Broach Tools

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

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.

Rotary-Cut Broaches
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.

Hexagonal Rotary-Cut Broach
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.
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Radial Rotary-Cut Broach
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 by flats.
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Spline, Rotary-Cut Broach
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.
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Keyway Broach
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.
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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.

Multiple Spline Keyway Broach
When several keyways are spaced around a hole, the resulting sections is a multiple-spline cut. A single keyway broach can be used to cut all the splines by indexing the workpiece around a fixture. However, high procuction work usually requires a multiple-spline broach. This tool is equivalent to a series of keyway broaches combined in one tool, with the cutting teeth spaced around the tool diameter. These teeth can be straight sided, involute, helical, spline or a combination.
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Helical Broach
Helical splines (either straightsided or involute) can be broached with a helical broach. The teeth are ground in a helical path around the tool axis. The helix angle corresponds to that required in the work.


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Spiral Tooth Broach
The spiral tooth tool for internal broaching basically is a round broach with teeth on a shear angle. The teeth are always engaged in the workpiece which can reduce vibration.

 

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Burnishers
Burnishers are broaching tools designed to polish (by cold-working) rather than cut a hole. The total change in diameter produced by a burnishing operation may be no more than 0.0005 to 0.001 in. Burnishing tools, used when surface finish and accuracy are critical and relatively short and are generally designed to push broaches.
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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
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.

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Spline-Burring Broaches
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.


SURFACE BROACHES
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
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 pine-tree configuration.
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Sectional Broaches
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.
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Carbide Broach Inserts
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


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Heavy-Duty Broaches
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
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.

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Progressive or Nibbling-Type Broaches
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
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
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
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.
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Strip Broaching
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.
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Rotary Broaches
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.

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