Rotary Broaching

Some years ago, I bought a rotating tailstock chuck at a Harrogate show. I thought it might come in handy for some future job. So when I needed to cut some Allen key hexagon holes, I thought of using the chuck as a rotary broach. It worked very well. The broach is milled from silver steel, being a 1/8" hexagon with a 2 degree (4 degree included) taper. Harden and temper to straw. Set the rotating chuck in the vertical slide and set it to 1 degree relative to the lathe mandrel centre line. Drill a 1/8 hole in the work and cone its entry with a centre drill. Then adjust the broach so it just enters the hole, apply oil, start the lathe (very gently) and use the tailstock to push the broach in.

You may notice that my broach has a spiral at one end. i didn't make it that way - I was a little careless in engaging the lathe chuck. But the broach still worked.

Image edited

Edited By JasonB on 19/03/2013 17:39:24

Otley,

My friend and I were discussing this subject at the weekend and watching video's as to how this works as I couldnt see or understand what happens, any chance you could explain it please.

Martin Perman

Hello Martin,

My pic seems to be a bit squashed!

Anyway, imagine trying to force a short length of hardened hexagon bar (a 'broach) into a pre drilled hole. If you apply enough force, the hexagon will enter the hole and cut a hexagonal form. But it will need a lot of force.

Now mount the hexagon broach in a holder which is free to rotate. Hold the work to be cut in the lathe chuck and start the lathe. Trying to push the broach into the work will still need a lot of force. So now angle the rotating broach holder. Just a little, say 1 degree. Now only one point of the hexagon broach is trying to cut at a time. As the work and the broach rotate, each point of the hexagon broach in turn takes a little peck at the work and much less force is needed to push the broach.

If the rotating broach is set to a 1 degree angle relative to the lathe axis, the hexagon cutter needs to have a similar taper angle, or say a degree more to give some clearance.

Edited By Otley on 19/03/2013 17:30:56

Edited By Otley on 19/03/2013 17:31:14

Aynone used an Armag profiling head?

**LINK**

Otley,

Thank you for that, I know have to tell my friend he was correct

Martin Perman

Nice one, Otley.

Neil

Very interesting, but one question. Does the hole end up with a 1deg pos taper or does the taper on the broach negate that ? Or even make it 1deg neg ?

Thank you Otley a good explanation I would like to know the maximum depth you can do.

Paul

prob relates to 4 degree included taper...the tool necks a bit but in practice relief at 4 degree then slows to 2 degree for rest of reach...

**LINK**...

The hole size is determined by the size of the cutting end of the broach and is constant throughout the depth of the hole. The holes I made were 1/8" across flats and .35" deep. A 1/8" Allen key is a nice fit to the bottom of the hole.

I think the technique is probably best suited to holes with depths about twice the diameter of the broach - I had to withdraw the broach after it was .2" deep and drill out the swarf before continuing. Then, making sure that the broach was properly entering the already cut hexagon was a bit tricky. I have also read that as the hole gets very deep, the broach can cut a slight spiral form.

I still cannot get over the way my hardened and tempered silver steel broach twisted so far without breaking...

Edited By Otley on 20/03/2013 10:36:31

Otley: That's a neat explanation, thanks. I believe that the technique is also called wobble broaching, which may help visualise the action of the cutting tool.

Andrew

Thanks for the simple explanation. Is there any reason why the broach could not be made from a good quality hex key, grind on relief and harden ?

Thanks Jason that link answers my question and the pictures on it are useful when I make one.

Paul

Too cold for the workshop today, so I thought I'd draw up a wobble broach for when it gets warmer.

If anyone wants to make one, make sure that the centre of the tool 'A' is on the centre line of the holder 'B'.

Otley,

PM sent

Andy

There is a very good article in MEW 157 describing the construction of a wobble broaching device

Graeme