10" Cushman Chuck

[Mgdoug3]

I have a 10" Cushman 36E3 3 jaw chuck with an L0 mount. It has bolt on reversible jaws. I like the chuck but it has some runout. I can tap the piece around and get it around 0.005". I had the idea of checking the backing plate but this chuck is all one piece. How can one tear it down and try to reduce runout? I have a 4 jaw chuck but it's 8".

I have checked the taper and made sure it was clean and smooth. I could loosen the collar and tap around the chuck but I would like to disassemble the chuck for cleaning. The lathe hadn't been used in years.

 

[pacifica]

It's probably worth disaaembling and cleaning but if wear is in the scroll due to years of use it would be hard to fix.

 

[Mgdoug3]

Thanks for the reply. Externally the chuck looks fine. When i get time, I'll tear into it.

 

[main_cogg]

I would check to see if the runout is consistent when the bolt on jaw tops are reversed. Unless the fit between the 2 pieces of the jaws is very good, you could easily get .005 runout. I would also switch the jaw tops to different bases and check runout, they may have been matched pairs from the factory. If the runout doesn't change then the problem is inside the chuck.

 

[talvare]

I can tap the piece around and get it around 0.005".

If you can tap the part and move it in the chuck jaws, I would suspect that maybe the jaws are bell mouthed. You can usually determine this condition by chucking a good piece of ground tool steel and checking to see if the jaws are only gripping the part at the back of the jaws. If this is the case, you should be able to see daylight between the gripping surface of the jaws and the part at the front of the jaws. This can usually be corrected by grinding the jaws in situ. Worth checking.

Ted

 

[Mgdoug3]

I tore apart the chuck and cleaned it. I noticed some runout on the back half so I chucked it in the lathe and matched it to the spindle. Put it back together and had 0.005 runout. Then I took emery cloth and to jaws that were the lowest on the indicator. After two tries of that I got to 0.003 and called it good enough. It's much better than what I started with.

I contacted Leblond and got the original sales receipt. This chuck didn't come with the lathe from the factory so there's a chance it was never trued in.

 

[Mgdoug3]

Just an update. The 0.003 runout was best case scenario and occasionally it would be 0.005. I switched to my 4 jaw chuck but it's only an 8". I like using the 4 jaw and it doesn't take much time to zero it in. I may just look for a 10" four jaw chuck. I know a guy who might have a tool post grinder. If I'm ambitious I'll see if he has one and regrind the jaws. It's my understanding even if I buy new replaceable jaws, they're need to be ground to fit.

 

[talvare]

If your friend doesn't have a tool post grinder, you can put together a temporary DIY grinder pretty easily. I ground the jaws on my 8" 4-jaw combination chuck by just cobbling together a mount from some scrap stuff I had laying around and mounted my die grinder on the tool post. It actually worked out very well. Just some food for thought. Here's a photo of what I did.
Ted

 

[Mgdoug3]

I think I made a rookie mistake. I don't think I cleaned the spindle when I trued up the backing plate. I put the 3 jaw on today and noticed the runout was worse the further you went from the chuck so the runout had to be in the chuck. Tore apart the chuck and and put the backing plate back on. Hopefully when I install it this time, runout is better.

 

[Mgdoug3]

Still no luck. I think the chuck is just wore out. I got online and saw I can buy a CME 10" 3 jaw for $350ish delivered. Looks just like my current chuck and the chuck is suppose to have a runout of 0.0015". Honestly if it's within 0.003 I'll be happy.

I have a job lined up soon and I could have used my smaller lathe and 3 jaw chuck but I like using the bigger Leblond. I have about 80 pieces to turn and dialing them in with a 4 jaw would be exhausting. The parts I need to make aren't high precise parts and a good 3 jaw chuck will make it a simple job.