Rotary broaching

Hi

I’m stuck. I made a wheel spindle, one end of which needs a 19mm hex cutting Into it . I bought a 19mm carbide rotary broach from a respected toolmaker, oversized the pilot hole to 19.2mm, chamfered it and then tried to cut. But nothing happens. The broach makes contact with the workpiece and spins with it but doesn’t cut whatsoever. I’ve tried what I consider heavy insertion force and still nothing. Has anyone done this before? How much insertion force does this require? Everything I’ve read just says rotary broaching is dead easy and requires little force

It’s 7050 aluminium. I’ve tried increasing the pilot Hole to just over 20mm but that’s as far as I dare go as the major diameter of the broach is only about 22mm

thanks as always

Dan

The broaches need to go into a special holder that allows them to "wobble" and work their way down into a hole. You can't just force them into the hole like a push or pull broach

Hi thanks. Yeah I forgot to say I do have a rotary broach holder - when you spin the broach independently you can see the 1 degree wobble

Following with interest as I had a similar problem. Is the broach holder a decent make, mine was a Chinese offering and just didn't work?

Tony

Yeah it’s a cheap Chinese one from eBay. Like I say though you can see the wobble so surely it should work? Who knows

I never did understand the principle really as surely once the part is in contact with the broach, nothing is ‘wobbling’, all that is happening is one part of the broach is ahead of the others. Obviously it’s clever people than me that came up with this though 🙂

This maker gives the pilot hole for their 19mm hex as 19.75mm so you were possibly small to start with, also check your chamfer

The wobble puts the force onto each corner in turn which is more effective than trying to cut them all at once

Edited By JasonB on 02/07/2023 15:05:36

Thanks. Chamfer is 45 degrees and just slightly wider than the widest point of the broach. I did debate going to 21mm for the pilot but I’ll only have 1/2 a mm all the way round to support the corners of the hex

Also interested in this as it is on the list to try. Surely though, the broach should not “wobble” when rotated, merely rotate on an axis one degree displaced from the rotation axis of the work. Presumably also the broach is advanced along the work axis, not its own rotation axis.

Edited By Macolm on 02/07/2023 15:37:58

We don’t need to reinvent the wheel, rotary broaching is a proven technique with decent tools.

Tony

Totally agree with Tony, a totally proven technique, if it doesn’t work you have a c*** tool. Hemmingway do a small broach kit and drawings, it works superbly well I have made a couple of them and had absolutely no problems broaching 5mm hexagon on a Myford lathe.

19mm is going to need a fair bit more pressure than the small sizes the Hemmingway unit can go upto. and would not be something to try on say a minilathe and 7050 is quite a strong alloy

Mike Cox wrote an illuminating article on the subject in MEW 241, page 24 et sequ. A read may help.

+1 Macolm's comment

Thanks - just done some reading as suggested and it sounds like it’s simply too big a size. The tool I have is quality, the holder perhaps less so but I can’t see any issue with it. I’ll have to have him make me a smaller broach so I can experiment with smaller ones.

It’s a good point about 7050 as well - i just looked up the Brinell hardness and it’s harder than some steels

How is the broach holder being held? In the tailstocks or in a toolpost? If it's in the tailstock then check the spindle to tailstock alignment. If it's offset a bit there might be enough bend in it all to cancel out the 1 degree axial offset.

If it's in the toolpost check that you actually do have the offset angle and that it's not turned to cancel it out and present the broach face flat to the work as it won't cut like that.

In either case you could try adding a little lateral offset and see if that helps.

The principle is very simple. If you look at Malcolm's sketch you'll see that the bottom edge of the tool is inside the hole and the top edge is outside. This means that the point at the top will be more to the left when it's made half a turn to get to the bottom. The end of the broach is the cutting face and it plows the metal into the hole as it turns. All you have to do is push it.

As Peter says the principle is very simple but perhaps more easily understood as the action is akin to skiving with a wood chisel type action.

The tool holder is being held in the tailstock (i turned a Morse taper on it). I have checked alignment but I’ll recheck it tomorrow. Thanks again

I suppose the bit I can’t my head round is how the spinning helps in any way. Plus once you have progressed beyond the opening, surely all of the tool is in contact with the part, just with the leading edge further in

That's correct, it's further in but still doing the same thing, shaving a small amount ahead of itself each rotation.

If it's in the tailstock try moving the tailstock off-set a tiny bit. If you were looking at Malcolm's sketch above move the tailstoks off-set in the downwards direction.