Square broach pilot size larger than broach?

kd4gij said:

I think a broached hole looks much better than a broken $144.00 dollar broach. Just sayen.

But the first time I used a square broach I did ask the same question.

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Broken from using the incorrect pilot size, I'm guessing? Thanks- at least I'm asking the right questions!

intjonmiller- really cool idea with the file!

Now if what you are working on isn't that thick and not to hard. There is a couple of good wright ups on making a rotary broach that could make a true square

I normally just drill a hole, then use a dremel to square it up enough for the carriage bolt to sit flush, but you've got a pretty good batch of them to do. I normally have only a couple to do at a time...

I have on certain occasions used one of those sq broaches with a pilot hole the size of the broach rather than the oversized hole. For a true square hole.
Just have to back up and clear chips. I seem to find myself using them on material way thicker than they say can be used. there again remove and clear chips, and go again.
At times when a not perfect square is needed and the material is way thicker than advised the pilot hole can be increased in size to make it easier on the broach and get it done with a single pass.

I even made an end for my press ram that accepts the end of the broach and has a set screw to hold it in place.
the ram can then be run up and down and chips cleared and oil applied.
My press is a little different than most. it's electric hydraulic and double acting cylinders.
A divot must be ground in the side of the broach for the set screw to be able to hold it for the up stroke.
Of course the item then needs to be held down to the press also.
Had some fun with that , but the press is not really rigid enough. A press die (if that's the terminology) would be nice.

Sounds like some really good ideas Russ! You caught my attention on the comment: "material way thicker than they say can be used." Mine didn't specify a max material thickness. I am cutting holes in 3/16 hot rolled flat stock and had planned on ganging up three or four pieces (held together with a tack weld). Are there known max thicknesses for these things?

There are minimums and maximums.
Minimum is probably just so at least a couple of teeth are engaged when cutting, or enough metal supporting the point for starting (I'm guessing....maybe someone can clarify)
Maximum is figured for available chip space in the shaft....once it fills up better stop pushing or bang!
Drilling the oversize pilot hole reduces the amount of chips. So if drilling on size and following min max specs you'll still have to back out and clear chips to make the hole.
I looked on the MSC site and they list those specs for their dumont sq broaches.
They show min 3/8 and max 1 1/4 for their 3/8 sq broach. They even show it takes 3720lbs to push at max cut..hmmm wonder what steel.
The ganged up plates would probably work good. Some of the chips will probably escape making it even better.
I doubt you'll need a tack weld... nothing or a vise grip would probably work fine.

Every descriptive doc or video I've seen on the subject shows the round-square final hole. Like others have said, the "true square" holes are probably done by another process, like punch and die.

rgray said:

I doubt you'll need a tack weld... nothing or a vise grip would probably work fine.

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Beyond the other stuff that you said, which was very informative, this is really smart. I didn't even consider vice grips. Great idea! I also didn't consider looking to the manufacturer for more specs. I bought the thing from McMaster Carr and just assumed that the specs listed were all the specs on it. I have a lot to learn. Much thanks!!

BigMo said:

Doesnt seem right....lets think about that for a min....pilot hole is round....vrs the broach will be square....theres a correlation

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The following process has always worked for me. Provided your required hole is a standard HSS tool bit size. Make a holder from a length of square HSS tool bit.

Put a short length of steel in the lathe two or three x dia. of the diagonal measurement of the square hole Drill a hole the dia of the diagonal size in the center, deep enough to hold the tool bit but allowing enough to stick out to punch your hole.

Drill a hole in the part where you want the square hole, I always drill on size, but you can go larger if you wish.

Grind both ends of the tool bit square, then hollow grind one end. Place the square end into the holder, set the job up on the bed of a hydraulic. press, with the hollow ground end over your pre drilled hole, and simply push the tool through, never had a failure. Then drill and tap for a grub screw to hold the tool bit.

I have used this method many times to make a square hole across the end of a boring bar to hold a HSS tool, I have even used it to make a diagonal hole about 45 deg. so it can bore to end of the hole, To do this you must drill a pilot hole to get the square tool bit started, same size as diagonal of the tool.

It is of course important that it is set up square.