All broaching tools
are "special designs" in that they
are generally made for a single user and a
specific machining operation. However, certain
types of broaches have become especially well-known
for the type of work they do.
Internal broaches are either pulled
or pushed through a starter hole. The machines can
range from fully automated multi stationed verticals
to horizontal pull types to simple presses.
Rough forgings, malleable~ iron castings with a hard
skin, and sand castings with abrasive surface inclusions
are cut with one of three types of rotary-cut broaches.
The design idea is somewhat similar
to that of a chipbreaking slot, but the cutting edge
has been drastically reduced and the slots between
the teeth have become much deeper. Rotary-cut broaching
teeth are heavier, to withstand the heavy cutting
load, and are spaced in staggered fashion along the
axis of the broach to generate the entire circumference
of the hole. The tools are designed to take deep cuts
underneath a poor-quality surface. Once this surface
has been penetrated, the balance of the broaching
tool proceeds to semi-finish and finish underlying
metal in the normal manner.
The hexagonal rotary-cut broach is used for small
diameter holes, remove little stock. Depth of
cut is limited to the distance across the flats.
The radial rotary-cut broach removes more stock
than the hex-type tool because the cutting portions
of the teeth are connected by arcs rather than
Spline, rotary-cut broaches offer a grater degree
of flexibility than either of the other tool types
and also permit maximum stock removal. The amount
of stock removal is governed primarily by the
capacity of the broaching machinem rather than
by any tooling limitations. Rise per tooth may
be as much as 0.050 in. on such broaches.
Almost all keyways in machine tools and parts
are cut by a keyway broach - a narrow, flat bar
with cutting teeth spaced along one surface. Both
external and internal keyways can be cut with
these broaches. Internal keyways usually require
a slotted bushing or horn to fit the hole, with
the keyway broach pulled through the horn, guided
by the slot.
If a number of parts, all of the same
diameter and keyway size, are to be machined, an internal
keyway broach can be designed to fit into the hole
to support the cutting teeth. Only the cutting teeth
extend beyond the hole diameter to cut the keyway.
Bushings or horns are not required.
Burnishing buttons sometimes are included
behind the finishing-tooth section of a conventional
broaching tool. The burnishing section may be added
as a special attachment or easily replaced shell.
These replacement shells are commonly used to reduce
tooling costs when high wear or tool breakage is expected.
They are also used to improve surface finish.
Shell broaches can be used on the roughing semi-finsihing
sections of a broach tool. The principal advantage
of a shell broach is that worn sections can be
removed and resharpened, or replaced, at far less
cost than a conventional single-piece tool. When
shells are used for the finishing teeth of long
broaches; the teeth of the shell can be ground
to far greater accuracy than those of a long conventional
broach tool and the tool can continue to be used
by replacing the shell.
Spline-burring broaches are quite short and are generally
designed as push broaches. They remove burrs created
by machining work done after the splines have been
formed. For example, a hole might be drilled and tapped
into the spline for a grease fitting, leaving burrs
that could create assembly problems. These broaches
are made slightly undersize on the spline width and
may be equipped with round teeth to remove burrs from
inside the bore.
Special Sizing Broaches
Special sizing broaches are pulled or pushed through
a semi-finished hole to take out the last few thousandths
of stock faster and more efficiently than a fine-feed
boring tool can.
These Broaches are used to remove material
from an external surface are commonly known as surface
broaches. Such broaches are passed over the workpiece
surface to be cut, or the workpiece passes over the
tool on horizontal, vertical or chain machines to
produce flat or contoured surfaces.
While some surface broaches are of solid
construction, most are of built-up design - with sections,
inserts, or indexable tool bits that are assembled
end-to-end in a broach holder or sub-holder. The holder
fits on the machine slide and provides rigid alignment
and support. The first tooth of each insert or section
of the assembly is ground to conform with the last
tooth of the preceding insert or section. Burnishing
inserts are sometimes provided at the end of the holder
to perform their function after the other teeth have
completed their operations, but such tools are very
vulnerable to metal pickup and can cause tearing.
Most surface broaches are special and
designed for a specific application, but some standard,
general-purpose broaches are available. Broach holders
can often be standardized in several sizes to hold
various surface broaches. Only the more common of
the many different surface broaches available are
discussed in this section.
Pine-tree broaches cut the complex serrations
used to lock turbine blades into their rotors.
Common practice is to use a set of broaches; the
first cuts a straight-sided V-notch in the rotor
rim and is followed by one or more serrated broaches
that progressively widen the notch to the full
Sectional broaches are used to broach unusual
or difficult shapes - often in a single pass.
The sectional broach may be round or flat, internal
or external. The principle behind this tool is
similar to that of the shell broach, but straight
sections of teeth are bolted along the axis of
the broach rather than being mounted on an arbor.
A complex broaching tool can be built up from
a group of fairly simple tooth sections to produce
a cut of considerable complexity.
Broaching tools with brazed carbide broach inserts
are frequently used to machine cast-iron parts.
Present practice, such as in machining automotive
engine blocks, has moved heavily to the use of
disposable, indexable inserts, and this has drastically
cut tooling costs in many applications
Carbide tool bits and the sectional-broach idea are
combined into heavy-duty broaches for cutting deeply
into heavily scaled surfaces. The carbide-tipped tool
bits are arranged in a staggered pattern on the face
of a tool holder. Each tooth is preset by means of
an adjustable screw and locked in place in the tool
holder by a setscrew. (See above)
Slab broaches are simple tools for producing flat
surfaces come closest to being truly general-purpose
broaches. A single slab broach can be used to
produce flat surfaces having different widths
and depths on any workpiece by making minor adjustments
to the broach, fixture, and / or machine.
Progressive or Nibbling-Type
Progressive or nibbling-type broaches are for cutting
through hard surfaces and for heavy stock removal.
These broaches have two sets of narrow roughing teeth,
with each set positioned at an angle with respect
to the centerline of the broach holder, thereby forming
an inverted vee. Each tooth or insert takes a shear
cut, generally to full depth, but covers only a small
portion of the workpiece surface. This is similar
to a single-point tool on a sharper or planer progressively
generating a flat surface on the workpiece.
Full-width teeth for semi-finishing
and finishing are located behind the roughing teeth
on progressive broaches so that the entire surface
in cut in one pass. For narrow surfaces, the teeth
or inserts at the starting end are V-shaped. On subsequent
teeth, the vees gradually widen until the full required
width of the surface is cut. The final teeth are flat,
similar to those on a slab broach.
Slot broaches are for cutting slots but are not as
general purpose in function as slab broaches. Adjustments
can easily be made to produce different slot depths,
but slot widths are a function of the broach width.
When sufficient production volume is required: however,
slot broaches are often faster and more economical
than milling cutters. In broaching, two or more slots
can often be cut simultaneously.
Spline punches, special types of broaches with only
one tooth, are used for shaping holes through which
conventional broaches cannot pass. One example is
internal gear teeth in a blind hole. The gear teeth
are rough cut by drilling and shaping, or milling,
then one or more spline punches are forced into the
work to produce the tooth form.
Blind-hole broaching violates two broaching principles:
the tool does not pass completely through the
workpiece, and it must be withddrawn badkward
over the broached surface. But it can be done
when necessary. The job usually involves a series
of short push broaches, each slightly larger in
diameter than the preceding tool. These short
push broaches are mounted on a circular indexing
table that rotates under or over the workpiece,
the broaching machine pushes the workpiece over
the tool, withdraws it, and then waits for the
next broaching tool to index into position.
Strip broaching also violates the principle that
a broach tool should not return through the workpiece,
or else tool life will be reduced and the surface
finish of the workpiece will be marred. In strip
broaching, the broach tool is returned through
the workpiece hole without stopping the machine
to unload. Strip broaching is most commonly used
for round-hole broaching of large quantities of
low-cost parts when machining costs must be held
to an absolute minimum. Strip broaches can be
combined with burnishing buttons that slightly
increase the hole diameter to provide a small
amount of clearance, permitting the tool to be
withdrawn without damaging the finished surface
or dulling the cutting teeth.
Rotary broaches are special types of surface broaches.
They are not commonly used, but they do offer advantages
when producing work with external radial forms. In
the most common setup, the broach tool is mounted
on a rotating faceplate and the work is clamped into
a hydraulic fixture. The tool makes one revolution
to cut the desired shape. Circular slots can be cut
by a rotary broach that is turned around its own axis.
Ring or Pot Broaches
In pot broaching, one or more workpieces are generally
pulled or pushed up or pushed down through the bore
of a pot broach subholder that is normally stationary
on a vertical machine. There are three basic types
of pot broaches having internal cutting tooth configurations:
ring, stick, and combination ring and stick.