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The forming of gear teeth has traditionally been a time-consuming heavy stock removal operation for which close tooth size, shape, runout, and spacing accuracy are required.

A gear machining process in which material from a work gear is removed by pulling or pushing a broach along the axis of the work gear is called broaching. The actual cutting action of any single broach tooth is very much similar to that of a single form tool. Each section of the broach has as many teeth as there are tooth spaces on the gear.

Gear Broaching Tools 73

Gear broaching is primarily applicable to high-volume jobs requiring high production rates. Gears can be broached accurately since a number of the elements are controlled by the accuracy of the broach. In addition, broaching can produce a fine finish on the gear tooth profile. Occasionally, it may be used in fairly small lot production to obtain an accu- racy level on a critical part that cannot readily be made with a high degree of accuracy by other methods. However, a gear broach will cut only parts that have identical gear specifications.

5.7.1 Broaching Internal Gears

A broach can be used to machine almost any internal form. The part teeth can be either equally or unequally spaced, and tooth forms can be either symmetrical or asymmetrical. The tooth forms must be uniform in the direction of the broach axis, and the wall of the part must be strong enough to withstand the broaching pressure.

Small internal gears can be cut in a single pass of a broach. Large internal gears can be made by using a surface type of broaches to make several teeth at a single pass. Indexing of the gear and repeated passes of the broach can produce a complete gear. Gears as large as 1.5 m (60 in.) in diameter are made by this process.

Today, the broached internal helical running gears in automotive automatic transmis- sions are exclusively produced by full-form finishing high-speed steel broaches. Full-form finish broaching provides fine surface finishes, precision involute form, accurate tooth thickness and precision tooth spacing, and lead.

In some cases, broaches can be designed with a removable finishing section that fin- ishes the complete tooth form. This “side-shaving” section of the broach finishes the entire profile of each gear tooth. A full form-finish section solves the profile problems caused by off-center drift of the broach, attributed to poor maintenance of machine alignment and perpendicularity or nonuniform tooth dulling.

In particular cases, gear broaches are designed with the so-called burnishing button. Burnishing button is a broach tooth without a cutting edge. A series of buttons is some- times placed after the cutting teeth of a broach to produce a smooth surface by material compression.

Pull broaches and push broaches are used for machining gears.

Push broaches are normally short and therefore less expensive than pull broaches, which are longer. However, one pull broach can sometimes remove as much stock as several push broaches. Most internal broaching is carried out with pull broaches. Not only do they remove more stock than push broaches, they are also capable of raking much longer cuts.

Gear broaching usually requires a part with a through hole. However, blind holes can be broached if there is sufficient clearance beyond the broached section and if a series of punch-type broaches are used.

Broaching can produce close tolerances on profile, spacing, lead, and size.

5.7.2 Broaching External Gears

The fastest way to produce medium and high production external gears is by pot broach- ing. The basic process consists of a tool with internal teeth that are held in a pot being passed over a round part, and producing external shapes in the form of splines and invo- lute gear teeth.

In the past, external gears were broached by either pushing the workpiece down through a stationary pot broach or mounting the workpiece on a stationary post and pushing the

pot broach down over it. This resulted in problems relating to chips packing in the chip gullets when using the push-down stationary pot broach method. Slow workpiece load- ing, attributed to the use of a plug-type approximate locator for each part, is a problem when broaching parts with the push-down stationary workpiece method.

Two more methods used were push-up pot broaching and pull-up broaching.

Push-up pot broaching consists of pushing the workpiece upward through a fixed pot broaching tool to produce external teeth under ideal conditions, assuring quick and com- plete chip removal from the broach teeth. Chips fall by gravity away from the tool and the work gear. The coolant is flushed into the tool area through a quick-disconnect coupling. Automatic loading is simplified because the part is elevated by the process and gravity is utilized to unload parts onto a conveyor [10].

Pull-up broaching is used when the part diameter and broach length are such that the required post diameter and length do not provide sufficient strength to rigidly support the workpiece. In this method, the work gear is pulled up through a stationary pot broaching tool with a pull rod. This process permits broaching parts with deeper teeth and wider faces by using longer tools.

Pot broaching tools are classified into two types: (1) the ring type for parts where tooth form and spacing are critical and (2.) the lower-cost stick type, which is used where accu- racy permits.

The ring-type pot broaching tool is made up of a holder with a series of individual preci- sion ground keyed high-speed steel rings or wafers, each of whose internal cutting teeth are individually backed off. These rings can be sharpened via either the conventional face sharpening method or the progressive method, in which several face sharpenings are per- formed followed by inner diameter grinding of all rings. Ring-type pot broaches are used for precision running gears and splines where close tooth tolerances are required.

The stick-type pot broaching tool, which has lower-cost individual high-speed steel slab broach inserts, includes a holder that supports a series of keyed ground rings that locate the broach sticks in internal ground slots. On thin parts, the teeth in the stick inserts can have wide, long-life tooth lands, and can also be axially staggered to provide a helical pat- tern to balance the cutting forces.

These tools are particularly adapted to short face-width splines and gears, and are often used in combination.