Post by springcrocus on May 14, 2020 11:45:08 GMT
I've probably covered this before but this is for anyone who may not be familiar with broaching. I've just been making some washers with square holes for some valves and thought I'd share it.
Basically, if you need a square, broached hole then you can sometimes use the back end of a high speed steel tap if the size is right. You just need to grind off the point, then very slightly hollow-grind it with the edge of the wheel across the flats.
I made a holder for the washers by making a recess in a lump of brass a few thou larger than their O/D and also drilled a clearance hole a few thou over the across-points size. The tap is held backwards in a drill chuck.
This one hits the bottom of the chuck but you may need a bit of packing to achieve this, rather than try to rely on the grip of the chuck. The 1/8" thick brass washer is set in the recess.
After locking the tailstock, the broach is just pushed through with the handwheel. My tailstock would normally move backwards with this pressure so I have a bedway clamp behind the tailstock. The washer is wedged on the broach after winding back.
The broach is knocked out in the vice and reset for the next one. Although there are only two finished here, the whole lot took less than five minutes. And I can still use the tap.
I have broached square holes up to 5/16" A/F this way in the past and always keep broken taps for jobs like this. There are quite a lot of taps in the drawers that I have ground this way and I've also broached mild steel up to 3/16" A/F although it needed a lubicant.
Or, of course, you can spend ages filing the things if you want. I hope someone finds this useful.
Basically, if you need a square, broached hole then you can sometimes use the back end of a high speed steel tap if the size is right. You just need to grind off the point, then very slightly hollow-grind it with the edge of the wheel across the flats.
I made a holder for the washers by making a recess in a lump of brass a few thou larger than their O/D and also drilled a clearance hole a few thou over the across-points size. The tap is held backwards in a drill chuck.
This one hits the bottom of the chuck but you may need a bit of packing to achieve this, rather than try to rely on the grip of the chuck. The 1/8" thick brass washer is set in the recess.
After locking the tailstock, the broach is just pushed through with the handwheel. My tailstock would normally move backwards with this pressure so I have a bedway clamp behind the tailstock. The washer is wedged on the broach after winding back.
The broach is knocked out in the vice and reset for the next one. Although there are only two finished here, the whole lot took less than five minutes. And I can still use the tap.
I have broached square holes up to 5/16" A/F this way in the past and always keep broken taps for jobs like this. There are quite a lot of taps in the drawers that I have ground this way and I've also broached mild steel up to 3/16" A/F although it needed a lubicant.
Or, of course, you can spend ages filing the things if you want. I hope someone finds this useful.
Steve
