Metal Removal Processes

1. Metal Removal Processes Dr. Ramon E. Goforth Adjunct Professor of Mechanical Engineering Southern Methodist University

2. Outline of Lecture • Basic information on material removal • Factors involved in material removal • Independent variables • Dependent variables • Machining Processes • Machining Economics • Machines Lecture 10 Lecture 11 Lecture 12

3. Basic Cutting Processes • Rotating part - turning • Creates round shapes • Stationary part - milling, drilling, sawing, etc

4. Basic Turning • Part of cylindrical cross section clamped in a "chuck" so that it can rotate about its axis • Part is rotated at fixed speed • A cutting tool is brought to bear on the moving surface of the part cutting of material • The "chuck" is a kind of vice which has rotational symmetry

5. Turning Process Parameters f d N

6. Turning Parameters • Tool Geometry • Rake angles • Side rake angle - more important than • Back rake angle • Cutting edge angles

7. Turning Parameters • Tool Geometry • Tool Materials • Feeds and speeds, N,d,f • (see table 22.4 for recommendations) • Cutting fluids • Material Removal rates • = p Davg d f N • Where Davg is the average diameter, d is the depth of cut, f is the feed rate and N the rotational speed • Forces and power used • Surface finish (scallops)

8. Power used • Power used is the material removal rate, MRR, times the specific energy

9. Feed Marks in Turning • Scallops created • The depth depends on the feed rate, surface velocity and tool shape Scallops

10. Machining Processes for Round Shapes • Turning • Facing • Boring • Produces circular internal profiles in hollow workpieces • Drilling • Produces round holes • Reaming • Produces more accurate holes than drilling • Parting • Threading • Knurling

11. Machining Processes for Round Shapes Kalpakjian p 663

12. Turning Guidelines • Avoid long skinny parts • Request wide accuracy and surface finish parameters • Avoid sharp corners and tapers • Avoid major dimensional changes • Design blanks to be as close to final dimensions as possible

13. Turning Guidelines • Allow for travel of tools across surfaces of workpiece • Design features so that standard tools can be used • Choose machinable materials • Minimize overhang of tool • Support workpiece • Use machines with high rigidity

14. Non Round Machining Processes • The operation • Clamp the workpiece onto a stationary bed or one that can move in multiple directions slowly • Bring a rotating tool to bear on the surface to be shaped • Move the rotating tool over the part or move the part past the rotating tool to shape it

15. Non Round Machining - Slab Milling • Milling • Slab/Peripheral • Cutter rotation axis parallel to workpiece surface • Conventional/up • Maximum chip thickness at end of cut • Low impact of tool with workpiece • Climb/down • Maximum chip thickness at beginning of cut • High low impact of tool with workpiece

16. Non Round Machining - Face milling • Axis of rotation perpendicular to workpiece surface • Large multi-insert cutter

17. Non Round Machining - Face Milling • Difference between climb and conventional face milling Action of an insert in face milling Climb Milling Conventional milling Parameters in face milling

18. Non Round Machining

19. Generic Milling formula • Cutting (peripheral) speed, • V = p D N • where D is the cutter diameter and N its rotational speed • Feed per tooth, • f = v/Nn • where v is the linear speed or feed rate of the workpiece, and n is the number of teeth • Undeformed chip thickness, (chip depth of cut), • tc = 2 f (d / D) • Where f is the feed per tooth, d is the depth of cut

20. Generic Milling formula • Cutting time, t = (l + 2lc)/ v • where v is the feed rate of the workpiece, l is the length of the workpiece and lc is the extent of the cutter’s first contact with the workpiece • Material removal rate, MRR • MRR = lwd/t = wdv • assuming the lc<<l and where w is the width of the cut • Power is equal to the MRR times the specific energy

21. Feed Marks from Milling

22. Design Guidelines for Milling • Design for standard cutters • Use chamfers instead of radii • Avoid internal cavities and pockets with sharp corners • Design workpieces with sufficient rigidity

23. Other Non Round Machining Processes • Drilling • Straddle milling • Planing • Broaching • Sawing • Generally used for cutting off pieces to be worked on by other processes • Filing and finishing • Gear machining

24. Drilling Practices • Type of drill bit, drill point geometry • Type of machine • Drill, press, radial drills, gang drills, NC controlled • Capabilities of drilling and boring operations (p 633) • HP used = Spec. Energy times MRR (pD2fN/4)

25. Drilling Operations and Drill bits

26. Drilling Guidelines • Design holes perpendicular to the surface • Do not design interrupted/overlapping holes • Design bottoms to match standard drill-point angles • Through holes are preferred over blind holes • If need large diameter holes design in smaller hole for casting • Design to minimize fixturing • Avoid reaming blind or intersecting holes

27. Machining Economics • Cost per piece decreases with cutting speed • Tool cost increases with cutting speed • Tool change time increases with cutting speed • Total cost goes through a minimum • Time spent removing material usually small fraction (<5%) of total time on machine Kalpakjian p 775/698

28. Machining Economics

29. Metal Removal Machines

30. Basic Lathe

31. Turning Machine Components • Bed • Supports all other major components • Top part has two ways • Carriage • Slides along the ways • Consists of the cross-slide, tool post and apron

32. Turning Machine Components • Headstock • Fixed • Contains the motors, pulley and belts to drive the spindle • Spindle has fixtures for attaching the workpiece • Tailstock • Can slide along the ways • Supports the other end of the workpiece • Feed rod and lead screw • Provides motion to the carriage and cross slide

33. A Manual Lathe

34. Turning Machines • Lathes • Tracer • Automatic • Automatic bar machines • Turret • Vertical • For very large diameters • Boring • Vertical • Horizontal (like a milling machine) • Computer controlled

35. Turret Lathe

36. MORI SEIKI SL-3 SLANT BED CNC LATHE

37. Vertical Boring Mill

38. Milling Machines • Column and Knee type • Horizontal spindle • Vertical spindle • Bed type • Skin mills • Other types • Planer type • Rotary tables • Duplicating machines • Profiling milling • More than three axes

39. #4 VERTICAL MILLING MACHINE W/SLIDING HEAD

40. Machining and Turning Centers • Combines turning with milling • Computer control essential • Multiaxis capabilities • Replacing simple lathes or milling machines

41. NC Turning Center

42. Giddings & Lewis dv15-l smart turn twin-spindle vertical production center

43. Drilling Machines • Drill presses • Radial machines • CNC Three axis drilling machine

44. Trends • High speed machining • Dry machining • Combining milling, drilling and turning operations • New, stiffer and highly damped machine tools • Graphite epoxy, ceramics (high modulus) • Modular machines • Multiple loading stations • More sensors • More and more automation • Automated program generation

45. Summary • There are many different types of machining operations • That is what makes it so versatile and attractive to industry • The basic cutting process is the same in all • Must consider the cutting operation as a system • Actual cutting time is a small fraction of the total time to create a part by machining