Anyone Know The Origin Of This Thread Size?

[npalen]

The thread size of this Jacobs style chuck is listed at 45/64"-16 as shown. I'm wondering if anyone know where this odd size originated.

 

[BnRidge]

The Jacobs chuck invention:

Arthur I. Jacobs and His Super Chuck

Arthur I. Jacobs designed a great drill chuck and it has been in use for over 100 years. One of the parts inside the chuck is intentionally broken. It may be the only item you own that works and is broken at the same time.

evolventdesign.com

 

[CliffordK]

Do you have the chuck?

How close is it to fitting a 3/4 - 16 bolt?

 

[ruffdog]

I have seen those chucks/mounts in shops but never knew why they were made that way. Odd thread size but not hard to make.

https://bbs.homeshopmachinist.net/forum/general/36423-jacobs-chuck-delimna

 

[npalen]

I don't have the chuck but I don't think it would be close to fitting correctly as it would be almost 0.050" smaller than 3/4"-16. Others have speculated that the 45/64"-16 thread was requested by a machine manufacturer to fit their spindle.

Just trying to figure out where this odd thread size originated.

 

[bcp]

I suspect it is to limit its use to what it is intended for. They didn't want someone threading it onto a bolt and using the chuck to hold a chisel or punch.

Bruce

 

[etpm]

I thought the thread might have been an old thread spec so I looked in my 1918 edition of Machinery's Handbook. It was not there. I suspect Jacobs used that thread so that the chuck would only thread onto what Jacobs wanted it to. In my 45 plus years machining I have seen very many special threads designed such that the threaded part would only fit what the manufacturer wanted it to fit, so no surprise at your chuck.
Eric