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The basicsBroaching 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 isrequired 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 tighttolerances, produce precision finishes, and minimize the need for highly skilled machine

operators.

ToolingTooling 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. Broach tools 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 complexcross-sections and tooth designs may be found, However, the basic axial, multi-toothed tool

shape remains

Exterior or surface broachingFor exterior or surface broaching, the broach tool may be pulled or pushed across a

workpiece surface; or the surface may move across the tool. I nternal broaching requires astarting 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 aslong 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 withlimited depth can also be broached with punch broaches which are pushed with limited travel.

Standard NomenclatureWhatever 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 endand fr ont pilotare passed through the starting hole. Then the pull endis locked to thepull headof the broaching machine. The fr ont pilotassures correct axial align-

ment of the tool with the starting hole and serves as a check on the starting hole size.

LengthThe 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

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finishing teeth. Broaching tools can be as small as 0.050 in. or as large as 15 to 20 in. in

diameter.

The rear pilotThe rear pil otmaintains 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 finishteeth. Often a notched tail orretr iever endis added to the tool to engage a handling mechanism

that supports the rear of the broach tool.

CONVENTIONAL PULL (HOLE) BROACHING TOOLThese 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, ortoothrise, 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 landand faceintersect to form a cutting edge.

The face is ground with a hookor face anglethat is determined by the workpiece material. Forinstance, Soft steel workpieces usually require greater hook angles; hard or brittle steel, smaller

hook angles.

The landThe land supports the cutting edge against stresses. A slight clearanceorbackoff angleis

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 notalter the tooth size.

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Distance between cutting teethThe distance between teeth, orpitchis determined by the length of cutand influenced by the

type of workpiece material. A relatively large pitch may be required for roughing teeth toaccommodate 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. Thegreatest contributing factors to vibration are poor tooth engagement and extremely hard

workpieces. Such problems must be anticipated by the broach designer.

Tooth riseThe tooth ri seortaperis 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 therise is too small to permit the teeth to bite into the workpiece, a glazed or galled finish will result.

Tooth GulletThe depth of the tooth gulletis related to the tooth rise, pitch, and workpiece material. The

tooth root radiusis usually designed so that chips curl tightly within themselves, occupying aslittle space as possible.

Chip loadAs 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 loadthat is determined by the design

of the broach tool and the predetermined feed rate.

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 befreely 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 produceflakes. Ductile or malleable materials produce spiral chips.

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Flat-bottomed gulletLong cuts in ductile materials or interrupted cuts producing two or more chips, would soon fill

a circular gullet with chips. The solution is a f lat-bottomed gulletwith extra-wide spacing. Thisprovides room for two or more spiral chips or a large quantity of chip flakes.

Chipbreakers

Notches, called chipbreakers, are used on broach tools to eliminate chip packing and tofacilitate 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.

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 toolto 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.

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.

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Generating Form/NibblingSome broach designs generate the tooth profile in a nibbling pattern. This process is called

generating f orm. Each tooth of the broach increases in size. Thus the nibbled profile is theenvelope 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 prepared.

Full-formfinishing broaches are available to improve the accuracy and surface finish of the partproduced by nibbling ~ type broaches.

Shear angleBroach designers may place broach teeth at a shear angleto 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.

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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 requiresmore manufacturing time and higher cost. A so-called criss-cross type of shear facilitates milling

and grinding of the teeth.

Side reliefWhen 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 thesides of the slot and cause rapid tool wear unless clearance is provided. This is done by grinding

a side relief angleon 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 taperAnother 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 inthe 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.

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. Thistechnique is more practical and economical than backing off or relieving the individual teeth