In order to internally broach a metal part, a pre-existing hole must be made. The broach then changes the shape and size of the starter indent into a hole with a keyway, spline, hexagonal, square or custom designed profile. A broaching tool that has successively larger and larger teeth pierces through the pre-existing hole by pulling or pushing through the metal part.
Internal broaching machines are automated, often CNC operated and able to quickly and efficiently produce holes in metal parts and products. Sometimes called shell broaches, internally cut broaching is done mostly for holes with large diameters that are to be mounted on an arbor.
Broaching is most effective on softer metals, including aluminum, brass, bronze and graphite. Many different tools and machinery parts are formed by the broaching process, including key holes, gears, pulleys, screw heads and other hardware. Because the broaching machines can get pricey but the tools and parts they produce are necessary in so many industries, broaching job shops are facilities that provide a variety of broaching services.
Particularly when going to a broaching shop, it is cost effective, highly repeatable and able to produce large volumes of metal parts with little to no variance or error. Broaching tools may become dull over time if used often, and are therefore periodically reground to retain their original sharpness.
Drilling the pre-existing starter hole is referred to as blank prepping. During this process, a simple, shallow hole is drilled and must be .005 to .015 inches larger than the smallest diameter of the broach. There are two main types of internal broaching; push broaching and pull down broaching, each with their own methods of forming a wide hole in metal.
Push broaching is done with vertical machines, and the broach tool is pushed linearly either up or down through the metal work-piece. Short broaches are used to avoid buckling, which limits the amount of metal that is able to be removed.
This type of internal broaching is suitable for light work and low runs, and the machinery is less expensive than pull down broaching. Pull down broaching, on the other hand, is only able to push the tool down on a vertical machine.
It keeps the tool in tension and prevents buckling, and is used with long, thin broaches to make deep holes.