2008年6月16日 星期一

4) Milling Cutters

Milling Cutters
a. General.
(1) There are different types of milling machine cutters. Some cutters can be used for
several operations, others can be used for only one operation. Some cutters have
straight teeth, others have helical teeth. Some cutters have mounting shanks, others
have mounting holes. The machine operator must decide which cutter to use. To make
this decision, he must be familiar with various types of cutters and their uses.
(2) Standard milling cutters are made in many shapes and sizes for milling both regular
and irregular shapes. Various cutters designed for specific purposes also are available.
(3) Milling cutters generally take their names from the operation which they perform.
Those commonly recognized are: (1) plain milling cutters of various widths and
diameters, used principally for milling flat surfaces which are parallel to the axis to the
cutter; (2) angular milling cutters, designed for milling V-grooves and the grooves in
reamers, taps, and milling cutters; (3) face milling cutters, used for milling flat surfaces
at right angles to the axis of the cutter; and (4) forming cutters, used for the production
of surfaces with some form of irregular outline.
(4) Milling cutters may also be classified as arbor-mounted, or shank-mounted.
Arbor-mounted cutters are mounted on the straight shanks of an arbor. The arbor is
then inserted into the milling machine spindle.
(5) Milling cutters may have straight, right-hand, left-hand, or staggered teeth. Straight
teeth are parallel to the axis of the cutter. If a helix angle twists in a clockwise direction,
the cutter has right-hand teeth. If the helix angle twists in a counterclockwise direction,
the cutter has left-hand teeth. The teeth on staggered-tooth cutters are alternately
left-hand and right-hand.
b. Milling Cutter Nomenclature. Figure 23 shows two views of a common milling cutter with
its parts and angles identified. These parts and angles are common to all types of cutters in
some form.
(1) Pitch. The pitch refers to the angular distance between like parts on the adjacent
teeth. The pitch is determined by the number of teeth.
(2) Face of Tooth. The tooth face is the forward facing surface of the tooth which forms
the cutting edge.

(3) Cutting Edge. The cutting edge is the angle on each tooth which performs the
cutting.
(4) Land. The land is the narrow surface behind the cutting edge of each tooth.
(5) Rake Angle. The rake angle is the angle formed between the face of the tooth and
the centerline of the cutter. The rake angle defines the cutting edge and provides a path
for chips that are cut from the workpiece.
(6) Primary Clearance Angle. The primary clearance angle is the angle of the land of
each tooth, measured from a line tangent to the centerline of the cutter at the cutting
edge. This angle prevents each tooth from rubbing against the workpiece after it makes
its cut.
(7) Secondary Clearance Angle. The secondary clearance angle defines the land of
each tooth and provides additional clearance for the passage of cutting oil and the
chips.
(8) Hole Diameter. The hole diameter determines the size of arbor that is necessary to
mount the milling cutter.
(9) Keyway. A keyway is present on all arbor-mounting cutters for locking the cutter to
the arbor.
(10) Spiral or Helix Angle.
(a) Plain milling cutters that are more than 3/4 inch in width are usually made
with spiral or helical teeth.
(b) A plain spiral-tooth milling cutter produces a better and smoother finish, and
requires less power to operate.
(c) A plain helix-tooth milling cutter is especially desirable where an uneven.surface or one with holes in it is to be milled.
(11) Types of Teeth. The teeth of milling cutters are either right-hand or left-hand,
viewed from the back of the machine. Right-hand milling cutters cut when rotated
clockwise; left-hand milling cutters cut when rotated counterclockwise.
(a) Saw Teeth. Saw teeth similar to those shown in figure 23 are either straight or
helical in the smaller sizes of plain milling cutters, metal slitting saw milling cutters,
and end milling cutters. The cutting edge is usually given about 5° primary
clearance angle. Sometimes the teeth are provided with offset nicks which break
up the chips and make coarser feeds poss Formed Teeth. Formed teeth are
usually specially made for machining irregular surfaces or profiles. The possible
varieties of formed-tooth milling cutters are almost unlimited. Convex, concave,
and corner-rounding milling cutters are of this type. Formed cutters are sharpened
by grinding the faces of the teeth radially. Repeated sharpenings are possible
without changing the contour of the cutting edge.
(c) Inserted Teeth. Inserted teeth are blades of high-speed steel inserted and
rigidly held in a blank of machine steel or cast iron. Different manufacturers use
different methods of holding the blades in place. Inserted teeth are more
economical and convenient for large-size cutters because of their reasonable
initial cost and because worn or broken blades can be replaced more easily and at
less cost.
(12) Kinds of Milling Cutters.
(a) Plain Milling Cutter (figure 24). The most common type of milling cutter is
known as a plain milling cutter. It is merely a metal cylinder having teeth cut on its
periphery for producing a flat horizontal surface (or a flat vertical surface in the
case of a vertical spindle machine). When the cutter is over 3/4 inch wide, the
teeth are usually helical, which gives the tool a shearing action which requires less
power, reduces chatter, and produces a smoother finish. Cutters with faces less
than 3/4 inch wide are sometimes made with staggered or alternate right-and
left-hand helical teeth. The shearing action, alternately right and left, eliminates
side thrust on the cutter and arbor. When a plain milling cutter is considerably
wider than its diameter, it is often called a slabbing cutter; slabbing cutters may
have nicked teeth that prevent formation of large chips.
(b) Metal Slitting Saw Milling Cutter (figure 25). The metal slitting saw milling
cutter is essentially a very thin, plain milling cutter. It is ground slightly thinner
toward the center to provide side clearance. It is used for metal sawing and for
cutting narrow slots in metal.
(c) Side Milling Cutters (figure 26). Side milling cutters are essentially plain
milling cutters with the addition of teeth on one or both sides.
1 A side milling cutter has teeth on both sides and on the periphery. When
teeth are added to one side only, the cutter is called a half-side milling cutter
and is identified as being either a right-hand or left-hand cutter. Side milling
cutters are generally used for slotting and straddle milling.
FIGURE 24. PLAIN MILLING CUTTERS.
FIGURE 25. SIDE AND METAL SLITTING SAW MILLING CUTTERS.
2 Interlocking tooth side milling cutters and staggered tooth side milling
cutters (figure 26) are used for cutting relatively wide slots with accuracy.
Interlocking tooth side milling cutters can be repeatedly sharpening without
changing the width of the slot that will be machined. After each sharpening,
a washer is placed between the two cutters to compensate for the ground-off
metal. The staggered tooth cutter is the most efficient type used for milling
slots where the depth exceeds the width.
FIGURE 26. SIDE MILLING CUTTERS.
(d) End Milling Cutters.
1 End milling cutters, also called end mills, have teeth on the end as well as
the periphery (figure 27). The smaller end milling cutters have shanks for
chuck mounting or direct spindle mounting. Larger end milling cutters (over 2
inches in diameter) are called shell end milling cutters and are mounted on
arbors like plain milling cutters. End milling cutters are employed in the
production of slots, keyways, recesses, and tangs. They are also used for
milling angles, shoulders, and the edges of workpieces.
FIGURE 27. END MILLING CUTTERS.
2 End milling cutters may have straight or spiral flutes. Spiral flute end
milling cutters are classified as left-hand or right-hand cutters, depending on
the direction of rotation of the flutes. If they are small cutters, they may have
either a straight or tapered shank.
3 Several common types of end milling cutters are illustrated in figure 27.
The most common end milling cutter is the spiral flute end milling cutter,
which contains four flutes. Two fluted end milling cutters are used for milling
slots and keyways where no drilled hole is provided for starting the cut.
These cutters drill their own starting holes. Straight flute end milling cutters
are generally used for milling soft or tough materials, while spiral flute cutters
are used mostly for cutting steel.
(e) Face Milling Cutter. Face milling cutters are cutters of large diameter having
no shanks. They are fastened directly to the milling machine spindle with
adapters. Face milling machine cutters are generally made with inserted teeth of
high-speed steel or tungsten carbide in a soft steel hub.
(f) T-Slot Milling Cutter (figure 28). The T-slot milling cutter is used to machine
T-slot grooves in worktables, fixtures, and other holding devices. The cutter has a
plain or side milling cutter mounted to the end of a narrow shank. The throat of the
T-slot is first milled with a side or end milling cutter and the headspace is then
milled with the T-slot milling cutter.
FIGURE 28. T-SLOT MILLING CUTTER.
(g) Woodruff Keyslot Milling Cutter. The woodruff keyslot milling cutter (figure 28)
is made in straight-shank, tapered-shank, and arbor-mounted types. The most
common cutters of this type, under 1 1/2 inches in diameter, are provided with a
shank. They have teeth on the periphery and slightly concave sides to provide
clearance. These cutters are used for milling semicylindrical keyways in shafts.
(h) Angle Milling Cutters. The angle milling cutter has peripheral teeth which are
neither parallel nor perpendicular to the cutter axis. Common operations
performed with angle cutters are cutting teeth in ratchet wheels, milling dovetails,
and cutting V-grooves. Angle cutters may be single-angle milling cutters (figure
29) or double-angle milling cutters. The single-angle cutter contains side-cutting
teeth on the flat side of the cutter. The angle of the right or left cutter edge is
usually 30°, 45°, or 60°. Double-angle cutters have included the angles of 45°, 60°,
and 90°.
FIGURE 29. SINGLE-ANGLE MILLING CUTTERS.
(i) Concave and Convex Milling Cutters. Concave and convex milling cutters
(figure 30) are formed tooth cutters shaped to produce concave and convex
contours of one-half circle or less. The size of the cutter is specified by the
diameter of the circular form the cutter produces.
(j) Corner-rounding Milling Cutter. The corner-rounding milling cutter (figure 30) is
a formed tooth cutter used for milling rounded corners on workpieces up to and
including one-quarter of a circle. The size of a cutter is specified by the radius of
the circular form the cutter produces, as with concave and convex cutters.
FIGURE 30. CONCAVE, CONVEX, AND CORNER ROUNDING MILLING CUTTERS.
(k) Gear Hob. The gear hob (figure 31) is a formed-tooth milling cutter with
helical teeth arranged like the thread on a screw. These teeth are fluted to
produce the required cutting edges. Hobs are generally used for such work as
finishing spur gears, spiral gears, and worm wheels. They may also be employed
for cutting ratchets and spline shafts.
FIGURE 31. GEAR HOB.
(1) Special Shaped-formed Filing Cutter. Formed milling cutters have the advantage of
being adaptable to any specific shape for special operations. The cutter is made for
each specific job. In the field, a fly cutter is made to machine a specific shape. The fly
cutter (figure 32) is often manufactured locally. It is a single-point cutting tool similar in
shape to a lathe or shaper tool. It is held and rotated by a fly cutter arbor. The cutter can
be ground to almost any shape, form, or contour that is desired. The cutter can be
sharpened many times without destroying the shape of the cutter or the cut being made.
There will be a very limited number of times when a special formed cutter will be
needed for cutting or boring operations, this is why a fly cutter is the most practical
cutter to use in this type of situation.
FIGURE 32. FLY CUTTER ARBOR AND FLY CUTTERS.
c. Selection of Milling Cutters. The following factors should be considered in the choice of
milling cutters:
(1) Type of Machine To Be Used. High-speed steel, stellite, and cemented carbide
cutters have the distinct advantage of being capable of rapid production when used on a
machine that can reach the proper speed.
(2) Method of Folding The Workpiece. For example, 45° angular cuts may either be
made with a 45° single-angle milling cutter while the workpiece is held in a swiveled
vise, or with an end milling cutter while the workpiece is set at the required angle in a
universal vise.
(3) Hardness of The Material To Be Cut. The harder the material, the greater will be
the heat that is generated during the cutting process. Cutters should be selected for
their heat-resisting properties.
(4) Amount of Material To Be Removed. A course-toothed milling cutter should be
used for roughing cuts, whereas a finer toothed milling cutter may be used for light cuts
and finishing operations.
(5) Number of Pieces To Be Cut. For example, when milling stock to length, the choice
of using a pair of side milling cutters to straddle the workpiece, a single-side milling
cutter, or an end milling cutter will depend upon the number of pieces to be cut.
(6) Class of Work Being Done. Some operations can be accomplished with more than
one type of cutter, such as in milling the square end on a shaft or reamer shank. In this
case, one or two side milling cutters or an end milling cutter may be used. However, for
the majority of operations, cutters are specially designed and named for the operation
they are to perform.
(7) Rigidity and Size of The Workpiece. The milling cutter used should be small
enough in diameter so that the pressure of the cut will not cause the workpiece to be
sprung or displaced while being milled.
(8) Size of The Milling Cutter. In selecting a milling cutter for a particular job, it should
be remembered that a small diameter cutter will pass over a surface in a shorter time
than a large diameter cutter fed at the same speed. This fact is illustrated in figure 33.
d. Care and Maintenance of Milling Cutters. The life of a milling cutter can be greatly
prolonged by intelligent use and proper storage. General rules for the care and maintenance
of milling cutters are given below:
(1) New cutters received from stock are usually wrapped in oilpaper which should not
be removed until the cutter is to be used.
(2) Care should be taken to operate the machine at the proper speed for the cutter that
is being used; excessive speed will cause the cutter to wear rapidly from overheating.
FIGURE 33. EFFECT OF MILLING CUTTER DIAMETER ON WORKPIECE TRAVEL.
(3) Whenever practicable, the proper cutting oil should be used on the cutter and the
workpiece during the operation, since lubrication helps prevent overheating and
consequent cutter wear.
(4) Cutters should be kept sharp, because dull cutters require more power to drive
them and this power, being transformed into heat, softens the cutting edges. Dull cutters
should be marked as such and set aside for grinding.
(5) A cutter should never be operated backward because, due to the clearance angle,
the cutter will rub, producing a great deal of frictional heat. Operating the cutter
backward may result in cutter breakage.
(6) Care should be taken to prevent the putter from striking the hard jaws of the vise,
chuck, clamping bolts, or nuts.
(7) A milling cutter should be thoroughly cleaned and lightly coated with oil before
storing.
(8) Cutters should be placed in drawers or bins in such a manner that their cutting
edges will not strike each other. Small cutters that have a hole in the center should be
hung on hooks or pegs, large cutters should be set on end. Tapered and straight shank
cutters may be placed in separate drawers, bins, or racks provided with suitable sized
holes to receive the shanks.
3. Conclusion
Milling cutters play an important role in performing milling machine operations. Knowing which
cutter to select and use for a specific operation, will at times, determine the overall quality of the
final product. The knowledge gained in this task on milling cutters will assist you in determining the
type of cutter(s) to employ for a specific operation, to include the nomenclature, selection, use and
care of milling cutters when tasked to perform milling machine operation

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