# Rotary broach



## Malave16

Here's a rotary broach that i just made. It not my idea, the gentleman that goes by arduinoversusevil on youtube is were i got the idea from. Its made out of stock from some center punches that came with my lathe.
The trickiest part was turning the offset. I did it on the 3 jaw by putting .140" shims on one of the jaws. The hole for the bearing is just the profile of the bit i used to drill them, nothing special. I just made sure that the ball would stick out of the hole just under halfway on all the holes. The holes for the bit were supposed to be temporarly round until i could make them square with the broach, but they work just fine like that. Profiling the tools was the biggest pain. I used a dremel with a ball attatcment. So far i only used it on a drill press.  It makes square holes. I was also considering making more dies for it using actual tool bits, like an actual allen key sharpened to make hex holes maybe torx and the like. They are hardened, so it should work. Any questions please ask.


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## Karl_T

Impressive. I didn't know it was fairly simple to make one of these.

Did you get prints for dimensions or draw it up yourself?


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## Mark_f

I wonder how thick a piece of metal you can do this to.? woul this be a viable way to make a square hole in say a boring bar?


Mark Frazier


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## Malave16

Thats the sole purpose why i built it, to make boring bars with it. So far i made one with the 1/4 bit trough 1" cold rolled. It worked fine. About 1/2 way trough had to stop and clear the chips, buy it went trough.


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## Ebel440

Does it hold itself together somehow or is it just held together with the spindle force?


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## Malave16

Got the idea from youtube. No prints, i kinda eyeballed everything


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## Malave16

Ebel440 said:


> Does it hold itself together somehow or is it just held together with the spindle force?



Just spindle force will hold it together. A bit of duct tape will make it usefull on the lathe, but not worth the risk


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## chips&more

That’s a clever way to make a rotary broach! Nice Going, Dave.


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## Mark_f

I have heard of these, but I never saw one of these work. Seems like it would vibrate like a jack hammer or be hard on the equipment. If not maybe I should try it. 


Mark frazier


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## chips&more

mark_f said:


> I wonder how thick a piece of metal you can do this to.? woul this be a viable way to make a square hole in say a boring bar?
> 
> 
> Mark Frazier



Mark, the tool bit does gyrations/rocks about a solid angle. Just as long as the tool bit has side clearance for the rocking/tilting it will go as deep as that clearance. The downfall is the more clearance on the bit, the weaker it gets, so you will need to compromise…Good Luck, Dave.


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## ogberi

Another item for my to-do list.  Is it tricky to get started in the hole?  What's the grinding procedure for the 1/4" bits?


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## Malave16

mark_f said:


> I have heard of these, but I never saw one of these work. Seems like it would vibrate like a jack hammer or be hard on the equipment. If not maybe I should try it.
> 
> 
> Mark frazier



With a moderate speed on the drill press vibration is not an issue at all. The only thing you get is a constant up and down of the quill. When i get a hold of my neighbor ill try it on his mill and post a vid.


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## Mark_f

Is that what the little "divots" on the corners of the bit are for? To give the relief for the gyrating movement? If so they limit the cutting speed and ability?  :thinking: Also, Do you have to drill a "starter" hole and if so does it need to be the size of the finished square hole? 

example" a 1/4" hole for a 1/4" square hole?


Mark Frazier


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## Ulma Doctor

:rubbinghands:
thanks for the great idea!!!!!
and I subscribed!!!


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## coolidge

My brother has a bunch of those for making hex sockets on a cnc lathe, they are expensive as hell, great job making your own!


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## jocat54

mark_f said:


> Is that what the little "divots" on the corners of the bit are for? To give the relief for the gyrating movement? If so they limit the cutting speed and ability?  :thinking: Also, Do you have to drill a "starter" hole and if so does it need to be the size of the finished square hole?
> 
> example" a 1/4" hole for a 1/4" square hole?
> 
> 
> Mark Frazier




Hi Mark,

The ones that I made (just 3/8 and 1/4 square) You have to drill a pilot hole and taper it to get the cutter centered in the hole.
I drilled a 17/64 hole for the quarter inch and that worked much than a 1/4 hole. Not a lot of precision needed on the holes I was doing. I ran the mill at a really slow speed and it just works it way down the hole.

I like the OP's idea of using hex wrench to make a cutter---never crossed my mind.


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## Malave16

mark_f said:


> Is that what the little "divots" on the corners of the bit are for? To give the relief for the gyrating movement? If so they limit the cutting speed and ability?  :thinking: Also, Do you have to drill a "starter" hole and if so does it need to be the size of the finished square hole?
> 
> example" a 1/4" hole for a 1/4" square hole?
> 
> 
> Mark Frazier



The divots on the side are for relief. They allow for the oscilating movement that produces the cutting. The pilot hole needs to be at least the size of the hole you want to make. You also want a chamfer at least as the widest point of the cutter.


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## broach

Great work Malave16! I make rotary broach holders commercially and am always impressed when I see people making their own or coming up with modifications on the design. Our three basic principles for setting up a rotary broach include chamfering the hole a little larger than the sharp corners, using a slightly oversize pilot hole diameter (10% for squares) and leave room for chips to accumulate. Hope that helps.


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## NormBourne

Help chaps,

As an electrician I'm an absolute beginner machinist, and I'm having trouble understanding the operation of the Broach described here. 

I've achieved a square hole before by bashing a HSS tool thro with a big hammer, the requirement was for carbon brush mountings in DC motors. The results were erratic to say the least.

Mention is made of the tool gyrating, and looking at the designs it is pretty obvious that the tool is meant to be powered.

Could someone please explain ...!

Thank you,

Norm.


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## mce5802

Most can probably explain this better than me but here goes.
  The piece that has the eccentric turned on it has a socket drilled in it to receive a single ball from a ball bearing. This socket is drilled to run off center, hence the eccentric, and there's also a socket on the part that holds the toolbit that the same ball sits in, the two are held together only by the downward pressure on the quill. When the machine spindle rotates, the upper end of the toolbit rotates off center because the ball that drives it rotates with the eccentric hole. This causes a rocking action if you will on the bottom of the toolbit, and the end of the bit is ground concave, leaving, in effect, four cutting edges, each alternating as the bit rocks. Relief is ground on the sides of the bit to allow for the rocking motion as it gets deeper in the hole. Hope that makes sense. He also posted a link to a video. Great job, by the way, been going to make one myself but haven't got around to it.


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## randyc

Not too difficult to hold the three parts together:

http://www.hobby-machinist.com/threads/a-simple-compact-rotary-broach.33032/


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## NormBourne

What a clever, brilliant idea, no less so the explanation on the way it works, thank you so much Mike, it certainly sounds a lot better than a "bl...dy"big hammer.

Thanks also to Mark for the design, excellent..!    I am going to have a go building one.

Thanks,

Norm.


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## Suzuki4evr

How much should the offset be on the eccentric and can you grind the relief on a bench grunder? Lastly is HSS toolbits sufficient.?


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## Suzuki4evr

Can keysteel be used?


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## pacifica

S-7 tool steel, mill to size and shape, then harden. Not to hard to get precise squares, hexes .
Hss is best but most likely you will be grinding it with abrasives,need a good jig.
The corners of the bit are the weakest link, so the less material you remove on the work the better.


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## FLguy

I've made one and they work well; as far as broaching depth. That's dependent on the back angle you make and it's length. You still can't beat a "Slater" brand which I use in lathe, mill and yes, drill press. I make most of  the needed broaches, but for about, $28.00 is the starting price point on store bought.


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## Creativechipper

I gotta make one of these.
After hand filing a 1/4 square on a 1" pc of round steel, never again..lol


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