A few pics of this simple project. The morse tapered stop piece with threaded adjustment bar, and the wrench to adjust it-
Inserted into the spindle taper-
With chuck in place- here it's set to give about 1/2 inch of material in the jaws behind the face of the jaws.
The diameter of the face on the adjusting bolt is just under 1/2 inch. There will be another adjusting bolt made if I need it to be a smaller diameter. The bolt is a friction fit, which I got by choosing a galvanized bolt over a chromed one. I polished up the bolt so it wouldn't chew up the threads in the pvc piece, and gave it a bit of grease. This was a carriage bolt, chosen to give enough of a head that I could machine it to give me a decent sized flat on the end. I also machined away the squared portion directly behind the head, reducing that diameter to 5/16 for this 3/8 bolt. That way I can screw it right in til it touches the plastic. It goes about as deep as I'll ever need it to, but if I do need it to be deeper, I can just cut off some of the plastic. The threads go in a bit over an inch, and the bore is drilled out from the other end to 3/8. No need to thread it all the way.
I could have simplified this and just used a normal bolt, but I wanted to be able to grip material down to about a half inch. This would have meant turning off the hex on the bolt head, leaving no way to adjust it. Otherwise I could simply have used a socket wrench. An alternative would have been to re-grind a hex head for a smaller socket, but I chose the pin wrench instead. If I need to use a smaller diameter adjusting bolt, I'll be forced to grind a hex onto it so I can adjust it. No way I'm going to cheap out and put a simple slot on the end of it. I hate those slippery flat blade screwdrivers.
