Broaching Basics

The Basics
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
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.

Standard Nomenclature
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.

Length
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
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 designer.

Tooth Rise
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 result.

Tooth Gullet
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.

Chip Load
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 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 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.

Flat-Bottomed Gullet 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 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.

Generating Form/Nibbling 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 prepared.
Full-form finishing broaches are available to improve the accuracy and surface finish of the part produced by nibbling ~ type broaches.
Shear Angle 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.

Side Relief
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 Taper 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.