When Unconventional Methods Pay: Turning Performance

Making and finishing holes in parts is common work for machinists. That’s what we do all day, drill, ream, thread or bore it. These are part of the basic machining processes that every shop uses. However, there are those times when the common hole finishing processes are not sufficient for the job at hand.

Odd geometries, close tolerances, cost considerations and many other factors can create situations that require us to look for less traditional machining processes to finish a hole. There are several processes available for machinists and engineers to explore when the standard processes just don’t get the job done.

Ballizing

Ballizing or ball broaching is a process that is used to finish small diameter holes. This is done by forcing a hardened ball bearing made from tungsten carbide or chromeplated tool steel through a hole that is slightly smaller than the desired diameter. As the ball passes through the hole, it sizes the hole and burnishes the surface, leaving a very fine surface finish. Holes can be sized to very close tolerances and small tapers can be corrected with ballizing.

Ballizing is not new but it is some-what obscure. Old tool makers have done this on a small scale when they didn’t have the right reamer or needed to open up an undersized hole. I have been known to use this method to cheat the quality man while running off first article parts. Ballizing is not just a tool room gimmick, there are companies that make machines for production ballizing.

Broaching

Another alternative finishing method is broaching. While broaching is a very common machining method, broaching is often overlooked when it could be very effective. Most broaching is done by pushing or pulling a tool through the part to create keyways, polygonal shapes and other geometry like splines. Traditional push-pull broaching is an excellent way to create close tolerance holes and allows a shop to size round holes outside of a machining center when needed.

Rotary broaching

The most significant advances have come recently as tool manufacturers have introduced improved rotary broaching for turning centers. Rotary broaching is not new; screw machine shops have been using it for decades, but the new tools are better and the process is becoming more widely accepted as an alternative to pushpull type machines. Broaching on the machine eliminates secondary processing thereby improving quality and reducing labor costs. Rotary broaching is one of the best tooling investments a modern shop can make.

Honing

Like broaching, honing is not new and is certainly a familiar process to most machinists and engineers. Honing is common in shops that create sealing surfaces like those in hydraulic cylinders or hydraulic valves. While honing is typically done on dedicated machines, it can be done on machining centers eliminating the need for special machines.