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pntrbl
06-17-2010, 12:07 PM
First let me say I've never broached anything in my life, but I'd like to make two square holes to hold tap shanks of the .193 and .164 size. I have a mill and a spindexer so I can rotate flats at 90 degrees and cut them accurately, but I've also never hardened anything with any degree of success either.

For a one shot, throw the cutter away when I'm done broach, is hardening necessary?

SP

Dr Stan
06-17-2010, 12:12 PM
For a one shot, throw the cutter away when I'm done broach, is hardening necessary?

SP

Yes, the cutting tool must be harder than the material being cut.

TGTool
06-17-2010, 12:53 PM
I don't know what you have in mind for the broach design specifically, but if it's a one-shot, consider going for the minimum material removal. That is, don't try to produce the complete flat-sided square hole, but remove enough in the corners to hold the tap and leave the "flats" with some vestigial clearance hole.

hojpoj
06-17-2010, 02:00 PM
Wouldn't the time spent dickering with making a broach be better spent with a file? If you were doing multiple holes I could see value in making a one-off broach. In this situation I'd drill to the interior of the 4 corner of the square (1/16 or 1/32 bit), rough out the remaining meat inside, and clean up with a needle file.

How deep is the hole? What metal were you proposing to use? O-1 drill rod is hard to bugger up- but you'll still have to do some grinding after it's hardened.

Carld
06-17-2010, 04:10 PM
Unless it's a blind hole a square file it the way to go for two holes.

Dr Stan
06-17-2010, 04:20 PM
If someone you know has access to a wire EDM, or even a ram EDM, it would be a cinch.

tyrone shewlaces
06-17-2010, 09:43 PM
If someone you know has access to a wire EDM, or even a ram EDM, it would be a cinch.

Oh yea, there's one of those behind just about every corner store.

J Tiers
06-17-2010, 09:58 PM
Wouldn't the time spent dickering with making a broach be better spent with a file? If you were doing multiple holes I could see value in making a one-off broach. In this situation I'd drill to the interior of the 4 corner of the square (1/16 or 1/32 bit), rough out the remaining meat inside, and clean up with a needle file.

How deep is the hole? What metal were you proposing to use? O-1 drill rod is hard to bugger up- but you'll still have to do some grinding after it's hardened.

Filing the inside of a hole that you want square, in a piece of steel of any significant thickness (meaning depth is 2x the hole side or more), is one b*&ch of a job, particularly for small holes.

Takes a long time, and is hard to do. Probably best if you have a file rest made of hardened rollers.....and can hold the work rigidly in relation to them. Then you don't have to worry so much about holding it square to the work.

But, for some reason filing a bit off the inside of a square hole is much more difficult than filing it off the OUTSIDE of a piece to make a solid square of the same size. Cleaning out the corners without messing up the hole is hard, too. Files don't tend to make square inside corners very well...

Most similar things I have seen seem to be round holes which have been broached with a simple square broach to cut corners into the round hole. I did that once to make a radiator key, and it worked well. Making a square hole for a cutter in a "box tool" was a real pain, though, that one I filed, because I was being fussy and didn't want the tell-tale remainders of the round hole.... Usually the diameter is larger than the side of the square, so as to require cutting less material in the corners.

Dragons_fire
06-17-2010, 10:59 PM
why not grind a broach from a 1/4" HSS lathe bit? then you shouldnt need to harden it. i would drill the 4 corners with as small of a bit as you can, then drill out the rest, then use a broach from a HSS lathe bit

dp
06-17-2010, 11:05 PM
I wonder just for the sake of wondering if a tap of adequate diameter could be ground into a workable shop-made broach.

Mcgyver
06-17-2010, 11:26 PM
the trick to filing a very accurate sq or hex hole is once this get close, use prussian blue (fairly heavy) on the master and file off the blue using needle files or other small light file. The files are flexible enough that you can direct where they cut.

having said that, 3/16 is really small to be filling....and I'd bet its not a thing section either if you're using it to hold a tap.

you can make a broach but its a lot of work. here's a couple i've made over the years, posted here many times times before, but what the heck, you asked. they are from O1 and you will absolutely have to heat treat them (not a big deal)

http://i20.photobucket.com/albums/b201/michael0100/broaching/homemadebroaches.jpg

the small square one is irrc 3/16.

unless you have a sinker you forget to mention, the most practical way might be to fabricate. Mill a slot and solder a piece over top, put in the four jaw, turn the outside round and loctite into a drilled hole. ...or make in two halves like the V clamps on a tap handle.

Other ideas, all of which I've done at one point or another is to forge the square around a master made of something tough (drill rod hardened and tempered), the turn or other machine the outside to shape, broach them in the lathe or mill with a single point tool...back and forth like a shaper or slotter and drilling relief holes at the four corners and milling out the square shape.....again tough with 3/16

Don Young
06-18-2010, 12:08 AM
I have done similar things by heating the part red hot and driving a part of the correct shape into it. I would just use a tap. Just don't let it stick and turn into a shrink fit. If it turns out too tight, file it out some or make a bigger punch.

That is the way blacksmiths made square holes, I understand.

bpsbtoolman
06-18-2010, 09:21 AM
Years ago my father who was a tool designer for General Motors made a part for me at home that needed a hole for a standard allen wrench by drilling a hole a little larger than the width of the allen wrench and used an allen wrench to broach the 6 sided hole. Worked great.
There is no reason that a HS square tool bit could not be used to broach a square hole.
Walt