Thread: Rotary broaching

Hi

Been reading a article on rotary broaching , I don't quite understand how it works . Has anyone tried it and how was the result ?

The work piece and the tool , both rotate ?

Mike

I am not sure if this is what you are talking about but in my distant past I have seen it happen in the Auto Engineering factory I used to work at. It was used to create the internal splines for gearbox parts such as the synchro drum/barrel? for early model holden 3 speed boxes. Also the gear that slid on the outer spline on this.

I don't remember much but I think the broach rotated and the work stayed still. These splines were not straight but a helix shape.

Dean

Mike, I was about to make one but had to do a few urgent projects so it got put on hold.

The principle is quite simple. Say you want to broach a hexagonal hole. Drill a pilot hole just under the minimum flat size of the hexagonal. The tool holder is basically a bearing that is offset by 1 degree or so (if I recall correctly, I'm getting ready to leave so can't check). The tool is shaped to the final size and profile from tool steel and placed in the holder. It is ground with a clearance angle of the 1 degree working angle, plus clearance. Again, don't quote me on the figures, the clearance could easily be more than that without affecting performance. In use the hex (in this case) is presented to the hole and both rotate together. The back of the tool however scribes a circle of the 1 degree working angle. Imagine putting a pen on a piece of paper, the ball of the pen is the hex on the work, rotate the top of the pen around in a circle. The front of the tool basically "wiggles" with all this action behind it and with pressure on it the wriggling broaches away the work material and gradually works through the starter hole.

Hopefully that makes sense.

Here's a site that describes making one. I made some lathe live centres last week that used the same bearing principle, but obviously I cut the MT2 taper to sit in my tailstock and be perfectly concentric and not be angled as in this arrangement. But the bearing arrangement looks like it's the same from the brief re-read of that site and it works quite well.

Rotary broaching - mikesworkshop

Pete

Originally Posted by morrisman

Hi

Been reading a article on rotary broaching , I don't quite understand how it works . Has anyone tried it and how was the result ?

The work piece and the tool , both rotate ?

Mike

Many years ago when I was doing night courses at TAFE, a teacher demonstrated how to cut a square hole using a reground (broken) centre drill held in the tailstock. For the life of me I can't remember the exact details but I think the TS was offset to make the tool wobble somewhat, the workpiece had been drilled/bored so only the corners needed to be cut out and the tool was ground radially on its front face so it was concave looking at its cutting end, with lots of relief behind the square edges.

None of us could believe it would work, but it did.

A poor explanation I'm afraid but it's a 30 year old memory of something I saw once. But yes, both pieces rotated. I think the cutting action took place due to the wobble of the cutter.

PDW

Ok, I am beginning to grasp the theory of it .

The article I referred to is in MEW magazine . BTW thanks to Pete F for recommending the subscription to MEW , the price is well worth it e.g, access to the MEW catalogue of on-line digital issues

Mike

Yeah that's the one Mike, but I didn't have time to look it up. Which issue is it in?

Originally Posted by Pete F

Yeah that's the one Mike, but I didn't have time to look it up. Which issue is it in?

Hi

It's in the Jan. 2012 issue ..... Mike

Rotary broaching caught my attention when I needed to make 1.5mm hex sockets in a batch of small screws; the screw materials being cold worked stainless steel (!), and titanium. I was doing this by the (non-rotary) brute force method using a lathe that was large relative to the task - screw held in a headstock collet and broach in the tailstock, and was surprised by the force required to sink the broach. The operation was slow and some of the workpieces (and broaches) did not survive.

A precision machining business showed me how they could do a similar sized rotary broaching operation quickly and reliably on a small automatic lathe. Clearly this technique used much less force than I had needed, apparently due to the rotary broach cutting only on a small part of the edge at any time. However the cost quoted was way out of reach for my prototype batch, so I soldiered on and did the lot by the brute force method. The reduction in cutting force, and the speed and ease of the operation seem to be the major advantage of rotary broaching.

Cheers,
Bill