Re: Rotary Broach

[Buggy Chief]

I am going to try and Rotary broach a hex in the end of a screw I made to accept Allen key. What size should I make the pilot hole? I researched and looks like "Minor" diameter, but what is minor diameter on a Hex? Thanks.

 

[macardoso]

From what I have seen of rotary broaching, and I have never actually done it, the drill size is a smidge larger than the width across the flats of the hex. In this way the broach has less to clean up, the chips are individual ears, but the hex is imperfect with slight remnants of the original drill hole on the flats of the hex.

This is probably a process you need to sample with and figure out what process works well for you. I've also seen that if you grove the bottom of the hole with a very tiny grooving tool (probably a Micro100 solid carbide bar), then the chips can cut free and fall out.

 

[rwm]

I have used the minor diameter with success. A smidge larger would not hurt.
Robert

 

[Buggy Chief]

Thanks Y'all for the input. Great info on smidge larger than minor diameter.

 

[Flyinfool]

If using a commercial rotary broach the recommended hole size is always a bit bigger than the dimension across flats by around .010. That center area contributes nothing the torque holding ability hole and drastically reduces load, or force required, on the cutting tool. The bottom clearance should be just a bit more than the dimension across points to help all of the chips to break off.

 

[Buggy Chief]

thanks Flyinfool....very helpful

 

[MikeInOr]

I have no advice but I would love to see a picture of your broaching setup if you have the time to take a few?

 

[Buggy Chief]

I have no advice but I would love to see a picture of your broaching setup if you have the time to take a few?

Sure...Once I get it set up, I will let you know. Thanks.

 

[MrWhoopee]

I've broached a number of screws without bothering with rotary. Drill the screw to the minor dimension of the hex + a smidge. Cut a piece off the end of the appropriate size hex wrench and grind the end square leaving sharp corners. Grasp firmly in the drill chuck mounted in the tailstock and push it into the hole. I've never tried it on anything larger than about 1/4 in., but it works, producing a snug fit.

It won't take much to try it.

 

[ddickey]

You should also countersink your hole to just the OD of the broach.