Unlike other broaching operations, blind hole broaching does not allow the tool to pass completely through the metal part. Instead, it produces an indent or slot in the metal.
Every aspect of this type of broaching, especially the tooling design and execution, are very difficult and are only done by skilled, experienced broachers. Although broaching styles that pierce from one side to the other, such as internal broaching require careful machining as well, it is not quite as difficult as a set-up that must only go part way through a surface.
Blind broaching is reserved for cutting slots, holes and imprints that are round, square, hexagonal or splines in hardware and parts for the aerospace and construction industries. A common example of a blind hole broached product is the internal hexagon on headless setscrews.
Applications for a broached blind hole are rare, but it is the only process able to produce holes with the necessary shapes and dimensions for some metal parts. It’s also an economical and efficient means of internal spline production. Broaching blind holes is cost effective, highly repeatable and produces no variance among the products.
The blind holes produced by broaching machinery have a good surface finish when the part’s material is hardened, smooth edges without burrs or cracks. The broaching machine is either hydraulically or electromechanically driven and, depending on the volume of work-pieces, is manually or automatically loaded and unloaded.
Blind broaching is either straight, meaning the tool containing the teeth is long and thin and moves linearly, or helical, which means the tool is rounded and moves in a circular motion; helical broaching is the most common. It uses a series of circular indexing tables that rotate under or over the work-piece.
The machinery pushes the work-piece up over the tool, where a shape punches down, withdraws and then turns to the next shape, which is bigger than the last. The next tool is positioned over the small punch, and is made bigger.
Some broaching machinery has dozens of successively larger tools that slowly create a larger blind hole. Each tool must be withdrawn backward over the broached surface, unlike most other broaching processes, which cut straight through the work-piece.