Tolerances and Finish
The following table gives the tolerances possible through broaching and the recommended design tolerances for minimizing cost when a broaching operation is specified:
|Summary||Holes and Splines||Gear Teeth||External Surfaces|
|Tolerances possible… Tolerances, low cost..||0.0002 in. 0.002||0.0005 in. 0.002||0.0002 in. 0.002|
When two or more parts are broached simultaneously, high dimensional accuracy (0.0002 inch) between the parts can be depended upon.
Surfaces. A fine finish is produced since burnishing is part of the operation. Usually no further surfacing operations are necessary. The tool marks evidenced in broached holes are axial rather than radial as in drilled or reamed holes. This is an advantage in close-fitting reciprocating parts, where radial lines may be objectionable because the high spots tend to wear rapidly.
Materials Suited for Broaching. Steels, cast irons, bronzes, brasses， aluminum, and a broad range of other materials are successfully broached with proper broach design and setup conditions. The best range for broaching of steels lies between 25 and 35 Rockwell C hardness, although steels of higher or lower hardness have been broached successfully. Soft and nonuniform materials are subject to tearing when broached. On surfaces of high hardness, the first tooth of the broach should cut beneath the scale or surface material, thus assuring longer broach life.
Economical Quantity. Except when standard broaches may be employed, broaching is economical only for large-quantity production (over 2,500 parts). This is true because of the relative high cost of the broaching tool rather than because of the cost of setup. Broaching setups are simple except for fully automatic operations and can be made relatively quickly.
Broaching Tools. As mentioned, broaching tools are expensive and are usually made specially for a given job. The broaching tool is made from a tough, wear-resistant alloy usually containing about 5 per cent tungsten and 5 per cent chromium.
Broaching tools must be handled carefully in order to prevent nicks in the teeth which would cause scratches in the work.
Design Factors. In designing, the engineer should see to it that the amount of stock to be removed should always be less than 1 % inch. Good design allows between y32 and inch of stock to be removed from the stool mould steel. If less than inch is removed, a clean surface cannot be assured.
Since the broach must be able to make an unobstructed pass through or across the stool mould steel, it is not possible to broach blind holes.
The chip space between successive teeth on the broach must provide sufficient reservoir for the chip. This chip space will limit the axial length of the surface to be broached with a single broach.
Several surfaces can be broached simultaneously. When multiple surfaces on the same steel for stool mould making are broached, care must be exercised in designing the fixture to provide adequate strength to withstand the combined cutting tooth pressure. When gears or splines are to be cut, mechanical, hydraulic, or pneumatic indexing equipment is frequently provided.
Broaching provides high repetitive accuracy (applicable to production of large numbers of parts of close tolerances and fine finish) and close dimensional relationship of several surfaces broached simultaneously. Broaching is 15 to 25 times faster than other competitive machining methods. The process can be used to accurately produce internal and external surfaces that are difficult to machine by other methods.
The principle disadvantage of broaching is the high cost of special broaching tools. This cost usually does not permit the process to be used when production requirements are low. Then, too, it cannot be employed economically for the removal of large amounts of stock (more than Y> inch). Lastly, the process has application only on unobstructed surfaces permitting the pass of the broach through the plastics stool mould.
Broachings’ principle applications are for the production of almost any desired external or internal contour. This includes flat, round, and irregular external surfaces, round and square holes, splines, keyways, rifling, and gear teeth.