NC State UniversityDepartment of Materials Science and Engineering 1

MSE 440/540: Processing of Metallic Materials

Instructors: Yuntian ZhuOffice: 308 RBII

Ph: 513-0559

ytzhu@ncsu.edu

Lecture 13: Machining I

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Lab tour: 2:30pm, Oct. 27, Daniel Hall 120

Department of Materials Science and Engineering 2

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• 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

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Machining Operations

• Most important machining operations:– Turning– Drilling– Milling

• Other machining operations:– Shaping and planing– Broaching– Sawing

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• 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

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• 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

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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

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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

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• Simplified 2-D model of machining that describes the mechanics of machining fairly accurately

Orthogonal Cutting Model

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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

c

o

tt

r

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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

sincos

tanr

r

1

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• (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

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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

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Four Basic Types of Chip in Machining

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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

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• Generating shape: (a) straight turning, (b) taper turning, (c) contour turning, (d) plain milling, (e) profile milling

Generating Shape

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• Forming to create shape: (a) form turning, (b) drilling, and (c) broaching

Forming to Create Shape

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• Combination of forming and generating to create shape: (a) thread cutting on a lathe, and (b) slot milling

Forming and Generating

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Turning Operation

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More Operations Related to Turning

• (d) Form turning, (e) chamfering, (f) cutoff

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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

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More Operations Related to Turning

• (g) Threading, (h) boring, (i) drilling

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Operations Related to Drilling

• (a) Reaming, (b) tapping, (c) counterboring

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More Operations Related to Drilling

• (d) Countersinking, (e) center drilling, (f) spot facing

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Two Forms of Milling

• (a) Peripheral milling and (b) face milling

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Types of Peripheral Milling

• (a) Slab milling, (b) slotting, (c) side milling, (e) straddle milling, and (e) form milling

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Types of Face Milling

• (a) Conventional face milling, (b) partial face milling, (c) end milling, and (d) profile milling using an end mill

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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

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• Similar operations, both use a single point cutting tool moved linearly relative to the workpart

Shaping and Planing

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Shaper

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• A multiple tooth cutting tool is moved linearly relative to work in direction of tool axis

Broaching

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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

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Power Hacksaw

• Linear reciprocating motion of hacksaw blade against work

Rotating saw blade provides continuous motion of tool past workpart

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Geometric Factors Affecting Surface Finish

• Effect of (a) nose radius, (b) feed, and (c) ECEA

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Ideal Surface Roughness

where Ri = theoretical arithmetic average surface roughness; f = feed; and NR = nose radius

NRfRi 32

2

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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

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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

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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 38