Broaching is one of the most productive precision-machining
processes known. It is also a study in self-contradiction.
It's a high-production, metal-removal process that sometimes
is required to make one-of-a-kind parts. It's at it's best
when machining simple surfaces or complex contours. Its
recent successes include such dissimilar items as high-precision
computer parts and massive locomotive bull gears.
Broaching is similar to planning, competes with milling
and boring, and gives turning and grinding stiff competition.
Properly used, broaching can greatly increase productivity,
hold tight tolerances, produce precision finishes, and minimize
the need for highly skilled machine operators.
Tooling is the heart of any broaching process. The broaching
tool is based on a concept unique to the process - rough,
semi-finish, and finish cutting teeth combined in one tool
or string of tools. A broach tool frequently can finish-machine
a rough surface in a single stroke.
In its simplest form, a broach tool resembles
a wood rasp. It is a slightly tapering round or flat bar
with rows of cutting teeth located along the tool axis.
In advanced forms, extremely complex cross-sections and
tooth designs may be found, However, the basic axial, multi-toothed
tool shape remains
Exterior or Surface Broaching
For exterior or surface broaching, the broach tool may be
pulled or pushed across a workpiece surface; or the surface
may move across the tool. Internal broaching requires a
starting hole or opening in the workpiece so the broaching
tool can be inserted. The tool, or the workpiece, is then
pushed or pulled to force the tool through the starter hole.
The final shape may be a smoother, flatter surface, larger
hole, complex splined, toothed notched curved, helical,
or some other irregularly shaped section. Almost any irregular
cross-section can be broached as long as all surfaces of
the section remain parellel to the direction of broach travel.
The exceptions to this rule are uniform rotating sections
such as helical gear teeth, which are produced by rotating
the broach tool as it passes the workpiece surface. Blind
holes or holes with limited depth can also be broached with
punch broaches which are pushed with limited travel.
Whatever the actual tooth size and shape, standard nomenclature
is used to describe the essential parts of a broaching tool.
(See illustration below) When an internal pull broach is
used, for example , the pull end and front pilot are passed
through the starting hole. Then the pull end is locked to
the pull head of the broaching machine. The front pilot
assures correct axial align-ment of the tool with the starting
hole and serves as a check on the starting hole size.
The length of a broach tool or string of tools is determined
by the amount of stock to be removed, and limited by the
machine stroke, bending moments (in a push broach), stiffness,
accuracy, and other factors. A pull broach is usually limited
to 75 times the diameter of the finishing teeth. Broaching
tools can be as small as 0.050 in. or as large as 15 to
20 in. in diameter.
The Rear Pilot
The rear pilot maintains tool alignment as the final finish
teeth pass through the workpiece hole. On round tools the
diameter of the rear pilot is slightly less than the diameter
of the finish teeth. Often a notched tail or retriever end
is added to the tool to engage a handling mechanism that
supports the rear of the broach tool.
CONVENTIONAL PULL (HOLE) BROACHING TOOL
These are the basic shapes and nomenclature for
conventional pull (hole) broaching tools. Note chipbreakers
in first section of roughing teeth. These may be
extended to more teeth if the cut is heavy or material
difficult. Note also extra finishing teeth.
Cutting Tooth Sections
Broach teeth usually are divided into three separate sections
along the length of the tool: the roughing teeth, semi-finishing
teeth, and finishing teeth. The first roughing tooth is
proportionately the smallest tooth on the tool. The subsequent
teeth progressively increase in size up to and including
the first finishing tooth. The difference in height between
each tooth, or tooth rise, usually is greater along the
roughing section and less along the semi-finishing section.
All finishing teeth are the same size.
Individual teeth (see illustration below)
have a land and face intersect to form a cutting edge. The
face is ground with a hook or face angle that is determined
by the workpiece material. For instance, Soft steel workpieces
usually require greater hook angles; hard or brittle steel,
smaller hook angles.
The land supports the cutting edge against stresses. A slight
clearance or backoff angle is ground onto the lands to reduce
friction. On roughing and semi-finishing teeth, the entire
land is relieved with a backoff angle. On finishing teeth,
part of the land immediately behind the cutting edge is
often left straight so that repeated sharpening (by grinding
the face of the tooth) will not alter the tooth size.
Distance Between Cutting Teeth
The distance between teeth, or pitch is determined by the
length of cut and influenced by the type of workpiece material.
A relatively large pitch may be required for roughing teeth
to accommodate a greater chip load. Tooth pitch may be smaller
on semi-finishing and finishing teeth to reduce the overall
length of the broach tool. Pitch is calculated so that,
preferably, two or more teeth cut simultaneously. This prevents
the tool from drifting or chattering.
Sometimes a broach tool will vibrate when a
heavy cut is taken, especially when the cutting load is
not evenly distributed. Vibration may also occur when tooth
engagement is irregular. The greatest contributing factors
to vibration are poor tooth engagement and extremely hard
workpieces. Such problems must be anticipated by the broach
The tooth rise or taper is calculated from one tooth to
the next so that the thickness of the chip does not impose
too great a strain on individual teeth. A large tooth rise
increases power requirements. When all teeth are simultaneously
engaged in the workpiece, too large a tooth rise could cause
an increase in power requirements beyond the rated tonnage
of the machine. If the rise is too small to permit the teeth
to bite into the workpiece, a glazed or galled finish will
The depth of the tooth gullet is related to the tooth rise,
pitch, and workpiece material. The tooth root radius is
usually designed so that chips curl tightly within themselves,
occupying as little space as possible.
As each broach tooth enters the workpiece, it cuts a fixed
thickness of material. The fixed chip length and thickness
produced by broaching create a chip load that is determined
by the design of the broach tool and the predetermined feed
This chip load feed rate cannot be altered
by the machine operator as it can in most other machining
operations. The entire chip produced by a complete pass
of each broach tool must be freely contained within the
preceding tooth gullet. The size of the tooth gullet (which
determines tooth spacing) is a function of the chip load
and the type of chips produced. However, the form that each
chip takes depends on the workpiece material and hook. Brittle
materials produce flakes. Ductile or malleable materials
produce spiral chips.
Long cuts in ductile materials or interrupted cuts producing
two or more chips, would soon fill a circular gullet
with chips. The solution is a flat-bottomed gullet with
extra-wide spacing. This provides room for two or more
spiral chips or a large quantity of chip flakes.
Chipbreakers are vital on round broaching
tools, Without the chipbreakers, the tools would machine ring-shaped
chips that would wedge into the tooth gullets and eventually
cause the tool to break. Special chipbreaker designs can be
used to increase the maximum tooth rise of a broach without
overloading the machine. If deep slots are ground into the
lands of the cutting teeth, the depth of cut can be increased
on each tooth without fear of overloading.
Notches, called chipbreakers, are used on broach tools
to eliminate chip packing and to facilitate chip removal.
(See illustration below) The chipbreakers are ground
into the broach, parallel to the tool axis. Chipbreakers
on alternate teeth are staggered so that one set of
chipbreakers is followed by a cutting edge. The finishing
teeth complete the job.
The sections of the workpiece not machined by the first
tooth are picked up by the next tooth, or the next, by staggering
the array of slots along the tool axis.
Some broach designs generate the tooth profile in a
nibbling pattern. This process is called generating
form. Each tooth of the broach increases in size. Thus
the nibbled profile is the envelope of a series or thousands
of corner generations. A nibbling-type broach can produce
accurate teeth or forms with a good surface finish only
when machine alignment is carefully maintained providing
stringent broach maintenance and the blank is carefully
Full-form finishing broaches are available to improve
the accuracy and surface finish of the part produced
by nibbling ~ type broaches.
Broach designers may place broach teeth at a shear angle
to improve surface finish and reduce tool chatter. When
two adjacent surfaces are cut simultaneously, the shear
angle is an important factor in moving chips away from
the intersection of the cutting teeth.
Another method of placing teeth at a shear angle on broaches
is by using a herringbone pattern. An advantage of this design
is that it eliminates the tendency for parts to move sideways
in the workholding fixtures during broaching. A disadvantage
is its inherent complexity which requires more manufacturing
time and higher cost. A so-called criss-cross type of shear
facilitates milling and grinding of the teeth.
When broaching slots, the tool becomes enclosed by the slot
during cutting and must carry chips produced through the
entire length of the workpiece. Sides of the broach teeth
will rub the sides of the slot and cause rapid tool wear
unless clearance is provided. This is done by grinding a
side relief angle on both sides of each tooth with only
a small portion of the tooth near the cutting edge, called
the slot. The same approach is used for one-sided corner
cuts and spline broaches.
Back tapering can be accomplished by using
a magnetic sine table and raising the back end of the surface
broach with shims for finish grinding. The grinding wheel
is dressed to the proper form in relation to the amount of
back taper needed, and this form is ground into the broach.
This technique is more practical and economical than backing
off or relieving the individual teeth.
Another type of relief commonly used on form broaches,
such as internal spline and rack tooth forms, is called
back taper. The purpose of back tapering is to provide
a tapered tooth form in the direction of clearance (face
of form to heel of tooth) to minimize contact between
tooth flank and workpiece and thus reduce frictional
contact, rubbing wear, and metal pickup.