# South Bend Junior (1930) Threading Setup



## silverhawk (Apr 21, 2017)

Hey, guys, I'm entering into a new world here - that of thread cutting.  I have been piecing my Junior together (22-YB) for a few years, and I was able to pick up change gears.  However, I think I am missing something (a part, or my head, can't tell which at this point).

First of all, the change gears came with an idler gear, that was a little low on the diameter (it's the one on the left) :







You can see the gear used to transfer to the lead screw on the upper right.  Obviously, the one on the left won't fit to the small teeth on the gear on the right, so I ordered an 80-tooth one from eBay.  It arrived, and it, too, wouldn't fit :






I don't have one of those forking banjos, either (pardon my dust and chips - I was in the middle of a project):






So, I'm either missing something, or I just don't know how to assemble for cutting threads.  Does someone have pictures of how the change gears are set up?  I couldn't find charts for mine that didn't include the forked banjos, but I don't know how or where to set these things up.  If you can do it for two thread pitches, that would be most beneficial - 18 and 46 would tell me exactly how I should be doing it for fine or course threads.

Any ideas, diagrams or suggestions?


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## silverhawk (Apr 21, 2017)

So, I am going to apologize right now.  They way I function at work when I have questions is to just start poking things (or trying them out).  So, I had 20 minutes after work today.  I found the solution, but here's how I went about it.  I already had the threading chart from the model catalog from 1931.  I took a known value from that, an 18-pitch thread, and looked up the gears.  The gears were identified as a 32-tooth stud gear, and a 72-tooth screw gear.  That gave me two known values.  The only thing I had to play with was the compound gear/idler combination in between them.  So...

1 - I chucked up some aluminum because it is easy to turn by hand, and because it is easily visible with the scoring from bits
2 - I threw the 32-tooth gear onto the stud and the 72-tooth gear onto the screw
3 - I knew the compound wouldn't fit the 72-tooth gear, so I started treating the compound gear in the middle as a simple idler
4 - positioned the cutting bit up
5 - engaged the apron
6 - manually turned it by hand enough times to use a thread pitch gauge
7 - measured it
8 - ended up utterly flabbergasted that the pitch matched

First time out, and I had the right pitch :











So, you simply don't use the small gear on the compound idler, you just reverse the gear and bushing on the lead screw so it matches the larger gear, and you are set.  Here's the final arrangement for anyone who asks (and please ignore the messy chips as I'm still in the middle of the project)

Amazing!  And I am absolutely glad no one jumped in and called me an idiot for not trying earlier.  The forum is great!


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## wawoodman (Apr 21, 2017)

Smartly done!


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## silverhawk (Apr 21, 2017)

That last picture didn't show up.  So here it is again :






 After helping out with a pinewood derby, I had a half hour, so I tried some real thread cutting.  Started with aluminum :










The threads didn't look perfect, but they were somewhat functional.  Next, some steel 25/64" rod :










That allowed me to re-tap the plastic threaded insert and finish a second brass pan :






The threads here DID clean up much nicer using a file after the threading, and they needed to be a bit awkward for the insert (25/64-18 anyone?), but it worked.  Yes, I went through all of that to save $10 on a cheap plastic insert.  It was a great learning experience.


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## mbucklew11 (Apr 23, 2017)

I have a 1928 South Bend Lathe, and that is exactly how I believe you are supposed to do it. If you look on the Vintage Machinery website, there are a bunch of manuals for how to operate these old lathes. Here is a link to a 1931 manual.

http://vintagemachinery.org/pubs/detail.aspx?id=15132

Hopefully, I am not breaking any rules by sharing this. I'm glad you figured it out. The gears for these lathes are 14 1/2 degree pressure angle and 16 Diametral Pitch, 5/8" Bore and 1/2" wide. Later models they changed the gear specifications. You can search that and find change gears on Amazon. the best choice if you want a full set is watch ebay. 

The best way to find out the pitch diameter is to add 2 to the number of teeth and divide by the outside diameter of the gear. i.e. a 32 tooth gear with a 16dp is (32 + 2) / 16 = 2.125" outside diameter. 

Good Luck!

Sent from my SM-G900V using Tapatalk


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## silverhawk (Apr 23, 2017)

mbucklew11 said:


> I have a 1928 South Bend Lathe, and that is exactly how I believe you are supposed to do it. If you look on the Vintage Machinery website, there are a bunch of manuals for how to operate these old lathes. Here is a link to a 1931 manual.
> 
> http://vintagemachinery.org/pubs/detail.aspx?id=15132
> 
> ...



That is how I was able to find my full set and ensure they fit right. Before ordering, calculate the diametral pitch. 16dp with 14 1/2 pressure angle is right for juniors, and old 13" and heavy 10's also had the same pitch. But not newer 9 and 10k's - those are 18dp gears if I remember. Some modern ones (more recent than 1930's) have different pitches altogether, so calculate first before buying anything.


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