I have thought about making my own collet holder for my 1 3/4-8 spindle nose. A problem to address is how to gauge the internal thread to make sure it fits the spindle nose. Obviously, you wouldn't remove it from the machine to try it. I suppose one could buy a bolt that size but the sloppy fit of a commercial bolt may not give the class of fit required on the spindle nose.
The answer to this was given to me by a more experienced machinist: first make a duplicate of your spindle nose. Since it has external threads, you can easily measure it with precision -- I bought some thread wire gages so I could use a regular micrometer. A critical dimension is the diameter of the unthreaded shoulder, as it will determine the concentricity of the fit of the collet chuck. After making the spindle nose model, you can use it to check the fit of the internal threads as you turn them on the collet chuck. Then mark it and put it in a drawer for future use. Here is the drawing of the chuck I made for my Grizzly G0602 10x22 lathe:
Naturally all of the shoulder dimensions should be to match your lathe's spindle. Note that the sequence of turning operations is important. After squaring the ends of my billet of 4130, I chucked the part in the 4-jaw and carefully centered it, then cut the internal thread bore and threads. When it just fit the duplicate spindle nose, I reversed the part and mounted it directly on the spindle before finishing the internal taper and outside diameters.
I set the taper by indicating off of a commercial MT4-ER40 holder, adjusting the compound until it traced the internal angle to <.001 TIR. Note that you must have the indicator exactly at center height. Incidentally, when you have that set up, it is easy to rotate the holder and measure its runout, which was less than half a thousandths. Not having a tool post grinder, I turned the best finish I could then used fine sandpaper, emery cloth and finally buffing compound to get very smooth surfaces, particularly on the internal taper.
I finished the collet holder with holes for a tommy bar to snug it on and break it loose, and also threaded holes for the little clamps Grizzly uses to prevent unscrewing in reverse rotation.
The hardest job for me was turning the 1.75-8 internal threads. I left a .25" landing zone, but if your lathe doesn't have a slow speed (like 50 RPM) it is very hairy turning those threads going in. (My lathe, as manufactured, had a slow speed of 150 RPM, which for an 8 tpi thread amounts to 5/16" travel per second! Fortunately I had made the VFD/3ph motor conversion so I could run as slow as I wanted. Even better, though, is turning the threads on the backside in reverse, traveling from the inside out. I wound up making my own threading tool from 3/4" drill rod...but now I have an even nicer internal threading tool from Mesa Tools (great tools at low prices!). Here is a picture of the nearly finished chuck:
The tommy bar holes are there but this doesn't show the 2 threaded holes near the spindle for the clamp block (only needed for reverse operations).
Hope this helps,
Craig
