Rotary broaching

[old martParticipant @oldmart]

I made one with an er25 collet with a pair of bearings on a MT2 shank. The cutting end of the tooling must project exactly to the null point or the tool won't work. I marked the length on the body whan I made it.

[bernard towersParticipant @bernardtowers37738]

Daniel , there will be 6 points of contact as it rotates and each point comes into contact once per rev. Out of interest I have made two of these , one to my own design and one of Hemingways design and both work well.

Edited By bernard towers on 03/07/2023 21:51:22

[Daniel BrannanParticipant @danielbrannan48897]

Thanks Bernard. What’s the largest size broach you’ve used with them?

[Daniel BrannanParticipant @danielbrannan48897]

I think I’ve just had what alcoholics call a moment of clarity. Yours truly turned a morse taper on the tool holder – could I have eliminated the 1 degree offset in doing this? Which part of the tool would have the offset the body/shank or the rotating holder?

[Pete RimmerParticipant @peterimmer30576]

The axial off-set is in the bearing pocket in the MT shank. The broach holder itself is machined all concentric.

As Malcolm very insightfully put, you should see no wobble in the broach, it merely turns on its axisbut the axis is off-set to the axis of the holder making it cut.

Wobble broaches do not spin with the work, they are merely driven by a holder which wobbles. This is a wobble broach I made quickly to cut a 12mm hex pocket. It doesn't turn with the part as you can see.

Wobble broack working

Description

Edited By Pete Rimmer on 04/07/2023 06:56:22

[not done it yetParticipant @notdoneityet]

Posted by Daniel Brannan on 03/07/2023 22:16:02:

I think I’ve just had what alcoholics call a moment of clarity. Yours truly turned a morse taper on the tool holder – could I have eliminated the 1 degree offset in doing this? Which part of the tool would have the offset the body/shank or the rotating holder?

I did wonder whether the lathe was up to cutting, with a one degree offset. There is also, of course, the possibility that the one degree is out of expected tolerance? Perhaps the size of the broach is too much for the lathe? Everything needs to be sufficiently rigid for these cuts, which might be regarded as ‘heavy’ for an aged, small machine.

[old martParticipant @oldmart]

This is what mine looks like, the null point is 87mm in front of the body, as marked. The angle it was made has been forgotten, but 1 degree would seem a good guess, hence the reverse taper on the tools to stop them fouling the hole.

[bernard towersParticipant @bernardtowers37738]

Hi Daniel. 8mm hex so far, I don't tend to do big stuff.

[JasonBModerator @jasonb]

The broach will turn with the part if the part is being broached in the lathe

And this one shows how the broach "wobbles" putting the load onto each corner at a time

 

 

Edited By JasonB on 04/07/2023 16:18:59

[old martParticipant @oldmart]

When you design a rotary broach the exact angle and offset of the bearing housing must be calculated as well as the position of the null point where the cutting takes place.

[Daniel BrannanParticipant @danielbrannan48897]

I noticed one of those videos was by polygon – I actually contacted them today about their broaches and they said 19mm is probably too big to cut on a small lathe as there isn’t enough thrust.

However based on what we have discussed here ie assuming my tool holder isn’t holding it at an angle, in a last ditch attempt to make what I have work, I tried this: brazed the broach holder onto some square steel and mounted it in the tool post, with it set off by a degree. It was more successful than before in that it removed some metal, but it essentially just made a a mess. It seemed to make contact but as that part of the broach spun away on its axis, it then lost contact and did this repeatedly until it chewed it up. I’m still no clearer on how it’s supposed to work really

[Tony Pratt 1Participant @tonypratt1]

I gave up on my rotary broaching because a] I couldn't harden the broach to my satisfaction, b] a lot of axial force is required? and c] I wasn't convinced the cheapo broach holder was any good but I don't recall seeing any mention of a null point??

Tony

[old martParticipant @oldmart]

I made my broaches out of silver steel, hardened them and then put them at the top of the gas oven at regulo 9 on a teaplate. They just about got tempered enough to use, but the plate cracked.

[Tony Pratt 1Participant @tonypratt1]

Posted by old mart on 04/07/2023 17:27:35:

I made my broaches out of silver steel, hardened them and then put them at the top of the gas oven at regulo 9 on a teaplate. They just about got tempered enough to use, but the plate cracked.

Yes I used Silver steel but my little torch just could't get the broach hot enough.

Tony

[jimmy bParticipant @jimmyb]

What speed are you running?

 

Jim

Edited By jimmy b on 04/07/2023 17:37:01

[old martParticipant @oldmart]

I used backgear on the lathe and plenty of tapping fluid for steel.

[Daniel BrannanParticipant @danielbrannan48897]

Tried most of the speeds my lathe will do so everything from about 150 to 1000 rpm

[DC31kParticipant @dc31k]

I think old mart's suggestion of paying attention to the null point has perhaps not been given enough attention.

The terminology he uses might not be standard but the geometric concept behind the words is important. In a way it is similar to how setting a lathe tool above, on or below centre can have serious effects on its cutting ability.

Slater Tools, who know quite a lot about this process, stress the need to set up the tool before starting:

http://www.slatertools.com/support/internal-rotary-broach-toolholder-setup-instructions

Their broaching catalogue, here:

http://www.slatertools.com/resources/uploads/PDFs/ST-catalog-english.pdf

shows some interesting things and highlights that the length of the broach is important.

If you mix and match broach and holder as the OP has done, there seems to be considerable scope for poor cutting. With a tool that is too long or too short, the vertex they show in their diagram will not coincide with the cutting edge.

[Daniel BrannanParticipant @danielbrannan48897]

It’s a good point but this is the bit I can’t get my head round. Surely if the holder was as per that slater diagram, then the cutting tool would just rotate on its own axis so as long as you centre it, the null point is irrelevant. As opposed to if the hole in the centre of the holder where the tool clamps was drilled offset, then you’d get ‘wobble’ when it spun? But what do I know.

Anyway this was todays creation (sorry I couldn’t work out how to rotate the image):

[peak4Participant @peak4]

Daniel, I'm no expert on this at all; I've bought a toolholder, but not tried to use it yet.
It looks similar to yours, but with a parallel shank, which I assume is the way your one started out.

I think it's a non adjustable one, so the Slaters video above which speaks about null points may be misleading; or may not of course as I've no experience.

Have a look at the range of Slaters videos, and in particular the last one, about Swiss style toolholders.

See also

Bill

 

 

Edited By peak4 on 04/07/2023 22:37:37

[Pete RimmerParticipant @peterimmer30576]

Some of the detail in those Slater videos leaves a lot to be desired. Clocking for 0.001" TIR using a mag mount dial and rotating chuck is poor practice and their animation for rotary broaching in the lathe shows the broach wobbling as it would if it were done in a mill.

[Daniel BrannanParticipant @danielbrannan48897]

Having looked more closely at the slater site and videos it backs up the idea about null point. They sell two types – non adjustable which require a specific tool length and adjustable which can be setup and crucially they are all offset at the inner drilled hole for the tool itself

 I’ve asked for a quote, just for a laugh

Edited By Daniel Brannan on 05/07/2023 08:56:20

[bernard towersParticipant @bernardtowers37738]

My most successful broaches are ground from HSS blanks and have successfully broached 1.5mm squares, nobody more surprised than me!

[mark costello 1Participant @markcostello1]

We used one on a medium size turret lathe, name escapes Me. We broached 3/8" hex 3/8" deep in 17-4 SS. It took all the grunt the machine had taking 45 seconds to a full minute to go that deep. Don't remember the name, the bar capacity was around 1" and tool shanks were 1". Cannot find a picture. Occasionally when the tool went dull, the machine would pull the tool end off when backing out. Not a real beast but not shabby either.

[old martParticipant @oldmart]

Seeing the photo of the large tool in the rotary broach makes me understand why the broaching was a failure. You would need tons of pressure and the broach is bigger than the shank of the tool. Think of the cutting end of the broach being rotated with the workpiece and moving like your finger would if you pressed it onto a surface and rocked it about in a rotary motion. The tip of the finger does not move about sideways at all. Iwould take the workpiece and the broach cutter, not the broaching tool to some local engineering workshops and ask them if they can do the job for you. The hex could be cut on a shaper if all else fails.

Edited By old mart on 05/07/2023 19:22:41

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