Jacobs 56B chuck incompatible with Atlas MFC spindle

[JPMacG]

I just purchased a Jacobs 56B chuck on Ebay. It is in like-new condition. I intended to use it on my Atlas MFC horizontal milling machine. The chuck has a 1" x 10 tpi thread, as does the Atlas spindle. But the chuck will not screw onto the spindle. It locks up at about 1/2 turn. I'm guessing the spindle threads and chuck threads are made to different class-of-fit standards.

Has anyone else observed this problem?

 

[markba633csi]

Interesting- I would have guessed the Atlas spindle to be a bit undersized and would fit easily.
You sure the chuck is 1" X 10?

 

[JPMacG]

Yes. Jacobs engraved "1"x10 PI" on the base of the chuck. And I cheked the TPI with a thread gage and the ID with a caliper.

 

[brandon428]

Worth running a thread chaser on the chuck. I bought an Atlas part on eBay at one point that was in "good used" condition but also got stuck before I gave it a thread cleaning. I recall getting farther than 1/2 turn, but didn't make it halfway on.

 

[JPMacG]

The threads look perfectly clean - almost like the chuck was never used. I ran a probe (bent wire) along the thread groove and did not feel anything amiss. The chuck is hardened, so chasing or re-tapping may be difficult.

 

[JPMacG]

I found another issue with the Jacobs 56B chuck. The counterbore (registry) diameter on the chuck is 1.031". The registry diameter on the Atlas spindle is 1.000". This means that the chuck would be located to the spindle only by the threads. So the runout would likely be poor.

This chuck is not designed to be used on an Atlas spindle.

 

[Orangetruckman]

JPMacG, thank you for sharing your experience. I’m sorry your plans fell through. It gives me and hopefully others another thing to think about before buying used items-the register size, not just the thread size.