Making a rotary broach. Twice

I have wanted a rotary broach for a few years. (A rotary broach is a tool for drilling non-round holes, such as hex or square holes.) I looked at a lot of plans online. I spent hours reading forum posts. My youtube viewing history for rotary broaches reaches back to 2018! A couple months ago, I finally bought the Hemingway rotary broach kit.

It’s not really that expensive as things go. Shipping to the US is not horribly expensive, but I had even cheaper shipping because some of my colleagues working in one of our UK offices were coming to the US and had some room to bring me the kit, so I just had to pay domestic shipping.

It was a good idea. I’m glad I got it, and would make the same choice again. And this picture of my new rotary broach isn’t the Hemingway design, even though it is superficially similar on the outside…

The Kit

Hemingway shipped a full set of detail drawings, quite detailed instructions, and all the hardware and stock necessary — as long as you don’t make any mistakes. They included enough 8mm “silver steel” drill rod for about three actual broaches as well. (For those inside the US, 8mm drill rod is harder to find than imperial sizes, so an option to ship more drill rod would have been convenient.)

The instructions, though long, were unclear on a few points. When I asked one question (how to harden the silver steel) on a weekend, I got a very fast response from Hemingway that it was a water-quenched steel. I would expect that they would be just as quick to answer any other question.

There were a few problems:

  • The instructions, though generally very clear, have a few places that they aren’t clear. In particular, I found a step for measuring the spindle that made no sense to me.
  • The instructions go into great detail about how to deal with not having tilt on the dividing head, and not having angle blocks or sine plate, not mentioning the obvious, cheap, simple solutions. My set of angle blocks was not expensive and made things so much easier.
  • Boring 8mm to a 20mm depth to a square end precisely is actually tricky. A lot of boring bars just don’t go that narrow. And getting that fit perfect is hard. It’s also important.
  • The instructions suggest precision in several cases where precision is completely useless, and don’t mention precision in a few places where it is important.
  • The drawings (dimensioned in both metric and imperial) don’t indicate tolerances anywhere.

I ended up modeling it in FreeCAD to make sure I understood how the parts fit together, and that really helped me understand how the instructions work. Electrons are cheaper than steel.

I made a couple mistakes that were all my own fault as I insisted on working in metric on my imperial lathe without DROs. I made the spindle bore too loose a fit, so that the drill rod rattled in it. In boring out the body that the spindle runs in, I bored one section about .001" too wide, so that the larger radial bearing rattled. I shimmed the bearing with shim stock, but that wasn’t a good option for the spindle bore.

I thought of remaking at least the spindle and maybe the body, but something bothered me. The design includes axial preload on a radial bearing. It has a nut that bears on the outer race of a radial bearing, and then the inner race of that same radial bearing bears on the spindle, which then bears on a thrust bearing. (A second radial bearing is in the tail.) I found that when I tightened the nut as instructed, the radial bearing turned stiffly. This would probably be fine in use because the pressure of using the broach would axially unload the radial bearing, but I didn’t like it anyway.

It’s definitely possible to make this design work, and a few folks with real skills (unlike me) have done this, including Stefan Gotteswinter, Randy Richard, and Inheritance Machining, with their own variations on the design.

Glutton for punishment that I am, I didn’t just grab some more 12L14 or 1144 from the stock pile and use what I had learned to remake one or two parts, more quickly and higher quality.

I Did It My Way

I used what I had learned and started over, mostly from scratch.

  • I ordered some 4140 steel after seeing how beautiful the similar Cr42 steel that Stefan Gotteswinter used looked. I have never used it before, so this seemed like a great opportunity to expand my repertoire.
  • I ordered a set of metric reamers by 0.5mm and a set of metric drills by 0.1mm. I also ground a flat-bottom 9/32" drill (I have several spares) as an undersize drill to get a flat-bottom hole before drilling 7.9mm and boring 8mm. I also ordered a set of metric counterbores instead of continuing to bore with end mills.
  • I opened up a fresh document in FreeCAD and made a different design from scratch, and made it parametric and driven from a configuration table so that it can be configured for different lengths and diameters of broaches, using a different bearing layout that simplified the body design, and in which there is no axial preload through a radial bearing.

In my design, I followed Stefan Gotteswinter’s example of using a separate shaft and using retaining compound to lock it to the back collar. The shaft was a dead 3/8" endmill with the flutes chopped off with a dremel. I used a different bearing arrangement, with an F5-12M thrust bearing behind the spindle, and two identical radial bearings around the body of the spindle. I designed a bushing/sleeve so that the nut on the front bears on the forward radial bearing’s outer race, which bears on the sleeve, which bears on the rear radial bearing’s outer race, which bears on the body that holds the nut, so that the whole thing is quite rigid. The only axial load on the radial bearing is when actually removing the broach from the work, which is not a significant load.

I learned first-hand that 4140 is different from free-machining steel. When cutting it with inserts intended for ferrous materials, it has a mirror finish, but when cutting it with sharp ground inserts meant for aluminum, it has a satin finish. This is definitely different from free-machining steel for which the ground inserts usually leave a beautiful finish. It also seems to work harden easily.

One of the first pieces I made was the shoulder for the shaft. I made that three times for my new design. The first time, I discovered that my old (cheap) set of metric drills, while it was fine for aluminum and mild steel, was not meant for 4140. The hole started feeling weird, and I pulled the drill out and it had a 5° or so bend in it, and the hole was shot. The second time, I was working late and night and cut a tab instead of a slot. That didn’t really matter; I could have compensated later (if I remembered), but I was annoyed with myself and determined to make it to plan. Third time was the charm.

While boring the body, I broke a boring bar almost instantaneously when chatter developed, and when I thought I could fix up the chowdered surface by very careful use of another boring bar, I instead learned again just how easy it is to snap a boring bar. At least now I have some extra screws for carbide inserts for when I drop a screw in the chip tray some day…

However, I got the sleeve so perfect on the first try that it was a perfect sliding fit in the body, and the body boring job was so good that there is no radial bearing play, yet I can tap the body face down to slide the bearings out. Could not be a better fit! (I made a design error and made the bore for the thrust bearing too wide, but fortunately I also made it too shallow, so I was able to rescue that with a 12mm endmill instead of starting over.) And the spindle bore was a piston fit for the drill rod for the actual broaches (the only remaining part I was using from the Hemingway kit).

I made a few (mostly cosmetic and temporary) mistakes as I worked, but I ended up with a broach holder in which the only appreciable play is between the balls and the races in the radial bearings.

Fortunately, the ugly fail on the side of this picture (I ran the cutter in the wrong direction and probably hadn’t clamped it as tight as I had meant to) is in material to be removed later in the manufacturing process:

The Broach

It was finally time to make my first actual broach!

The Hemingway instructions say that dishing the end of the broach doesn’t matter. I don’t believe that after all the HSS tooling I’ve ground and experienced various relief angles, but I saw the idea of dishing the end by just starting a drill bit into it. I used a 120° spotting drill and put a cone in the end of the rod, and then pulled out the dividing head. I set it at -2.5° (that is, pointing 2.5° under horizontal), chucked the rod, and put in an endmill.

Then I discovered that the chuck got in the way. It was just too big. Fortunately, I realized that I could remove the chuck and use my MT3-ER40 adapter in the dividing head’s MT3 taper, which gave me the access I needed. Unfortunately, I ended cutting the broach about .002" under nominal size (I was shooting for a slightly oversized 6mm broach and ended up undersize) but decided to finish it to try it out. I hardened it by heating it with propane to cherry red and then swirling it in water, then tempered it back to straw. I touched up the faces with an arkansas stone.

I drilled a 6mm hole in a piece of 6061 aluminum and loaded the broach into the holder. It broached smoothly and I locked the screws now that it had self-located to the correct offset.

I can kind of fit a 6mm hex wrench in there, but I’ll start over again to make new broaches the right size.

Then I took the body off the shoulder, put 638 retaining compound on the sliding surfaces, assembled it with red loctite on the screws, again drilled 6mm and broached, and this time tightened the screws for good.

A day later, after the retaining compound had fully cured, I turned the whole permanently assembled unit down to size (removing the previously-visible evidence of a mistake). I did this with the ground and polished aluminum insert for low cutting force, but that didn’t leave it with the shine I wanted to see. I then polished it with 80, 200, 500, and 1200 grit to bring out a nice shine.

And I ordered 8mm drill rod in both O1 and A2 from McMaster so that I can make more broaches, and enough so that I can start over if I make a mistake. :smiling_face:

Despite the fact that I went down the design-your-own rabbit hole, I’m glad I started with the Hemingway kit, and I recommend it. It’s a better design than several others I have found on the internet, and they are responsive to support questions.

Epilog

I have made drawings for the scaled-up design to take 1/2" broaches 1.75" long (another common size), and before even starting on making that, I have come up with an idea for a floating broach holder that doesn’t need adjustment, but instead self-centers, like the Hemingway design does before being locked in place. I think that it would be possible to have a single holder accommodate both the 8mm x 28mm micro broaches and the 8mm x 1.25" (really, mixed units…) standard length without any adjustment. But I haven’t put that in CAD yet to be sure. And I’m not sure I will. Probably I’ll move on to other tasks.

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I had to look up what a rotary broach was. Very cool.

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Thanks for letting me know that I buried the lede! I’ll edit my post so that the next person doesn’t have to go looking. :smiling_face:

You did a wonderful job broaching the subject!

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Typo? 9/32"?

Heh, that would indeed be an unusual size. Fixed now, thanks.

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That design was substantially more complicated; it involved adding several more parts to the overall set, would have been bulky and a bit fussy.

Now I have a design that is instead actually simpler than the one I made already.

I expect that I’ll end up making this new design, if only because I can’t work out any other way to have confidence whether it will work in practice.

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If you eventually want another person to test build your design and provide feedback then I’m up for it.

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I would love that! :heart:

It probably makes sense for me to make at least one variant on the design first, after the new bearings I have ordered arrive. Among other things, that will determine whether my brilliant idea will actually work. :smiley:

My drawings demonstrate that there is a lot I don’t know about technical drawing. I did a few weeks of learning paper drafting in a “graphic arts” class as a high school freshman; this was long enough ago that other class skills included offset printing, wet darkroom (though that I already knew thanks to my father), and pantograph engraving… So anything you can do/say to help me make drawings that someone other than me can interpret will be very helpful.

Because you teach machinists, you might also have significant feedback on instructions, once I write them. I’d be unsurprised if after you read instructions I write, your feedback not only helps me improve the instructions but also teaches me something new about machining. :teacher:

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The 120° spotting drill for dishing the face made a broach that went through aluminum like the proverbial knife through hot butter, but didn’t last long in steel. Since that broach was undersize, I wasn’t trying to take care of it anyway; it was basically trash for testing. I’m also not sure that I tempered it well.

After a bit of a break for some other tasks, I finally set up for cutting more broaches. I decided to try one with a hole for pressure relief for tight broaching, primarily for the challenge of drilling a high aspect ratio hole. I bored this 1/16" (bit I had handy, size doesn’t matter much) hole 31mm deep (a bit of leeway for a 28mm long broach) in this 8mm O1 rod:

That hole also made it easier for me to dish the front out; I didn’t have to be precise about the center location.

I realized that a boring bar mounted backwards was the best tool for a shallow dish in the end due to the nose relief (or, in this presentation, I suppose edge relief):

I cut about a 2.5° angle at first, decided that wasn’t enough, and cut about a 4° angle the second time. It looks big on the protractor, but it’s barely visible across the 8mm stock. Should be enough for cutting steel.

Got my dividing head set up the same way as before for cutting the flats on the broach, though I have decided to save that for another day, to avoid stupid mistakes from being tired. Like maybe cutting another undersized broach! :smiling_face:

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It’s an impressive achievement :heart:

OK, on to cutting the flats. Decided to try to make a proper 6mm broach to start. That should be about 50 microns oversize, so shooting for 6.05mm across the flats.

First cuts:

After several careful passes:

Note: It would be better if I had an end mill with a rounded corner radius, to avoid those sharp corners. Those are of course a weak point due to stresses. But I don’t have that right now in a reasonable size for this. Yet.

Nailed it! (used the HOLD button for an easier picture, though!)

The nub after parting off 28mm with a 2mm blade shows that the center hole with a nearly 20:1 aspect ratio didn’t wander:

Sorry, no pictures of heating it cherry-red with a propane torch before dunking it in oil to harden it, but here it is post-dunk:

Note that the pressure relief hole was convenient; I stuck a thin piece of stainless wire through the hole with a bend on the end, and used that to hold it in the flame, and then to stir it in the oil to make it cool evenly.

Now in the oven tempering at about 200°C for about 2.5 hours. Started that right after hardening it.

At some point in the next few days, I’ll hone this broach and give it a try, to see how it compares to the first broach I made.

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I honed the tip as sharp as I could and did a series of tests, cutting both aluminum and mild steel (A36 hot rolled). I tried oversize holes and undersize holes. (The purpose of the center hole was to allow those undersize holes.)

The 6.05mm broach still cut a little bit tight for a 6mm hex wrench. It worked great in aluminum, but after a bunch of cuts in steel, the wrench didn’t fit as well, and the tip looked like this:

That’s not recoverable. I don’t think I hardened it well enough.

Also, the grub screw damaged the shaft and made the broach hard to remove from the holder spindle.

So I think the next one should be 6.1mm, and I need to learn better how to harden these. I probably need some rockwell hardness testing files to determine whether I’ve hardened properly. And I think I left it to temper too long, and a bit too hot. And I need to test that the red-hot part is not magnetic before quenching.

I have already ordered a radiused end mill to use for cutting the next broach.

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Drilling a 32mm deep hole with a 1/16" (1.6mm) drill bit—an aspect ratio of 20:1—takes patience. I was trying to take only ~1mm pecks with air blowing at the tip to clear chips from the drill between pecks, and I still scrapped three pieces by breaking drill bits in them. It’s a good thing I have a few extra 1/16" drill bits! (It was only in retrospect I realized I could have waited until I parted it off and drilled from the other end and not scrapped any parts. :roll_eyes:)

I ended up drilling a #1 center hole with the tailstock, then used a toolpost drilling setup to peck with the carriage instead of winding the tailstock in and out.

Since I want to eventually make down to around 3mm broaches, all 28mm long, I am ordering some 0.6mm drills. Those I wouldn’t drill all the way through; I’d drill the business end past the flutes of the broach, then drill from the back to meet the hole after parting off.

I used the new solid carbide radius end mill that just arrived. You can see the 0.02" (~ 0.5mm) radius here. I cut these flats 6.1mm across this time.

I took a file to the shoulder above the flats. As usual, the phone tries really hard to find texture in the smooth surface—but compare to the smooth precision ground rod surface to see how close smooth this really is.

I found this chart:

image

The last one I did at 208°C ~= 400°F which might have been hotter than I wanted. I also am not sure I got it heated through; I didn’t test it with a magnet. I also probably let it cool too much before starting the tempering cycle in the oven, and didn’t pre-heat the oven.

This time, I pre-heated the oven to 300°F (~150°C) before I started heat treating, and did the heat treating right next to the oven. I had a rare earth magnet (harvested some years ago from a hard drive) handy to test it. I was surprised how long it retained magnetism when it was glowing cherry-red. I turned up the gas to provide more heat to make sure the whole thing was hot through. Then I stirred it vigorously in the oil, cleaned excess oil with a thick wad of paper towel to protect my hand, and quickly set it in the pre-heated oven set to run for two hours. And… apologized to several family members for the smell of vaporized oil.

Testing with a file, this one seems harder than the previous two. Hopefully this time I got it right.

Honing is hard enough to do well with a 6mm broach. I’m finding myself wondering how I’m going to hone a 3mm broach. Maybe I’ll have to 3d print some sort of holder?

I’m also wondering if the damage to the previous broaches going into steel was in part due to me having so much leverage with the tailstock and overdoing it. Maybe I should use the toolpost instead, where I can feel pressure and take it easy going by hand, or use power feed for a precise feed per revolution.

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I switched from using the tailstock to the carriage, and set my slowest metric (default) feed rate of .02mm/revolution (about .0008"). For this test, I drilled with a 6mm drill, did not chamfer the hole at all, and then cut with power feed for the carriage.

First, aluminum:

Those inner surfaces cut by the broach are reflective, and you can see the difference with the drill surfaces, where you can see that the drill did cut a bit oversize as twist drills so often do.

Next, steel:

The holder itself wobbled slightly; I’m guessing that it started with imperfect alignment and then held it the rest of the way through, and that a lead-in chamfer would have resolved that.

So far the broach does not appear to have been damaged. I’m confident I was pushing a lot faster with the earlier broaches. I’ve seen suggestions up to 0.002" / 0.05mm per revolution and I could try that, but I’m not doing production work, so I think I’ll spend my time making more broaches before I experiment to find out how hard I can push this.

After hardening, sharpening, and honing, this 6mm nominal broach measures about 6.07mm across the flats, and a hex wrench is very slightly loose, so there’s room for a few hundreds of a milimeter of sharpening before this broach is dead.

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Beautiful results!

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The stock for this project is sitting next to my lathe but I’ve been so scattered and spread thin that I haven’t attempted a build. My Summer is chock full so I’m not sure when it’ll happen, but it’s still on my to-do list!
In the meantime, the results photos you posted look very promising.

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