Group Project: Rotary Broach-- Building complete, all shipped out!

rwm said:
"Really? My understanding in these rotary broach situations (not the 'wobble broaches' that use the ball bearing) that the idea is that the broach itself rotates concentric to the 'body' of the tool, which is tipped at a 1 degree by its mount to the tailstock. That way it presents only 1 side of the broach itself at any one point. That 'point' catches inside the material, and as it 'spins' the in/out motion is caused entirely by that 1 degree angle moving it around at the slight incidence angle."

Yes, I think that plan can work as long as the TIP of the broach remains centered on the axis of rotation. That means that once you set the bearing angle, each tool has to have the exact same length. I have no idea how to calculate and measure that or what the tolerance is for being off angle. It looks to me like the original designer tried to measure that using a dowel pin in place of a broach? I suspect a lot of these might work due to lack of rigidity in the tooling allowing for some degree of wobble even if you can't see it.
My humble opinion is that the single bearing is much easier.
R
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Yep, that seems to be the case of all of the "rotary style" of broaches vs a wobble. As far as "a lot of these might work due to lack of rigidity", you could very well be right.

GunsOfNavarone said:
I'm really leaning toward this design, simple bearings as well, compact design....thoughts?
YouTube Broach
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This is almost exactly what I was proposing above (and my thoughts above were heavily influenced by that!). That one uses 2 bearings, the needle bearings near the front and a thrust-washer-bearing near the back. Actually, he uses _3_ bearings, but I think the rear most ball bearing is perhaps superfluous: https://www.etsy.com/listing/831959563/rotary-broach-plan?ref=shop_home_active_1

You can see the plans at 21 sec of that video, and on the preview of that site.

I'd be OK having each of us spend the $13 and each making a part for that one, though I'm a touch concerned of its apparent lack of adjustability to get the dowel-pin running true.
GunsOfNavarone said:
How to grind the cutting bits
Broach grinding
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I was actually JUST watching that a bit ago. Sadly I didnt see the part that I most care about, getting the 'dish' in the end.
 
1 downside to his design is that it uses 3/8" bits instead of the 1/2" bits. This can limit the size of what you can do, since you're limited by the size of the rod (you can BUY bigger ones, but they are expensive, and making them would require a cylinder grinder to form the shank.

A 1/2" rod limits you to a .433" across flats, 3/8 limits to .324.
 
After watching that video (like 5 times!) I see how he achieves his result. His back plate introduces a 1deg angle on the broach. Then he offsets the base and indicates so the tip of the broach is centered on the axis of rotation. That should work fine. If your broach length varies, you could indicate it and adjust the offset to get centered on the hole. If all your broaches are exactly the same length you would only need to indicate it the first time.
I would love to see you guys successfully execute this!
Robert
 
rwm said:
After watching that video (like 5 times!) I see how he achieves his result. His back plate introduces a 1deg angle on the broach. Then he offsets the base and indicates so the tip of the broach is centered on the axis of rotation. That should work fine. If your broach length varies, you could indicate it and adjust the offset to get centered on the hole. If all your broaches are exactly the same length you would only need to indicate it the first time.
I would love to see you guys successfully execute this!
Robert
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Right, yes, that is exactly what the design I posted does too, except it has some adjustment for the offset. The video only has the offset adjusted in the range of the clearance holes I think...

Purchased bits tend to be all 1.750", so I was going to try to replicate that (in case we wanted to just buy off the shelf ones), but I like the idea of being able to indicate in to adjust the cutter.
 
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Another interesting project version we might take some cues from:

I like that he split the mount and the body and put the angle in the body instead. This might be a touch harder to make, but it makes mounts interchangeable. Not sure it is worth it though for the lathe-like work on the mill... Also, the ER collets for the mount is interesting!
 
Also, re-taught myself a bit of free cad today and came up with the attached. The only annoying one is that the 'dish' in the end and the large-diameter of the hex aren't linked, so I'd need to adjust those manually together.
 

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I love the optical centering microscope in that video!
Robert
 
Yeah the centering needs to be easier and quick. I hate having to adjust a independent jaw chuck, some of these designs would be possibly worse. How do these $1000 ones work? I'm sure they are the bee's knees.
 
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