A Shop-Made Compact Rotary Broach

Hi guys.

I don't understand it, but my broach is not working/cutting. I made it the same as the drawings in post #19 by @Cobra . All I did was convert to metric and made the body of the broach and cutter 3mm bigger but in relation to the drawing sizes. The one difference is that I made the cutter more snug fit to the cutter holder rather than the 0.0500 clearance between cutter and body. Don't know if that could add to the cutter not cutting. I gave the cutter a 2° relief and dished it too. Cutter is nice and hard. It is a 10mm square broach and I drilled a 11mm hole and gave a good countersink and the picture below shows the only marks it makes,doesnt matter how fast or how much force I give.
20230719_130349.jpg
20230719_115100.jpg20230719_115117.jpg

What could I possibly have done wrong. I feel a bit lost. Somewhere I read it takes about 10 seconds to make a square hole........ye na...not my my broach.

PLEASE HELP.
 
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a frew things I can think of.
How much of an off center is your broach holder? There needs to be a wobble.
How hard is that material? Did you try it in Aluminum to make sure it is working before trying in steel?
For a fixed broach like that the material must be a certain length to center itself. Adjustable broaches, you center depending on the length of the broach itself.
that hole does not look big enough. It looks like even the flat sections are cutting more than desired.
 
woodchucker said:
a frew things I can think of.
How much of an off center is your broach holder? There needs to be a wobble.
How hard is that material? Did you try it in Aluminum to make sure it is working before trying in steel?
For a fixed broach like that the material must be a certain length to center itself. Adjustable broaches, you center depending on the length of the broach itself.
that hole does not look big enough. It looks like even the flat sections are cutting more than desired.
Click to expand...
- Broach off center=0.0500". There is a wobble.
- Material about 1/2" mildsteel
- Tried a piece of Delrin...same effect.
- The picture might deceive you because the hole is 11mm and the broach across thr flats is 10mm.
 
The offset......if I convert 0.0500 to metric,it is 1.27mm from center to the offset point. So in other words when I made the offset in the lathe, I dialled in the workpiece to zero and then offset it to 0.0500 (1.27mm) using my DRO radius fuction. Is that correct or must that 0.0500 actualy be dubbled?
 
Suzuki4evr said:
The offset......if I convert 0.0500 to metric,it is 1.27mm from center to the offset point. So in other words when I made the offset in the lathe, I dialled in the workpiece to zero and then offset it to 0.0500 (1.27mm) using my DRO radius fuction. Is that correct or must that 0.0500 actualy be dubbled?
Click to expand...
that's correct it still must be on center.
since you tried on delrin you've done a soft piece and it doesn't work.
I'm at a loss, can you take a high speed video, so we can see what's going on in slo motion?
maybe we'll be able to see what it's doing.
 
I was looking to see if I can see the cutting action, and I wasn't.
How much force do you think you have on that? These require a lot of force. You need to hold that tool to the fire so to say.
Have you tried a hex , a square will require much more force since it has longer points out to the edges that need to work.
you tool looks good in the pics
your offset looks ok
your relief looks ok

I probably would have built in a touch more offset, but I don't think that's the issue. More equals more of the tool doing the work, as it is it would still cut, just slower.
 
I'm a little baffled, especially with the Delrin.

Maybe there is a little bit of gold in here for you?
 

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  • Somma Tool - Offset Rotating Broach Information.pdf
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