Rotary Broach

Rotary broaching is a process that creates internal shapes with sharp corners like the internal hex of a screw with a tool that gets pushed into the predrilled workpiece. The broach is at a slight angle and is driven by the workpiece (In case of the lathe), so it is “wobbling” and that motion changes the leading edge of the tool all the time and creates something like a cutting action.

There are commercial tools for this process available (Slater Tools for example) but they are not exactly cheap and affordable for the hobbyist.

At the same time there is a large number of shopbuilt versions of this kind of tool out there and there is also a set of plans from Hemingway availible.
Together with another Kit I ordered the compact rotary broach. It consists of a set of plans and some material:

I have a series of videos showing the build process and use of the rotary broach availible on youtube:

Part 1:
Compact Rotary Broach Part 1

Part 2:
Compact Rotary Broach Part 2

The first part to be machined is the nut that holds everything together. I choose to machine it from some 42CrMoS4 that I had left from another project. The Thread is a M20x0,5mm finethread. After threading the parts went onto the mill for drilling and then back to the lathe to be parted off:

The spindles that hold the actual broach are machined from the same 42CrMoS4 steel, as it is very tough:

I decided to build the shank in two parts, with a hardened 10mm dowel pin that has a flange-like part glued on with Loctite 648:

The face of the shank gets drilled for two screws to hold the body of the rotary broach and a key machined for alignment:

The housing was turned from a piece of freecutting mild steel that was supplied with the kit - It has to have three counterbores with exact depth and diameter for the bearings and the M20x0,5 fine thread for the nut:

Grinding two flats on the housings to help with alignment in the further process of building the rotary broach:

Aligning a vice at 1° using the sine bar and machining the rear end of the housing. The keyway fits the shank:

Drilling and tapping the M4 thread to clamp the broach in the spindle:

First test, I ground a 4mm hexagonal broach out of a round 8mm HssCo10 blank and gave it a try on a piece of brass - Predrilled with a 4mm drill and then followed up with the

A normal counterbore tool would be to big to counterbore the screwholes on the back of the shank, so I ground a stepped drill:

After the parts are aligned and screwed together, the outer diameter of the tool can be turned down to a uniform look:

I made a complete set of hexagonal broaches from 2mm up to 6mm:

Broaching a 5mm hex on the milling machine in mild steel - The result is pretty good, but the predrilled hole was a bit to big, so the broach did not clean up the whole hex:

4mm hex in brass, 4mm and 6mm hex in steel:

The completed rotary broaches, a set of hexagonal broaches, some test pieces and the pin spanner:

All in all a very interesting small project that can be accomplished in one weekend – If you are brave you can do it with a lathe and a milling attachment, but a milling machine will make your live easier.