Thread: Chuck accuracy

Good eveing all,

Feels like summers struck early today...Almost to hot to play in the shed.

Been battling chuck run out or what ever you call it for a while with my Hercus.My first 3 jaw out of centre was measured in 1/8" plus variation. Its on the shelf under the lathe for 'oneday'. I now have a nice unamned second hand 5" 3 jaw but still have issue's with a noticable "out of round".

If I insert a piece of polished steel rod (out of a bubblejet printer which I am advised is made to very fine tolerances) I got a run out or out of center or what ever the correct term is of several thou on my dial guage. Enough to be visually obvious. Same on a number of materials I tried.

I set the dial gauge on the spindle and on a dead center in the spindle and it was almost undetecable variation. My conclusion - the lathe is OK

Next step was to clean all the threads, etc and re machine the backing plate - found it was it was out a few thou.

This brought the error back to about 4 thou. I tried rotating the chuck on the back plate and improved to about 3 thou, regardless of what was in the chuck and how I rotated that.

Now before you real machinsts suggest a four jaw = I have done that for more accurate stuff, but I am intrigued as to what is an an acceptable variation and what else I might to to improve the three jaw - other than buying a new one.

Love to hear your thoughts.

Cheers

Ian

.003" is fine for what you seem to be doing as you said for more accurate work you are using the 4 jaw.
Unless you are prepaired to pay a lot of money for a very good quallity 3 jaw what you have is ok.

As pipeclay says 3 jaws are not designed for accuracy...

That makes me feel a whole lot better.

Thanks PipeClay,

Just was'nt sure what is 'normal'.

Ian