NC State UniversityDepartment of Materials Science and Engineering 1
MSE 440/540: Processing of Metallic Materials
Instructors: Yuntian ZhuOffice: 308 RBII
Ph: 513-0559
Lecture 13: Machining I
NC State University
Lab tour: 2:30pm, Oct. 27, Daniel Hall 120
Department of Materials Science and Engineering 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
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
https://www.youtube.com/watch?v=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 RMR can be computed as
RMR = 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; 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
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
sincos
tanr
r
1
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
http://www.youtube.com/watch?v=Q7QUiCJJmew
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=U99asuDT97I
https://www.youtube.com/watch?v=9OsNUi_o6C4
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 Ri = theoretical arithmetic average surface roughness; f = feed; and NR = nose radius
NRfRi 32
2
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 38