I know what you mean about 3 jaw chucks, they normally are not that accurate. I was lucky and got a accurate one with the machine, too bad the outside jaws are missing.
Started with assembling the parts of the holder. Put bearing grease for the trust bearing
And mounted the broach holder with light tapping with a plastic hammer. I bought some shims to set the trust bearing to tension, I needed 0.6mm in total. Its running easy and there is no any play on the head
Tool finished!! Put the broaching holder in the tail stock and mounted a piece of 10mm silver steel in the holes. The total run-out is 0.02mm, not bad for a home made tool, I am happy! If I had took the effort to make a collet for drilling and reaming the 10mm hole it could have been even better….
Then I cutted 11 pieces (1 trial piece, hope 10 good ones) of 10mm silver steel and chamber them in the lathe on one side
And milled a flat surface on them, 8mm mill, 1mm deep
I decided to make a 6mm Hex broach, first made 2 parallel side so I could check the dimensions easily
During the machining I made a mistake by having the collet holder not flat in the vice, so now it will be a 5mm broach!
Here it is milled and I tried to harden it. Heated until red and dropped in a bucked with old motor oil. Then heated again until just becoming red and dropped it in cold water.
To bring the relieve I took my Clackson grinder and try to set-up the angle with a protractor (the grease on my jacked is just to make it water resistant). Its a great machine but a real pain to set-up angles accurate.
To get the broach back parallel in the collet I used a parallel and flat table (Tacchella bed), have the flat side of the broach on the parallel and tighten the collet
Then tried to grind the broach by pressing the collet holder agains a flat and sliding it up and down. At the second face I hit the stone with the tip…..so now it will become a 4mm broach!
Took the already finished dividing head of the Tacchella and bolted it to the Clackson table.
I set the angle by eye (big mistake, but I got impatient)….and grinded the broach. This worked out well. Here it is after grinding, and with a way to large relieve angle.
Chucked up a piece of aluminum and drilled a 4mm hole and gave it a light chamber. Put the broach in the tailstock and brought it in position with the 2 screws fixing the bearing holder to the MT3 shaft loose. Let the lather run on very low rpm and put some pressure with the tailstock. Switched off the lathe while keeping the pressure on the tool. Then tightened the 2 screws on the back, this should be center. This is the procedure described by Ken. Started the lathe and hold my breath. I was able to keep the tailstock moving in nice and steady. I checked several times if I was not pressing the tailstock backward, but no, the broach is CUTTING!
Put in an allan key and it a very nice and tight fit. My goal has been achieved, I drilled a hexagonal hole!
Then tried a piece of brass. It worked ok for the first 3 to 4mm and then the broach got rotated or damaged I think. Also the hardening did not worked out well, I guess with my method you just harden the surface and I reduced the size with grinding. The broach has now some rounded corners. Can it be an option to harden it after grinding?? So now this broach will become the 3mm version
Thank you so much Ken for sharing your design, its a perfect tool and but a little but of practice and experience it work great
Thanks for watching, regards Jeroen