1. NC State University Department of Materials Science and Engineering1 MSE 440/540: Processing of Metallic Materials Instructors: Yuntian Zhu Office: 308 RBII Ph: 513-0559 ytzhu@ncsu.edu Lecture 13: Machining I

2. NC State University Cutting action involves shear deformation of work material to form a chip, and as chip is removed, new surface is exposed: (a) positive and (b) negative rake tools Machining

3. NC State University Machining Operations Most important machining operations: –Turning –Drilling –Milling Other machining operations: –Shaping and planing –Broaching –Sawing

4. NC State University Single point cutting tool removes material from a rotating workpiece to form a cylindrical shape Turning and Drillng  Used to create a round hole, usually by means of a rotating tool (drill bit) with two cutting edges https://www.youtube.com/watch?v= Mn9jpqI8rao&feature=related

5. NC State University Rotating multiple-cutting-edge tool is moved across work to cut a plane or straight surface Two forms: (c) peripheral milling and (d) face milling Milling

6. NC State University Cutting Tool Classification 1.Single-Point Tools –One dominant cutting edge –Point is usually rounded to form a nose radius –Turning uses single point tools 2.Multiple Cutting Edge Tools –More than one cutting edge –Motion relative to work achieved by rotating –Drilling and milling use rotating multiple cutting edge tools

7. NC State University Cutting Conditions in Machining Three dimensions of a machining process –Cutting speed v – primary motion –Feed f – secondary motion –Depth of cut d – penetration of tool below original work surface For certain operations (e.g., turning), material removal rate R MR can be computed as R MR = v f d

8. NC State University Simplified 2-D model of machining that describes the mechanics of machining fairly accurately Orthogonal Cutting Model

9. NC State University Chip Thickness Ratio where r = chip thickness ratio; t o = thickness of the chip prior to chip formation; and t c = chip thickness after separation Chip thickness after cut is always greater than before, so chip ratio is always less than 1.0

10. NC State University Determining Shear Plane Angle Based on the geometric parameters of the orthogonal model, the shear plane angle  can be determined as: where r = chip ratio, and  = rake angle

11. NC State University (a) Chip formation depicted as a series of parallel plates sliding relative to each other, (b) one of the plates isolated to show shear strain, and (c) shear strain triangle used to derive strain equation Shear Strain in Chip Formation

12. NC State University Chip Formation More realistic view of chip formation, showing shear zone rather than shear plane Also shown is the secondary shear zone resulting from tool ‑ chip friction

13. NC State University Four Basic Types of Chip in Machining

14. NC State University Four Basic Types of Chip in Machining  Ductile materials  Low ‑ to ‑ medium cutting speeds  Tool-chip friction causes portions of chip to adhere to rake face  BUE forms, then breaks off, cyclically Serrated Chip

15. NC State University Generating shape: (a) straight turning, (b) taper turning, (c) contour turning, (d) plain milling, (e) profile milling Generating Shape

16. NC State University Forming to create shape: (a) form turning, (b) drilling, and (c) broaching Forming to Create Shape

17. NC State University Combination of forming and generating to create shape: (a) thread cutting on a lathe, and (b) slot milling Forming and Generating

18. NC State University Turning Operation

19. NC State University More Operations Related to Turning (d) Form turning, (e) chamfering, (f) cutoff

20. NC State University Methods of Holding Workpiece in a Lathe (a) Holding the work between centers, (b) chuck, (c) collet, and (d) face plate http://www.youtube.com/watch?v=Q7QUiCJJmew

21. NC State University More Operations Related to Turning (g) Threading, (h) boring, (i) drilling

22. NC State University Operations Related to Drilling (a) Reaming, (b) tapping, (c) counterboring

23. NC State University More Operations Related to Drilling (d) Countersinking, (e) center drilling, (f) spot facing

24. NC State University Two Forms of Milling (a) Peripheral milling and (b) face milling

25. NC State University Types of Peripheral Milling (a) Slab milling, (b) slotting, (c) side milling, (e) straddle milling, and (e) form milling

26. NC State University Types of Face Milling (a) Conventional face milling, (b) partial face milling, (c) end milling, and (d) profile milling using an end mill

27. NC State University Types of Face Milling (e) Pocket milling and (f) contour milling https://www.youtube.com/watch?v=U99asuDT97I https://www.youtube.com/watch?v=9OsNUi_o6C4

28. NC State University Similar operations, both use a single point cutting tool moved linearly relative to the workpart Shaping and Planing

29. NC State University Shaper

30. NC State University A multiple tooth cutting tool is moved linearly relative to work in direction of tool axis Broaching

31. NC State University Broaching Advantages: Good surface finish Close tolerances Variety of work shapes possible Cutting tool called a broach Owing to complicated and often custom ‑ shaped geometry, tooling is expensive

32. NC State University Power Hacksaw Linear reciprocating motion of hacksaw blade against work  Rotating saw blade provides continuous motion of tool past workpart

33. NC State University Geometric Factors Affecting Surface Finish Effect of (a) nose radius, (b) feed, and (c) ECEA

34. NC State University Ideal Surface Roughness where R i = theoretical arithmetic average surface roughness; f = feed; and NR = nose radius

35. NC State University Work Material Factors Built ‑ up edge effects Damage to surface caused by chip Tearing of surface when machining ductile materials Cracks in surface when machining brittle materials Friction between tool flank and new work surface

36. NC State University Effect of Work Material Factors Multiply theoretical surface roughness by the ratio of actual to theoretical roughness for the given cutting speed to obtain estimate of actual surface roughness

37. NC State University HW assignment Reading assignment: Chapters 17 Review Questions: 15.4, 15.5, 15.11, 16.2, 16.5, 16.6, 16.13, 16.14 Problems: 15.1, 15.3, 15.4, 15.6, 15.10, 16.1, 16.2, 16.6, 16.8, Department of Materials Science and Engineering37