Mastering Rotary Broaching on a Lathe: Precision Techniques Explained

In this article, we’ll be exploring the concept of “broaching” in metalworking: what broaching is, how it works, and its role in the industry. Most importantly, we’ll look at different techniques for broaching, and a particular technique for broaching on a lathe, known as rotary broaching.

What is Broaching?

Have you ever looked at the teeth on a gear or the grooves on a cylinder, or even the shape on the head of a screw, and wondered how they were made? Chances are those indentations or patterns were created through a technique known as broaching.

Broaching is a grinding technique, using a toothed tool or head to remove material from a part and create the desired pattern of notches or grooves. The most common forms of broaching involve the use of a broaching tool, a specific grinding head that usually features a number of raised bits at different heights.

Different Broaching Techniques

The basic method described above, using a specialised tool, is known as linear broaching. The broach moves down the length of the workpiece. Rather than use multiple passes, the broach may have a sequence of teeth, set like steps at different heights (the height of a tooth is known as the RPT or rise per tooth).

As the broach moves down the workpiece, each tooth removes more material, until the final tooth reaches the desired depth. For a surface broach, the broaching tool moves down the exterior of a workpiece, such as when creating a gear or cog. Broaches can be used to create internal patterns as well.

Broaching can also be performed with rotary tools such as lathes, a process known as rotary broaching. Rotary broaching involves a spindle-mounted head, the actual broaching tool, which meets the workpiece at a 1-degree axial tilt.

The spindle then rotates both the workpiece and the broach, which cuts the desired shape one corner at a time. This creates a visible “wobble” effect to an observer, leading to another name for rotary broaching – wobble broaching.

Pros and Cons of Rotary Broaching

In a nutshell, rotary broaching excels at cutting smaller, shallower patterns. Think of a hex-head screw – a shallow, clearly-defined pattern. A rotary broach can create those patterns cleanly and efficiently. When the pattern is a blind hole, a push-pull grind such as a linear broach is impossible, and a rotary broach may be the only option.

The single biggest advantage of rotary broaching is that it can be performed on non-dedicated machinery. You can use an available lathe, mill, or screw machine instead of a dedicated linear broaching machine. Using a lathe instead of a linear broach provides flexibility, particularly for smaller or one-off jobs.

On the negative side, because of the 1-degree offset, there is often some inaccuracy to rotary broaching. This may not always be significant, but it can be a factor in situations requiring high precision.

There always needs to be a degree of tapering or drafting with rotary broaches, similar in concept to the taper of a drill bit, where the walls of the cut slope inwards. This prevents the broach from becoming stuck.

In the end, rotary broaching can be an important part of lathe work. Knowing the advantages of rotary broaching provides another tool for the metalworker’s toolbox, and expands the services you and your lathe can offer.

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