Cutter

axis
.,,,,,..-
/

I
/ "' \
I \
\
.----4------1 --IL- Finished

///
/ surface

{a)

Cutter axis

Finished
surface

-----
1-~L____l__L__
/
(b)
FIGURE 4-114
(a) Peripheral and ( b) face milling.
 \ Cutting
forces

Cutting
forces

(a) (b)

U
RE 4-115 d (b) climb (do wn) milling.
(a) Conven tional (up) an
FIG
 + High
rake
- -

Forces approximately parallel

to body of cutter tooth

FIGURE 4-116
Climb-milling and high-radial-rake
milling cutters.
 Grinding wheel

I
_j
I
I
\
\ I - -._
I
\
\
\
\
Curvature of tooth
forms relief
I \
\
I
\

-----+-----
...

(a) (b)

FIGURE · 4-118
Methods of resharp_ening milling cutters. (a) Form relief. Relief is
provided at the time of manufacture; the face of the tooth is ground on
~- radial line to resharpen. ( b) Profile relief. Relief is ground behind the
cutting edge in the form of a land.
 ',

\-
f \

v
(c)

(e) (f)
(d)

(h) (i)
FIGURE 4-121 (c)
Standard milling cutters: (a) double-angle, (b) Woodruff k~Y, (g)
concave and convex, (d) plain, (e) side, (f) staggered-tooth side &
interlocking side, (h) metal-slitting sa\iv, (i) screw-slotting. (Pratt
Whitney.)
 profile relief is more frequently used and is found on end mills, plain and side mills,
face and shell mills, angle cutters, etc. Profile-sharpened form cutters, because of
~=~ifficulties involved in resha?pening, are generally used only for simple forms. Form
t i·ef is used o~ gear-tooth cutters, concave and convex cutters comer-rounding cutters,
re1 tt . I . . '
and special cu ers mvo vmg ~mphcated forms. Form-relieved milling cutters can be
des1•gned and manufactured
. . to null
. almost any form if the part can be positioned and held
flJ that the entire form IS acceSSible to the cutter. Figure 4-121 shows several examples of

~andard milling cu~te~s. The following standard-cutter listing gives the name of the
cutter, general descnpt1on, and type of relief.

Plain mil~ing cutter (profile relief) A cutter of plain cylindrical form having teeth
on the circumferential surface only. Teeth may be either straight or helical. This
cutter is used for slabbing.
Sfde milling ~utter (profile relief) A cutter of cylindrical form having teeth on the
cir_cumferen~1~ surface and also on both sides. These cutters are frequently used in
pairs for mlllmg both ends of the work at the same time. This is called straddle
milling.
Half-side milling cutter (profile relief) A cutter similar to a side milling cutter but
with teeth on the one side only. In both the side and the half-side cutter, the side
teeth extend a portion of the distance from the circumference toward the axis.
Interlocking side milling cutter (profile relief) Similar to a side milling cutter
except that it is composed of two interlocking sections that can be adjusted by the
use of shims to form a complete cutter of exact width.
Staggered-tooth milling cutter (profile relief) A cutter of cylindrical form having
cutting teeth on the circumferential surface only, the teeth being alternately of
opposite helix or angle. This type of cutter has side teeth extending a short distance
toward the axis for the purpose of chip clearance only; these side teeth are not j
ground for cutting purposes.
Single-angle milling cutter (profile relief)) A cutter having teeth on the conical
surface and with or without teeth on one or both of the flat sides. Designated by
the included angle between the conical face and the larger flat surface.
Doubie-angle · milling cutter (profile relief) A cutter having two intersecting
conical surfaces with teeth on both. The angle of teeth may or may not be
symmetrical with respect to a plane at right angles to the axis.
Metal-slitting saw (profile relief) A plain milling cutter wi~h ~d~s relieved or
"dished" to afford side clearance. It has more teeth than a plam m1llmg cutter and
generally has a thickness less than 136 in. .
Screw-slotting cutter (profile relief) A cutter used for .shallow cuts as 10
screwdriver slits. It has fine teeth on its circumference and 1s not ground on the
.
End mill (profile relief) A cutter with teeth on the circumf~rent1~ sur~ace and
be straight or tapered. The teeth may be helical or parallel
one . end . The shan k may . . d - ·th d t h 1· gl
to the axis of rotation. A spiral end m11l 1s an en m111 w1 a mo era e e IX an e.
Shell end mill (profile relief) Similar to an end mill with a hole for an atbor
· t ead o f h av1ng
ins · a shank . Generally· driven from a key slot across the back of the
face. The teeth may be straight or spiral. . . .
T-slot cutter (profile relief) Similar to an end mill but designed for cutting T slo1.s.
 Woodruff key-seat cutter (profile relien Either a shank or arbor cutter designed
to cut semicylindrical keyways in shafts for Woodruff keys.
Gear cutter (form relien A formed cutter specially designed to cut one space at a
time in gears.
Multiple gear cutter (form relien A single cutter unit made to mill several gear
spaces at one p~. Generally used for roughing out gear blanks.
Convex cutter (form relien A formed cutter designed_ to_ mill a C?ncave surfat>e
equal to a half-circle or less. Size is designated by specifying the diameter of the
circular form. ·
Concave cutter (form relien A formed cutter shaped to mill a convex surface of
circular contour equal to half a circle or less. Size is designated by the diameter of
the circular fonn.
Nomenclature of milling-cutter elements The Metal Cutting Tool Institute lists the
following as the important milling~utter elements:
Angulat flute A space between two cutter teeth which forms a cutting edge lying
in a plane intersecting the tool axis at an angle. It is unlike a helical flute in that it
fonns a cutting edge which lies in a single plane (see Fig. 4-122.a).
Axial iunout The total variation in an ~ial direction of a cutter element from a
true plane of rotation.
Body The carrier or head for holding blades, or that part of a solid or tipped
cutter exclusive of the teeth or shank.
Chamfer (1) A beveled surface to eliminate an otherwise sharp corner; (2) a
relieved angular cutting edge at a tooth corner.
Clearance The additional space provided behind the relieved land of a cutter
tooth to eliminate undesirable contact between the cutter and workpiece (see Fig.
4-122b).
Comer angle On face-milling cutters, the angle between an angular cutting edge of
a cutter tooth and the axis of the cutter, measured by rotation into an axial plane
(see Fig. 4-122c).
Entrance angle The angle formed between a centerline on the cutter which is
perpendicular to the direction of feed and a radial line through a point on the
cutting edge where the tooth first contacts the workpiece (see Fig. 4-122d).
Face cutting edge That edge of the tooth on a face mill which travels in a plane
perpendicular to the axis. It is the edge which sweeps the milled surface in the
normal operation of a face-milling cutter (see Fig. 4-122c).
Face cutting edge angle The angle of concavity between the face cutting edge and
the face plane of a face mill. It serves as relief to prevent the face cutting edges
from rubbing in the cut (see Fig. 4-122c).
Fillet The bottom surface of the flute (see Fig. 4-122b).
Flute The chip space between the back of one tooth and the face of the following
tooth (see Fig·. 4-122b).
Form cutter Any cutter, profile-sharpened or cam-relieved, shaped to produce a
specified form on the work.
Form tool As related to milling cutters, a tool used to sbape a cutter blank or to
produce the form on a cam-relieved cutter.
Heel (1) The back edge of the relieved land (see Fig. 4-122e) • (2) the inner end of
a face-cutting edge (see Fig. 4-122c). '
 Angular')-..._/
flute
·- - - - ---t-----=---:_-....,-.--- -,......-

(a)

Radial relief angle t Peripheral

-----==-,?Tl cutting edge
Clearance surface
Axial
Land relief
angle
Radial rake angle J
' positive shown --+-- Tooth
Offset-_s;;~h~
Heel Concavity-
Fillet---- ·11.

(b)
Radial rake angle
positive ~ho~n Face cutting-edge angle
Carbide t i p
- ~
Face cutting edge Peripheral cutting edge
Blade orner angle
Cutter b o d y ~ Lead ·
. . .a..:::::::::::::,.,
Bolt circle
=.{
_tleel
Enlarged view
of bl~de
cuttmg ....!..-~~
-+-+--,-- edges 1

Axial rake Drive slot

1
angle
positive shown
Effective diameter

(c)
 -= I
( e radial rake
-Nega t \

Entrance \ •Face
' ridge

/ angle , \\.\-K land

(d)

- 1- Neck Flute Heel

Cu~~n~-

E d
nh
gas
I Conventional_ Raised
land
_ lan<;f
Helix angle

(e)

/ - - Face width -1
I Helical teeth
Helical rake
angle, left-hand
helix shown

(f)

Nested
2 2

(g)
 Primary

I
Relieved
- -- 1--
Cylind ricu l _ Offset relf
land
land
9Cf

(h)

Tangential
rake

(i)

Radial rake angle

positive shown ;--
Radial relief Tooth face
Tooth
Flute Axial relief

Fillet~
~~

(j)
 Radial
relief

r<: ~:::~,
Internal . External
back taper back taper

+- --M
'-/y:~:i:
Axial -,--..-.,.._

Concavity
___j__ Flat _l .
"- relief Eccentnc
relief
V
-

(k)

/ _ Wearland
____,. ..

(I)

FIGURE 4-122
Milling-cutter elements; (a) angular flute, (b) peripheral milling cutt~r,
(c) face-milling cutter, (d) face milling, (e) end-milling cutter, (f) plai~
milling cutter, (g) nested and interlocking cutters, (h) types of la~ds, (z)
hook of cutter tooth, (j) form-relieved cutter, (k) types of rehef, (l)
wear land. (The Metal Cutting Institute.)

Helical A term describing a cutting edge or flute which progresses uniformly

around a cylindrical surface in an axial direction.
Helix angle The cutting-edge angle which a helical cutting edge makes with a
plane containing the axis of a cylindrical cutter (see Fig. 4-122n.
Interlocking Mating cutter sections on which side projections on one section mesh
with those on an adjacent section to provide cutting-edge overlap (see Fig. 4-122g).
K land A relatively narrow land on the face of a tooth from the cutting edge
inward which is at a lesser rake angle than the main face of the tooth. It is the
surface of the tooth on which the chip is intended to impinge so as to reduce
contact between the chip and the whole tooth face (see Fig. 4-122d). -·
 i:: d The narrow surface of a profile-sharpened cutter tooth immediately behind
cutting edge (see Fig. 4-122h).
~ylindrical land nan:ow portion of the peripheral land, adjacent to the cutting
e having no radial rehef.
d
eg' Th rt' f th
Believed e po ion ° e land adjacent to the cutting edge which provides a
relief. .
Lead ,(1). The axial advanc! of a helical cutting edge in one tum around the axis;
(2) the relleved angular cutting edge between the comer angle and the face cutting
edge of a face mill (see Fig. 4-122c).
Nesting The axial overlapping of cutters of different diameters without the use of
interlocking projections (see Fig. 4-122g).
Radial runou~ The total variation in a radial direction of all cutting edges in a
plane of rotation.
Rake The angular relationship between the tooth face or a tangent to the tooth
face at a given point and a given reference plane or line.
Axial rake Applies to ~ngular (not helical) flutes. The axial rake at a given point
on the f~ce of th~ flute 1s the angle between the tool axis and a tangent plane at the
given point (see Fig. 4-122c).
Effective rake The complement of the angle between the direction of motion of
any point on the cutting edge and the direction of chip flow from the same point.
Effective rake is therefore the rake resulting from three factors: (1) the cutter
geometry, (2) the actual path of the cutting edge, and (3) the actual direction of
the chip flow. ·
Helical rake Applies to helical teeth (not angular). The helical rake at a given
point on the flute face is the angle between the tool axis and a tangent plane at the
given point (see Fig. 4-122/).
Hook A concave condition of a tooth face. The rake of a hooked tooth face must
be determined at a given point (see Fig. 4-122i).
Negative rake Describes a tooth face in rotation whose cutting edge lags the
surface of a tooth face (see Fig. 4-122d).
Positive. rake Describes a tooth face in rotation whose cutting edge leads the
surface of the too.th face (see F.ig. ·4-122j).
Radial rake The·angle between the too.th face and a radial line passing through the
cutting edge in a plane perpendicular to the cutter axis (see Fig. 4-122J). .
True rake See Effective rake.
Relief The result of the removal of tool material behind or adjacent to the cutting
edge to provide clearance and prevent rubbing.
Axial relief The relief measured in the axial direction between a plane
perpendicular to the axis and the relieved surface. It can be measured ~Y the
amount of indicator drop at a given radius in a given amount of angular rotation.
Back taper A slight reduction of the outside_ diameter _from fr~nt to back of the
e~n~ially cylindrical surface in which the cutting edges he (see Fig. 4-122k).
Cam relief The relief from the cutting edge to the ~ac~ of the tooth prod~ced by
a cam-activated cutting tool or grinding wheel on a rehevmg (back-off) machine (see
Fig. 4-122k). -
Eccentric relief A convex relieved surface behind the cutting edge (see Fig.
4-122k).
 .
Primary re 1·,ef The relief immediately behind the cutting edge.
Relief angle The angle formed between a relleved surfa~ and a given Plane
tangent to a cutting edge or to a point on a cutting e~e (see Fig. 4-122k)._
•n
Radial relief Relief in a radial direct.ton measu~9, the pla~e of ~otation. ~t can
be measured by the amount of indicator drop at a given radius m a given amount of
angular rotation (see Fig. 4-122k).
Secondary relief See Clearance, the preferred term.
Shank The projecting portion of a cutter which locates and drives the cutter frorn
the machine spindle or adapter.
Taper shank A cutter shank made to fit a specified ( conical) taper socket.
Tooth face The surface of the tooth on which the chip impinges (see Fig.
4-l22b).
Wear land The cylindrical or flat land worn on the relieved portion of the cutter
tooth (see Fig. 4-1221).

Selection of cutter geometry and design The major elements to consider when selecting
a milling cutter for a particular application are size, teeth, flutes, relief, and material. The
following discussion gives a brief general description of these elements and how they may
be influenced by operating conditions.

Size The face width of the cutter must be wide enough to provide adequate support
behind the cutting edges. The cutter diameter should be kept as small as possible because
small-diameter cutters require less torque and deflect less. The diameter will depend upon
the depth of the flute and the diameter of the hole in the cutter. A general rule for
diameter is to provide a minimum ratio of 3: 1 between cutter diameter and hole
diameter. Larger-diameter cutters ~ay be required when interference between the outer
arbor support and workpiece occurs.

Teetfz The solution of the correct number of teeth in a milling cutter is a compromise
depending upon working material, type of milling cutter, and the surface tmish required.
A milling cutter with a relativ~ly large number of small teeth around its periphery will
promote smoother cutting because more teeth are in the cut. When other cutting
conditions are constant, a cutter of many teeth will also have a finer feed per tooth,
which results in a better surface finish., However, a cutter of many teeth may not have
adequate chip space to prevent chip interference. A soft material that allows a heavy feed
rate and produces long cont_inuous chips requires a greater chip space. An effective way of
providing chip space is by ·reducing the number of teeth in the cutter and increasing the
size of the individual tpoth. This provides a stronger tooth and is therefore suitable for
heavy roughing cuts when a large volume of material is being removed.
A milling cutter should have enough teeth to ensure uninterrupteq contact with the
workpiece and yet not so many that there is too little space between the teeth for chip
disposal. A rule of thumb is to select a cutter so that a minimum of one tooth will always
be in the cut. This will ensure that one tooth will not leave the workpiece before another
enters and will promote a smooth cutting op~ration. Additional teeth may be added to
 ve the surface finish, but the .trend is to specify a cutter having a coarse tooth when
ifflPro and wide cuts are necessary. In this instance, it has been found that coarse-tooth
bea\lYrs with fewer teeth will stand up longer than fine-tooth cutters with many teeth.
cut_te does not hold true when using narrow plain mills and slitting saws. (The main
'fb15 tage of specifying a fine-tooth milling cutter is that a smdother surface finish is
a::ced without reducing the feed rate.) ·
P Regarding the number of teeth and type of cutter, fewer teeth can be used on a
'rnill because the tooth contact is.usually longer. In other words, each tooth is in the
face • d·oft··1me.
t for a longer .peno
cu Milling cutters designed for machining aluqiinum alloys require additional chip
ace, which in this instance is often provided by increasing the size of the cutter flute.
ibis weakens the tooth but is acceptable because milling soft aluminum does not require
rnaxirnum tooth strength.

Flutes The.flutes of 11, milling cutter can be straight, helical, or angular. Helical flutes are
used on the majority of cutters designed for wide peripheral cuts. Helical teeth form the
chip -at an angle with respect to the direction of feed. This provides a shearing action, and
the helix.angle of the flute is S<>metimes incorrectly referred to as the "shear angle" of the
cutter. I

When milling-cutter teeth are cut on a helix, the entire cutting edge does not come
into contact :with t,he workpiece at ~ne ~ime. The hel~ causes more than one tooth to be
in contact with the wotkpiece along a single contact Jine parallel to the cutter axis. Each
individual tooth picks up the chip gradually until a .maximum chip load is attained. The
maximum .chip load conti~ues for a considerable part of the tooth travel and then
,, J , L I

decreases gr~ually as: the tooth leave~ the workpiece. This action provides smooth and
continuo~s cutting and ~a~ly .
decreases the tendency to chatter.
' ' '
The·helix may be either right- or left-hand. To determine whether the flute helix is
right- or lef~-11.a~d.,..visu~jze it as a c~mmon screw thread (another form of helix). If it is a
right-hand thread, it is' a',, rj;ght-han4. helix and
' ' .
yice versa. Standard end mills are made in
various combinations of hands of rotation and hands of helix. A right-hand cut and
right-hand helix is excellent for milling slots and shoulders, in that chips are augered away
from 'the finished work -surface:. However, there is a tendency for th~m to suck in or grab
with some types of work materials because the tooth tends to '·act like a thread. A
right-hand cut, left-hand helix would help offset this condition because the helix would
force th.e ' cutter in the opposite axial direction. Right-hand-cut-left-hand-helix end mills
are also well suited for profile operations when the cutter is used to cut on its periphery
only. They tend to push the workpiece down toward the heavy part of the machine and
provide a much· steadier cutting action.
The cutting action of a tooth formed by a straight flute is intermittent. When the
tooth enters the workpiece, the whole length of ~he tooth takes the full load and the
cutting forces increase rapidly. The forces continue to increase until the tooth leaves the
cut and then suddenly drop. The shock load produce~ by this type of cutting generally
reacts through the drive mechanism of the machine to produce chatter.
 The majority of form-relieved cutters have straight flutes because ~f ~he difficul
in sharpening form relieved cutters. Narrow plain milling cutters and shttmg saws m
have straight flutes. In general, however, .the use of milling cutters with straight flu
should be avoided.
Angular flutes on milling cutters are a compromise between helical and straig
flutes. They are best suited to cutters with ·symmetrical fonns and narrow face width
The face of the tooth is at an angle to the cutter axis and therefore the radial rake Vari
from one side of the cutter to the other. They are not recommended for wide cutters fo
this reason. Special sharpening equipment is not needed for angular-flute cutters. It i
only necessary to align t ae swivel table of the cutter grinder with the angle of the toot
and sharpen through thE.- gash in a straight line.

Relief Relief has already been discussed in general tenns. In review, two methods ofi
relief are provided which actually define two basic cutter types. These cutter types are
the fol'Il)- and profile-relieved cutters, and the names are descriptive of their design. The
following discussion will be concemed with the specifics of relief.
The relief is not affected when resharpening form-relieved cutters because only the
faces of the teeth are ground. The relief is built in at the time of manufacture.
Resharpening profile-relieved cutters consists of removing enough from the top of the
teeth back of the cutting edge to remove the wear land. The angle of relief to be
sharpened into the cutter depends upon the diameter of th·e cutter, the cutting-tool
material, and the material to be milled. The angle must be greater for small .cutters than
for large ones in order to prevent heel drag.
Relief angles sho1Ild be just enough to eliminate heel drag. This practi~ leaves more
metal for heat dissipation and ensures maximum strength at the cutting edge. Excessive
relief angles tend to increase the likelihood of chatter and may add to the causes of
failure during heavy milling operations.
The hardness of the material ~as an effect on -the correct relief angle. Larger relief
angles may be used with softer materials because maximum strength at the cutting edge is

Table 4-23 AVERAGE RELIEF ANGLES FOR WORK MATERIALS OF VARYING

HARDNESS
Work material
Cutter Tool material
Cast Non-ferrous
Steel iron and
non-metallic

peripheral or OD cutting edges* High-speed steel 5-10° 5-10°

Cast alloy
7-12°
4-6° 4-6° 5-10°
side or end cutting edges Cemented carbide
All
4-6° 4-6° 5-10°
1-4° 1-40 2-7°
* Smaller-diameter cu
SdURC tters require lqer relief ana].es.
E: Barber Colman "Milllna Cutter Handbook."
 _ _ Relief on new Relief on cutter
cutter (primary) after repeated
sharpenings ; heel
-- ---
drag is result
--- --------,
',

Cutter tooth Additional secondary

relief Cutter tooth

FIGURE 4-123
The width of land must be narrow enough to prevent heel drag.

not required. Average relief angles for work materials of varying hardness are given in
Table 4-23.
The width of land (the narrow surface immediately behind the cutting edge that is
ground to the relief angle) will depend upon the type and size of the milling cutter. The
land must be narrow enough to prevent the heel from dragging on the finished workpiece
surface. This may vary from 0.005 to 0.010 in. on small end mills and up to i in. on
large-diameter cutters, with i"1. in. ton in. as an average. The land may become too wide
as a ~suit of repeated sharpenings, and the heel of the tooth will drag on the workpiece.
To control the width of the land, a clearance angle (sometimes called secondary relief) of
twice the relief angle (approximately) is ground as shown in Fig. 4-123.
The relief on side cutting edges may be less than that on peripheral cutting edges
because side cutting edges do not cope with a radial surface. Heel drag is not a problem
because it does not exist. Relief on side cutting ed~es is usually one-quillter to one-half
that of the peripheral cutting edges.
The relief on end teeth of end and face mills may also be less for the same reason.
Multiple-tooth high-speed-steel end mills are generally ground with about 4° relief,
although two-flute' end mills may have as much as 7 because they are frequently plunged
' 0

endwise into solid stock.

End and side teeth are often sharpened with radial taper by removing from 0.001 to
0.002 in. more material ne~ the center of the cutter than at the outside. This prevents
the teeth from rubbing on the finished workpiece surface.
It has been previously noted that relief must be increased as the diameter of the
milling cutter becomes smaller. The smaller cutters must have extremely large relief aniles
in order to prevent heel drag. For example, a ¼-in. cutter has from 12 to 15° relief, and
!-in. cutter has from 16 to 19° relief. Table 4-24 gives relief angles for the smaller cutte
The necessary increase in relief on small cutter.s results in a decrease in cutting-ed
strength because of the removal. of material behind the cutting edge. To increase t
strength of the cutting edge on smaller-diameter milling cutters, eccentric relief is often