Hex broach options?

icecats · Sep 14, 2019

Hello,
I need to cut a 1/2" hex hole in mild steel to join a part to a shaft. I understand that the proper way to do this would be broaching. These broaches are quite expensive so I was wondering if anyone had input on how to make a broach. I was thinking of following a similar procedure to Clickspring in his broach making video.
The problem is that I can't find 1/2" hex tool steel. Would hardening mild steel work to cut mild steel? I only need the broach to last 1 hole.
Any other ideas on how to accomplish this?
Thanks

DDoug · Sep 14, 2019

More important...What machine doo you have to drive/locate/hold said tool ?

kctallguy · Sep 14, 2019

What material are you trying to broach???

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EmGo · Sep 14, 2019

icecats said:
I need to cut a 1/2" hex hole in mild steel

If you are just doing one part, then drill six holes at the corners, drill out the middle, then file the flats. If you have an nc machine, mill it out with a 1/8" end mill (I'd still drill out the corners first.)

If you are doing more, buy the broach. It's not expensive if you are doing even three or four parts. Thirty seconds max in even a hand-pumped cheapo hydraulic press.

kctallguy · Sep 14, 2019

If it's a single part and you have access to an indexer and a Bridgeport you could broach the hex by hand with a cheap HSS tool bit using the quill and indexing accordingly every 60 degrees... Another option is to relieve the corners with a drill so you could machine the hex normally in a vertical machining center or Bridgeport... just trying to help...

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sagande · Sep 14, 2019

Not the cheapest options, but a Wire, or Ram EDM?

implmex · Sep 14, 2019

Hi icecats:
I have had success re-hardening a cut-off Allen key , grinding a small dish on the end face to get some positive rake and forcing it through on the lathe with the tailstock for one-time quick and dirtys.
Haven't done it in decades now that I have a wire EDM and a sinker EDM, but that's the rude and crude version.

If you oil quench an Allen key from critical temp, they'll usually get up into the mid 50's Rockwell C which is plenty hard for a one-time broach of mild steel.
Draw it back to very light straw after quenching it and have at it.

Emanuel Goldstein's advice to drill out the corners before you drive in the broach is excellent advice.
So drill the corners, drill the middle, bore it out to just over diameter across the flats, bore a little 0.025" deep pocket the diameter across the points to help center the broach, line it all up in the lathe, squirt on some Rapidtap and reef the piss out of it. (or put it in the hydraulic press or even the mill vise and save your lathe!!)
Flip it around, push out the broach with a dowel pin and Voila...a good lookin' hex.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
www.vancouverwireedm.com

will hound · Sep 14, 2019

Hello Icecats! another fast way of getting your 1/2 inch hex done is to buy a 1/2 inch Impact Socket, machine out your part to the OD of your Impact socket, install, clean up , done !

MattiJ · Sep 15, 2019

implmex said:
If you oil quench an Allen key from critical temp, they'll usually get up into the mid 50's Rockwell C which is plenty hard for a one-time broach of mild steel.
Draw it back to very light straw after quenching it and have at it.

www.vancouverwireedm.com

YMMV good hex keys like bondhus are closer to 60 Rockwell C even "out of the box".
Cheapo asian imports could be in low fifties but I have also one set that is harder than Bondhus.
Pretty much as good as it ever gets with "ordinary" steels when you have optimum heat treatment and controlled rolling process. Still somewhat ductile and deform instead of suddenly snapping off.

lazz · Sep 15, 2019

Dunno if this helps...
This guy did a sorta DIY rotary broach video...

Caution AVE's language can be colorful and offensive ...

Illinoyance · Sep 15, 2019

I notice the video showed a tool with no back taper. In conventional rotary boaches the tool oscillates about 1* about the centerline of the hole. The broach has a 2* taper per side to eliminate interference. The end of the broach is usually ground concave. The starting hole should be about 5% larger than the across flats dimension of your broach. The hole needs to be countersunk so the corners of the broach are inside countersink.

If you have a press of sufficient capacity you can push a section cut from an Allen key through a round hole. The hole needs to be oversized and you should have a short counterbore to center the
allen key.

MichaelP · Sep 15, 2019

Illinoyance said:
I notice the video showed a tool with no back taper.

I think you missed it. Watch starting at 3:10 min.

macchi7 · Oct 1, 2019

Depending on the accuracy you are looking for you could wire erode, spark erode or if you can find one, it's an ideal job for a slotter

Illinoyance · Oct 1, 2019

EmanuelGoldstein said:
If you are just doing one part, then drill six holes at the corners, drill out the middle, then file the flats. If you have an nc machine, mill it out with a 1/8" end mill (I'd still drill out the corners first.)

If you are doing more, buy the broach. It's not expensive if you are doing even three or four parts. Thirty seconds max in even a hand-pumped cheapo hydraulic press.

Do like he said except for the filing. Shove a cutoff from an Allen key through the hole to clean up the corners.

Lewie · Oct 2, 2019

The holes in the "corners" and a small milling cutter in a Bridgeport did these nicely in aluminum. would be ok in steel just a bit slower.
...lewie...

JCByrd24 · Oct 2, 2019

Lewie et al,

In your example it appears you drilled the corner holes centered on the corners of the hex. While that is one option, there are two somewhat better and more aesthetic approaches.

1) Offset the corner drilled holes towards the center so that the OD of the drilled hole lands on the corner of the hex. This looks better and provides better contact to the male hex, but still allows a full sharp male hex to go into the hole and use of a larger end mill than the corner drill for cleanup.

2) Offset the corner drilled holes even further towards center so that the OD lands on the tangent of the hex. This would be the most aesthetic but would also require a radius on the male hex or another process to produce the female hex corners.

I built a spreadsheet to calculate bolt circles and added offsets for both of the above based on tool diameter for us non-CNC guys. If anyone wants it shoot me a PM.

crossthread · Oct 2, 2019

Icecats...in your original post you asked if hardening mild steel would broach mild steel, the answer is no. You realize that you can't harden mild steel. You would have to case the mild steel to add enough carbon to harden. I hope I'm not overstating the obvious and I am certainly not trying to be pedantic. All of the above suggestions are an excellent way of making a hex socket without expensive broaching equipment. It's slow but you only have one to do.

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Hex broach options?

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sagande

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implmex

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lazz

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Illinoyance

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macchi7

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Lewie

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JCByrd24

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