South bend chuck on a Logan

[bcfoster89]

Hello,
I recently purchased an 8" 4 jaw Logan chuck for my 11" Logan lathe. The back plate on the chuck is 8tpi which should match up with the Logan 2.25-8tpi spindle but it only threads on a few threads before tightening up and stopping. Is there anything different between the South bend and Logan threads or thread profile? If so, is there an easy way to fix this?

 

[Jim F]

I had the same issue with a chuck from an Atlas, I chucked the pate in a 4 jaw and cut the threads to fit.

 

[bcfoster89]

It's a Logan 1284 chuck, so I'm surprised it doesn't fit. Maybe the back plate is not original? Do new generic 2 1/4-8tpi back plates fit the Logan or are they going to be the same as the one I have?

 

[jwmay]

I didn’t do a ton of searching, but it looks like the logan 1284 chuck was a plain back chuck. I’d probably buy a new backplate with correct threads.
Unfortunately, the only way to know for certain is to buy or make a replacement.

I’ve got an Atlas lathe, but every new 1 1/2-8 backplate I’ve purchased new has fit perfectly. This goes for several very inexpensive and dare I say, controversial sources of machining equipment. Ie; Ebay, China, India, some dude in a trench coat in an alley, etc.

The point of saying so, is just that I think you’ll be fine buying a new chuck back plate.

 

[OTmachine]

I didn’t do a ton of searching, but it looks like the logan 1284 chuck was a plain back chuck. I’d probably buy a new backplate with correct threads.
Unfortunately, the only way to know for certain is to buy or make a replacement.

I’ve got an Atlas lathe, but every new 1 1/2-8 backplate I’ve purchased new has fit perfectly. This goes for several very inexpensive and dare I say, controversial sources of machining equipment. Ie; Ebay, China, India, some dude in a trench coat in an alley, etc.

The point of saying so, is just that I think you’ll be fine buying a new chuck back plate.

And you possibly will have to machine the register to fit.

 

[mattthemuppet2]

I've read of this problem several times - the thread diameter and pitch is supposed to be the same between SB and Logans, but there are some small differences that means they don't always interchange. I think from memory the SB backplates have slightly shallower threads? Either way, the solution is to buy a 1-1/2x8tpi tap and run it through or single point the threads a smidge deeper.

 

[Jake M]

Hello,
I recently purchased an 8" 4 jaw Logan chuck for my 11" Logan lathe. The back plate on the chuck is 8tpi which should match up with the Logan 2.25-8tpi spindle but it only threads on a few threads before tightening up and stopping. Is there anything different between the South bend and Logan threads or thread profile? If so, is there an easy way to fix this?

Use some dye, sharpie mark, or something to VERIFY THE PROBLEM before you touch anything based on my say-so. I can't recall where I read this so I can not cite it, nor can I be certain that I have the brands correct, or if the problem was a to b or b to a......

Anyhow, disclaimers out of the way, I saw someone out there on the internet (which is never wrong....), who had a similar problem, and what turned out as simply that one manufacturer cut their threads with the with the peaks and valleys pointier and one rounder. Their answer was to use a dremel drum sander to relieve the peaks in the female chuck thread. Depending on who did what, your problem might be the reverse. In that case I'd avoid reducing the spindle threads and probably just run a tap down the chuck threads, as they'd be 99 percent there, and should guide the tap quite well.

But absolutely, put some die/marker on the spindle threads and see what's binding up on who. That will drive your decision to repair what you have or to make (or purchase) replacement parts.

 

[Shiseiji]

Hello,
I recently purchased an 8" 4 jaw Logan chuck for my 11" Logan lathe. The back plate on the chuck is 8tpi which should match up with the Logan 2.25-8tpi spindle but it only threads on a few threads before tightening up and stopping. Is there anything different between the South bend and Logan threads or thread profile? If so, is there an easy way to fix this?

The issue is the probably the length of un-threaded part of the spindle. The 11" Logan's is .375" The Southbend 2.25-8tpi is either .094" or .17" The backing plate needs to have the threads modified so the spindle nose doesn't "bottom out" in the backing plate preventing the back of the chuck from contacting the registration face.
The Logan site spindle specifications table isn't listed in the site Table of Contents, but it has a list of many. Scott Logan recently posted the drawings for the 11" spindle.

In Logan FAQ 2.4. Is a Logan spindle nose the same as a South Bend nose? is how to modify a 9" Southbend 1.5- 8tpi backing plate to fit the 10" Logan. The process would be the same for your 11"

Ron

 

[bcfoster89]

Thanks for all the info! It doesn't look like my spindle matches the dimensions on the spindle specs table. It's a Logan model 922. I'll draw up the dimensions and post tomorrow.

Also, I'm not sure it's the register, if I flip the back plate around it still only threads on one or two threads before it tightens up. I might have to pickup and chase the threads deeper.

 

[Shiseiji]

Probably a miscommunication on my end, but I have an additional thought.

I have the experience of single pointing a 13 tpi when a 14 tpi was required. It did exactly this. It would thread on for ~ 1 and 1/2 threads then stop.
You may just have an odd spindle threaded to a spec so it was compatible to already owned equipment. Or the backing plate and it was forgotten about.

I first suggest you double check the tpi. on both the spindle and backing plate.

Assuming it's not a thread mismatch.

The problem as I understand you is the backing plate stops screwing on before it contacts the register. if I have this correct, it's because a backing plate for a Southbend only has to thread on .17" max until the backing plate mates with the registration face. So lets guess the threads are cut with a small margin so the backing plate will fully thread on. Say .1" so max travel is .27" before the spindle nose bottoms out at the end of the threads.

To mate with the Logan registration face, the backing plate must travel A minimum of .375" .
.365 - .27 = .095" difference and so the nose of the spindle hits the end of the threads in the backing plate and stops before the backing plate mates with the registration.

After you double check the tpi, I suggest you join the group moderated by Scott Logan and give him the measurements from your 922. I have read of some odd spindles showing up from time to time and Scott is the best factual source there is. https://groups.io/g/Lathe-List

Lots of experience there in addition to Scott, about the only unwritten rule is because Scott sells the manuals, people generally don't post other sources.

Ron