Friday, April 21, 2017

South Bend Junior - How To Set Up For Screw Cutting

I have a problem.  My lathe came without quite a few parts.  I've purchased change gears, and those change gears came with a thick idler gear :

(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 :

I was not sure how I was supposed to set these up, so I looked up the gearing for cutting an 18-pitch thread :

I needed a 32-tooth gear on the stud and a 72-tooth gear on the lead screw.  I reversed the screw gear and bushing so it would line up with the large gear on the idler (the same one connecting to the stud gear).  This turned it from a compound gearing to an idler gear.

Next, I chucked up a small chunk of cheap aluminum, because scoring from a tool cutter would show up easily, and it was soft enough I could rotate the chuck by hand.  Then, I turned it a few times to get enough for a thread pitch gauge :

And measured it :

18 on the pitch!  Success!  Next, cut some actual threads.  Again, I started with aluminum.

Next, I threw on my 25/64 bar I needed for a custom tap (yes, I had to learn all of this for a tap, which was for a stupid brass pen).

I removed it from the chuck and used a dremel to cut some relief in it (some of the reliefs I cut backwards, so beware if you make a tap), heated it up red-hot, and quenched it.  I didn't care how hard it really was, because it had a single purpose - to cut some threads in a plastic insert to finish a brass pen :

For anyone who doesn't know about South Bend Juniors, that's how screw cutting is set up!

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