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Transcript of Ch23 With Text
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Page 22-1
Chapter 23Machining Processes Used to Produce
Various Shapes
Alexandra Schnning, Ph.D.Mechanical Engineering
University of North Florida
Figures byManufacturing Engineering and Technology
Kalpakijan and Schmid
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Introduction
What is milling?A manufacturing process in which a rotating,
multitooth cutter removes material whiletraveling along various axes with respect to the
workpiece.Other processes will be discussed, such as Planing, shaping, broaching, sawing, filing, and gear
manufacturing.
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Milling operations
Slab milling (Figure a) Arbor, cutter
Face milling (Figure b) Spindle and cutter
End milling (Figure c) Spindle, shank, end mill
Figure 23.3 A typicalpart that can beproduced on a millingmachine equippedwith computercontrols. Such partscan be madeefficiently andrepetitively on
computer numericalcontrol (CNC)machines, without theneed for refixturing orreclamping the part.
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Slab Milling
Also called peripheral millingThe axis of cutter rotation is parallel to the workpiece
surface to be machinedThe cutter has a teeth along its circumference where
each tooth acts as a single point cutting toolConventional vs. Climb milling
Conventional milling Also called up-milling: the rotation of the cutter is such that it first
engages the workpiece at the bottom. The cut is not a function of the surface characteristics Common method of milling. Proper clamping is necessary to prevent the upward rotation of the
cutter.
Climb milling Also called down-milling: the rotation is such that the cutter first
engages the workpiece at the top. Cutting forces holds the workpiece in place. However, a rigid setup is
important since there are high impact forces.
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Conventional and Climb Milling
Figure 23.4 (a) Schematic illustration of conventional milling and climb milling. (b) Slab milling operation,showing depth of cut, d, feed per tooth,f, chip depth of cut, tc, and workpiece speed, v. (c) Schematicillustration of cutter travel distance lcto reach full depth of cut.
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Milling Parameters
The velocity at the point ofcontact
The chip thickness can be found
Feed per tooth
Cutting time
Material Removal Rate
Definitions of symbols
Definition of Symbols tc: chip thickness f: feed per tooth of cutter
D: depth of cut
N: angular speed in rpm
n: number of teeth on cutter periphery
v: linear speed (feed rate) t: cutting time
L: length of the workpiece
Lc: extent of the cutter's first contactwith the workpiece (illustration on next
page)
w: width of cut
v D
v
t c2 f d
D
f v
N n
rad
slength
N = angular speed in rpm. This
version of the formula is used in the
book. It includes the unit conversion
from rpm to radiance per minute.
The units of the velocity aredistance/min.
It is assumed that lc
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Illustration of Lc
Lc
Lc: extent of the cutter'sfirst contact with the
workpiece
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Summary of Milling Parameters and Formulas
TABLE 23.1
N = Rotational speed of the milling cutter, rpmf = Feed, mm/tooth or in./tooth
D = Cutter diameter, mm or in.n = Number of teeth on cutterv = Linear speed of the workpiece or feed rate, mm/min or in./minV = Surface speed of cutter, m/min or ft/min
=D N
f = Feed per tooth, mm/tooth or in/tooth=v /N n
l = Length of cut, mm or in.t = Cutting time, s or min
=( l+lc) v, where l
c=extent of the cutters first contact with workpiece
MRR = mm3
/min or in.3
/min
=w d v, where wis the width of cut
Torque = N-m or lb-ft(F
c) (D/2)
Power = kW or hp
= (Torque) (), where = 2Nradians/min
Note: The units given are those that are commonly used; however, appropriate units must
be used in the formulas.
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Example
A slab milling operation is beingcarried out on a 12-in long, 4 inwide annealed mild steel block ata feed f=0.01 in/tooth and adepth of cut d=1/8 in. The cutteris D=2in in diameter, and has 20straight teeth, rotates at N =100rpm, and is wider than theblock to be machined. Calculate
the material Removal rate,estimate the power and torquerequired for this operation, andcalculate the cutting time. Given:
w=4in L=12 in f=0.01 in/tooth d=1/8 in D=2 in n = 20 teeth N = 100 rpm Cutter width > block width
Specific energy: 1.1 hp*min/in3
Table 20:2 (annealed mildsteel)
Find Material Removal Rate Power Torque Cutting time
secondst 37.5t
L Lc( )
n60
inLc 0.5Lc D d
lb fTorque 577.7Torque PowerN
330002
hpPower 11Power spec_enrg MRR
in3
minMRR 10MRR w d v
in
min
v 20v f N n
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Face Milling
Cutter is mounted on a spindlehaving an external axis ofrotation perpendicular to theworkpiece surface
Workpiece moves along astraight path at a linear speed, v.
Direction of cutter
Conventional milling (Fig. c) up-milling Climb milling (Fig. b)
down-milling
Leaves feed marks on themachined surface
Terminology in figure Lead angles: 0-45o. Low anglelow vertical force
Figure 23.8 Terminology for a
face-milling cutter.
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Face Milling
Figure 23.6 A face-milling cutter with indexable inserts.
Source: Courtesy of Ingersoll Cutting Tool Company.
v
d w
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Cutter and Insert Position in Face Milling
Figure 23.10 (a) Relative positionof the cutter and insert as it firstengages the workpiece in facemilling, (b) insert positionstowards the end of the cut, and (c)examples of exit angles of insert,showing desirable (positive ornegative angle) and undesirable(zero angle) positions. In all
figures, the cutter spindle isperpendicular to the page.
Third example in figure c: the
insert exits the workpiece
suddenly as opposed to exiting
with an angle.
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End milling
The cutter, called an endmill, rotates about an axis
perpendicular to theworkpiece surface(typicallycan be at an
angle)Ball nose: A type of end
mill in which the bottomsurface is rounded Used in the production of
curved surfaces for diesand molds
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Other Milling Operations and Cutters
Straddle milling: two or morecutters are mounted on anarbor and are used to machinetwo parallel surfaces on theworkpiece Easier to keep tolerances than if
milling one surface at a time
Form milling: producescurved profiles. Also used inmachining gear teeth.
Circular cutters can be usedfor slotting and slitting. Slitting saws are typically