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True the chuck a on hobby Mini-lathe

Run out on my mini-lathe

I had the prospect of cutting a long thread on some 20mm diameter silver steel bar. I was mounting the stock in the chuck and using my dial indicator to make sure it was rotating true. Try as I might, I could not get the stock to rotate without quite a bit of runout - at least 30/1000th of an inch.

So I started to investigate the runout of my chuck. Putting the dial indicator on the edge of the chuck backplate, I could see that there was less than a thousandth of an inch of runout. This Made me feel that I should be able to get the runout on bar in the chuck pretty good.

Removing the chuck and indicating the front of the faceplate: Horror! It showed over 15/1000th error. My first stage was to carefully skim the face of the backplate true. The backplate looked quite poorly machined. I skimmed the smallest amount possible, just enough so that the tool removed metal all around the backplate, but leaving a few low spots.

Re-attaching the chuck I experimented with different positioning of the chuck, and different positions for the chuck key. Indicating on my steel bar 6 inchs from the chuck, I was still get between 5 and 10/1000th runout. An improvment.

Next, I decided to be more drastic. Grinding! When using grinding tools on a lathe, it is important to cover everything as much as possible to keep dust out. Cleaning the chuck afterwards is also a good idea!

I rigged my dremel tool onto the toolpost using a large clamp. Using a small grinding tool, I adjusted everything so the grinding tool was close to the centre line of the lathe. I set the chuck jaws so the gap was smaller than the hole in the chuck.

The dremel was set to max speed, and the lathe was also set to max speed (without vibration). I carefully adjusted the crossslide until the first sparks were seen and ran the grinding tool up and down the centre line of the lathe, almost just cleaning the jaws of the chuck.

Repeating this to just remove the most tiny amount possible. You are only trying to clean the metal, taking off perhaps only 1 or 2/1000ths of a inch.

The high speed of the lathe keeps the jaws forced open against the chuck scroll.

Indicating on the steel bar again - runout was less than 2/1000th of an inch when measured 8 inches from the chuck. Magic!