WT5912TECHNOLOGY EDUCATION &WORKSHOP PRACTICE 2:MATERIALS AND CONSTRUCTIONUNIT 2 – WEEK 3 Machine Anatomy and ConstructionCourse Involved: Graduate Diploma in Technology Education Studies DetailUniversity of LimerickDepartment of Design & Manufacturing TechnologyLecturer/Teacher: Mr. Joseph LysterAcademic Year 2012: Spring SemesterTechnical Support: Mr. Joe Murray & Mr. Richie HennessyNotes Prepared by: Mr. Joseph LysterAvailable on www.slideshare.net/WT4603 UNIVERSITY of LIMERICK OLLSCOIL LUIMNIGH
MACHINE ANATOMY WT5912Surface Planer Thicknesser Rip/Panel Saw -Machine Parts -Machine Design -Machine Function -Machine Safety/Use -Machine Processes -Machine Maintenance
SURFACE PLANER When planing wooden material a number of factors combine to generate the flat surface. Number of cutting knives in the block Speed of the revolving block Feed speed of the material Knife cutter design Chip breaking aids Nature and species of the material
MACHINE DESIGN: CIRCULAR CUTTER BLOCK• Reduced noise levels• Better balance• Safer clamping mechanism• Can run head at higher speeds (RPM)• Can produce better finish• Easier and quicker maintenance Department of Manufacturing & Operations Engineering
MACHINE DESIGN: CIRCULAR CUTTER BLOCK KNIFE CLAMPING MECHANISM Department of Manufacturing & Operations Engineering
MACHINE DESIGN: CIRCULAR CUTTER BLOCK CUTTER PROJECTION Use of a limiter to achieve limited cutter projection Department of Manufacturing & Operations Engineering
MACHINE DESIGN: CIRCULAR CUTTER BLOCK CUTTER PROJECTION The cutter projection and the shape of the block face cause the severed chip to bend back causing a crack across its width This makes long grain riving less likely (Chip breaker not shown)
MACHINE DESIGN: PLANER KNIVESPlaner KnivesImpor tant factor s when selecting a planer knife Suitability for cutter block Material being processed Finish required Volume being machined Clamping and setting mechanisms Planers can have 2,3,4,6……. Cutter knives. Most smaller machines such as those found in schools will have 2,3 or 4 knives.
MACHINE DESIGN: PLANER KNIVES Knives can be made from Chrome Vanadium steel alloy. This is suitable for machining softwoods and non abrasive hardwoods. However with advances in machining technology better materials have been developed to machine wood and wood composites. Chrome Vanadium knives dull quickly on harder more dense material. This requires more sharpening, setting up and leads to a lot of time wastage.
MACHINE DESIGN: PLANER KNIVES High Speed Steel (HSS) is a cobalt steel alloy with a small percentage of Tungsten added. It is more suitable for machining all types of wood than the chrome steel compounds. Abrasive stock should be machined using solid or tipped cutters. Tungsten Carbide (TC) is the best tool compound for machining manufactured boards. For general work HSS cutters are preferred to TC Cutters can be re-sharpened easily. A keener edge can be achieved on HSS giving a better finish
MACHINE DESIGN: PLANER KNIVES The reason for this is that steel compounds are smelted and shaped by rolling and forging while the metal is close to melting point. The molecules of the compound flow and align themselves in response to this pressure giving the material maximum strength and edge holding capabilities Department of Manufacturing & Operations Engineering
MACHINE DESIGN: PLANER KNIVES Tungsten carbide is a sintered compound. The fine grain powder from which the cutter will eventually will be made is compressed into a mould ( the ‘blank’ un-edged cutter required) under extremely high temperature (1500 C) and pressure to form a solid block. Tungsten does not flow – it retains a granular structure and will chip rather than deform if abused. Department of Manufacturing & Operations Engineering
MACHINE DESIGN: PLANER KNIVES Because if its brittle nature TC cutters require a more obtuse sharpness angle than the HSS cutters (more support for the cutting edge). This makes it less satisfactory for cutting softwoods than HSS knives which can be ground to a more acute cutting angle. Department of Manufacturing & Operations Engineering
MACHINE DESIGN: PLANER KNIVES Cutter Cutter Knife Knife H SS TC Large grinding angle Smaller grinding angle to support cutting edge produces keen edge Department of Manufacturing & Operations Engineering
MACHINE FUNCTION: KNIFE CUTTER GEOMETRY Rake or Cutting angle Angle created between the face of the cutting knife Cutting and the centre of the cutter Angle block Can have a wide range Softwoods 27 to 35 Hardwoods 15 to 25 Department of Manufacturing & Operations Engineering
MACHINE FUNCTION: KNIFE CUTTER GEOMETRYBevel or Lip Angle Angle formed to give the cutting edge Bevel Minimum of usually Angle 35 Greater for tipped cutters Department of Manufacturing & Operations Engineering
MACHINE FUNCTION: KNIFE CUTTER GEOMETRYClearance Angle Angle formed between a line tangential to cutting circle and the bevel angle of the Cutter Circle knife Diameter Must be present Clearance Angle Has a bearing on the life of the cutting edge Usually 10 to 15 Department of Manufacturing & Operations Engineering
MACHINE FUNCTION: KNIFE CUTTER GEOMETRYPeripheral Cutting Speed Cutter Rotation A constant speed in the range of 35-45 m/s will give best results Increase in speed may Cutter Circle cause loss of dynamic Diameter balance due to vibrations Poor finish Increased noise levels Work Movement Direction Department of Manufacturing & Operations Engineering
MACHINE FUNCTIONPITCH DISTANCE Combination of a rotary Pitch cut and a linear feed will leave the surface of t the material with a Work Piece Fast Feed Rate series of arcs on it called Curtate Trochoids Pitch The pitch and depth of Work Piece t these arcs will Slower Feed Rate determine how smooth t = Cutter arc depth on the surface finish will machined surface be Department of Manufacturing & Operations Engineering
MACHINE FUNCTIONP I T C H D I S TA N C E 2mm to 3mm for non obvious joinery and painted external work. 1mm to 1.5 mm for internal painted work. 0.5mm to 1mm for hardwood joinery and furniture. Department of Manufacturing & Operations Engineering
MACHINE FUNCTIONPITCH DISTANCEThe SI unit of time is the second , but the minute is acceptable.Feed rate on wood working machines is expressed in metres per minute. (m/min)The formula for the pitch of the cutter marks is given by: f p = ------- nRwhere p = pitch of cutter mark f = fe e d r a te n = n u m b e r o f e f fe c t i v e c u t te r s R = r e v o l u t i o n s p e r m i n u te o f b l o c k Department of Manufacturing & Operations Engineering
MACHINE FUNCTIONPITCH DISTANCE The unit for “p” will be metres (m) f m/min m min p = ---- = --------- = ----- x ------ = m nR 1/min min 1 Department of Manufacturing & Operations Engineering
MACHINE FUNCTIONPITCH DISTANCEProblem 1 Calculate the cutter pitch of a 4 cutter block revolving at 4200 rev/min with a feed speed of 24m/min. F 24 24p = ------- = ------------ = --------- = 0.0014m = 1.4mm nR 4 x 4200 16800 (Internal painted work) Department of Manufacturing & Operations Engineering
MACHINE FUNCTIONPITCH DISTANCE If a graded surface is specified and the machine has a multi - speed feed gearbox, the same formula is used but “f” is expressed in terms of n ,p, and R. f p = ------- f = nRp nR Department of Manufacturing & Operations Engineering
MACHINE PROCESSES: CHIP FORMATION Department of Manufacturing & Operations Engineering
MACHINE PROCESSES: CHIP FORMATION Chip breaking aid and pressure bar prevent riving and splintering Department of Manufacturing & Operations Engineering
MACHINE MAINTENANCECutter and Machine MaintenanceInvolves:1. Grinding and setting of knives2. Roller and pressure bar setting3. Prevention of resin build up on table and rollers.4. Attention to: bearing wear feed complex adjustments rise and fall table Department of Manufacturing & Operations Engineering
MACHINE MAINTENANCEGrinding The grinding angle of a cutter can vary between 30 to 35 This is increased to 40 for hardwoods (cutting edge lasts longer)Overheating May produce micro cracks in the cutting edge which can run into gaps when the cutter is used. May cause the cutter to bow due to expansion. Department of Manufacturing & Operations Engineering
MACHINE MAINTENANCEOverheating can be avoided By taking light cuts. By ensuring that the grind wheel is ‘dressed’ when required to ensure that the face is open and not glazed when grinding the knives. By using a ‘soft’ grinding wheel on HSS cutters – the soft structure of the wheel allows its grains to break away as soon as they are blunt revealing sharper ones. By wet grinding – this is the grinding of cutters while partially submerged in a mixture of water and soluble oil. The water is a coolant to prevent frictional heat developing and to disperse it should it occur. The oil prevents rust in the cutters and it provides a degree of cutting lubrication . Department of Manufacturing & Operations Engineering
MACHINE MAINTENANCESetting Cutters in Block Before setting the following points should be checked. The out feed table and cutter block must be clean and free from dust resin. Method of adjusting cutters. Area where setting device is used from should be free from resin and damage. Straightness of cutters. Cutters correctly balanced both in weight and end for end. Department of Manufacturing & Operations Engineering
MACHINE MAINTENANCE Setting of knives will greatly depend on the type of cutter block Knife cutter projection Chip breaker Knife parallel to table All knives in the same peripheral cutting circle(Refer to machine manual for setting) Department of Manufacturing & Operations Engineering
MACHINE MAINTENANCESetting devicesThere are a number of cutter setting devices.This device and procedure will often be supplied with the machine.They can be loosely placed into the following four categories:1. Bridge device2. Precision cutter setter device3. Pin locater device4. Wooden straight edge device5. Cutter s require accurate setting in the block because if the knives are not revolving in the same cutting circle a poor finish will be produced. Department of Manufacturing & Operations Engineering
CIRCULAR SAWING MACHINESNOTE Circular sawing machines are high risk woodworkingmachineryPupils should not be permitted to use this machine.BS 4163:2000The machine should be included in a planned maintenanceprogram that should include electrical safety tests.Read Circular Sawing Machines (Week 6 Notes)
MACHINE DESIGN: SAW BLADE TOOTH CONFIGURATION The shape of the saw blade tooth and the way the teeth are grouped also affect the way the blade cuts. The configuration of the teeth on a saw blade has a lot to do with whether the blade will work best for ripping, crosscutting, or laminates. Of course, no matter which tooth design youre looking at, more teeth will give you a smoother cut than fewer teeth.
MACHINE DESIGN: SAW BLADE TOOTH CONFIGURATION A cross cut blade will do theA ripping blade will have a Flat best job with an AlternatingTop Grind (FTG) for fast cutting Top Bevel (ATB), cutting acrosswith the grain. the grain like a knife and producing a ver y smooth cut. A blade with Triple Chip Grind (TCG) is good for all-purpose cutting and also gives you a ver y clean cut. TCG blades are also good for cutting non - ferrous metals and plastics.
MACHINE DESIGN: SAW BLADETOOTH CONFIGURATION In general, blades with more teeth yield a smoother cut, and blades with fewer teeth move material faster. A 250mm blade designed for ripping wood can have as few as 24 teeth, and is designed to quickly move material along the length of the grain. A rip blade isnt designed to yield a mirror -smooth cut, but a good rip blade will move through wood with little ef fort and leave a clean cut with a minimum of scoring.
MACHINE DESIGN: SAW BLADETOOTH CONFIGURATION A crosscut blade is designed to give a smooth cut across the grain of the wood, without any splintering or tearing of the material. A crosscut blade will usually have from 60 to 80 teeth. More teeth mean that each tooth has to cut less material. The result is a cleaner cut on edges and a smoother cut surface. With a top-quality crosscut blade, the cut surface will appear polished.
MACHINE DESIGN: SAW BLADE HOOK ANGLE In both Rip and Cross -cutting saws the Hook angle determines The feel of the cut The quality of the finish The power consumed The approach angle of the saw varies according to the relative position of the tooth in the downward cutting arc. This angle alter s from the top plane of the timber where the tooth top makes fir st contact to compress the timber before the tooth point engages, to a plane where the tooth angle and the timber face are parallel.
MACHINE DESIGN: SAW BLADE HOOK ANGLE The amount of Hook determines the degree to which the tooth will drive into the timber during the cut. The effect is of the timber being drawn forward. The greater the hook angle the greater this tendency. Too great of a hook angle will result in Harsh cut Tearing Poor finish Less rigid tooth Vibration.
MACHINE DESIGN: SAW BLADE HOOK ANGLE A blade with high positive hook angle (+20 ) will have a ver y aggressive cut and a fast feed rate. A low or negative hook angle will slow the feed rate and will also inhibit the blades tendency to "climb" the material being cut. A blade for ripping wood on a table saw will generally have a high hook angle, where an aggressive, fast cut is usually what you want. Radial arms saws and sliding compound mitre saws, on the other hand, require a blade with a ver y low or negative hook angle, to inhibit overly fast feed rate, binding, and the blades tendency to tr y to "climb" the material
MACHINE DESIGN: SAW BLADE HOOK ANGLE On most saw blades, the tooth faces are tipped either toward or away from the direction of rotation of the blade, rather than being perfectly in line with the centre of the blade. Hook angle is the angle formed between the tooth face and a line drawn from the centre of the blade across the tip of the tooth. On a blade with a positive hook angle, the teeth are tipped toward the direction of the blades rotation. A negative hook angle means that teeth tip away from the direction of rotation, and a zero degree hook angle means that the teeth are in line with the centre of the blade.
MACHINE DESIGN: SAW BLADE GULLET The gullet is the space cut away from the blade plate in front of each tooth to allow for chip removal. In a ripping operation, the feed rate is faster than in crosscutting and the chip size is bigger, so the gullet needs to be deep enough to make room for the large amount of material it has to handle. In a crosscutting blade the chips are smaller and fewer per tooth, so the gullet is much smaller. The gullets on some crosscutting blades are purposely sized small to inhibit a too-fast feed rate, which can be a problem, especially on radial arm and sliding mitre saws.
MACHINE DESIGN: SAW BLADE GULLETThe gullets of a combination blade are designed to handle both ripping and crosscutting. The large gullets between the groups of teeth help clear out the larger amounts of material generated in ripping. The smaller gullets between the grouped teeth inhibit a too-fast feed rate in crosscutting
MACHINE DESIGN: SAW BLADE CLEARANCEWork clearance must be provided. The saw tooth provides this clearance. The „Kerf‟ produced by the teeth must be wider than the supporting saw plate. Steel saws had the kerf formed by bending or „setting‟ alternate teeth laterally. With tipped saws the tips are wider than the saw plate and thus create the clearance. Clearance or relief bevels are ground on the sides and the top of each tooth.
MACHINE DESIGN: SAW BLADE PLATE TENSION A flat disc will remain flat and true if turned at a slow speed. When variable stresses are created on this disc due to: Braking effect of sawing Heating effect of friction Outward pull of centrifugal force the outer rim area of the disc will expand. If the whole area of the disc can expand at the same rate the disc will remain flat and true. This does not happen with a saw blade. The central region of the blade is clamped between the collars and does not expand.
MACHINE DESIGN: SAW BLADE PLATE TENSION Only the teeth of the saw blade should make contact with the work and a por tion of the energy expended in cutting will unavoidably be conver ted to heat. The peripher y of the blade will therefore tend to warm up more quickly than the main plate body. This will cause the peripher y of the blade to expand. If this is not taken into account the blade will distor t. To prevent this saw blades are „tensioned‟ during the manufacturing stage. Rim speed will determine the amount of tension required in a par ticular saw blade. Thinner saw blades require greater tension. Faster saws require more tension.
MACHINE DESIGN: SAW BLADE PLATE TENSION Traditionally this was done by highly skilled labour but modern saw manufacturers use machine operated roller s to achieve a faster more uniform result. This within limits allows the plate to expand uniformly in uneven temperature gradients. A blade which has lost its tension will be seen to be throwing from side to side. This is most noticeable as the blade slows down af ter the machine is switched of f. If this is the case the blade should be removed and sent for ser vicing. This can be reduced by cooling the blade tip while in operation by packing.
MACHINE DESIGN: SAW BLADE PLATE TENSION TC tipped blades have an extremely long life and to assist the tension factor, slots are incorporated around the edge of the blade. These allow a degree of individual expansion between segments on the plate edge. They also break up harmonic frequencies, which build up during the sawing process.
MACHINE DESIGN: BLADE GUARDTOP BLADE GUARD Covers the top edge of the saw blade. Deflects waste. Prevents accidental contact with the uppermost teeth of the blade. It can also limit the ef fects of material rejection.