South Bend Model C Question

Nyala

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I have an old South Bend Model C lathe that is in good shape and I would like to learn some single point threading on it. I bought one of those 3-D printed thread dials and it seems to work very good. However, the thread dial needs a cross fit set screw to keep it from moving out of position. The lathe is drilled and tapped for the set screw but I can't figure out what the screw size is.

#10 is too small and 1/4 is too big, so it appears that it might be a #12 screw with either 24 or 28 TPI. I have dozens of taps around to try to check the thread size but oddly, none were a #12.

Does anyone out there know the correct size of the set screw to lock down the thread dial?

Thanks.

Denny
 
Mine is 1/4 -20. Ask the seller of the 3D dial. The set screws on the crossfeed and compound are 12-28, you could test it with one of those.
 
Thanks for the suggestion. I'll pull one of those screws and test it. I suspected it might be a #12 because the nothing else fit but I couldn't find a #12 screw or tap last night for anything.
 
Good luck with the threading. It is probably the most satisfying turning process there is, IMO. BTW- you don’t really need to use a thread dial for threading. It takes a bit longer, but you can simply reverse the motor to reset the carriage position between passes. (Keep the gear train fully engaged and don’t try to reverse the carriage by throwing the tumbler gear though - that was my rookie mistake when I was learning !).

If you have a lot of parts to thread, the thread gage would come in handy to save time.

Rick
 
Thanks for all the help.

I pulled a gib screw from the compound and checked the size - #12-28. It fit perfectly into the set screw hole for the thread dial, so now I'll order some of those. Hard to find here locally.

Being this will be my first go at threading, and that I need to make four screws about 3" long, I decided that a thread dial might ease the learning process. Fortunately I have lots of scrap for practice and a nearby metal company that I'm sure will sell me all I want - if need be.

Thanks for all the help. You guys solved my problem - at least the most immediate one. I have more that I'll address in the near future as I learn this new skill.

Denny
 
I'm surprised they would use that pitch but it is what it is. You could make your own setscrew if you have the right change gears.

3" is pretty long for your first threading project. You will likely need to support it with a dead center or half dead center like this:
http://www.abmtools.com/product/half-dead-center-lathe-centre/ easy to make unhardened but use lots of oil.

How are you going to slot the head?

Don't forget to make a spring pass every few cuts and let us know how you make out.

Good luck!
 
They might have used this size set screw because it was what was popular then. This lathe was built sometime between 1937 and 1941. It is small, 9x20, with only 1/2hp motor, that's easy to stall with too big of a cut, and lacks a lot of modern features but it's ok for small projects and i've built many. I just ordered a QCTP for it. I have been using the lantern style but that's a pain.

I never heard of a half dead center, I'll have to look at that more carefully. I have live and dead now. Thanks for the tip.

I doubt that I'll need to slot the head. I intent to put a knurled head on it for finger tightening only. I do have a slitting saw if I should change my mind.

I'll do lots of practice first. What's not to like about machining, even if it is for practice.

I recently bought a bench mill. My wife asked what my intended projects were. I told her that I didn't have any immediately but the end project might be to simply learn how to use a mill. I don't mind practice.

Attached is my little piece of the world.

Denny
 

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