holes. As hole tolerance requirements increase and surface finish needs become finer, many deburring processes are eliminated from potential consideration. Welding, plating, and soldering needs eliminate those processes which impregnate abrasive particles, and require- ments to have very small or very precision edge radii further eliminate processes from consideration. PROCESSLIMITATIONS As seen in Tables 1 and 2, if the deburring process is allowed to change the hole size by 0.001 inch (25.4 pm> and the resulting edge is allowed to have up to 0.010 inch (254 pm) radius or chamfer, then five of the deburring processes may reasonably be expected to deburr hole intersections. Table 1 assumes however, that the hole has and requires a surface finish of 32 microinch (0.81 pm), an initial burr size equivalent to 0.003 inch thick by 0.003 inch high (76.2 x 76.2 pm) 303 stainless steel, and that at least one of the intersecting holes is 0,5-in& in diameter (12.7 mm). It further assuIy1es that all the burrs and associated raised metal must be removed. As the allowable hole size change or maximum edge break is reduced, begin dropping out as feasible processes quickly realistic possibilities. Similarly, as
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-5 Table 2. Codes Used in Table 1 Code Deburring Process A B C D E F G H I J K L M N 0 P & R S T U v W X Vibratory Barrel Tumbling Spindle Finishing Centrifugal Barrel Finishing Abrasive Jet Sanding Brushing Hand Abrasive Flow Mechanized Mechanical Thermal Energy Chemical Electrochemical Electropolish Ultrasonic Torch or Flame Yelting Water Jet Electrochemical Vibratory Electrochemical Brush Chemical Vibratory Liquid Hone Chlorine Gas Magnetic Loose Abrasive Plasma shown in the right half of Table 1, when the allowable standard deviation or repeatability of hole size change decreases, the number of feasible deburring processes diminishes. In some cases the indicated burr size is the major reason a process is not feasible. In others, the material and process are not compatible, In most cases, however, it is the combination of material, burr size, diameter change, and allowable final edge break th,t limits process capability.
-4- Two subtle points are contained in Table 1. Obviously, some parts are produced with 0.003 inch (76.2 pm) burrs and require- ments of not changing hole size more than 0.0001 inch and not exceeding edge chamfers of 0.001 inch (25.4 pm). The manufac- turers of these parts do remove the burrs, even though Table 1 indicates no process will do so. Table 1 defines the capa- bilities of individual pro:esses, In difficult situations nanufacturers, by necessity, must combine processes. Thus, by first reaming the hole to make the burrs smaller and then using one of the mechanized deburring processes, it is possible to meet the indicated constraints. Another table of capabilities then exists in principle {although not yet in print) which describes deburring process capabilities for these smaller burrs. Similar tables also could be constructed for different materials, surface finish, and hole size. Table 3 illustrates some basic process capabilities when there is no concern for anything other than the size change resulting from deburring. The second subtle point in Table 1 is that resulting edge breaks or chamfers are in part a function of edge geometry. Consider Figure 1. As seen there, when two holes intersect, the angles formed by the intersection vary from acute to obtuse angles. The diameter and nature of the intersection (center line to center line or just slight breakthrough) dictate the variation in angles. The effect of these variations in angles can be appreciated by viewing Figure 2. To produce a 0.005 inch (127 pm) edge break on a 90 degree edge requires removing 0.0021 inch (53.3 pm) of stock material. On a 120 degree edge, the same stock removal would produce a 0.015 inch (381 pm> radius, and on a 30 degree edge the resulting radius would be approximately 0.0005 inch (12.7 pm). The mechanized processes basically try to produce the same stock loss all over. As a result, on the three dimensional inter- section shown in Figure 1, considerable variation will exist in edge break, chamfer, or radius as the line of edge intersections is traversed. Figure 3 illustrates how edge angles on exterior surfaces are affected by vibratory deburring. The net result of all these observations is that for cost effec- tive deburring it is necessary to minimize burr size, allow the maximum possible hole size change, and allow edge breaks of at least 0.0'; inch (127 pm). PROCESSESTYPICALLY USED ON HOLES SMALLERTHAN 0.060 INCH (1.5 mm} The following list summarizes the processes now in commercial or potential use for intersecting holes smaller than 0.060 inch (1.5 mm) in diameter.
-5- Table 3. Basic Capabilities of &burring Processes--Stock Loss* Typical Working Range Stock Loss (p) (Inch) Beburring Process 0.25 2.54 25.40 254.00 (0.00001) (0.0001) (0.001) (0.010) ABRASIVE Abrasive Jet Abrasive Flow Semisolid Carriers Liquid Carriers Water Jet Loose Abrasive Barrel Centrifugal Barrel Magnetic Spindle Finishing Vibratory Recipro Finishing Flow Finishing Orboresonant Sanding (Edges Only) MECHANICAL Mechanized Hand Brushing THERMAL Flame Thermal Energy (TEM) Plasma Hot Wire EDM Resistance Heating
-6- Table 3 Continued. Basic Capabilities of Deburring Processes-- Stock Loss* - Typical Working Range Stock Loss (p) (Inch) Deburring Process 0.25 2.54 25.40 254.00 (0.00001) (0.0001) (0.001) (0.010) CHEMICAL Cnemical Ultrasonic Chlorine Gas Chemical Loose Abrasive Barrel Centrifugal Barrel Magnetic Loose Abrasive Spindle Finishiqr Vibratory ELECTROCHEMICAL Electrochemical Electropolish Electrochemical Brush Electrochemical Loose Abrasive Barrel Centrifugal Barrel Spindle Vibratory I *Based on the removal of 76.2-pm-thick (0.003 in.) burrs from the edges of 303SE stainless steel directly exposed to the deburring process. These capability estimates assume that surface finishes of 0.81 pm (32 pin.) are required and exist on the workpiece prior to deburring. Stock losses are defined as size changes which occur on exposed surfaces; the values are overall size changes of diameters or thickesses. These estimates assume a maximum allowable edge break of 254 pm (0,010 in.), Less stock loss will occur if smaller burrs are present.
Dl Cl 81 A! VIEW SEEN LOOKING SIDEVVAYS Figure I. Three-Dimensional Configuraticn of Hole Intersections
RADIUS DIFFICULT TYPICAL RADIUS 3 I------’ T--‘-.“-‘--- --, I RADIUS S I ‘T -.__0.002 50.8 0.003 76.2 0.005 127.0 I 0.0150.010 254.0381.0 - 0.0003 o.ooou Ii*: --l---l 0.0007 17:a 0.0015 38.1 0.0022 55.9 LESS DIFFICULT * STOCK REMOVALREQUIRED TO PRODUCEIWOICATED RADIUS Figure 2. Effect of Geometry on Edge Radiusing In Commercial Use Other Possibilities Thermal Energy Method Chemical Abrasive Jet Electropolish Abrasive Flow Chlorine Gas Electrochemical EDM Manual Resistance Heating . -
-9- 254.0 228.6 203.2 =$ 177.8 Sd i? 152.4 s z g 127.0 wl .g 101.6 isi 76.2 so.8 25.4 0 WORKPIECE HARDNESS: Rc24 -! o.oog - 0.008 - 0.007 z z -- 0.006 Eu - 0.00s 2 ze - 0.004 !g cr - 0.003 2 -- 0.052 1 0.001 0 1 2 3 4 5 TIME IN VIBRATORY FINISHER (HOURS) Figure 3. Effect of Edge Angle and Vibration Time on Edge r Radiusing of Phosphor Bronze Workpiece Thermal Energy Method The thermal energy method (TEM) utilizes the high temperature short duration shock wave, accompanyin Q xhe detonation of two gases to vaporize or burn off burrs," The fact that it uses gas as a removal tool makes it particularly applicable to many hard-to-reach areas. In this case, the ability of the gas to conform to almost any shape implies that the combustion heat wave will attack the burr. This is one of the fastest processes available for deburring and should be considered for any intersecting hole applicatiokls. It currently is used to deburr the majority af the carburetors produced in the United States. The method has been used to produce radii of 0.002 to 0.060 inch (0.05 to 1.5 mm) on steel parts and 0.002 to 0.010 inch (0.05 to 0,225 mm) on aluminum parts. For uniform initial conditions, edge radii should be crnsistent within k0.002 inch (250 pm). The process has some limitations: c
-16- g The burr must be 15 times thinner than other walls or features; 0 Complete deburricg may not occur on some features; and 0 It is not effective on many materials. While a TEM installation is expensive, the cost for each part can be as low as $0.05. Deburring job shops using this process are available in some parts of the country. The best applications to date appear to be for steel, zinc, and some aluminum and brass materials. Abrasive Jet Deburring Miniature abrasive jets are used frequently for deburring complex intersecting holes. These miniature blasters (Figure 4) have nozzle openings as small as 0.003 inch (76.2 pm) in diameter. By varying the nozzle diameter, nozzle configuration, and nozzle tip-to-part distance, a wide variety of deburring conditions can be met. Figure, 5 illustrates the effect of nozzle-to-part distance, 2nd Figure 6 illustrates some of the nozzle variations in commercial use. This process has been used to deburr hypodermic needles having 0.028 inch (0.71 mm) openings, intersections of miniature slots and threads, intersecting holes, and external features. In most applications, deburring by this process requires less than 1 minute for each intersecting hole.' Intersecting holes and holes intersecting threads represent two areas of particular noteworthy applications of this process. Threads which have been machined through are a particularly difficult deburring problem. Because the blasting process d conforms to feature contours, this process is particularly applicable to such features. The process is limited by the depth of the intersection and size of holes to be deburred (Figure 7). Table 4 illustrates some of these limitations based on existing nozzle size. In this instance, it is assumed that deburring action is ineffective on metals when the nozzle is more than 0.5 inch (12.7 mm) from the intersection. If burrs are thin enough, this assumption might be overly restrictive. In addition to the process variables already mentioned, the aggressiveness of the process can be increased by changing abrasive material, abrasive size, the ratio of abrasive to air, and the air pressure. Some miniature blasters utilize pressures up to 190 psi (1.31 MPa). This is twice the pressure utilized by some units and this 2:l difference results in four times faster cutting. 12
Figure 4. Miniature Blasting Machine Any blasting process affects surface finish, but in most situa- tions it is the textul-e rather than the average roughness which changes, This process can readily deburr many parts and not worsen a 16 microinch (0.4 pm) finish. Complete burr removal is a function of time, the variables described, and initial burr size. In most instances, however, burr size can be controlled by a reaming operation to remove heavy burrs prior to blasting. While the process is most applicable to hard metals, 303 se stainless steel (l&30) can be easily deburred by this approach. Since media size is typically 25 pm (0.001 inch) or larger the process probably would not be used on cross holes smaller than 0.005 inch. This process is normally a hand held operation, but it is possible to fixture the operation or combine it with one of the miniature robots.
-12- 5.00 0.64-----MM------ -- -- : I i 10.00 1.50---------------.w.-- II 2.00-1----1----w---- DIAMETE 7-4 TIP OF CUT DISTANCE (MM) Figure 5. Cutting Action of 0.018 Inch (0.46 mm> Diameter Nozzle Figure 6. Commercially Available Nozzle Variations With Side and End Views.
A RD COMMERCIAL NOZZLE c SPECIAL NOZZLE EXTENSIQN Figure 7. Examples of Nozzle Placement 6 STANDARD COMMERCDAL NOZZLE D ENTIRE RIGHT ANGLE HEAD COMPLETELY iNSERTED IN HOLE Equipment costs are approxiately $1500 (1979) and media costs are frequently on the order of 10 to 25 dollars per day. Conventional Abrasive Flow Deburring The abrasive flow deburring prooess is a method of abrading surfaces by extruding a viscous, semisolid, abrasive-laden media across areas or surfaces. In order to contain and direct the flow of media, the workpiece is confined within a fixture between the two vertically opposed media chambers (or cylinders) of the machine (Figure 8). Under hydraulic pressure, the media is extruded from one media chamber through the fixtured workpiece and into the cpposite media chamber. In most applications, the media is then extruded back through the part into the original
-14- ? /T2LcDsl.e4. Approximate Hole Diameter and Depth Limitations of Miniature Blasting Nozzles* Smallest Access Maximum Hole Diameter Depth of Nozzle Allowable Cross Hole Approach** (inch) (mm) (inch) (mm) A 0.040 1 0.250 6.4 B 0.155 3.9 0.575 14.6 C 0.040 1 2.00*** 50 D 0.400 10 l.OO*** 25 *This table assumes that the maximum effective- ness of the process is lost when the nozzle is more than 0.5 inch (12.7 mm) from the area to be deburred. **See Figure 7 for Explanation of Nozzle Approach. ***Nozzles this size are in use; actual maximum is probably larger. media chamber. The amount of abrasion that is accomplished with a given media formulation and process pressure is directly related to the quantity of media flow. Depending upon the application, adequate flow quantities range from a few cubic inches to many comp'iete cycles. The process can be thought of as a flowable file with capabili- ties ranging from a light buff to coarse stock removal. By using various compound formulations, machine adjustments, and fixture designs, selective processing can be accomplished on many types of components. A basic principle of this process is the media's ability to be significantly abrasive only at the point of greatest restriction in its flow path. This process is comple2$ly automatic and, as in the case o.f abrasive jet deburring, several variables can be changed to provide aggressiveness. In most applications the deburring will improve surface finish and provide longitudinal flow lines rather than the circumferential marks left by conventional drilling and reaming. This later process byproduct is a major advantage in many cases involving flui flOW*
-5 R MACHINE CHAMBER ‘TOLOWER IUlACHIRECWILPBER Figure 8. Fixturing for Simple Part With Intersecting Holes Some advantages and disadvantages of this process are listed below. Advantages Ability to deburr all metals High degree of accuracy and repeatability System flexibility Automatic controlled process Selective deburring capability Ability to deburr a wide range of part sizes Easy changeover from one part to another Multipart processing capability Short process cycles Minimal fixture costs Improved surface finish Ability to deburr areas not accessible to normal deburring techniques Can do many holes at one time; ideally suited to cross hole intersections 1
Disadvantages Requires major capital equipment Requires hclding fixture Requires cleaning to remove particles and carrier Increases hole size The process has been used in a number of instances to deburr holes of 0.040 inch (1 mm) in diameter or smaller.* Typical cycle times are 30 to 60 seconds. Total deburring and cleaning time per part will be twice to five times this amount in many instances, In one case, 16 small intersecting holes were deburred in a cycle time of 12 seconds. If burrs are small, edge radii can be limited to less than 0.003 inch (76.2 pm). Large radii can also be produced, The process can be used on thin wall components if care is taken. One example cited in Reference 2 involved the deburring of an aluminum part having 180 slots EDM'd through a 0.006 inch (152 pm) wall. Each slot was 0.006 inch wide and 0.060 inch long (152 pm by 1.5 mm). Few material compatibility problems have been observed with this process. The process uses silicone materials and inert abra- sives. Some users, however, note that silicone oils are never totally removed by cleaning processes. These oils tend to migrate, such that a single drop will spread out and cover all surfaces in an assembly. On electromechanical assemblies this can increase the resistance of electrical contacts. While not normally a problem for many users, it can be when allowable contact resistance must be kept below 200 milliohms. The process does change hole size. While these changes can be large (up to 0.008 inch [203.2 pm]) on small holes in soft material, the process is relatively repeatable. Several studies of these size increases have been reported.3"6 Despite the indicated limitations, this process is one of the few which provides the quality normally desired at hole intersec- tions. Equipment costs are $25,000 minimum for fully automated rn-achines. Simpler machines are available for much less than this cost * Several job shop deburring facilities also use this process. Abrasive Flow Debldrring Using Water as a Carrier- In the previous process a putty-like silicone rubber is generally used to carry the abrasive particles over edges. At least two deburring facilities have used water rather than silicone rubber. By this method, abrasive is suspended in a tank of water by means of constant agitation and then. forced through the part. A simple laboratory unit, which is also suitable for production, can be built for 250 dollars.
-1-F This process is not as fast as conventional abrasive flow &burring nor will it remove as many large burrs, but it does work, and has been used on gyroscope components having miniature holes, as well as other applications involving small intersecting holes. Electrochemical Deburring Electrochemical deburring is another process which is ideally suited to cross hole intersections. By this method a tool is positioned adjacent to the area to be deburred, and a salt solution is passed by the tool and edge to be deburred, while an electric current flows (Figure 9). In a matter of 5 to 60 seconds most commercial burrs are removed and a smooth radius is produced. Radii of 0.005 to 0.020 inch (127 to 500 pm) can be produced easily. Figure 10 illustrates the effect burr size has on deburring time. The process is automated except for part loading and is simple to use. It does require a fixture for each part. These fixtures can be built for 100 to 3,000 dollars, depending upo:* part complexity and volume of parts to be produced. This process has been used to deburr 0.040 inch (1 mm) diameter holes and can be used for smaller holes. The size of the access hole (d deburre 8 Figure 11) limits the size of holes which can be however. As seen in Figure 11, the access hole must be at least'0.020 inch (0.508 mm) in diameter to provide a suitable gap for electrolyte flow. This is still not a feasible working diameter because it assumes a zero diameter electrode. In most situations, the electrode stem must be at least 0.010 inch (0.25 mm) in diameter and this must have an insulating coating of at least 0.002 inch (50.8 pm). Thus, in most applications it is necessary to have an access hole of at least 0.034 inch (0.86 mm). If the intersection is at some depth, a stiffer stem would be desired to prevent damage during loading. This process normally leaves eitiler a white or a black residue on parts. This is readily removed by miniature brushes or blasting with a soft abrasive. Size changes only occur at the edge of parts if the tooling is designed correctly. Figure 12 illu- ' strates the size change which occurred at the edge of one 120 dp miniature gear. In this case 0.001 inch (25.4 pm) of material was removed for a distance of up to 0.005 inch (127,tf pm) from the edge. While a near sharp edge was left in this application, an observable radius is produced in most applications, This process is applicable to most steels, stainless steels, and aluminum. Materials such as sintered tungsten cannot be used in this process without special chemicals. Some titanium alloys can be deburred but the results may not be either asthetic or repeat- able. The literature essentia&ly ignores the subject of workpiece metals applicable to this process,
-18- DC POWER SOURCE CONTACT POINT COMBINATION CLAMP AND ELECTRICAL CONTACT WORKPIECE J FIXTURE rc ELECTROLYTE RESERVOIR FLOW CONTR F!L?ER PUMP Figure 9. Schematic of Electrochemical D&w-ring System I
-19- 6 .Dl5” i (381 mm) I- .QlO” kj (-254 mm) ifi > Q .OQ5” f.127 mm) AVERAGE ECD BURR REMOVAL RATE FOR IRON BASE ALLOYS / / 10 20 ,30 40 DEBURRING CYCLE TllME IN SECONDS Figure 10. Burr Removal for Iron Base Alloys Other Automated Processes While other processes mentioned previously can be used for inter- secting holes, hole sizes are generally affected. The reader is encouraged to review Reference 1 for basic details about these processes. Manual Deburring Most manufacturers rely on some form of manual effort to remove burrs or verify that intersecting holes are in fact burr free. The success of this approach is, of course, a function of worker conscientiousness, hole geometry, and time allowed. It is also expensive. It does, however, have several advantages. It doesn't tie up equipment required for precision machining. -e It can concentrate all action at edges. /
TRODE INSULATED STEM ’ ~~~~&~S’BASE D 2 d2 + P(GAP) GAP = Q.010 TO 0.030 INCH (0.25 TO 0.75 mm) Figure 11. Relationship Between Tool, Process, and Part Diameters Q It does not require tooling, tape, and machine available lead time (short turn around time for small. quantities). e It does not normally require expensive holding fixtures. Q It can quickly re-orient parts with respect to tools since no fixtures are involved (an N/C approach may require 5 axes). It adapts to all edge conditions (versus machine pre- programmed motion). It can accommodate tool shortages by substituting tools or technique. Manual or hand deburring involves many approaches and an esti- mated 10,000 tools are commercially available to assist in this 22
Figure 12. Configuration of an Edge After Electrochemical Deburring process. As discussed elsewhere, there are 19 basic types of hand deburring tools and 60 subcategories,' Because of the geometry of intersecting holes, many of the hand deburring tools are not useful in this application. Those tools which are normally used for this application include Drills Bur Balls Abrasive Filled Rubber Dental Points Abrasive Filled Nylon Fibers Pin Vises, and Reamers Cross Hole Deburring Brushes Abrasive Cord and Tape Mounted Dental Points Miniature Hand Stones Dental Motors c 23
-22- j. ln most applications, reamers are used first to cut out the large b~mr and insofar as possible leave a sharp edge. When burrs are too thick for reamers to remove by hand, a drill is used. The helix angle on drills reduces the initial torque to shear the burrs. If the largest hole is 0.0625 inch (1.57 mm) or larger, then a bur ball is normally used to chamfer or break the edges of the intersection. While bur balls can be manufactured as small as 0.004 inch (101.6 pm), the shanks are normally much larger, which may present a problem when two very small holes intersect. As seen in Figure 13, the relationship between absolute minimum size of the larger hole must be, D4>d3/2 + L2tanO + d4 - ZOsO For a standard commercial bur ball of 0.040 inch (0.1 mm) dia- meter and an intersection depth of 0.5 inch (12.7 mm), the minimum large hole size would be 0.101 inch (2.56 mm). Actually, this calculated size is considerably smaller than required, because the tooth pattern on most balls is designed to cut with the end of the ball rather than the side (Figure 14). In prac- tice, the ball diameter should be twice the diameter of the hole to be deburred to provide an approximate 45 degree chamfer. In addition, the number of teeth on these tools can vary signifi- cantly between suppliers. when this orientation of tooth cut is a problem, some users resort to ball-shaped mounted dental stones. These are produced commercially in several grades of hardness in sizes down to 0.0938 inch (2.38 mm) diameter. Bur balls and miniature mounted stones will in themselves produce burrs or, at the minimum, leave chamfered but sharp edges. When these are not allowable, the standard practice is to use minia- ture cross hole deburring brushes, abrasive filled rubber dental points, abrasive cord, or tape or abrasive filled nylon fibers to provide a radius or blend at the intersection. The cross hole deburring brushes are stainless steel tube.brushes available in sizes down to 0.024 inch (0.61 mm). While these do not provide a smooth blend, they do remove any fine burrs and most loose particles. Figure 15 illustrates the size of these brushes. Abrasive filled rubber dental points represent the fastest manual approach to provide radiused edges of small holes (Figure 16). These are available in at least two grades of aggressiveness and three shank configurations. Because of the limited shank length, they cannot be used on intersections deeper than 0.5 inch (12.7 mm) unless the access hole is 0.875 inch (22.2 mm) or larger (Figure 17). If a sharp burr exists when these are used, it will cut the rubber point and destroy its usefulness in a matter of a few revolutions. These tools are available from most --.
TYWCAL BUR BALL DIMENSIONS Ll ,,,,;; (,NC$ (INC:; (INCH) (DEGREES)~-~ - - 0.09380.040 .020 1.75 5 0.020 a16 0.062 0.75 5 INDUSTRIAL DENTAL MINIMUM HOLE SIZE D4mw=d3+LsTANB+d4 2 2COSB Figure 13. Bur Ball Dimensions Related to Intersecting Hole Locations and Size dental supply houses, as are a number of mounted dental stones, diamond plated dental stones, and dental rotary burs. In some situations the use of an abrasive string or cord provides satisfactory radiusing. These products are available in dia- meters down to 0.012 inch (0.30 mm) and their limpness allows them to be used in a variety of unusual geometries. The abrasive does not last long in this application, however, when used on stainless steel parts. In use, the string is drawn back and. forth over the hole intersection to blend the edge (Figure 18). The fibers in abrasive filled nylon brushes have some excellent characteristics for deburring holes. In this application, a radial brush is disassembled to yield 50 or 100 fibers each 0.5 to 1 inch (12.7 to 25.4 mm) long (Figure 19). One of these fibers is trimmed to provide a pointed end. This fiber is then inserted +.n a bench motor to provide a long wearing and abrasive chamfering tool. This approach has been used to provide radii on holes 0.007 to 0.030 inch (0.178 to 0.75 mm). 25
-24- Figure 14. Cup Bur for Finishing Ends of Rods These same fibers are used to polish 0.020 inch (0.5 mm) diameter holes. By using a long fiber and extending it through the hole intersection and rotating the motor fast enough, the fiber will whip. This whipping will help the abrading force, Several adaptations of this approach have been used on holes 0.5 inch (12.7 mm) or larger. Instead of a single nylon fiber, a cluster of very long steel wires will accomplish the same result. One company makes a similar tool using a cluster of fibers, each having a ball of tungsten carbide bonded to the fiber end (Figure 20). This ball adds weight and thus force to the abra- sion by fiber and the carbide ball is in itself a very abrasive material. By replacing the carbide ball with hard metal stars, a much more aggressive action is possible (Figure 21). In some cases the use of a tube type brush with abrasive silicon carbide balls on the fiber ends will accomplish the same result faster (Figure 22). This type tool is commonly used to hone automotive engine and brake cylinder walls. These brush-like tools are available as standards in diameters as low as 0.25 inch (6.4 mm). A simple similar approach is to use narrow strips of abrasive paper looped through a slotted mandrel (Figure 23). This pseudo flap wheel can be individually tailored to each part at little cost. When large radii are not required, the nonwoven abrasive filled nylon wheels can be used. These products can be cut with
-25” Figure 15. Cross Hole Deburring Brushes scissors to provide any diameter "brusht' desired (Figure 24). If the access hole is 0.5 inch (12.7 mm) diameter or larger, pencil thin air motors and dental motors can be used with a variety of abrasive rubber wheels or stones (Figures 25, 26, and 27). The cross hole deburring brushes and the mounted poLnts will scratch hole walls. In most situations these shou!j not worsen 16 microinch (0.4% pm) finishes. The abrasiv$? fi;..Led rubber dental tools and the abrasive filled nylon fibeis will polish to less than 8 microinches (0.20 pm). Although not generally known, hand stones are available in diameters down to 0.084 inch (2.38 mm). These stones are excellent for many hard-to-reach places on small parts. As shown in Figure 28, these stones are made in a variety of shapes and one is only 0.010 inch (0.25 mm) thick. 27
-26- Figure 16. Abrasive Filled Rubber Dental Points Economic deburring almost demands the use of motorized tools. For miniature parts, and intricate or hard-to-reach areas, this will normally necessitate the use of dental motors, As seen in Figure 17, these can be inserted in holes 0.875 inch (22.2 mm) in diameter. They are available with speeds of 6000 to 400,000 rpm and with integral air or water coolant on tools. Some even have a built-in light aimed at the tool. A variety of interchangeable handpieces are available. An estimated 500 different dental cutter or stones are used in these tools. Verification that intersections are burr free can be as difficult as the deburring itself. One company reportedly spends $500 per month on miniature light bulbs just to inspect for burrs in these hard-to-see areas. Table 5 lists some of the size limitations on some of the more commonly used deburring and inspection tools. The economics of the hand deburring approaches discussed have apparently only been evaluated on individual piece parts and not reported in the technical literature. Figures 29 and 30 illu- strate the effect of access hole size on deburring time for some difficult-to-reach intersection. 28
-27- Figure 17. Rubber Dental Point in Right Angle Dental Motor NONDEBURRINGAPPROACHESTO REDUCINGCOSTS The deburring approach represents only one-fourth of the approach to minimizing intersecting hole deburring costs. There are other aspects: e Designing to Prevent the Problem, 8 Preventing Burr Formation, and Q Minimizing Burr Size. Design Approaches While each part has its own design constraints, there are some potential design alternatives: Design cast or molded holes having generous radii, Design laminated structures which can be disassembled for deburring and inspection, Design coined laminated assemblies, 29
-28- ? Figure 18. Abrasive-Coated Cord Used to Deburr Intersecting Holes Figure 19. Brush Disassembled to Provide Individual Fibers A4 30
Figure 20. Abrasive Balls on Long Flexible Fibers Figure 21. Wintered Tungsten Carbide Stars on Stainless Steel Cable for Heavy Duty Cutting e Drill holes through then plug to provide blind holes, e Design large counterbore in blind end for access then plug in assembly, Utilize the largest hole size possible, Utilize the largest hole tolerances possible, and Utilize nonductile workpiece materials. Burr Prevention As discussed in References 1 and 2, burr prevention is a sound practice, but it is essentially limited to the use of electro- chemical machining processes. They can produce holes as small as 31
-3o- Figure 22. Abrasive Balls on End of Nylon Fibers Hone and Deburr 0.010 inch (0.25 mm) and can produce curved and noncircular cross sections. These processes typically produce radii rather than sharp edges at intersections. Unfortunately these processes are not applicable to all materials or to small job shop quantities. Burr Minimization Several approaches are used to minimize the size of burrs at hole intersections. This minimization may eliminate the need to perform deburring, or at least reduce deburring time, APPROACHESTO BURR MINIMIZATION Q, Utilize appropriate drill geometry Utilize appropriate drill feedrate Utilize appropriate drill sharpness Utilize appropriate drilling sequence Fill the first bole with a removable backup material Ream the holes 32
Figure 23. Pseudo Flap Wheel The subject of appropriate drilling conditions for burr minimiza- tion has been the subject of many research studies.la2 In all cases helix angle, point angle, feedrate, and sharpness play a major and controllable role in burr size. In any cross hole situations, the manufacturing engineer needs to practice the concepts outlined in the indicated references. Sacrificial backup material has been used often for minimizing burrs, In cress hole situations, its use is somewhat limited because its removal may be more difficult than burr removal. Low melting metal alloys, plastic alloys, and special metal pins or tubes are used in these applications. Low melting point alloys have been very effective in some applications and useless in others. In some instances these materials adhere too tightly to allow complete removal, In other cases they are simply too soft to provide adequate support. Figure 31 illustrates the effect of backup material hardness on both burr thickness and height. Harder backup obviously minimizes burr size, Frequently the larger hole is drilled first. In this case it is possible to plug this hole with a solid pin and then drill the cross hole (Figure 32). If the pin fits tightly in the hole and is as hard as the workpiece, then the resulting burrs must be very small. If the pin is significantly smaller than its hole (more than 0.002 inch 150.8 pm) difference for example) or -0 33
-32- Figure 24. Cut Abrasive Filled Nonwoven Nylon to Fit Hole Size softer, large burrs can occur. When large burrs form it may be difficult to withdraw the pin because the burr acts as a locking feature. If the intersecting holes are the same size, then the drill will sever the sacrificial rod and a portion of it will be left in the hole. by acid etching. This piece can only y removed in many cases Burrs are the result of ductile deformations. When the workpiece is not ductile, only small burrs can, form, In many cases a permanent or temporary heat treat operation will l‘educe the ductility. CONCLUSION While many approaches have been described in this report, in many instances deburring intersecting holes can be difficult. Success 34 -- --~ --_.-.. .~.._.--- .-----__.~
3c,
Figure 26. Dental Tools Used in Deburring requires both in-depth analysis and experimentation. size, tolerances, and allowable roughness decrease, culty increases dramatically. As hole the diffi- 36
Figure 27. Dental Tools Used in Dental Air Motor 37
Figure 28. Miniature Hand Stones
-37- . . ..- Table 5. Dimensional Limitations of Some Deburring and Inspection Tools Diameter of Large Hole (inch) Maximum Depth Tool/Press is 0.01 0.1 1.0 Effective (inch) Dental Motors 4.0 Hand Held Tools 6.0 Abrasive String 36.0 Dental Mirrors 6.0 Sight Pipes 12.0 Borescope 18.0 Otoscope 6.0 Fiber Optic Displays 36.0 39
-3% 0.062-IN.-DIA HOLE, 4 PLcs (I.57 mm) / / (25.4 mm) 1.00 INCH (TYPICAL)-W / Lxc // (BLIND HOLE) HOLE DIAMETER VARIED FROM 0.062 TO 0.5o9 IN. (1.57 TO 12.7 mm) DEBURR HOLE N r#iERSECTIONS 4 PLCS. 3,002-IN. (0.051 mm) MAX BREAK Figure 29. Geometry of Intersecting Hole Specimen
-39- DIAHETErl OF CQOSSHOLE (mm) 0 2.5b 5. I 7.6 10.2 12.7 15.2 - - ..-. ..--c- _._____- T' _._..__.- ,_ ---T"y----T- -1-1 7 I I I I I I 0 3. iO0 0.200 0.300 0. uoo 0.500 0.600 DIAMETER OF CROSSHOLE (IN.) Figure 30. Effect of Cross-Hole Diameter on Deburring Time . 41
-4% 203.2 177.8 ‘i52.4 y 127.0 3 b- x * 76.2 50.8 25.4 303Se STAINLESS STEEL WORKPIECE 008 006 0 10 20 30 40 50 BACKUP MATERIAL HARDNESS (RC) Figure 31. Effect>f Backup Material Hardness on Burr Size . 42
-41- ROD OR OTHER BACKUP Figure 32. Fill First Hole With Backup to Provide Near Burr Free, Near Sharp Edges at Hole Intersections 43
‘42- REFERENCES l- L, K. Gillespie (ed), Advances in Deburring, SME, 1978.- "L. K, Gillespie (ed), Deburring Capabilities and Limitations, SME, 1976. "L. K0 Gillespie, Extrude Hone Deburring with X-Base Media, Bendix Kansas City Report BDX-613-1546, March 1976 (Amble from NTIS). 'Julius Brothers, "Production Research Observations on Abrasive Flow Deburring," SME paper MR77-438, 1977. 5Takashi Miyatani, Jungi Nakata, Tameyasu Tsukada, and Koya Takazawa, "Influence of Finishing Conditions on Work Accuracy by Extrude Hone Process," SME paper MR77-437, 1977. 'L. Michael Heglin, Laser Drillingfi General Electric Company, Aircraft Engine Group, Report AEml-TR-75-44, 1975. 7L. K. Gillespie, A Training Manual for Precision Hand Deburring, Part 1, Bendix Kansas City Report BD-13,2400, May 1980 (Available from NTIS). *L.'K. Gillespie, "Deburring Precision Miniature Parts," Precision Engineering, Volume 1, Number 4, October 1979, pp. 89-198. 44

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Deburr small-intersecting-holes

  • 1.
    MM . TE IQ MSMR ‘CM PE @(All Rights Resewedl SMALLlNTERSECTINGHOLES BY LaRouxK. Gillespie StaffEngineer BendixCorporation ABSTRACT Beburring intersecting holesis oneof the mostdifficult deburring tasksfacedbymanyindustries. Only 14of the 37majordeburring processesare applicableto mostinter- sectingholeapplications. Onlyfive of theseare normally applicableto smallor miniature holes. This paper summarizes sic processcapabilitiesandtechniques used asa function of holesizesandintersection depths.
  • 2.
    ABSTRACT Deburring intersecting holesis one of the most difficult deburring tasks faced by many industries. Only 14 of the 37 major deburring processes are applicable to most intersecting hole applications. Only five of these are normally applicable to small or miniature holes. This paper summarizes basic process capabilities and techniques used as a function of hole sizes and intersection depths. INTRODUCTION Effective deburring of intersecting holes is a function of many variables. For example, the following must be considered in any analysis of the 'best' deburring techniques. Hole Sizes Hole Tolerances Hole Surface Finishes Type r,E Holes (Blind or Thru) Initial Edge Condition Material Compatibility (Deburring Side Effects) Part Schedules Depth of Intersection Wall Thicknesses Desired Edge Condition Workpiece Material Quantity of Intersections Accessibility of Intersection Few deburring processes will &burr small holes. As hole tolerance requirements increase and surface finish needs become finer, many deburring processes are eliminated from potential consideration. Welding, plating, and soldering needs eliminate those processes which impregnate abrasive particles, and require- ments to have very small or very precision edge radii further eliminate processes from consideration. PROCESSLIMITATIONS As seen in Tables 1 and 2, if the deburring process is allowed to change the hole size by 0.001 inch (25.4 pm> and the resulting edge is allowed to have up to 0.010 inch (254 pm) radius or chamfer, then five of the deburring processes may reasonably be expected to deburr hole intersections. Table 1 assumes however, that the hole has and requires a surface finish of 32 microinch (0.81 pm), an initial burr size equivalent to 0.003 inch thick by 0.003 inch high (76.2 x 76.2 pm) 303 stainless steel, and that at least one of the intersecting holes is 0,5-in& in diameter (12.7 mm). It further assuIy1es that all the burrs and associated raised metal must be removed. As the allowable hole size change or maximum edge break is reduced, begin dropping out as feasible processes quickly realistic possibilities. Similarly, as
  • 3.
  • 4.
    -5 Table 2. CodesUsed in Table 1 Code Deburring Process A B C D E F G H I J K L M N 0 P & R S T U v W X Vibratory Barrel Tumbling Spindle Finishing Centrifugal Barrel Finishing Abrasive Jet Sanding Brushing Hand Abrasive Flow Mechanized Mechanical Thermal Energy Chemical Electrochemical Electropolish Ultrasonic Torch or Flame Yelting Water Jet Electrochemical Vibratory Electrochemical Brush Chemical Vibratory Liquid Hone Chlorine Gas Magnetic Loose Abrasive Plasma shown in the right half of Table 1, when the allowable standard deviation or repeatability of hole size change decreases, the number of feasible deburring processes diminishes. In some cases the indicated burr size is the major reason a process is not feasible. In others, the material and process are not compatible, In most cases, however, it is the combination of material, burr size, diameter change, and allowable final edge break th,t limits process capability.
  • 5.
    -4- Two subtle pointsare contained in Table 1. Obviously, some parts are produced with 0.003 inch (76.2 pm) burrs and require- ments of not changing hole size more than 0.0001 inch and not exceeding edge chamfers of 0.001 inch (25.4 pm). The manufac- turers of these parts do remove the burrs, even though Table 1 indicates no process will do so. Table 1 defines the capa- bilities of individual pro:esses, In difficult situations nanufacturers, by necessity, must combine processes. Thus, by first reaming the hole to make the burrs smaller and then using one of the mechanized deburring processes, it is possible to meet the indicated constraints. Another table of capabilities then exists in principle {although not yet in print) which describes deburring process capabilities for these smaller burrs. Similar tables also could be constructed for different materials, surface finish, and hole size. Table 3 illustrates some basic process capabilities when there is no concern for anything other than the size change resulting from deburring. The second subtle point in Table 1 is that resulting edge breaks or chamfers are in part a function of edge geometry. Consider Figure 1. As seen there, when two holes intersect, the angles formed by the intersection vary from acute to obtuse angles. The diameter and nature of the intersection (center line to center line or just slight breakthrough) dictate the variation in angles. The effect of these variations in angles can be appreciated by viewing Figure 2. To produce a 0.005 inch (127 pm) edge break on a 90 degree edge requires removing 0.0021 inch (53.3 pm) of stock material. On a 120 degree edge, the same stock removal would produce a 0.015 inch (381 pm> radius, and on a 30 degree edge the resulting radius would be approximately 0.0005 inch (12.7 pm). The mechanized processes basically try to produce the same stock loss all over. As a result, on the three dimensional inter- section shown in Figure 1, considerable variation will exist in edge break, chamfer, or radius as the line of edge intersections is traversed. Figure 3 illustrates how edge angles on exterior surfaces are affected by vibratory deburring. The net result of all these observations is that for cost effec- tive deburring it is necessary to minimize burr size, allow the maximum possible hole size change, and allow edge breaks of at least 0.0'; inch (127 pm). PROCESSESTYPICALLY USED ON HOLES SMALLERTHAN 0.060 INCH (1.5 mm} The following list summarizes the processes now in commercial or potential use for intersecting holes smaller than 0.060 inch (1.5 mm) in diameter.
  • 6.
    -5- Table 3. BasicCapabilities of &burring Processes--Stock Loss* Typical Working Range Stock Loss (p) (Inch) Beburring Process 0.25 2.54 25.40 254.00 (0.00001) (0.0001) (0.001) (0.010) ABRASIVE Abrasive Jet Abrasive Flow Semisolid Carriers Liquid Carriers Water Jet Loose Abrasive Barrel Centrifugal Barrel Magnetic Spindle Finishing Vibratory Recipro Finishing Flow Finishing Orboresonant Sanding (Edges Only) MECHANICAL Mechanized Hand Brushing THERMAL Flame Thermal Energy (TEM) Plasma Hot Wire EDM Resistance Heating
  • 7.
    -6- Table 3 Continued.Basic Capabilities of Deburring Processes-- Stock Loss* - Typical Working Range Stock Loss (p) (Inch) Deburring Process 0.25 2.54 25.40 254.00 (0.00001) (0.0001) (0.001) (0.010) CHEMICAL Cnemical Ultrasonic Chlorine Gas Chemical Loose Abrasive Barrel Centrifugal Barrel Magnetic Loose Abrasive Spindle Finishiqr Vibratory ELECTROCHEMICAL Electrochemical Electropolish Electrochemical Brush Electrochemical Loose Abrasive Barrel Centrifugal Barrel Spindle Vibratory I *Based on the removal of 76.2-pm-thick (0.003 in.) burrs from the edges of 303SE stainless steel directly exposed to the deburring process. These capability estimates assume that surface finishes of 0.81 pm (32 pin.) are required and exist on the workpiece prior to deburring. Stock losses are defined as size changes which occur on exposed surfaces; the values are overall size changes of diameters or thickesses. These estimates assume a maximum allowable edge break of 254 pm (0,010 in.), Less stock loss will occur if smaller burrs are present.
  • 8.
    Dl Cl 81 A! VIEW SEEN LOOKINGSIDEVVAYS Figure I. Three-Dimensional Configuraticn of Hole Intersections
  • 9.
    RADIUS DIFFICULT TYPICAL RADIUS 3 I------’ T--‘-.“-‘--- --, IRADIUS S I ‘T -.__0.002 50.8 0.003 76.2 0.005 127.0 I 0.0150.010 254.0381.0 - 0.0003 o.ooou Ii*: --l---l 0.0007 17:a 0.0015 38.1 0.0022 55.9 LESS DIFFICULT * STOCK REMOVALREQUIRED TO PRODUCEIWOICATED RADIUS Figure 2. Effect of Geometry on Edge Radiusing In Commercial Use Other Possibilities Thermal Energy Method Chemical Abrasive Jet Electropolish Abrasive Flow Chlorine Gas Electrochemical EDM Manual Resistance Heating . -
  • 10.
    -9- 254.0 228.6 203.2 =$ 177.8 Sd i? 152.4 s z g127.0 wl .g 101.6 isi 76.2 so.8 25.4 0 WORKPIECE HARDNESS: Rc24 -! o.oog - 0.008 - 0.007 z z -- 0.006 Eu - 0.00s 2 ze - 0.004 !g cr - 0.003 2 -- 0.052 1 0.001 0 1 2 3 4 5 TIME IN VIBRATORY FINISHER (HOURS) Figure 3. Effect of Edge Angle and Vibration Time on Edge r Radiusing of Phosphor Bronze Workpiece Thermal Energy Method The thermal energy method (TEM) utilizes the high temperature short duration shock wave, accompanyin Q xhe detonation of two gases to vaporize or burn off burrs," The fact that it uses gas as a removal tool makes it particularly applicable to many hard-to-reach areas. In this case, the ability of the gas to conform to almost any shape implies that the combustion heat wave will attack the burr. This is one of the fastest processes available for deburring and should be considered for any intersecting hole applicatiokls. It currently is used to deburr the majority af the carburetors produced in the United States. The method has been used to produce radii of 0.002 to 0.060 inch (0.05 to 1.5 mm) on steel parts and 0.002 to 0.010 inch (0.05 to 0,225 mm) on aluminum parts. For uniform initial conditions, edge radii should be crnsistent within k0.002 inch (250 pm). The process has some limitations: c
  • 11.
    -16- g The burrmust be 15 times thinner than other walls or features; 0 Complete deburricg may not occur on some features; and 0 It is not effective on many materials. While a TEM installation is expensive, the cost for each part can be as low as $0.05. Deburring job shops using this process are available in some parts of the country. The best applications to date appear to be for steel, zinc, and some aluminum and brass materials. Abrasive Jet Deburring Miniature abrasive jets are used frequently for deburring complex intersecting holes. These miniature blasters (Figure 4) have nozzle openings as small as 0.003 inch (76.2 pm) in diameter. By varying the nozzle diameter, nozzle configuration, and nozzle tip-to-part distance, a wide variety of deburring conditions can be met. Figure, 5 illustrates the effect of nozzle-to-part distance, 2nd Figure 6 illustrates some of the nozzle variations in commercial use. This process has been used to deburr hypodermic needles having 0.028 inch (0.71 mm) openings, intersections of miniature slots and threads, intersecting holes, and external features. In most applications, deburring by this process requires less than 1 minute for each intersecting hole.' Intersecting holes and holes intersecting threads represent two areas of particular noteworthy applications of this process. Threads which have been machined through are a particularly difficult deburring problem. Because the blasting process d conforms to feature contours, this process is particularly applicable to such features. The process is limited by the depth of the intersection and size of holes to be deburred (Figure 7). Table 4 illustrates some of these limitations based on existing nozzle size. In this instance, it is assumed that deburring action is ineffective on metals when the nozzle is more than 0.5 inch (12.7 mm) from the intersection. If burrs are thin enough, this assumption might be overly restrictive. In addition to the process variables already mentioned, the aggressiveness of the process can be increased by changing abrasive material, abrasive size, the ratio of abrasive to air, and the air pressure. Some miniature blasters utilize pressures up to 190 psi (1.31 MPa). This is twice the pressure utilized by some units and this 2:l difference results in four times faster cutting. 12
  • 12.
    Figure 4. MiniatureBlasting Machine Any blasting process affects surface finish, but in most situa- tions it is the textul-e rather than the average roughness which changes, This process can readily deburr many parts and not worsen a 16 microinch (0.4 pm) finish. Complete burr removal is a function of time, the variables described, and initial burr size. In most instances, however, burr size can be controlled by a reaming operation to remove heavy burrs prior to blasting. While the process is most applicable to hard metals, 303 se stainless steel (l&30) can be easily deburred by this approach. Since media size is typically 25 pm (0.001 inch) or larger the process probably would not be used on cross holes smaller than 0.005 inch. This process is normally a hand held operation, but it is possible to fixture the operation or combine it with one of the miniature robots.
  • 13.
    -12- 5.00 0.64-----MM------ ---- : I i 10.00 1.50---------------.w.-- II 2.00-1----1----w---- DIAMETE 7-4 TIP OF CUT DISTANCE (MM) Figure 5. Cutting Action of 0.018 Inch (0.46 mm> Diameter Nozzle Figure 6. Commercially Available Nozzle Variations With Side and End Views.
  • 14.
    A RD COMMERCIAL NOZZLE c SPECIALNOZZLE EXTENSIQN Figure 7. Examples of Nozzle Placement 6 STANDARD COMMERCDAL NOZZLE D ENTIRE RIGHT ANGLE HEAD COMPLETELY iNSERTED IN HOLE Equipment costs are approxiately $1500 (1979) and media costs are frequently on the order of 10 to 25 dollars per day. Conventional Abrasive Flow Deburring The abrasive flow deburring prooess is a method of abrading surfaces by extruding a viscous, semisolid, abrasive-laden media across areas or surfaces. In order to contain and direct the flow of media, the workpiece is confined within a fixture between the two vertically opposed media chambers (or cylinders) of the machine (Figure 8). Under hydraulic pressure, the media is extruded from one media chamber through the fixtured workpiece and into the cpposite media chamber. In most applications, the media is then extruded back through the part into the original
  • 15.
    -14- ? /T2LcDsl.e4. ApproximateHole Diameter and Depth Limitations of Miniature Blasting Nozzles* Smallest Access Maximum Hole Diameter Depth of Nozzle Allowable Cross Hole Approach** (inch) (mm) (inch) (mm) A 0.040 1 0.250 6.4 B 0.155 3.9 0.575 14.6 C 0.040 1 2.00*** 50 D 0.400 10 l.OO*** 25 *This table assumes that the maximum effective- ness of the process is lost when the nozzle is more than 0.5 inch (12.7 mm) from the area to be deburred. **See Figure 7 for Explanation of Nozzle Approach. ***Nozzles this size are in use; actual maximum is probably larger. media chamber. The amount of abrasion that is accomplished with a given media formulation and process pressure is directly related to the quantity of media flow. Depending upon the application, adequate flow quantities range from a few cubic inches to many comp'iete cycles. The process can be thought of as a flowable file with capabili- ties ranging from a light buff to coarse stock removal. By using various compound formulations, machine adjustments, and fixture designs, selective processing can be accomplished on many types of components. A basic principle of this process is the media's ability to be significantly abrasive only at the point of greatest restriction in its flow path. This process is comple2$ly automatic and, as in the case o.f abrasive jet deburring, several variables can be changed to provide aggressiveness. In most applications the deburring will improve surface finish and provide longitudinal flow lines rather than the circumferential marks left by conventional drilling and reaming. This later process byproduct is a major advantage in many cases involving flui flOW*
  • 16.
    -5 R MACHINE CHAMBER ‘TOLOWERIUlACHIRECWILPBER Figure 8. Fixturing for Simple Part With Intersecting Holes Some advantages and disadvantages of this process are listed below. Advantages Ability to deburr all metals High degree of accuracy and repeatability System flexibility Automatic controlled process Selective deburring capability Ability to deburr a wide range of part sizes Easy changeover from one part to another Multipart processing capability Short process cycles Minimal fixture costs Improved surface finish Ability to deburr areas not accessible to normal deburring techniques Can do many holes at one time; ideally suited to cross hole intersections 1
  • 17.
    Disadvantages Requires major capitalequipment Requires hclding fixture Requires cleaning to remove particles and carrier Increases hole size The process has been used in a number of instances to deburr holes of 0.040 inch (1 mm) in diameter or smaller.* Typical cycle times are 30 to 60 seconds. Total deburring and cleaning time per part will be twice to five times this amount in many instances, In one case, 16 small intersecting holes were deburred in a cycle time of 12 seconds. If burrs are small, edge radii can be limited to less than 0.003 inch (76.2 pm). Large radii can also be produced, The process can be used on thin wall components if care is taken. One example cited in Reference 2 involved the deburring of an aluminum part having 180 slots EDM'd through a 0.006 inch (152 pm) wall. Each slot was 0.006 inch wide and 0.060 inch long (152 pm by 1.5 mm). Few material compatibility problems have been observed with this process. The process uses silicone materials and inert abra- sives. Some users, however, note that silicone oils are never totally removed by cleaning processes. These oils tend to migrate, such that a single drop will spread out and cover all surfaces in an assembly. On electromechanical assemblies this can increase the resistance of electrical contacts. While not normally a problem for many users, it can be when allowable contact resistance must be kept below 200 milliohms. The process does change hole size. While these changes can be large (up to 0.008 inch [203.2 pm]) on small holes in soft material, the process is relatively repeatable. Several studies of these size increases have been reported.3"6 Despite the indicated limitations, this process is one of the few which provides the quality normally desired at hole intersec- tions. Equipment costs are $25,000 minimum for fully automated rn-achines. Simpler machines are available for much less than this cost * Several job shop deburring facilities also use this process. Abrasive Flow Debldrring Using Water as a Carrier- In the previous process a putty-like silicone rubber is generally used to carry the abrasive particles over edges. At least two deburring facilities have used water rather than silicone rubber. By this method, abrasive is suspended in a tank of water by means of constant agitation and then. forced through the part. A simple laboratory unit, which is also suitable for production, can be built for 250 dollars.
  • 18.
    -1-F This process isnot as fast as conventional abrasive flow &burring nor will it remove as many large burrs, but it does work, and has been used on gyroscope components having miniature holes, as well as other applications involving small intersecting holes. Electrochemical Deburring Electrochemical deburring is another process which is ideally suited to cross hole intersections. By this method a tool is positioned adjacent to the area to be deburred, and a salt solution is passed by the tool and edge to be deburred, while an electric current flows (Figure 9). In a matter of 5 to 60 seconds most commercial burrs are removed and a smooth radius is produced. Radii of 0.005 to 0.020 inch (127 to 500 pm) can be produced easily. Figure 10 illustrates the effect burr size has on deburring time. The process is automated except for part loading and is simple to use. It does require a fixture for each part. These fixtures can be built for 100 to 3,000 dollars, depending upo:* part complexity and volume of parts to be produced. This process has been used to deburr 0.040 inch (1 mm) diameter holes and can be used for smaller holes. The size of the access hole (d deburre 8 Figure 11) limits the size of holes which can be however. As seen in Figure 11, the access hole must be at least'0.020 inch (0.508 mm) in diameter to provide a suitable gap for electrolyte flow. This is still not a feasible working diameter because it assumes a zero diameter electrode. In most situations, the electrode stem must be at least 0.010 inch (0.25 mm) in diameter and this must have an insulating coating of at least 0.002 inch (50.8 pm). Thus, in most applications it is necessary to have an access hole of at least 0.034 inch (0.86 mm). If the intersection is at some depth, a stiffer stem would be desired to prevent damage during loading. This process normally leaves eitiler a white or a black residue on parts. This is readily removed by miniature brushes or blasting with a soft abrasive. Size changes only occur at the edge of parts if the tooling is designed correctly. Figure 12 illu- ' strates the size change which occurred at the edge of one 120 dp miniature gear. In this case 0.001 inch (25.4 pm) of material was removed for a distance of up to 0.005 inch (127,tf pm) from the edge. While a near sharp edge was left in this application, an observable radius is produced in most applications, This process is applicable to most steels, stainless steels, and aluminum. Materials such as sintered tungsten cannot be used in this process without special chemicals. Some titanium alloys can be deburred but the results may not be either asthetic or repeat- able. The literature essentia&ly ignores the subject of workpiece metals applicable to this process,
  • 19.
    -18- DC POWER SOURCE CONTACTPOINT COMBINATION CLAMP AND ELECTRICAL CONTACT WORKPIECE J FIXTURE rc ELECTROLYTE RESERVOIR FLOW CONTR F!L?ER PUMP Figure 9. Schematic of Electrochemical D&w-ring System I
  • 20.
    -19- 6 .Dl5” i (381 mm) I-.QlO” kj (-254 mm) ifi > Q .OQ5” f.127 mm) AVERAGE ECD BURR REMOVAL RATE FOR IRON BASE ALLOYS / / 10 20 ,30 40 DEBURRING CYCLE TllME IN SECONDS Figure 10. Burr Removal for Iron Base Alloys Other Automated Processes While other processes mentioned previously can be used for inter- secting holes, hole sizes are generally affected. The reader is encouraged to review Reference 1 for basic details about these processes. Manual Deburring Most manufacturers rely on some form of manual effort to remove burrs or verify that intersecting holes are in fact burr free. The success of this approach is, of course, a function of worker conscientiousness, hole geometry, and time allowed. It is also expensive. It does, however, have several advantages. It doesn't tie up equipment required for precision machining. -e It can concentrate all action at edges. /
  • 21.
    TRODE INSULATED STEM ’~~~~&~S’BASE D 2 d2 + P(GAP) GAP = Q.010 TO 0.030 INCH (0.25 TO 0.75 mm) Figure 11. Relationship Between Tool, Process, and Part Diameters Q It does not require tooling, tape, and machine available lead time (short turn around time for small. quantities). e It does not normally require expensive holding fixtures. Q It can quickly re-orient parts with respect to tools since no fixtures are involved (an N/C approach may require 5 axes). It adapts to all edge conditions (versus machine pre- programmed motion). It can accommodate tool shortages by substituting tools or technique. Manual or hand deburring involves many approaches and an esti- mated 10,000 tools are commercially available to assist in this 22
  • 22.
    Figure 12. Configurationof an Edge After Electrochemical Deburring process. As discussed elsewhere, there are 19 basic types of hand deburring tools and 60 subcategories,' Because of the geometry of intersecting holes, many of the hand deburring tools are not useful in this application. Those tools which are normally used for this application include Drills Bur Balls Abrasive Filled Rubber Dental Points Abrasive Filled Nylon Fibers Pin Vises, and Reamers Cross Hole Deburring Brushes Abrasive Cord and Tape Mounted Dental Points Miniature Hand Stones Dental Motors c 23
  • 23.
    -22- j. ln most applications,reamers are used first to cut out the large b~mr and insofar as possible leave a sharp edge. When burrs are too thick for reamers to remove by hand, a drill is used. The helix angle on drills reduces the initial torque to shear the burrs. If the largest hole is 0.0625 inch (1.57 mm) or larger, then a bur ball is normally used to chamfer or break the edges of the intersection. While bur balls can be manufactured as small as 0.004 inch (101.6 pm), the shanks are normally much larger, which may present a problem when two very small holes intersect. As seen in Figure 13, the relationship between absolute minimum size of the larger hole must be, D4>d3/2 + L2tanO + d4 - ZOsO For a standard commercial bur ball of 0.040 inch (0.1 mm) dia- meter and an intersection depth of 0.5 inch (12.7 mm), the minimum large hole size would be 0.101 inch (2.56 mm). Actually, this calculated size is considerably smaller than required, because the tooth pattern on most balls is designed to cut with the end of the ball rather than the side (Figure 14). In prac- tice, the ball diameter should be twice the diameter of the hole to be deburred to provide an approximate 45 degree chamfer. In addition, the number of teeth on these tools can vary signifi- cantly between suppliers. when this orientation of tooth cut is a problem, some users resort to ball-shaped mounted dental stones. These are produced commercially in several grades of hardness in sizes down to 0.0938 inch (2.38 mm) diameter. Bur balls and miniature mounted stones will in themselves produce burrs or, at the minimum, leave chamfered but sharp edges. When these are not allowable, the standard practice is to use minia- ture cross hole deburring brushes, abrasive filled rubber dental points, abrasive cord, or tape or abrasive filled nylon fibers to provide a radius or blend at the intersection. The cross hole deburring brushes are stainless steel tube.brushes available in sizes down to 0.024 inch (0.61 mm). While these do not provide a smooth blend, they do remove any fine burrs and most loose particles. Figure 15 illustrates the size of these brushes. Abrasive filled rubber dental points represent the fastest manual approach to provide radiused edges of small holes (Figure 16). These are available in at least two grades of aggressiveness and three shank configurations. Because of the limited shank length, they cannot be used on intersections deeper than 0.5 inch (12.7 mm) unless the access hole is 0.875 inch (22.2 mm) or larger (Figure 17). If a sharp burr exists when these are used, it will cut the rubber point and destroy its usefulness in a matter of a few revolutions. These tools are available from most --.
  • 24.
    TYWCAL BUR BALLDIMENSIONS Ll ,,,,;; (,NC$ (INC:; (INCH) (DEGREES)~-~ - - 0.09380.040 .020 1.75 5 0.020 a16 0.062 0.75 5 INDUSTRIAL DENTAL MINIMUM HOLE SIZE D4mw=d3+LsTANB+d4 2 2COSB Figure 13. Bur Ball Dimensions Related to Intersecting Hole Locations and Size dental supply houses, as are a number of mounted dental stones, diamond plated dental stones, and dental rotary burs. In some situations the use of an abrasive string or cord provides satisfactory radiusing. These products are available in dia- meters down to 0.012 inch (0.30 mm) and their limpness allows them to be used in a variety of unusual geometries. The abrasive does not last long in this application, however, when used on stainless steel parts. In use, the string is drawn back and. forth over the hole intersection to blend the edge (Figure 18). The fibers in abrasive filled nylon brushes have some excellent characteristics for deburring holes. In this application, a radial brush is disassembled to yield 50 or 100 fibers each 0.5 to 1 inch (12.7 to 25.4 mm) long (Figure 19). One of these fibers is trimmed to provide a pointed end. This fiber is then inserted +.n a bench motor to provide a long wearing and abrasive chamfering tool. This approach has been used to provide radii on holes 0.007 to 0.030 inch (0.178 to 0.75 mm). 25
  • 25.
    -24- Figure 14. CupBur for Finishing Ends of Rods These same fibers are used to polish 0.020 inch (0.5 mm) diameter holes. By using a long fiber and extending it through the hole intersection and rotating the motor fast enough, the fiber will whip. This whipping will help the abrading force, Several adaptations of this approach have been used on holes 0.5 inch (12.7 mm) or larger. Instead of a single nylon fiber, a cluster of very long steel wires will accomplish the same result. One company makes a similar tool using a cluster of fibers, each having a ball of tungsten carbide bonded to the fiber end (Figure 20). This ball adds weight and thus force to the abra- sion by fiber and the carbide ball is in itself a very abrasive material. By replacing the carbide ball with hard metal stars, a much more aggressive action is possible (Figure 21). In some cases the use of a tube type brush with abrasive silicon carbide balls on the fiber ends will accomplish the same result faster (Figure 22). This type tool is commonly used to hone automotive engine and brake cylinder walls. These brush-like tools are available as standards in diameters as low as 0.25 inch (6.4 mm). A simple similar approach is to use narrow strips of abrasive paper looped through a slotted mandrel (Figure 23). This pseudo flap wheel can be individually tailored to each part at little cost. When large radii are not required, the nonwoven abrasive filled nylon wheels can be used. These products can be cut with
  • 26.
    -25” Figure 15. CrossHole Deburring Brushes scissors to provide any diameter "brusht' desired (Figure 24). If the access hole is 0.5 inch (12.7 mm) diameter or larger, pencil thin air motors and dental motors can be used with a variety of abrasive rubber wheels or stones (Figures 25, 26, and 27). The cross hole deburring brushes and the mounted poLnts will scratch hole walls. In most situations these shou!j not worsen 16 microinch (0.4% pm) finishes. The abrasiv$? fi;..Led rubber dental tools and the abrasive filled nylon fibeis will polish to less than 8 microinches (0.20 pm). Although not generally known, hand stones are available in diameters down to 0.084 inch (2.38 mm). These stones are excellent for many hard-to-reach places on small parts. As shown in Figure 28, these stones are made in a variety of shapes and one is only 0.010 inch (0.25 mm) thick. 27
  • 27.
    -26- Figure 16. AbrasiveFilled Rubber Dental Points Economic deburring almost demands the use of motorized tools. For miniature parts, and intricate or hard-to-reach areas, this will normally necessitate the use of dental motors, As seen in Figure 17, these can be inserted in holes 0.875 inch (22.2 mm) in diameter. They are available with speeds of 6000 to 400,000 rpm and with integral air or water coolant on tools. Some even have a built-in light aimed at the tool. A variety of interchangeable handpieces are available. An estimated 500 different dental cutter or stones are used in these tools. Verification that intersections are burr free can be as difficult as the deburring itself. One company reportedly spends $500 per month on miniature light bulbs just to inspect for burrs in these hard-to-see areas. Table 5 lists some of the size limitations on some of the more commonly used deburring and inspection tools. The economics of the hand deburring approaches discussed have apparently only been evaluated on individual piece parts and not reported in the technical literature. Figures 29 and 30 illu- strate the effect of access hole size on deburring time for some difficult-to-reach intersection. 28
  • 28.
    -27- Figure 17. RubberDental Point in Right Angle Dental Motor NONDEBURRINGAPPROACHESTO REDUCINGCOSTS The deburring approach represents only one-fourth of the approach to minimizing intersecting hole deburring costs. There are other aspects: e Designing to Prevent the Problem, 8 Preventing Burr Formation, and Q Minimizing Burr Size. Design Approaches While each part has its own design constraints, there are some potential design alternatives: Design cast or molded holes having generous radii, Design laminated structures which can be disassembled for deburring and inspection, Design coined laminated assemblies, 29
  • 29.
    -28- ? Figure 18. Abrasive-CoatedCord Used to Deburr Intersecting Holes Figure 19. Brush Disassembled to Provide Individual Fibers A4 30
  • 30.
    Figure 20. AbrasiveBalls on Long Flexible Fibers Figure 21. Wintered Tungsten Carbide Stars on Stainless Steel Cable for Heavy Duty Cutting e Drill holes through then plug to provide blind holes, e Design large counterbore in blind end for access then plug in assembly, Utilize the largest hole size possible, Utilize the largest hole tolerances possible, and Utilize nonductile workpiece materials. Burr Prevention As discussed in References 1 and 2, burr prevention is a sound practice, but it is essentially limited to the use of electro- chemical machining processes. They can produce holes as small as 31
  • 31.
    -3o- Figure 22. AbrasiveBalls on End of Nylon Fibers Hone and Deburr 0.010 inch (0.25 mm) and can produce curved and noncircular cross sections. These processes typically produce radii rather than sharp edges at intersections. Unfortunately these processes are not applicable to all materials or to small job shop quantities. Burr Minimization Several approaches are used to minimize the size of burrs at hole intersections. This minimization may eliminate the need to perform deburring, or at least reduce deburring time, APPROACHESTO BURR MINIMIZATION Q, Utilize appropriate drill geometry Utilize appropriate drill feedrate Utilize appropriate drill sharpness Utilize appropriate drilling sequence Fill the first bole with a removable backup material Ream the holes 32
  • 32.
    Figure 23. PseudoFlap Wheel The subject of appropriate drilling conditions for burr minimiza- tion has been the subject of many research studies.la2 In all cases helix angle, point angle, feedrate, and sharpness play a major and controllable role in burr size. In any cross hole situations, the manufacturing engineer needs to practice the concepts outlined in the indicated references. Sacrificial backup material has been used often for minimizing burrs, In cress hole situations, its use is somewhat limited because its removal may be more difficult than burr removal. Low melting metal alloys, plastic alloys, and special metal pins or tubes are used in these applications. Low melting point alloys have been very effective in some applications and useless in others. In some instances these materials adhere too tightly to allow complete removal, In other cases they are simply too soft to provide adequate support. Figure 31 illustrates the effect of backup material hardness on both burr thickness and height. Harder backup obviously minimizes burr size, Frequently the larger hole is drilled first. In this case it is possible to plug this hole with a solid pin and then drill the cross hole (Figure 32). If the pin fits tightly in the hole and is as hard as the workpiece, then the resulting burrs must be very small. If the pin is significantly smaller than its hole (more than 0.002 inch 150.8 pm) difference for example) or -0 33
  • 33.
    -32- Figure 24. CutAbrasive Filled Nonwoven Nylon to Fit Hole Size softer, large burrs can occur. When large burrs form it may be difficult to withdraw the pin because the burr acts as a locking feature. If the intersecting holes are the same size, then the drill will sever the sacrificial rod and a portion of it will be left in the hole. by acid etching. This piece can only y removed in many cases Burrs are the result of ductile deformations. When the workpiece is not ductile, only small burrs can, form, In many cases a permanent or temporary heat treat operation will l‘educe the ductility. CONCLUSION While many approaches have been described in this report, in many instances deburring intersecting holes can be difficult. Success 34 -- --~ --_.-.. .~.._.--- .-----__.~
  • 34.
  • 35.
    Figure 26. DentalTools Used in Deburring requires both in-depth analysis and experimentation. size, tolerances, and allowable roughness decrease, culty increases dramatically. As hole the diffi- 36
  • 36.
    Figure 27. DentalTools Used in Dental Air Motor 37
  • 37.
  • 38.
    -37- . . ..- Table5. Dimensional Limitations of Some Deburring and Inspection Tools Diameter of Large Hole (inch) Maximum Depth Tool/Press is 0.01 0.1 1.0 Effective (inch) Dental Motors 4.0 Hand Held Tools 6.0 Abrasive String 36.0 Dental Mirrors 6.0 Sight Pipes 12.0 Borescope 18.0 Otoscope 6.0 Fiber Optic Displays 36.0 39
  • 39.
    -3% 0.062-IN.-DIA HOLE, 4PLcs (I.57 mm) / / (25.4 mm) 1.00 INCH (TYPICAL)-W / Lxc // (BLIND HOLE) HOLE DIAMETER VARIED FROM 0.062 TO 0.5o9 IN. (1.57 TO 12.7 mm) DEBURR HOLE N r#iERSECTIONS 4 PLCS. 3,002-IN. (0.051 mm) MAX BREAK Figure 29. Geometry of Intersecting Hole Specimen
  • 40.
    -39- DIAHETErl OF CQOSSHOLE(mm) 0 2.5b 5. I 7.6 10.2 12.7 15.2 - - ..-. ..--c- _._____- T' _._..__.- ,_ ---T"y----T- -1-1 7 I I I I I I 0 3. iO0 0.200 0.300 0. uoo 0.500 0.600 DIAMETER OF CROSSHOLE (IN.) Figure 30. Effect of Cross-Hole Diameter on Deburring Time . 41
  • 41.
    -4% 203.2 177.8 ‘i52.4 y 127.0 3 b- x * 76.2 50.8 25.4 303SeSTAINLESS STEEL WORKPIECE 008 006 0 10 20 30 40 50 BACKUP MATERIAL HARDNESS (RC) Figure 31. Effect>f Backup Material Hardness on Burr Size . 42
  • 42.
    -41- ROD OR OTHER BACKUP Figure32. Fill First Hole With Backup to Provide Near Burr Free, Near Sharp Edges at Hole Intersections 43
  • 43.
    ‘42- REFERENCES l- L, K. Gillespie(ed), Advances in Deburring, SME, 1978.- "L. K, Gillespie (ed), Deburring Capabilities and Limitations, SME, 1976. "L. K0 Gillespie, Extrude Hone Deburring with X-Base Media, Bendix Kansas City Report BDX-613-1546, March 1976 (Amble from NTIS). 'Julius Brothers, "Production Research Observations on Abrasive Flow Deburring," SME paper MR77-438, 1977. 5Takashi Miyatani, Jungi Nakata, Tameyasu Tsukada, and Koya Takazawa, "Influence of Finishing Conditions on Work Accuracy by Extrude Hone Process," SME paper MR77-437, 1977. 'L. Michael Heglin, Laser Drillingfi General Electric Company, Aircraft Engine Group, Report AEml-TR-75-44, 1975. 7L. K. Gillespie, A Training Manual for Precision Hand Deburring, Part 1, Bendix Kansas City Report BD-13,2400, May 1980 (Available from NTIS). *L.'K. Gillespie, "Deburring Precision Miniature Parts," Precision Engineering, Volume 1, Number 4, October 1979, pp. 89-198. 44