A Shop-Made Compact Rotary Broach

randyc said:
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.

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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.

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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 …

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Works in the lathe too, of course:

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And here’s a sketch:

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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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Nice, simple design, but the photobicket link is dead - could you post a PDF of the drawing? Thanks, Charlie
 
Can someone perhaps let me know the proses of heat treating O-1 drillrod at home. I have LP gas and oil,no heat sensor gun. Do you temper this stuff aswell. Please bear with me,never worked with it before.

Michael
 
If you heat until the part no longer attracts a magnet, you should be good to harden. I use a water quench for simple parts but oil will work too.You will want to temper the part. I temper by color for most parts but you can used a temperature controlled oven for more precision,
 
A clarification on my post #25 above. Ultimate hardness is determined by the speed at which the steel can be brought to a temperature below a p[oint at which tempering can occur. In terms of effectiveness, Brine is the fastest, followed by water, then oil, and air. The flip side of the coin is the internal stress created when cooling a part form 1400ºF to 200ºF. A faster quench is more likely to cause stress cracks.

In viewing the literature, oil quenching of O1 steel is capable of producing an as quenched hardness of 67 Rc. Tempering to 300ºF will reduce the hardness to around 63 or 64 Rc.This should be sufficient hardness for most common applications so a more aggressive quench shouldn't be necessary.

That said, I have not experienced issues with stress cracks quenching simple O1 parts in water. I temper the parts immediately after quenching and generally double temper the parts.

I generally do this as water is more convenient than setting up an oil bath. However a best practice would be to do a proper quench using oil as prescribed by steel manufacturers.
 
I would like the broach to be as hard as possible and last as long as possible, so I will go with the oil methods. Thanks for the information @RJSakowski .
 
With oil hardening steels, the temper should follow immediately after the quench. Have your oven, toaster, or hot plate warm and ready when the part comes out of the bath. I believe in going by color, but ramp and soak to a setpoint and checked with an IR thermometer is fine too. O-1 gets harder because of a case layer effect. It has less Mn, Mo, and C than other hardenable steels, but still gets quite hard. It should be very good for your application. The other option is to use an appropriate HSS blank and grind the bit. That will certainly outlast O1, but not by much when you consider the ease of working annealed O1.
 
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