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Angle the tailstock with live center to rotary broach.

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agfrvf

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There was a video on YT that was all in russian where the machinist performed a rotary broaching operation using the typical broach shape and live center. Anyone try this method?
 

Mark Needham

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Yes, and you need TONS of power to do anything above 1/4". I can just push a 1/2" through Al, but not Fe. You do not offset tail stock.
 

agfrvf

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Tried last night with no luck. Ground a 1/4" hss blank with some reliefs loosely based off of AvEs square broach with no luck.
 

brino

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Got a link to the original video?

All of the rotary broaches I have see have some kind of "off-centre" mechanism that tilts the tapered tool alternately in the starter hole.
It is then the leading edges that do the cutting alternately in kinda slicing motion.
The offset is usually built in to the broach holder that goes in the tail stock.
That's why they also call them a "wobble broach".

Here's AvE's take used in the mill:
....sort of a "proof of concept" only.

and a few links to better such projects here:
https://www.hobby-machinist.com/threads/shop-made-rotary-broach-holder-and-broaches.19945/
https://www.hobby-machinist.com/threads/a-shop-made-compact-rotary-broach.32219/#post-272748
https://www.hobby-machinist.com/threads/rotary-broach.74152/

I have put aside some largish ball bearings to build mine.....I just gotta find some time to do it.

-brino
 

Mark Needham

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When I said Yes, I meant that a home made rotary does work. Now, I did not say that it was perfect. No, I have not tried any russian method. ('aint seen it)
as to the pressure, believe me, when I said Tons (tonnes) I mean a shirt load of 'em. I destroted, (destroyed 'em also) 2 6204 bearings, doing a 1/2" hole in a 1" piece of aluminium. At the moment I am back to square file 101., best method so far.
edit:- http://www.docsmachine.com/projects/rotarybroach/rb01.html This is the style I made.
 

jbolt

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I designed and built one based on the commercial units. Not that difficult to make. The broaching bit is relief ground on the sides as well as hollow ground in the center. The holder for the bit is at a 2 degree angle to the axis centerline with the tip of the bit intersecting the centerline. The largest broach I have done is 1/2" hex 5/8" deep through 4140 steel. The pilot hole needs to be slightly larger, 2 to 3 thousands, to lower the cutting forces Otherwise the forces can be brutal on lighter duty machines. It also helps to put a 60 degree chamfer on the pilot hole.


Not shown but I also made a straight shank for use in the mill.
 

brino

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Thanks for posting that
....interesting....I have never seen a "spare" live centre held in the tool post before.
But I guess that's one way to do a controlled offset and not have to re-align the tailstock centre afterwards.
Kinda clever that there was no special, exotic tooling required....besides the cutter itself......no body or holder.

okay a few more thoughts......
first safety
  1. I don't think he wore any safety glasses. I can't image that. I'm so trained that I now put on saftey glasses to watch machining videos!
  2. it made me cringe a little to see him pulling the tool out of the work knowing the centre was right there to puncture him
3) I suppose he used some sort of on-line speech recognition program, but is the Russian word for "broach" really similar to the Russian word for "firmware"?

-brino
 
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agfrvf

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I may just get a MT2 axa tool holder for my live center now.
 

ThinWoodsman

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Interesting idea - thanks for posting the video.

Pretty beefy toolpost he has there. Dunno if an AXA would be up for it, but I'd guess it depends more on the carriage, which you can lock down.
 

brino

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I'd guess it depends more on the carriage, which you can lock down.
I was hoping it would allow you to use automatic feed......but I guess it comes down to how much power is needed to rotary broach and what lathe you have (size, rigidity, motor, etc).
-brino
 
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