Mini-chuck repair
 

Hi,

Thanks everybody for you responses at my topic "Cupertino, CA - local lathe owner for
small help?". I got lot of valuable information, I also got the invitation to use the
lathe (thanks, skuke). Most important thing that I understood that what I wanted to do to
fix this chuck was absolutely wrong. Nice article by brownsharp gave me some ideas what I
have to do (yes, I also consulted with my brother who owns a machine shop - unfortunately
6000 miles away).

Today I fixed the chuck by myself. There is what I did:

Central hole has been bored already too big. So I removed jaws, made some pattern with 60
degree angle and filed all the jaws a little - so the tip width became only 1/16". My
chuck thread is able to move those shortened jaws all the way forward so I recovered the
ability to hold small parts. There is no reason to do precise filing as jaw's tip angle is
not too important.

Now when I am able to bring all the jaws as close as 3/32" I put the chuck on to the
spindle (spindle TIR is 0.0005") and used the 1/4 drill rod as the probe. I indicated at
three points where jaws contact the probe. It is better to reset the indicator at the
highest point and then get the negative readings for other two points. Negative reading
means that jaw is going too far and has to be shortened.

I do not have granite/steel plate so I used sharpening stone (new, Japan made, big one). I
build the prismatic 90 degree construction using the machinist square as the guide and
pressed jaw against the stone by fingers. Electronic caliper has been very helpful - zero
it on original jaw and then shorten the jaw by half of the indicator reading. Do it for
two (or one) jaws and rebuild the chuck. Put it onto the spindle and repeat the process
untill result is good. In my case I got too much to cut and needed additional filing for
tip to keep tip narrow. So after 5 or 6 iterations I got TIR 0.0015" and could not move
further due to quality of the thread.

So my toy still be the toy but at least it is the decent toy now! I could use the collet
for small sizes with TIR 0.0005" and for bigger parts current precision is enough. Anyway
device saddle and crosslide lack the rigidity and precision but it means that I have
something to do with it!

There is the picture of the chuck fixed:
http://s89568048.onlinehome.us/images/P1.JPG
and it's original state:
http://s89568048.onlinehome.us/images/044.JPG

Thanks,
Arcady