MSE 440540 Processing of Metallic Materials Instructors Yuntian

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

Machining • 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 NC State University

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

Turning and Drillng • Single point cutting tool removes material from a rotating workpiece to form a cylindrical shape https: //www. youtube. com/watch? v= Mn 9 jpq. I 8 rao&feature=related § Used to create a round hole, usually by means of a rotating tool (drill bit) with two cutting edges NC State University

Milling • 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 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 RMR can be computed as RMR = v f d NC State University

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

Chip Thickness Ratio where r = chip thickness ratio; to = thickness of the chip prior to chip formation; and tc = 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 Φ can be determined as: where r = chip ratio, and α = rake angle NC State University

Shear Strain in Chip Formation • (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 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 • Generating shape: (a) straight turning, (b) taper turning, (c) contour turning, (d) plain milling, (e) profile milling NC State University

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

Forming and Generating • Combination of forming and generating to create shape: (a) thread cutting on a lathe, and (b) slot milling 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 http: //www. youtube. com/watch? v=Q 7 QUi. CJJmew 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 https: //www. youtube. com/watch? v=U 99 asu. DT 97 I https: //www. youtube. com/watch? v=9 Os. NUi_o 6 C 4 NC State University

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

Shaper NC State University

Broaching • A multiple tooth cutting tool is moved linearly relative to work in direction of tool axis 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 Ri = 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, 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 Engineering 37 NC State University