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1. MillingMilling

2. 24.1 Introduction24.1 Introduction
Milling is the basic process of progressive chip
removal to produce a surface.
Mill cutters have single or multiple teeth that
rotate about an axis, removing material.
Often the desired surface in obtained in a single
pass of cutter or workpiece with very good
surface finish.
Milling is particularly well suited and widely used
for mass production.
More flat surfaces are produced by milling than
by any other machining processes.

3. 24.2 Fundamentals of Milling 24.2 Fundamentals of Milling
ProcessesProcesses
Milling is classified in two categories:
◦Peripheral milling (also called Slab milling) -
the surface is generated by teeth located on the
periphery of the cutter body. The surface is
parallel with the axis of rotation of the
cutter.
◦End milling: also called facing milling, the
surface generated is at a right angle to the
cutter axis. Material is removed by the
peripheral teeth and the face portion providing
finishing action.

4. Peripheral MillsPeripheral Mills
FIGURE 24-1 Peripheral
milling can be performed
on a horizontal-spindle
milling machine. The
cutter rotates at rpm Ns ,
removing metal at cutting
speed V. The allowance
for starting and finishing
the cut depends on the
cutter diameter and depth
of cut, d. The feed per
tooth, ft and cutting speed
are selected by the
operator or process
planner.

5. Peripheral MillingPeripheral Milling
The milling variables, such as cutting speed V and
feed per tooth depend upon the work material,
the tool material, and the specific process.
The rpm of the spindle is determined from the surface
cutting speed V, the cutter diameter D (in inch) as
below:
N
s = (12V)/( D)
The feed of table fm, in inch per minute, is calculated:
f
m = f
t N
s n
Where ft, feed per tooth, and n is the number of teeth
in the cutter.
The cutting time is:
T
m
= (L + L
A
)/f
m

6. Peripheral MillingPeripheral Milling
The length of approach is:
L
A = SQRT[D
2
/4 – (D/2-DOC)
2
]= SQRT(d(D-d))
The MRR is:
MRR = Volume/T
m = (LWd)/T
m = Wf
md in
3
/min
where W is width of the cut in inch, d is the depth of
cut in inch.
If ignoring L
A, the values for f
t are given in Table 24-1,
along with recommended cutting speeds in feet per
minute.

7. Suggested Starting Feeds and Speeds using HSS and Suggested Starting Feeds and Speeds using HSS and
Carbide CuttersCarbide Cutters

8. Face Mills Face Mills
FIGURE 24-2 Face
milling is often
performed on a
spindle milling
machine using a
multiple-tooth cutter
(n = 6 teeth)
rotating Ns at rpm
to produce cutting
speed V. The
workpiece feeds at
rate fm in inches
per minute past
the tool. The
allowance depends
on the tool diameter
and the width of
cut.

9. Face MillingFace Milling
The rpm of the spindle is determined from the surface
cutting speed V, the cutter diameter D (in inch) as below:
N
s
= (12V)/( D)
The feed of table fm, in inch per minute, is calculated:
f
m = f
t N
s n
Where f
t
, feed per tooth, and n is the number of teeth in
the cutter.
The cutting time is:
Tm = (L + L
A
+ L
0
)/f
m
The MRR is:
MRR = Volume/T
m = (LWd)/T
m = Wf
md in
3
/min
For a setup where the tool doesn’t completely pass over
the workpiece,
L
0
= L
A
= SQRT(W(D – W)) for W < D/2
L
0
= L
A
= D/2 for W>=D/2

10. Face Milling ExampleFace Milling Example
For a 4” diameter, six-tooth end mill, using
carbide inserts (Fig 24-3), the work
material is low-alloy steel, annealed (BHN
= 200). Please determine rpm at the
spindle and the feed rate of the table.

11. Face Milling ExampleFace Milling Example

Using cutting data recommendations, select V = 400 sfpm
with a ft = 0.008”/tooth.
N
s
= (12V)/( D) = (12 x 400)/ (3.14 x 4) = 392 rpm
The feed rate of table is:
f
m = f
t N
s n = 0.008 x 6 x 392 = 19”/min
If slab milling were being performed (see Fig. 24-4) with the
same parameters being selected, the cutting time for face
cutting is more than for slab milling because the allowances
A
0
for face milling are greater than for slab milling.
In milling, power consumption is usually the limiting factor.

12. Vertical and Horizontal CuttersVertical and Horizontal Cutters
FIGURE 24-3 Face milling
viewed from above with vertical
spindle-machine.
FIGURE 24-4 Slab or side
milling being done as a up
milling process with horizontal
spindle-machine.

13. End MillingEnd Milling
FIGURE 24-5 End milling a step feature in a block using a flat-bottomed, end mill cutter in a vertical
spindle-milling machine. On left, photo. In middle, end view, table moving the block into the cutter. On
right, side view, workpiece feeding right to left into tool.

14. End Milling ExampleEnd Milling Example
In Fig 24-5, an end mill with 6 teeth on a 2” diameter
(carbide cutter) is used to cut a step in 430F
stainless. d = 0.375” and the depth of immersion
is 1.25”. The vertical milling machine tool has a 5-
hp motor with an 80% efficiency. The specific
horsepower for 430F stainless(BHN = 300) is
1.3hp/in
3
/min. Can the step be cut in one pass or
in multiple passes?

15. End Milling ExampleEnd Milling Example
The maximum amount of material that can be removed per pass
is usually limited by the available power.
H
p = HP
s x MRR = HP
S x f
mW DOC
max
= HP
s
f
m
x DOI x DOC
max
From Table 24-1, select f
t
= 0.005 ipt, V = 250 fpm.
N
s
= (12 x 250)/(3.14 x 2) = 477 rpm
f
m
= f
t
x n x N
s
= 0.005 x 6 x 477 = 14.31”/min
The actual table feed rates for selected machine are 11”/min. or
16”/min. Select f
m
= 11”/min.

16. End Milling ExampleEnd Milling Example
DOC
max
= (0.8hp)/(HP
s
f
m
DOI)
= (0.8x 5)/(1.3 x 11 x 1.25 ) = 0.225”
So two cutting passes are needed because 0.375/0.225 =
1.6.
The first pass: DOC
1 = 0.225” rough cut
The second pass: DOC
2 = 0.15”
For DOC
2
= 0.15” the f
t
would be only slightly increased to
0.0057ipt
F
t = (0.8hp)/(HP
s n N
s DOC DOI)
= (0.8 x 5)/(1.3 x 6 x 477 x 0.15 x 1.25)
= 0.0057ipt

17. Up Versus Down MillingUp Versus Down Milling
Up milling or Conventional milling
◦The cutter rotates against the direction of feed
of the workpeice.
◦The Chip is very thin at the beginning and
increased along its length.
◦The cutter tends to push the work along and lift
it upwards from the table. The action tends to
loosen the workpiece from the fixture .
◦In the up milling, chips can be carried into the
newly machined surface, causing the surface
finish to be poorer than in down milling .

18. Up Versus Down MillingUp Versus Down Milling
Down milling or Climb milling
the cutter rotates in the same direction as
the direction of feed
◦Advantage:
The work piece is pulled into the cutter, eliminating
any effects from looseness of the work table feed
screw.
There is less tendency for the machined surface to
show toothmarks, and the cutting process is
smoother, with less chatter.
The cutting force tends to hold the workpiece
against the machine table, permitting lower
clamping force.
◦Disadvantage:
The maximum chip thickness is at the point of tooth
contact with the work piece . Dulling the teeth more
quickly, especially for workpiece with a hard surface.

19. Climbing versus Conventional MillsClimbing versus Conventional Mills
FIGURE 24-6 Climb
cut or
down milling versus
conventional cut or
up milling
for slab or face or end
milling.

20. Milling Surface FinishMilling Surface Finish
Milling is an interrupted cutting process .
◦Impact loading
◦Cyclic heating
◦Cycle cutting forces
As show in Fig. 24-7, the cutting force, Fc, builds
rapidly as the tool enters the work at A and
progresses to B, peaks as the blade crosses the
direction of feed at C, decreases to D, and then drops
to zero abruptly upon exit.
The interrupted-cut phenomenon explain in large part
why milling cutter teeth are designed to have small
positive or negative rake angles , particularly
when the tool material is carbide or ceramic.
Cutters made from HSS are with positive rakes, in
the main, but must be run at lower speeds.

21. Facing MillFacing Mill
FIGURE 24-7 Conventional face milling (left) with cutting force diagram for Fc (right) showing the
interrupted nature of the process. (From Metal Cutting Principles, 2nd ed., Ingersoll Cutting Tool
Company.)

22. 24.3 Milling Tools and Cutters24.3 Milling Tools and Cutters
There are a variety of mills used, the most
common being face mills and end mills
◦End mills are either HSS or have indexable
inserts (Figure 24-8)
◦End Mills come in a variety of geometries
Plain End Mills
Shell End Mills
Hollow End Mills

23. Other Mill Cutter TypesOther Mill Cutter Types
Face mills have indexable inserts along
the periphery
Face Mills come in a variety of geometry
(Figure 24-9)
◦Center hole for arbor mounting
◦Side mill (Figure 24-10)
◦Staggered-tooth
◦Straddle milling
◦Interlocking slot cutters
◦Slitting cutters

24. Typical Cutter ProblemsTypical Cutter Problems

25. End Mill GeometryEnd Mill Geometry
FIGURE 24-8 Solid end mills are often coated. Insert
tooling end mills come in a variety of sizes and are
mounted on taper shanks.

26. Facing Mill GeometryFacing Mill Geometry
FIGURE 24-9
Face mills come
in many
different designs
using
many different
insert
geometries
and different
mounting
arbors.

27. Side MillingSide Milling
FIGURE 24-10 The
side-milling cutter
can cut on sides
and ends of the
teeth, so it makes
slots or grooves.
However, only a few
teeth are engaged
at any one point in
time, causing heavy
torsional vibrations.
The average chip
thickness, hi, will be
less than the feed
per tooth, ft . The
actual
feed per tooth fa will
be less than feed
per tooth selected,
Ft .

28. Arbor MillingArbor Milling
FIGURE 24-11 Arbor (two views) used on a horizontal-
spindle milling machine on left. On right, a gangmilling
setup showing three side-milling cutters mounted on an
arbor (A) with an outboard flywheel (B).

29. Helical MillsHelical Mills
FIGURE 24-12 The chips are
formed progressively by the
teeth of a plain helical-tooth
milling cutter during up milling.

30. Shaped CuttersShaped Cutters
Form Relieved Cutters are used when
intricate shapes are needed.
T-slot cutters are used to produce slots
in material. An end mill is use first to
produce the initial groove
A woodruff keyseat cutter is used to
produce a slot in a shaft and come in
standard sizes
Fly cutters are single toothed face mill
cutters, with adjustable radii.

31. Relieved CutterRelieved Cutter
FIGURE 24-13 Solid
form
relieved milling cutter,
would be
mounted on an arbor
in a
horizontal milling
machine..

32. 24.4 Machines for Milling24.4 Machines for Milling
The four most common types of manually
controlled milling machines are listed
below in order of increasing power (and
therefore metal removal capability):
◦1. Ram-type milling machines
◦2. Column-and-knee-type milling
machines
a. Horizontal spindle
b. Vertical spindle
◦3. Fixed-bed-type milling machines
◦4. Planer-type milling machines

33. Machines for MillingMachines for Milling
Milling machines whose motions are electronically
controlled are listed in order of increasing
production capacity and decreasing flexibility:
◦1. Manual data input milling machines
◦2. Programmable CNC (Computer Numerical
Controlled) milling machines
◦3. Machining centers (tool changer and pallet
exchange capability)
◦4. Flexible Manufacturing Cell and Flexible
Manufacturing System
◦5. Transfer lines

34. Basic Mill ConstructionBasic Mill Construction
Most mills consist of column-and-knee designs
◦The column is mounted on a base and the spindle
mounted on a knee extending from the column.
◦The knee has vertical movement
◦The material in mounted on a table with longitudinal
movement, and the table is mounted on a saddle
with transverse movement
Most common of this type mill is the Ram mill
which has a motor and pulley system mounted on
the top of the column.

35. Major Components of a Plain Column-and-Major Components of a Plain Column-and-
Knee-Type Milling MachineKnee-Type Milling Machine
FIGURE 24-14 Major components of a plain column-and-knee-type milling machine, which can
have horizontal spindle shown on the left, or a turret type machine with a vertical spindle, shown
on the right. The workpiece and workholder on the table can be translated in X, Y, and Z
directions with respect to the tool.

36. Ram-type Knee-and Column Ram-type Knee-and Column
MachineMachine
FIGURE 24-15
The ram-type
knee-and-
column milling
machine is one
of the most
versatile and
popular milling
machine tools
ever designed.

37. Bed Type Milling MachineBed Type Milling Machine
Made for deep cuts and heavy material
removal, the bed only had horizontal
movement
Once the bed is set up, the spindle height
is not changed during operation.
These machines are very common due to
their ease of use.

38. Bed Type MillBed Type Mill
FIGURE 24-16
Bed-type
vertical-spindle
heavy-duty
production
machine tools for
milling usually
have three axes
of motion.

39. Planer Type MillPlaner Type Mill
Planer type mills can have several heads
to remove large amounts of material
while the material is fed slowly into the
machine.
Systems are setup typically for single
pass operations.
These are advantageous for large work
pieces requiring heavy material
removal.

40. Large Planer-type Milling MachineLarge Planer-type Milling Machine
FIGURE 24-17 Large
planertype milling
machine. Inset shows 90°
head being used.
(Courtesy of Cosa
Corporation.)

41. Milling Machine SelectionMilling Machine Selection
When purchasing or using a milling
machine, consider the following issues:
◦1. Spindle orientation and rpm
◦2. Machine capability (accuracy and
precision)
◦3. Machine capacity (size of workpieces)
◦4. Horsepower available at spindle (usually
70% of machine horsepower)
◦5. Automatic tool changing

42. HW for Chapter 24HW for Chapter 24
Review Questions:
2, and 9 (pages 674)
Problems:
1, 2, 6. (page 675)
Note:
For HW 1: Number of teeth n = 8.
For HW 6:
Material of workpiece: Cast iron, medium hardness, d = DOC =
0.214”

43. HW for Chapter 24HW for Chapter 24
Problems:
1 (page 675)

(Calculate the table speed f
m
)

44. HW for Chapter 24HW for Chapter 24
Problems:
2 (page 675)