Tips and Tricks for Threading Aluminum Parts

Quick take: Aluminum threading looks forgiving until chip control, built-up edge, and thread quality start slipping. This page works best when it is read with broader tapping-survival and calculator references so the setup is grounded in both material behavior and basic thread math.

Related references: Seven Ways to Avoid Breaking Taps, Tapping Deep Holes: Guide to Chip Evacuation and Tool Life, and Comprehensive Threading Calculators.

Just about every machine shop makes parts out of one aluminum alloy or another. Just about every shop also threads holes in those parts using taps. While taps are a highly efficient way to make threads in a hole, they come with potential problems, such as poor or inconsistent thread finishes, thread galling, and the ultimate nightmare — a tap broken off in a part. Here are some common mistakes to avoid and a few tips that have helped me.

I have found that the softer the aluminum, the tougher it is to tap because the material acts like chewing gum if not properly lubricated. Just like when machining aluminum, keeping the chip hardened is important to prevent a myriad of problems. The use of oil or flood coolant helps with preventing galling. Inversely, the harder the alloy, the easier it is to tap.

Proper toolholding also goes a long way in preventing downtime caused by tap failure, in whatever mode it presents itself. Toolholder choice is usually dictated by factors involved in the tapping operation such as the number of holes to tap, the machine being used to do the tapping, as well as the capabilities and limitations of your shop, among others.

For example, if you are about to thread a lot of parts with holes drilled by a different machine, we use a Tapmatic tapping head as it has radial play to allow for better alignment of the tap to the hole, along with an internal clutch to automatically reverse the rotation of the tap as you lift it back out of the part. Most of the time, the drilling and tapping will occur in the same machine and the same setup, so rigid tapping or power tapping with the tool in the spindle, as in a milling machine or CNC mill, is an obvious choice.

A common problem that presents itself is the cutting tool selection, and the use of the wrong tap for the job. There is, after all, an enormous list of different taps, grades of substrate, classes of fit, etc., from which to choose. Each of these facets need to be considered when selecting a tap for a particular part.

Let’s take blind holes as an example. The selection of a gun tap or spiral point tap may be the types to avoid in this case. When you need to reach further down, and when you want to prevent chips from compacting in the bottom of the hole and causing premature tap failure, consider switching to a spiral-fluted tap, which is designed to bring the chips up and out of the hole. The chip evacuation helps extend the tool’s life and reduces the chances of tap breakage.