# A Shop-Made Compact Rotary Broach



## randyc

I’ve been looking at some shop-made rotary broaching tools for about a year, thinking that one could be very handy when a small internal feature is (occasionally) required. Finally I decided to make one and the result is shown below with the tool installed in a vertical mill.





Most shop-made rotary broaches that I’ve seen use a pair of radial ball bearings and a ball thrust bearing. Most of these tools are also pretty darned large and would look ridiculous on my 8 x 30 vertical (definitely overkill for my needs).

I made the compact design shown above by using a single hardened ball for alignment and thrust. The ball thrust bearing runs in wheel bearing grease. The body of the tool is made from drill rod, torch-hardened moderately. (I would have used pre-hardened 4140 if I’d thought of it.)

The broaching tool was also made from drill rod but is harder than the tool holder. This is the second one made; the first was ruined while torch-hardening the finish machined part. The second one was turned to the finish diameters but only rough milled to the cutting shape.

After hardening and tempering, I used a solid carbide end mill to carefully bring the broach cutting surfaces to finish dimension (I have no surface grinder). Before using the tool and after removing cutter marks from the sides, I put it in the mill and gently lowered the rotating tool against a hard Arkansas oil stone to polish the face.

I didn’t make a radiused face like most of the other shop-made broaches found on the internet. This was intentional due to comments written by a guy (manufacturing engineer, foreman, production machinist ?) who worked for Volkswagen for a number of years and was closely associated with their rotary broaching operations.

He wrote that, not only was the “dished” cutting face unnecessary but that it diminished the time between sharpenings because the sharper edges broke down and the breakage, although not visible, was easily seen under magnification as cratering and erosion of the cutting edge.

This is important because the cutting tool is tapered ! Any sharpening of the face reduces the dimensions of the finished work. I decided to give his opinion a try. I figured I can always carefully “dish” the face later if it didn’t work properly, right ? This is the result of the first test drive.





The hole is octagonal, .340 inches across the flats, .500 deep, .750 square CRS material. I made the pilot hole .005 oversize then countersunk generously to provide a good start for the broach. The broach was run at 1000 RPM (just because) using black pipe threading oil.

It is visible in the photo that the feed was erratic. I might try using the boring crank rather than the quill lever next time as well as experimenting with spindle speed. Total broaching time was on the order of ten seconds.

The shape and dimensions of the prototype broach were arbitrary, determined by whim. More practical applications include producing “D” holes, hex holes, square holes and splined holes.

It seems like this tool will be handy. I can quickly make up custom broaches from 0.500 or 0.750 drill rod as the need arises. (I don’t envision producing an internal feature larger than 0.375.) For my purposes, heat treatment is not at all critical so a torch and some motor oil will suffice. I doubt that any broach that I make will be required to produce more than 10-20 parts.

(An example anecdote: I made a tracer attachment for my smaller lathe about ten years ago. It worked well although was sort of a PITA to set up. To date I have made a total of five parts with the system.)

Here is the complete tool; the house key gives an indication of tool size …






  Works in the lathe too, of course:





  And here’s a sketch:





  The shank of the tool is offset, as can be seen.  The offset and the length from the ball to the cutting edge of the broach is intended to produce a cutting angle of 1.5 to 2 degrees.  The angular clearance on the sides of the broach must be just slightly greater than that.  Too much clearance and the broach won’t “track”, producing a spiral cavity.

Edited to add:  the sockets for the .500 ball are made with a .500 ball end mill secured in the tail stock drill chuck of the lathe.  The lathe should be slowed way down and the feed from the tailstock moderate else the socket will NOT be spherical.


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## Ulma Doctor

excellent design!!!!
i have wanted a rotary broach for years but i haven't made one yet.
your design looks like it's right up my alley!
thanks for the info and drawing!!!
:thumbsup2:

mike)


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## randyc

Cheers, you have probably noted this already but the four set screws do not touch any surfaces during normal operation.  They exist only to prevent the broach holder and the ball from separating from the shank when the tool is not in use.  Three screws would be fine ...

It's a fun project, inexpensive and doesn't take much time.


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## FOMOGO

Nice. Thanks for posting. Will have to put that one on the list. Mike


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## rwm

Very nice.
Why do the sockets for the ball need to be spherical? Wouldn't a simple countersink center the ball just as well? Is it a force issue?
R


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## randyc

rwm said:


> Very nice.
> Why do the sockets for the ball need to be spherical? Wouldn't a simple countersink center the ball just as well? Is it a force issue?
> R



You're absolutely correct, rwm !

A center drill or countersink could locate the ball just fine and - for my modest needs - would last as long as me, ha-ha.  But the very much increased spherical contact area won't wear as much as the center drilled hole .  (The two parts that bear against the ball are not hardened very much -)


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## Greedpetey

love the simplicity of the design, I think I'll try my hand at one. What is the ID of the shank? I didn't see that called out on the drawing. 
-Gideon


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## cs900

what a great idea! thanks for sharing. Looks easy enough that even I could make one!


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## talvare

Randy,

Thanks for posting your design. I just finished making my rotary broach copying your design. I up-sized some of the components a little although it probably wasn't necessary. I initially turned the offset for the drive shank at .092" which would provide the 2 deg. cutting angle. For some reason, this just didn't work for my tool. I ended up re-doing the offset to .180" and it now seems to work well.

Thanks again for sharing.

Ted


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## Silverbullet

Thanks this is one that's going up to the front of the list. I see square holes in my future , tools are us. Thanks really all kidding aside it's a great easy tool to build and use. I've looked at brooches and there costs just make me winch in pain. Great job lots more will be thanking you too.


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## glenaman

gone to make one soon thanks .


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## HBilly1022

Bookmarked and thanks very much for posting the drawing. I need to get some drill rod and now have an excellent reason to get it soon.


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## benmychree

Brooches are things that ladies wear on their clothes, broaches, on the other hand, are used to make multi faceted features in holes.


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## homebrewed

Simply elegant!


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## Thriller

Thanks very much. its on my project list


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## Sailors

Very nice design, I think I’m going to try this. Thanks for posting


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## A41capt

I know this is a relatively old thread, but then again, I’m a relatively new home shop machinist!  I have a couple of quick questions regarding the unlisted dimensions in your drawing if I may ask:

First, what is the clearance dimension between the broach holder and tool body?  Next, what is the centerline dimension for the 4-40 set screws in the tool body and broach holder?  These may not be critical, but in order to maintain that 1 to 2 degree taper in cutting angle, it seems to me that they would have some stated tolerance for the pivot around the ball bearing.  Additionally, I am considering Kasenit as a surface hardener on the bearing surfaces of the holder and body.  Is this overkill?

Thanks in advance for the info, and an even bigger thanks for providing the drawing.  I’ve found that I have many applications for a tool like this (including odd shapes, etc.), and as a hobbyist building small IC engines in my garage workshop, I can’t possibly justify buying such an expensive tool.

John W


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## extropic

A41capt said:


> I know this is a relatively old thread, but then again, I’m a relatively new home shop machinist!  I have a couple of quick questions regarding the unlisted dimensions in your drawing if I may ask:
> 
> First, what is the clearance dimension between the broach holder and tool body?  Next, what is the centerline dimension for the 4-40 set screws in the tool body and broach holder?  These may not be critical, but in order to maintain that 1 to 2 degree taper in cutting angle, it seems to me that they would have some stated tolerance for the pivot around the ball bearing.  Additionally, I am considering Kasenit as a surface hardener on the bearing surfaces of the holder and body.  Is this overkill?
> 
> Thanks in advance for the info, and an even bigger thanks for providing the drawing.  I’ve found that I have many applications for a tool like this (including odd shapes, etc.), and as a hobbyist building small IC engines in my garage workshop, I can’t possibly justify buying such an expensive tool.
> 
> John W


 The OP has made one post (4 years ago) in the last seven years so I doubt he'll answer your questions.

I'll try: Regarding the missing dimensions = wing it. Their function should be pretty clear.

Regarding hardening, I would, but before hardening I would add center drill points at the bottom of both halves of the spherical sockets 
(as lubricant reservoirs). Also, I would mount a 1/2" ball on a handle and use it to lap the spherical sockets to insure circular elements. In reality, the contact between the pivot ball and the socket halves will be narrow circular bands. Worst case would be the ball contacting at the bottom (single point) of a socket or at three points. More contact area and smooth, lubricated, hard surfaces = less wear/damage.


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## Cobra

These sketches are from a broach I made a while back.
Hope they help you.
Jim


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## Winegrower

A couple years ago in a burst of enthusiasm I made a similar rotary broach.   I broached a couple hex holes to test, worked actually quite impressively.   Since then, I've used it by actual count zero times.

But a fun project in any event.   And if I ever need a hex or other shape, I'm covered.


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## A41capt

extropic said:


> The OP has made one post (4 years ago) in the last seven years so I doubt he'll answer your questions.
> 
> I'll try: Regarding the missing dimensions = wing it. Their function should be pretty clear.
> 
> Regarding hardening, I would, but before hardening I would add center drill points at the bottom of both halves of the spherical sockets
> (as lubricant reservoirs). Also, I would mount a 1/2" ball on a handle and use it to lap the spherical sockets to insure circular elements. In reality, the contact between the pivot ball and the socket halves will be narrow circular bands. Worst case would be the ball contacting at the bottom (single point) of a socket or at three points. More contact area and smooth, lubricated, hard surfaces = less wear/damage.


Thank you for the suggestions.  I will certainly do what you propose as far as the lubricant reservoir and lapping in the ball bearing.  Thanks again!

John W


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## A41capt

Cobra said:


> These sketches are from a broach I made a while back.
> Hope they help you.
> Jim


Excellent, thank you for the drawings Jim!  I certainly appreciate the help, as a newbie home shop machinist (about 20 years of self education on some worn out American machinery), I can use all the help I can get!

Still working on my accuracy through metrology, and learning the weaknesses and strengths of my machines.

John W


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## extropic

@A41capt

Sorry, I didn't notice (earlier) that you're new on H-M. Welcome.

Also, thanks for dredging up this old thread. I hadn't seen it before and it's now on my project list (with customizations).

Please start a build thread when you get to it.


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## wachuko

Added to the wishlist of tools I would like to make... 

I just checked what it would cost to buy one... anywhere from 300-500.00+


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## A41capt

extropic said:


> @A41capt
> 
> Sorry, I didn't notice (earlier) that you're new on H-M. Welcome.
> 
> Also, thanks for dredging up this old thread. I hadn't seen it before and it's now on my project list (with customizations).
> 
> Please start a build thread when you get to it.


I’ve been lurking since 2014, didn’t figure I had anything much to offer to a group of experienced folks such as are found here.

I’m not much on build threads, I’ve completed several small engines and done a bunch of custom gunsmithing for myself (generally restoration of antique firearms, and a custom Mauser build or two), but I’m not much on documentation.

Who knows, this might be just the project to start a build thread about!  Thanks for the welcome, and once again, thanks to everyone that has helped my education along through their wonderfully instructive posts.

John W


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## extropic

A41capt said:


> I’ve been lurking since 2014, didn’t figure I had anything much to offer to a group of experienced folks such as are found here.
> 
> I’m not much on build threads, I’ve completed several small engines and done a bunch of custom gunsmithing for myself (generally restoration of antique firearms, and a custom Mauser build or two), but I’m not much on documentation.
> 
> Who knows, this might be just the project to start a build thread about!  Thanks for the welcome, and once again, thanks to everyone that has helped my education along through their wonderfully instructive posts.
> 
> John W


Lurking for 7 or 8 years, but just posted. LOL
I'm guessing you do some research (reading) rather than immediately blurt out a question. Old school (like me).

Anyway, jump in and join in. IMHO, the community loves build threads.

Don't forget the golden rule. If there are no pictures, it didn't happen. 

Coincidence: I just noticed you registered on H-M 2 days after I did. Small world.


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