We would like to expand a 3/8" hex to a 1/2" hex in one of our gears.
Our team has the 1/2" AndyMark Hex broach which says it’s suitable for mild steel. The steel gears are 4140 which is a high strength low alloy steel. Thoughts on if this is possible (or wise)? I don’t really want to ruin our broach, its expensive!
That broach is for a half inch pilot hole. If you can get the gear to 1/2 inch bore on a lathe you should be okay geometry wise, but 4140 is not the same as mild steel, so…
Maybe look for other options COTS for gears.
I guess don’t press your luck.
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At the very least you need to open it up to half inch round first. However, 4140 is much harder than mild steel, particularly if it’s been heat treated. I wouldn’t try it. The only time I’ve had to do this, we ended up going to our sponsor who had a wire EDM.
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You can also look into milling the opening larger, which, if you have the capability, may stand a better chance.
You can do the triangle pattern like the SDS flywheels. Obviously take your time getting things perfectly centered on the mill.
Another option would be to bore out the hex to accommodate a press in hex bore hub with an interference fit. Again assuming access to a lathe here to make everything concentric.
File a piece of steel with a good, sharp file. Then try and file your 4140 gear and see if it bites into the hex bore you have reasonably well. If the file doesn’t bite in that it has been hardened and you will not be able to drill and broach it. If the file does bite then you know it isn’t hardened and might be possible to drill and broach. If you wish to attempt this drill your starting hole to at least 0.516" as this will make the job easier for your broach. Use lots of oil and make VERY SURE your broach is near perfectly perpendicular in the bore. Go slow and if your broach flexes at ALL, STOP. As long as the gear isn’t hardened in the bore and only about 1/2 inch thick it will probably be fine.
Take your time, and if you don’t fully understand what I wrote you probably shouldn’t try.
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AndyMark sells what I need, so I’ll probably end up going that route, just trying to make something already “in house” work! I might still try and mill it out as per your suggestion.
Just the other day I drilled out a gear from a tough box to one half an inch and used a 1/2-in hex broach and a small hydraulic press worked just fine.
I did step it up one bit at a time. I.e I didn’t go straight to half inch on the drill
I would say if it’s soft enough to drill it’s soft enough to brooch.
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The other thing that’s tricky sometimes is the length through bore versus the pitch of the broach. You ideally want to have 3 or more broach flutes/steps engaged at all times, but with narrow gears, that’s not always possible.
I once broached a steel sprocket. It must have been hardened. I had a 3 ton arbor press bolted to a bench that was anchored to the concrete slab. I had a 7 foot pipe over the press handle, and I was hanging off of it. I thought surely I was going to explode the broach. Miraculously it survived. I wouldn’t try it again though.
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