Lathe mod

[dikman (Richard)]

I changed the motor on my larger lathe to a 3-phase with a VFD, so I now have a variable speed lathe, adjustable from the front panel. Should have done it years ago.
I've also been unhappy with the accuracy of the 3-jaw chuck, so I ground the jaws and took the twist out of the bed. Final result is runout of 0.0004" over 13" (was about 0.015 before).

[bojan]

Which VFD you used, and how did you ground the jaws?
With my smaller chuck (Ø80mm, ~$75) there is no way to improve the accuracy with grounding, because it seems the spiral is offset, so position of the center depends on diameter... I am using it now as vice for round things when drilling.

[doug mc]

I was a machinist for 50 years. Get a four jaw and center your work. They hold whatever you are machining better and safer.

[dikman (Richard)]

I have a large 4-jaw chuck, of course, and yes, it is the only choice where the best accuracy possible is needed. The 3-jaw is just more convenient for most jobs, particularly now that it's a lot more accurate than it was. A 3-jaw, however, will never be as accurate as a 4-jaw.


As for grinding the jaws, I made a holder to clamp a Dremel in the toolpost and made a ring to fit over the outside part of the jaws so that the jaws could be tensioned against it. This was the method recommended on a couple of youtube vids. The chuck was rotated at about 100 rpm and a suitable grinding wheel fitted to the Dremel (rotating opposite to the chuck) and slowly wound in and out so that it only just touched the jaws. Coating the jaws with a black marker helps to see what is being ground off. I found that tensioning the jaws with the ring, however, gave inconsistent results as it caused the grind to be at a slight angle, front to back.
As my chuck has very little wear in the scroll I tried something different, I cranked up the chuck speed to near maximum and used centrifugal force to exert pressure on the jaws, then slowly fed the grinding wheel in and out. This actually worked.


I've bought two VFD's recently, a 2HP from a Victorian company called Conon (about $200) for my belt grinder, and a cheaper 1HP from an ebay seller ($104) for the lathe. The more expensive one has a cooling fan, the cheaper one has a large heatsink instead. Both work fine.

[bojan]

Originally Posted by dikman

.... I cranked up the chuck speed to near maximum and used centrifugal force to exert pressure on the jaws, then slowly fed the grinding wheel in and out. This actually worked.

Hmm, that was interesting idea

[dikman (Richard)]

A couple of photos. First one is the $200 VFD on the belt grinder, second is the "generic" $100 VFD (widely available on ebay, I found good reports for it) and lastly the Dremel attacking the chuck jaws. It shows what I meant about using a ring to pre-tension the jaws.