Experiments in making a very simple rotary broach holder

I can’t seem to stop experimenting with rotary broaches. But this one just looks like a round shaft with a slightly off-center hole in the end, so a cutaway view in FreeCAD probably shows the design better.

I had the idea that you only really need a ball or two and no other bearings, just a bushing. And I hoped that with a countersink screw as an air valve, this could be set up to extract the broach from the work pneumatically, with suction.

After two weeks of thinking through iterations on this, I finally made the experiment.

Didn’t take long, and I learned two things.

  • My idea that this could be made with only a drill press appears bunk
  • The pneumatic tool extraction doesn’t work

However, I can therefore make the design even simpler, leaving out the useless air valve screw. And as long as you don’t mind removing the broach from the work after cutting by hand (probably not a big deal for a few cuts anyway), it works really well.

What I learned

I had the idea that I could cut from the center at a 1° angle, making the front face the focus, and then just cut back as far as I wanted to allow for the broach to stick out. I cut off a chunk of 1/2" stock, put it in the drill chuck, and held rough sandpaper to it while it ran. It clearly marked where the center was. I tried to center punch it based on those markings. Then I set it up at a 1° angle and started drilling. With this stubby drill, I drilled only a short way in:

My initial idea was that this hole would be centered on the center of the part, and when I cut this off 8mm down the shaft, the hole would be centered 0.16mm off the center.

When I measured, however, failure #1 ended up being an accidental success. Instead of being centered, the hole happened to be about 0.16mm off center, and in the correct direction. So instead of drilling 8mm too deep and then cutting it off, I just drilled it to depth. 6mm twist drill 32.5mm deep, 7.9mm twist drill 21mm deep, 8mm reamer to the shoulder.

Then I put it on its side and drilled, countersunk, and tapped M3.

I found that way oil didn’t work well enough, so I switched to grease for everything. Grease to hold the balls in place, but also grease to lubricate the broach. And it turned very smoothly!

But after broaching a test piece, the broach came off in the test piece. It wasn’t hard to remove by hand, but the pneumatic assist didn’t work.

While I used a 1° angle block to set the angle, I checked whether a digital level would work. Yes, if you set up the shaft and v-block lightly in the vice, you should be able to tap it in close enough to work.

Referenced to the vise:

Stuck to the v-block:

Simplification from successful failure

The design got even simpler in the test. Just start the hole accurately, drill two different sizes into the end, insert bearings, and use.

Using only one bearing, the smaller diameter hole could be shorter by a bearing diameter; the idea is that the bearings being a loose fit, the top bearing is actually a few microns off center and can roll around, reducing friction. But it will work without it.


Doing this without CAD

I used CAD to explore and illustrate, but it’s not necessary to use it to work out how to make your own broach. Let’s define some terms:

  • D: Diameter of broach shaft
  • OAL: Over-all length of broach
  • S: Stickout: Amount of broach sticking out of the holder
  • d: Depth of broach inside holder OAL - S
  • r: Radius of ball bearings
  • ᶿ: Half-angle of smaller drill bit tip (e.g. 59° for a typical 118° drill)
  • m: Minimum diameter of stock with 1mm wall

Now some calculations:

  • Offset from center of 1° angled part to start drilling: S · sin(1)
  • Diameter of first drill: At least 2·r — should create a loose fit for bearings. About 0.25mm or 0.01" oversize is fine. The bearings should be able to turn.
  • Depth of first drill: d+ 4·r + ((r/sin(ᶿ))-r) from surface to tip
    • If you want to use only one bearing, subtract the bearing’s diameter (2·r)
  • Diameter of second drill: D-0.1mm
  • Depth of second drill: d + 1mm from surface to shoulder
  • Diameter of reamer: D (or oversize nominal D)
  • Minimum diameter of stock with 1mm walls: D + 2mm + 2·((OAL + 1mm) · sin(1°))

Worked example

Let’s say you are in the US and it’s easier to find US stock, and you have some really large broaches to make, so you will be starting with 5/8" tool steel drill rod.


  • D: 5/8" drill rod for broach shaft, 15.875mm
  • OAL: 75mm
  • S: 25mm
  • d: 50mm
  • r: , 5/16" ball bearings, 7.9375mm/2 = 3.9687mm
  • ᶿ: 59°
  • m: 15.875 + 2mm + 2·((75mm+1mm) · sin(1°) = 20.5mm


  • Set stock at 1° angle to quill, find center.
  • With a spot drill, then 5/16" twist drill: offset towards the low side 25mm·sin(1°) = 0.44mm and drill 50 + 4·3.9687mm + .66mm = 66.54mm (58.6mm if using only one bearing)
  • With a 39/64" drill (correct size for 5/8" reamer), drill 51mm
  • Ream 5/8"
  • Grease bearing(s), drop them in

Super interesting. The reduction in part count and build complexity is sweet.

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I realized this is also easy to make with only a lathe, no mill.

  • Clamp the stock in a toolholder oriented with Z, like a boring bar, and set the toolpost at a 1° angle (clockwise).

  • Indicate from the chuck to center the OD, adjusting toolholder height and cross slide, then move the cross slide by the offset (0.16mm in the design) toward you.

  • Put drills and reamer in the chuck and drill/ream as described for the mill.

  • Leave in the toolholder for use, rotated square and re-centered on the OD, or use in the tailstock.