Chip Formation (Traditional Machining) In any traditional machining process, chips are formed by a shearing process Ref: Manufacturing Processes for Engineering Materials by S. Kalpakjian, Addison Wesley, 2nd Ed., 1991 Shear Plane Shear Plane Shear Plane
Chip Types Continuous Built Up Edge (BUE) Discontinuous Segmented BUE Ref: Manufacturing Processes for Engineering Materials Fig 8.4, p 478.
Tool Geometry The shape and orientation of the cutting tool greatly affects the chip formation mechanics
Rake Angle Of particular importance is the rake angle that the tool makes with the workpiece normal Positive Rake Neutral Rake Negative Rake Workpiece Normal + + Cutter Velocity Workpiece Normal 0 Cutter Velocity - Cutter Velocity Workpiece Normal
Cutting Parameters (Vertical Milling) Depth of Cut - measured along workpiece normal Step over Distance - (also called radial depth of cut)- Measured in tangent plane of workpiece and perpendicular to cutter travel or workpiece feed s is step over distance d is depth of cut f is feed direction of workpiece s w f
Feeds & Speeds Ref: From Machinery’s Handbook 21st ed
Feeds & Speeds
"For ordinary twist drills (HSS- high speed steel) the feed rate used is...
0.001-0.003 in/rev for drills smaller than 1/8 in. (dia.);
0.002-0.006 in/rev for 1/8 to 1/4 in. dia. drills;
0.004-0.010 in/rev for 1/4 to 1/2 in. dia. drills;
0.007-0.015 in/rev for 1/2 to 1 in. dia. drills; and,
0.010-0.025 in/rev for drills larger than 1 inch. (dia)
The lower values in the feed ranges should be used for hard materials such as tool steels,
superalloys, and work hardening stainless steels; the higher values in the feed ranges
should be used to drill soft materials such as aluminum and brass."
In the “typical operating range”, tool life (T) and cutting speed (V) are related according to Taylor’s Equation
where n & C are experimentally determined constants
FW Taylor studied the effects of the feed, depth of cut, and
1) Cutting Speed is the dominating factor in determining tool life
2) Feeds and Depths of Cut are the dominant forces in determining
the force acting on the tool
Taylor recommended using the maximum allowable feed
and depth of cut, then selecting V to balance tool wear with
cycle time for the process
Cutting Speeds Cutting Rates- Often given speeds in SFM (surface feet/min), but control spindle rotation in RPM (rev/min). Note: Use the maximum effective cutting diameter of tool Formula for spindle RPM comes from basic kinematics v= x r
Cutting Diameter To select the correct radius (or diameter) to use in the formula-- Determine what the spindle is rotating Find the perpendicular distance from the axis of rotation to the furthest point where cutting occurs Double it to get the diameter Axis of Revolution Cutting Edge d Lathe- part turns(NOT tool) r is from center to tool if turning down - d is workpiece diameter Flat Nosed End Mill d=cutter diameter d Axis of Revolution Cutting Edge d Axis Ball Nosed End Mill if ball is not “buried” in workpiece, then d will be less than cutter diameter i.e. NO cutting occurs at full tool diameter
Feed Rates Feed Rates are commonly given as Advance Per Tooth (APT) To get the feed rate in surface inches per minute use: More properly one wishes to control the chip load or nominal chip thickness t l . If the cutter is NOT fully loaded, one must increase the feed (APT) to keep the same chip load (t l ). Most tabulated values of the APT assume a fully loaded cutter- they are really listings of the required chip load t l . Feeds on lathes and drills can be in ipr (inches per revolution): N is no longer required in formula:
Chip Load and Advance Per Tooth Step over distance (radial depth of cut) at least 1/2 tool diameter chip load (t )= APT Step over distance (radial depth of cut) less than 1/2 tool diameter chip load (t ) < APT APT APT l l t l t l
Shallow Cuts with Ball Nosed End Mill Decrease in Effective Cutting Diameter Decrease in Chip Load Notice how the chip load (t l ) is less than the APT for a shallow cut
R CTF - Ball Nose @ Small Depth of Cut Ref: Figure O-51, Kibbe, et al. Machine Tool Practices 5 th Ed, Prentice Hall,1995.
Feeds w/ Radial Chip Thinning Factor Proper feeds come from finding the required advance per tooth (APT) to get correct chip load (feed value commonly given in books) As we use it, the R CTF is a “first pass” improvement 1) R CTF s for FLAT end mill with small step over distance 2) R CTF s for BALL end mill with small depth of cut 3) Anything over tool radius is assumed to be fully loaded In some cases tables incorporate R CTF s and give true APT But usually what you look up in a table is really t l
Feeds & Speeds - Example 1 Estimate the cutting speed and feed rate required for a 3/4” diameter 2 flute HSS end mill in Cast Iron, with a depth of cut of 0.375” and a step over distance of 0.375.” The spindle rotational speed is given by: The machine feed rate is given by:
Feeds & Speeds - Example 2 Estimate the depth of cut, the cutting speed, and feed rate required when rough turning a bronze shaft, from a diameter of 2.000” to 1.800.” Refer to tables to get recommended speed and feed.
Feeds/Speeds for Example 2
Feeds & Speeds - Example 2 (Cont’d) Estimate the depth of cut, the cutting speed, and feed rate required when rough turning a bronze shaft, from a diameter of 2.000” to 1.800.” The recommended speed is The recommended feed is
Feeds & Speeds - Example 3 Estimate the cutting speed and feed rate required for a 1/2” diameter HSS 2 flute ball nose end mill in “medium” tool steel, with a depth of cut of 0.0625” and a step over distance of 0.250.” The ball end mill depth of cut is less than the radius. Therefore the effective diameter must be computed: Find speeds and feeds from table.
Feeds/Speeds for Example 3
Feeds & Speeds - Example 3 (Cont’d) Estimate the cutting speed and feed rate required for a 1/2” diameter HSS 2 flute ball nose end mill in “medium” tool steel, with a depth of cut of 0.0625” and a step over distance of 0.250.” The recommended speed is: Find the chip reduction factor from table.
R CTF - Ball Nose @ Small Depth of Cut for Ex 3 Ref: Figure O-51, Kibbe, et al. Machine Tool Practices 5 th Ed, Prentice Hall,1995.
Feeds & Speeds - Example 3 (Cont’d) Estimate the cutting speed and feed rate required for a 1/2” diameter HSS 2 flute ball nose end mill in “medium” tool steel, with a depth of cut of 0.0625” and a step over distance of 0.250.” The recommended feed rate is:
Machinability Machinability generally involves three factors 1) Surface Finish 2) Tool Life 3) Force and Power Requirements Machinability Ratings are the cutting speeds required to obtain a tool life of T=60 min-- (in general, for a given material, higher speeds decrease the tool life, & slower speeds increase it Standard is AISI 1112 steel- rating of 100 for a tool life of 60 min, use cutting speed of 100 SFM (AISI 1112) From example 8.5, Kalpakjian. Manufacturing Processes for Engineering Materials 2 nd Ed, Addison-Wesley 1991.
Power & Force Estimation Power, P, requirements can then be determined as... where MRR is the Material Removal Rate Torque, , is found from where is the spindle speed F p, the force in the direction of the cutting velocity, V, is
Specific Energies of Machining Ref: Shaw. Metal Cutting Principles , Clarendon Press 1984, p. 43 Material Aluminum Alloys 100,000 Gray Cast Iron 150,000 Free Machining Brass 150,000 Free Machining Steel (AISI 1213) 250,000 “ Mild” Steel (AISI 1018) 300,000 Titanium Alloys 500,000 Stainless Steels 700,000 High Temp. Alloys 700,000 u can be determined from where is the effective rake angle (in degrees) & t l is the undeformed (nominal) chip thickness (in inches)
Cutting Power - Example 1 Find the power for an 8” HSS face mill (10 teeth, e =30 o ) to remove 0.1” from Cold Drawn, Wrought Aluminum, with a step over distance of 4.0” at a speed of 600 fpm and an APT 0.022.” Compute the speed and feed. The material removal rate is:
Cutting Power - Example 1(cont’d) Find the power for an 8” HSS face mill (10 teeth, e =30 o ) to remove 0.1” from Cold Drawn, Wrought Aluminum, with a step over distance of 4.0” at a speed of 600 fpm and an APT 0.022.”
Cutting Power - Example 2 Estimate the work required to turn down an annealed 304 stainless rod 6 in long from a diameter of 0.500” to a diameter of 0.480.” (Assume e =13 o , & ipr=0.003”)
Factors for Chip production:
Factors that affect machining parameters:
depth of cut
radial depth of cut (if applicable)
speeds (tip and spindle)
This module is intended as a supplement to design classes in mechanical engineering. It was developed at The Ohio State University under the NSF sponsored Gateway Coalition (grant EEC-9109794). Contributing members include:
Gary Kinzel …………………………………….. Project supervisor
Chris Hubert and Alan Bonifas ..……………... Primary authors
Phuong Pham and Matt Detrick ……….…….. Module revisions
L. Pham …………………………………….….. Audio voice
Machinery’s Handbook 21st ed
Kalpakjian, S. and Addison Wesley, Manufacturing Processes for Engineering Materials , 2nd Ed., 1991
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