An overview of broaching and its advantages.

BY Chris Van De Motter
 

roaching is one of the most productive and precise metal cutting operations performed today. It's also one of the most misunderstood. 

Broaching resembles planing or shaping in that the broach tool presents a sharp cutting edge to the work piece and moves across it, removing a predetermined amount of material. What primarily distinguishes broaching from other metal cutting operations is the tooling used. A broach combines roughing, semi-finishing, and finishing teeth in a single line. Although many variations exist, the basic tool is an axial, multi-toothed rod, bar, or plate.' 

Broaching's strengths are that it produces parts at a high rate, removes heavy amounts of stock, roughs and finishes in one pass, and permits the machining of complex contours and simple shapes alike. 

Economical operation is another advantage of broaching. While initial tool costs generally are higher than for other metal cutting operations, the cost per finished part is lower because of the high production rates broaching permits. Coupled with automatic or semiautomatic parts-handling equipment, unskilled or semiskilled operators can be employed, further reducing operation costs. 

Broaches also make shallow cuts and perform finishing operations, often yielding smoother surfaces than can be attained with other metal-removal processes. Some broaches have burnishing sections that impart almost any finish desired, thereby eliminating the need for grinding. In terms of productivity, repeatability, accuracy, and surface finish, broaching surpasses milling in any one plane of a work piece. 

There are two basic types of broaches: surface (external) and internal. Surface broaches cut on the outside of the work piece. Internal broaches enlarge or change the shape of an existing hole. 

Surface Broaches 

The simplest surface broach is the slab broach, used for cutting flat surfaces. Considered a general-purpose tool, it squares the ends of parts or provides a reference surface for additional broaching or machining. 

In applications involving hard surfaces or heavy stock removal, free egress (or "nibbling") broaches are employed. They have sets of narrow roughing teeth positioned at a specific angle to the centerline and quickly remove material. Full-width teeth follow the roughing section and make semi-finishing and finishing cuts.

Slot broaches cut slots of various depths and widths. In operations requiring high production rates, slot broaching is faster and more economical than milling. Two or more slots can be cut simultaneously with proper tooling and fixtures. It's easy for standard slotting broaches to cut slots in the ends or sides of a work piece. But that's not the case when cutting a lot along the part's length. Often, heavier stock-removal rates are involved that demand application of a longer broach. 

Contour broaches cut concave, convex, cam-shaped, contoured, and irregular surfaces to extremely close tolerances. Broaching these surfaces requires that they be parallel and not present obstructions in the broach's path. 

Manufacturers of turbine engines often use dovetail (or "pine tree") broaches to create special forms in the compressor wheels that hold blades in turbine discs. This sort of broaching usually involves multiple passes, due to the heavy amount of stock removed and the complex forms involved. 

Pot broaches cut precision external forms such as involute spur gears, splines, slots, and special tooth forms. Pot broaching allows the task to be performed in a single pass, making it an economical, high production process. It has replaced hobbing and shaping in the manufacture of many types of external gears. During pot broaching, parts are pushed or pulled through the bore of the pot-broach holder. 

Straddle broaches incorporate two separate slab-broach inserts to cut similar (or identical) parallel surfaces on opposite sides of the work piece in one pass. Straddle broaching's advantage is that it maintains a more precise dimensional relationship between the two sides than would be possible with separate passes. 

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