NC State University Department of Materials Science and Engineering1 MSE 440/540: Processing of Metallic Materials Instructors: Yuntian Zhu Office: 308 RBII Ph: Lecture 13: Machining I

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

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

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 Mn9jpqI8rao&feature=related

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

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

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

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

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

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

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

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

NC State University Four Basic Types of Chip in Machining

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

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

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

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

NC State University Turning Operation

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

NC State University Methods of Holding Workpiece in a Lathe (a) Holding the work between centers, (b) chuck, (c) collet, and (d) face plate

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

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

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

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

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

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

NC State University Types of Face Milling (e) Pocket milling and (f) contour milling

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

NC State University Shaper

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

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

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

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

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

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

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

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, 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