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.

Honing not only creates very fine surface finishes, it makes very round holes. Because it is a process that removes small amounts of material it can be used to size holes with little risk to scrapping a part. I have seen it used in aerospace applications to size small holes in hydraulic circuits when parts had clearance requirements of 0.00004". The small cylindrical pins, usually 0.1250" to 0.1870", would be ground and the mating hole would be honed to provide the very close clearances. Honing in this case was the only option.

Roller burnishing

Roller burnishing is another finishing method that can be used to size and finish holes. Roller burnishing utilizes a tool with several cylindrical rollers that look like dowel pins mounted axially around a tool body. The tool is rotated like a reamer or boring bar and passed through a hole that is a few ten-thousandths of an inch less than the desired diameter. The burnishing action basically smooths out the tool marks left by the previous tool, and leaves a very fine finish while improving the geometry. Burnishing also induces compressive stress into the surface providing improved mechanical properties.

Abrasive flow machining

Deburring intersecting holes can be a challenge especially when size and finish are critical or there is a reason to have crisp intersections. Again, hydraulic components are good examples of parts where intersecting hole geometries are critical. In these cases, abrasive flow machining is a non-traditional process that can be applied.

Abrasive flow machining forces a viscous fluid, similar to caulk, containing abrasive grains through the internal passages of a component. As the fluid flows through the part the abrasive grains erode the burrs and polishes the surfaces. The cutting action is controlled by abrasive grit size, flow rate, pressure and time, and that gives the user a lot of control over material removal rates. This is also a very safe process that can be applied with little risk to altering the geometry of the part. Unfortunately, it does require specialized equipment and must be performed as a secondary process.

Ceramic brushes

Ceramic brushes are a newer deburring option that can be applied to internal holes where geometry is critical. Ceramic brushes are constructed from continuous ceramic filaments bundled together into bristles. These are mounted on a solid body made from aluminum that can be constructed in almost any configuration. The tool is placed in the hole and rotated at relatively high speeds that allow the edges of the ceramic filaments to create the cutting action. This process, which is very safe for use on critical geometries, not only removes burrs but provides some polishing action as well.

Parting thought

Machining small holes, safeguarding critical geometry and achieving fine finishes in holes can be challenging. We often get focused on changing our traditional tooling or doctoring the cutting parameters in our attempts to achieve the desired results. Sometimes the best answer is adopting a less traditional machining method like those mentioned above.