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Traditional Machining

Traditional Machining

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Traditional Machining

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  1. Traditional Machining

  2. Shear Plane Shear Plane Shear Plane 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

  3. Chip Types Continuous Built Up Edge (BUE) BUE Segmented Discontinuous Ref: Manufacturing Processes for Engineering Materials Fig 8.4, p 478.

  4. Tool Geometry The shape and orientation of the cutting tool greatly affects the chip formation mechanics

  5. Cutter Velocity Cutter Velocity Cutter Velocity - + + 0 Workpiece Normal Workpiece Normal Workpiece Normal Rake Angle Of particular importance is the rake angle that the tool makes with the workpiece normal Positive Rake Neutral Rake Negative Rake

  6. Tool Wear

  7. 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 f w s

  8. Feeds/Speeds

  9. Feeds & Speeds Ref: From Machinery’s Handbook 21st ed

  10. Cutting Speeds for Drilling (fpm) Material Cutting speed (fpm) Wrought Aluminum Alloys (Cold Drawn) 300 Free Cutting Brass (Cold Drawn) 175 Wrought Magnesium Alloys (Cold Drawn) 350 Mold Steels- P20 & P21 60 1040 Plain Carbon Steel (CD ,Hardness 175-225HB) 75 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." Ref: From Machinery’s Handbook 21st ed

  11. “Optimal” Feeds & Speeds • FW Taylor studied the effects of the feed, depth of cut, and • cutting speed: • 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 • 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 • Taylor recommended using the maximum allowable feed • and depth of cut, then selecting V to balance tool wear with • cycle time for the process

  12. Cutting Speeds Cutting Rates- Often given speeds in SFM (surface feet/min), but control spindle rotation in RPM (rev/min). Formula for spindle RPM comes from basic kinematics v= x r Note: Use the maximum effective cutting diameter of tool

  13. 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 Flat Nosed End Mill d=cutter diameter Lathe- part turns(NOT tool) r is from center to tool ifturning down- d is workpiece diameter Axis of Revolution Axis d d Cutting Edge Cutting Edge Axis of Revolution d 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

  14. Feed Rates Feed Rates are commonly given as Advance Per Tooth (APT) To get the feed rate in surface inches per minute use: Feeds on lathes and drills can be in ipr (inches per revolution): N is no longer required in formula: More properly one wishes to control the chip load or nominal chip thickness tl. If the cutter is NOT fully loaded, one must increase the feed (APT) to keep the same chip load (tl). Most tabulated values of the APT assume a fully loaded cutter- they are really listings of the required chip load tl.

  15. t l t l Chip Load and Advance Per Tooth APT APT Step over distance (radial depth of cut) less than 1/2 tool diameter chip load (t ) < APT Step over distance (radial depth of cut) at least 1/2 tool diameter chip load (t )= APT l l

  16. Shallow Cuts with Ball Nosed End Mill Decrease in Effective Cutting Diameter Decrease in Chip Load Notice how the chip load (tl) is less than the APT for a shallow cut

  17. RCTF- Ball Nose @ Small Depth of Cut Ref: Figure O-51, Kibbe, et al. Machine Tool Practices 5th Ed, Prentice Hall,1995.

  18. .05 .08 .12 .16 .20 .18 .10 .14 .25 .5 .8 1.0 .4 .7 .95 .3 .9 .6 RCTF- Peripheral Milling w/ Flat Nose .06 Ref: Figure O-49, Kibbe, et al. Machine Tool Practices 5th Ed, Prentice Hall,1995.

  19. 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 RCTF is a “first pass” improvement 1) RCTFs for FLAT end mill with small step over distance 2) RCTFs 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 RCTFs and give true APT But usually what you look up in a table is really tl

  20. Feeds/Speeds F

  21. 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:

  22. 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.

  23. Feeds/Speeds for Example 2 F F

  24. 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

  25. 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.

  26. Feeds/Speeds for Example 3 F

  27. 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.

  28. RCTF- Ball Nose @ Small Depth of Cut for Ex 3 Ref: Figure O-51, Kibbe, et al. Machine Tool Practices 5th Ed, Prentice Hall,1995.

  29. 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:

  30. 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 2nd Ed, Addison-Wesley 1991.

  31. Power & Force Estimation Power, P, requirements can then be determined as... where MRR is the Material Removal Rate where is the spindle speed Torque, , is found from Fp, the force in the direction of the cutting velocity, V, is

  32. Specific Energies of Machining u can be determined from where  is the effective rake angle (in degrees) & tl is the undeformed (nominal) chip thickness (in inches) 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 Ref: Shaw. Metal Cutting Principles, Clarendon Press 1984, p. 43

  33. Cutting Power - Example 1 Find the power for an 8” HSS face mill (10 teeth, e=30o) 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:

  34. Cutting Power - Example 1(cont’d) Find the power for an 8” HSS face mill (10 teeth, e=30o) 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.”

  35. 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=13o, & ipr=0.003”)

  36. Summary • Factors for Chip production: • rake angle • clearance angle • shear angle • Factors that affect machining parameters: • effective diameter • depth of cut • radial depth of cut (if applicable) • speeds (tip and spindle) • feed rate • material • tool material

  37. Credits • 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 • References: • Machinery’s Handbook 21st ed • Kalpakjian, S. and Addison Wesley, Manufacturing Processes for Engineering Materials , 2nd Ed., 1991 • Kibbe, et al. Machine Tool Practices5th Ed, Prentice Hall,1995 • Shaw. Metal Cutting Principles, Clarendon Press

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