Hex Broach Material?

[TTD]

I want/need to make a 5/8” hex push-broach, but not exactly sure what to make it from…long section of a hex key or 4140HT hex bar? I only need 2-3 holes in total…1 each in some “pucks” of 3/4” thick 12L14 round bar.

Not knowing any better, I’m currently planning on making it from a long Bondhus hex key (only because it’s the cheaper option), but don’t mind spending a bit more if 4140HT is the better choice.

I assume (?) the 4140 will most likely be completely hard throughout, while I’m not sure if the hex key will be the same or just a surface case-hardening.

I wish they made hex in HSS…

 

[f350ca]

Not sure what material would be best Tod. Wouldn't it be easier to rough machine the broach with the material annealed then harden and final grind the cutting edges

Greg

 

[benmychree]

Hex keys and 4140 are not nearly hard enough to cut steel; You'd have to make one out of tool steel and heat treat it to a hardness near 60 Rc to be able to do your job.

 

[JimDawson]

Real broaches are made from tool steel and are file hard. But, given that you only need 3 holes and the material is 12L14, you can probably get away with the Bondhus hex key. Not nearly hard enough for a long lasting tool, but I think it will get the job done.

 

[benmychree]

The Bondhus wrench that I just tested measured 50 -53 Rc; it will be difficult to machine and any over enthusiastic grinding will render the cutting edges too soft to cut due to loss of hardness.

 

[pacifica]

I made a hex key broach in o-1 tool steel then I hardened it to 58-60. It didn't hold up in steel, ok for brass. I have several commercial hss hex broaches and they work great in steel, alum and brass. The hss cuts nice clean hexes. It does depend on how deep you plan on going, anything over 1/4" deep needs a better quality broach.

 

[benmychree]

I made a hex key broach in o-1 tool steel then I hardened it to 58-60. It didn't hold up in steel, ok for brass. I have several commercial hss hex broaches and they work great in steel, alum and brass. The hss cuts nice clean hexes. It does depend on how deep you plan on going, anything over 1/4" deep needs a better quality broach.

Was that a multi toothed broach or eccentric rotary?

 

[rgray]

Here is a video that may help with the machining.
If broaching 3/4 inch depth I think a lot of backing out and clearing of the teeth would be required as the broach won't be able
to hold that quantity of chips.

 

[rgray]

Forgot to attach the vid.

 

[benmychree]

The question stated that he wanted to make a push broach, if that means that he wants to make a multi toothed broach, it would have no problem broaching through 3/4" of steel.