broaching small hex sockets

[jslocomb]

Hi all,
I need to make some "shoulder bolts" that I can't seem to buy. In more detail I need to take some 5/16 drill rod or other high quality stock, broach an allen head socket in the end, turn the body to a custom diameter and length, and then put a 12-24 thread on the end.

I can't find anything with the 12 - 24 thread that is modifiable to what I need so I'm wondering about the practicalities of making my own small hex broach.

I only need to make a few of these so high end production tooling is not necessary.

All thoughts and suggestions gratefully appreciated.

 

[TroyO]

No experience with it myself, but maybe you could set up a small cutting bit and nibble each flat one at a time the same way you can broach a keyway slot?

[ame]http://www.youtube.com/watch?v=XcU0LTavzDM[/ame]

I'll be curious what others come up with. My answer has always been just "Come up with something else" LOL.

 

[Lew Hartswick]

Can you start with a 1/4 inch shoulder screw and modify it?
...lew...

 

[Ken I]

You can always try rotary or "wobble" broaching.

See thread

http://www.homemodelenginemachinist.com/index.php?topic=14871.0

Regards,

Ken

 

[Dr Jo]

I wonder if you can drill a hex? I know it is possible to drill a square hole with a triangular shaped drill and a square drill guide.

Jo

 

[Mainer]

I've broached a couple of squares, never a hex, but I think TroyO has the right idea. Drill a hole slightly larger than the desired across-the-flats dimension desired -- not much -- and somewhat deeper than you want the finished hex to be. Make a small flat chisel cutter small enough to fit in the drilled hole and plane out one side of the hex at a time. I have done this with the work held centered on my rotary table, the broach held in the spindle, and operated with the quill handle. You may find that broaching the full width of a side in one step takes more force than you can muster with the quill. In that case, reduce the width of the broach and do part of a side at a time, or take this approach http://www.homemodelenginemachinist.com/index.php?topic=12639.0
I drilled 1/16" holes at each corner so they were just tangent to the inner sides of the hex, drilled out the center with a drill the same size or ever-so-slightly larger than the hex, then milled out the remaining material (not much) with a 1/16" end mill along each flat. That leaves a slight radius in each corner than I found didn't matter with the nuts I was using it on. Your mileage may vary....

 

[sorekiwi]

I dont know if this is of any help to you, but I will throw it out there anyway (never been afraid to say something stupid!!)

100 years ago a machinist I was working with wanted to broach a hex into the end of some drive pegs he had just made.

He took an old broken end mill and ground the 6 flats on it using a spin indexer on the surface grinder. Then he took that and pressed it into a suitably sized hole drilled in the end of the drive pegs with the hydraulic press.

He needed to pull his broach back out with a pair of pliers, but it wasnt ridiculously tight. I am thinking now that he must have ground some sort of relief on the end of his broach, but I dont remember him doing so. Unfortunately I wasnt watching exactly how he was doing all this, I just happened to be doing something else in the machineshop at that time.

 

[maverick]

The method that sorekiwi describes works fine for a small number of parts. When grinding the flats, put
a degree or two of relief on them. Dishing out the face with a small mounted wheel in a die grinder helps
alot. Drill the hole a little bigger than the broach and deep enough to provide room for the chips, then chamfer
it. Use a guide when pressing in the broach. My arbor press has a hole in the end of the ram that the
broach slips into and is secured with a set screw. With this setup the broach is easily pulled free of the part.
Slow but effective.

Maverick

 

[Antman]

Although I have so much less machining experience and knowledge than most of the other guys on HMEM I believe I can answer this one with a Lo-Tech approach. If all you want is a hex socket for an allen key, you can make a punch or a set of punches from an allen key. Any half-way decent allen key is chrome vanadium steel and plenty hard enough for mild steel. I have found (beginner’s luck) the best shape punch to start with is ground to a V shape on the end from one apex of the hexagon to the opposite apex. Or grind at an angle so that a flat or an apex is the deepest part of the punch. Finish with an allen key ground flat.
What you do is drill the same size as the allen key, e.g. for a 5mm across flats key drill 5mm. Then whack in your V shape punch with a couple of hammer blows. Rotate 60degrees whack again, rotate 60 whack and you should have an ok hex hole. Carry on until the hole is deep enough. You can clean up the bottom of the hole with a small chisel made from a Chee Po jewelers’ screwdriver. Watch your fingers and use eye protection! Hope this helps. I found this out from the Geometer series of articles from ME mag. Case harden then your hex socket won’t distort with use.
Ant