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- Jun 26, 2018
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How to grind the cutting bits
Broach grinding
Broach grinding
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."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
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_1I'm really leaning toward this design, simple bearings as well, compact design....thoughts?
YouTube Broach
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.How to grind the cutting bits
Broach grinding
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...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