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#15
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Re: Hex Broach Options?
Here's one possible way:
"Make up a fixture to hold the [item] you want to broach the hole in. Ideally the end should be vertical but horizontal will do. Take a good quality allen key and cut the short end off it. Drill a small hole about 4mm diameter through the key about 1/2 inch from one end - you'll see why later. Then heat the other end to bright yellow and quench in boiling water to get a superhard tip. Grind the end to get sharp cutting edges. Drill the hole in the piece to be broached at the size across the flats of the allen key. Make it a bit deeper than you need so there is somewhere for the swarf to go although if you want a bar magnet in there you'll have a smaller deep hole anyway which will be fine. Now you need a guide for the allen key to run in. Take a piece of steel about 3/4 inch thick and drill a hole through it just smaller than the size across the corners (not the flats) of the allen key. Then tap the allen key through it and you have just broached your first hole. Mount this piece of steel so the hole in it is central to the part to be broached. You now have a guide which will keep the allen key bang on centre all the time. Now just tap the allen key into the workpiece a bit at a time and pull back to blow out swarf. If the allen key sticks then use a small tommy bar in the hole you drilled through one end of it. If you want the broached hole to be as dead to size as possible then do this. Heat the workpiece in a flame to 200 degrees C or so until you have broached to full depth. This will expand the workpiece and the broached hole will be a tad undersize when cool. Then let it cool and contract again and take a final cut in one hit to the full depth. It's a lot of messing about to make one hole but you'll get it dead true and dead to size if you do it this way. Dave Baker at Puma Race Engines (London - England) - specialist flow development and engine work. ." |
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