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

I like bill's finished product, but trying to imagine those plans and how it works....it's not happening. Am I the only one that can't follow those?
Can we take a simple idea and make our own? I'm decent with F360, maybe I'll dink around there, it's easier to imagine the tool if you can see it in 3d.
 
ErichKeane said:
The wobble is introduced by the 1 degree angle in the base. The "body" and broach are supposed to remain concentric.
Click to expand...
I don't think this is correct. I don't think you can easily use axial needle bearings in this design. The base of the broach must necessarily move in a circle. The tip of the broach remains centered in the hole you are cutting. The easiest way to accomplish this is to have a single ball bearing at the base of the broach. This acts a a thrust bearing and allows rotary motion and angular motion away from the axis.
You could only use an axial bearing if the angle of the bearing was very precisely calculated and the length of each broach was exactly the same.
I recently made one of these using a single bearing design.

www.hobby-machinist.com

Rotary Broach

I finally got around to making a rotary broach for that special project that I have not yet envisioned: I used my Sherline tool cutter and grinder to grind the broaches out of O1 tool steel: I clamped a 1/2" bar in my 5C collet block and mounted the vise with 2.5 deg of angle as viewed from...
www.hobby-machinist.com www.hobby-machinist.com

As far as the broach goes, the hardest part is grinding the cutting edge and getting it to the correct size. To dish out the end I strongly recommend a Dremel wheel. It sounds primitive but works great.

Robert
 
rwm said:
I don't think this is correct. I don't think you can easily use axial needle bearings in this design. The base of the broach must necessarily move in a circle. The tip of the broach remains centered in the hole you are cutting. The easiest way to accomplish this is to have a single ball bearing at the base of the broach. This acts a a thrust bearing and allows rotary motion and angular motion away from the axis.
You could only use an axial bearing if the angle of the bearing was very precisely calculated and the length of each broach was exactly the same.
I recently made one of these using a single bearing design.

www.hobby-machinist.com

Rotary Broach

I finally got around to making a rotary broach for that special project that I have not yet envisioned: I used my Sherline tool cutter and grinder to grind the broaches out of O1 tool steel: I clamped a 1/2" bar in my 5C collet block and mounted the vise with 2.5 deg of angle as viewed from...
www.hobby-machinist.com www.hobby-machinist.com

As far as the broach goes, the hardest part is grinding the cutting edge and getting it to the correct size. To dish out the end I strongly recommend a Dremel wheel. It sounds primitive but works great.

Robert
Click to expand...
Yeah, it seems more the base the broach rides against is angled, causing an in/out movement, not side to side (off center)
 
GunsOfNavarone said:
I like bill's finished product, but trying to imagine those plans and how it works....it's not happening. Am I the only one that can't follow those?
Can we take a simple idea and make our own? I'm decent with F360, maybe I'll dink around there, it's easier to imagine the tool if you can see it in 3d.
Click to expand...

I do as well! But I have no idea what is going on in those plans either. If you are decent at F360 you are better than my near-zero-experience with FreeCAD, so feel free!

rwm said:
I don't think this is correct. I don't think you can easily use axial needle bearings in this design. The base of the broach must necessarily move in a circle. The tip of the broach remains centered in the hole you are cutting. The easiest way to accomplish this is to have a single ball bearing at the base of the broach. This acts a a thrust bearing and allows rotary motion and angular motion away from the axis.
You could only use an axial bearing if the angle of the bearing was very precisely calculated and the length of each broach was exactly the same.
I recently made one of these using a single bearing design.

www.hobby-machinist.com

Rotary Broach

I finally got around to making a rotary broach for that special project that I have not yet envisioned: I used my Sherline tool cutter and grinder to grind the broaches out of O1 tool steel: I clamped a 1/2" bar in my 5C collet block and mounted the vise with 2.5 deg of angle as viewed from...
www.hobby-machinist.com www.hobby-machinist.com

As far as the broach goes, the hardest part is grinding the cutting edge and getting it to the correct size. To dish out the end I strongly recommend a Dremel wheel. It sounds primitive but works great.

Robert
Click to expand...

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.

So the 'body' and the 'bit' still run concentric to each other.


GunsOfNavarone said:
Getting there...
.5" I.D. taper bearings
Click to expand...
The tools I was planning are .5 ID, so I was considering we needed at least .75 ID on our bearings. That gets us to about 2" diameter body. We could probably get away with only 1 taper bearing vs 1 of each in the other case though.
 
I read every word of this thread.
You guys rock.
Please keep posting your progress.
 
"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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