Hex broaching in thick aluminum stock

For reasons that are somewhat complicated we want to hex broach 1" aluminum square bar stock.

We have successfully broached thinner material.

Can someone please share knowledge about broaching in thick material and whether it is recommended?

Also we have access to either 6061 or 2024 so any advice on alloy for this purpose would be appreciated.

Because you are bound to ask, we are transferring torque from a hex shaft. Originally we were going to use square tube with laser cut inserts that had the hex shape. The issue that we are running into is compactness. Having external fasteners that hold the ‘torque plates’ inside of the tube is adding unwanted width to the mechanism.

would the hex stock go through the walls of the tube in the original design? Or would the hex only insert into the laser cut insert?

If it goes through the walls couldn’t you just allow the hex itself to locate the inserts? Then have them cut to fit snug inside the tube and be good?

We have been broaching 1/2" 6061-T6 bar this year without a lot of trouble. I suspect we’d get through 1" as well, but haven’t tried it.

Could you hold your inserts in place in the tube with tension pins, cut flush with your tube?

That’s a good question. Originally there was a round clearance hole in the tube.
The tube was also .063 wall.

Maybe we could broach the tube (and go up to .125 wall if necessary).

I should remember my own advice: Never answer the question before you ask “What problem are you trying to solve?”

That’s another good suggestion. We talked about that last night. We are/were worried about the ability to stay in the tube wall. (Again with thin wall tube.)

if you spread the load out enough 1/16" wall tube will hold up to a lot of force before failing. If I knew what you wanted to build, like I could actually see it I’d know for sure, but I think either of those options above would work.

As for broaching thick stock, you might have to flip your part over to remove the broach so you can clear the aluminum chips out of it so it won’t get stuck on you. But it should work fine if your press is big enough.

I have successfully broached 2 inches of aluminum(6061) and brass(unknown) with an 8 ton arbor press. They were fully hex broached, not an over sized hole. It requires a LOT of force and a great deal of CARE. Watch the broach very carefully for any deflection and bowing and use stupid amounts of lube compared to normal.

While some sort of insert would probably be better design wise I’m just going to comment on the broaching.

I’ve heard that it’s recommend your broaching length doesn’t exceed 2x your broaching diameter. So for 1/2 hex 1" would be the edge of that recommended range. However, 192 has done up to 1.5" long 1/2 hex in aluminum belt pulleys for our drivetrain without problems. This is done with cutting fluid on a hydraulic press. If you don’t have access to a hydraulic press this may not be possible with an arbor press. There will obviously be higher loads involved as you’ll have more teeth engaged at once so be extra careful the broach is kept straight. 6061 is probably the safer bet, softer should be easier to cut.

We use a push broach like this one to make hex holes for hubs and such. The material thickness rating on this one is 1/2" to 1-3/8". I know we have broached to the upper limit in 6061 aluminum before without issue, when making live shaft hubs for Colson wheels. In that case the hub was more like 2", and we counterbored each side some to limit our overall broach length.

I don’t know what tooling you have available (push, rotary, or pull broach), or what the limits of other tools are, but with the push broach and a big arbor press, 1" aluminum bar should not be a problem. Just lubricate well and make sure the broach stays straight to the hole.

In regards to thin wall tube, we have gotten in the habit of plugging the ends of the tube with snug-fit delrin (or even hardwood) blocks, for 2-3", to give a larger bearing surface for pivots, or prevent tube crush where bolting on a sprocket. No reason you couldn’t plug the end of a thin wall tube and broach through both the tube and the plug.

That’s the broach that we have. Seeing the 1 3/8" spec makes me feel better.


Are you making the trip to Wisconsin this year? That’s our home regional. Let me know if you guys need any help finding your way around etc. We’d be glad to lend a hand.

I would recommend countersinking the aluminum it helps a lot.

For clarification
Can the broaching be done with a manual arbor press or with only a hydraulic press into the 1" stock?

We have a manual arbor press and broached 1" think 6061 AL this year without a problem. Does take some muscle but not too bad.

Could you it cut instead? Our waterjet sponsor had no issues cutting a 1/2 hex hole through 4 inches of 7075.

Don’t know too much about broaching but if your still going to go your current route listen to everyone else above me.

Thanks Brian!

I mean… you can do what we did… wouldn’t necessarily recommend it.