Rotary Broaching a Hex Key Recess on a Mini Lathe

Whilst busy working on something entirely different, I wandered off on this hex hole cutting tangent. Essentially I wanted to be able to make an Allen key recess.

Rather than double the length of the original project, I’ve separated it out as a stand alone film.

Cutting small a small hex recess in a blind hole is relatively easy using a simple tool that can be knocked up from materials lying round the home shop.
Credit for at least 50% of the design goes to Mike at Mikes Workshop. Take a look at his web site. There’s some real gems in there for the bench-top machine tool user.

http://mikesworkshop.weebly.com/

Mike’s rotary broaching tool can be seen in action here:

https://youtu.be/T8EhGxH7z_w
Although I’ve yet to try it, the same technique can be used to make holes in various other shapes including polygons, splines, Torx etc.

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POST UPLOAD UPDATE:

Thanks for all the comments and feedback :-)

I've read them all, but I’ve not the time to respond individually. There’s some good ideas and advice for me to try next time.

In response to queries about the white material in my tin can brazing hearth:

It’s called ‘Insulfrax S Blanket’ ceramic fibre. It’s available from eBay (in the UK at least). There’s a thin piece of steel sheet in the bottom of the can (where the part sits) to help prolong the blanket’s life where the flame hits it directly.

Note: some types of ceramic fibre are bad for the lungs (in the same way as asbestos is) so I’d recommend at least gloves, mask and ventilation when handling (what ever the type of fibre -just to be safe). I only handle a small amount, infrequently, and Insulfrax S is body soluble so think my precautions are enough.
As for how the broach works:

Consider if the broach and brass were in-line on the same axis. You’d be in an arbor press situation, with all corners cutting simultaneously. The rotation would contribute nothing.

The key is the 1 degree offset and the relief on the broach.

This allows the small contacting part of the cutter to present a shearing action to the inside of the pilot hole as it rotates in the brass. As the work turns so does the cutter, presenting a different cut profile as the broach rotates.

The cutter has to be free to rotate or it would just act as a boring bar and inefficiently cut a circular hole.

I’ve not tried it, but I imagine as the number of sides increases, and the shape tends towards a circle, the less effective the tool will become.

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Made with subtitles -click the CC box.
Shot: HDC-HS700 1920x1080 50P AVCHD
Edit: FCP7 ProRes 422
Mic: C01U

Видео Rotary Broaching a Hex Key Recess on a Mini Lathe канала The Recreational Machinist