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South Bend Lathe Threading problem

I am learning to cut threads on my SB Heavy 10 Lathe. I put a piece of plastic rod in the chuck and a pen in the tool holder to simulate thread cutting. However I am having trouble matching up the actual TPI vs what the lathe is set for. At first I tried to replicate 10 tpi. I set the gear to the correct slot, put the sliding gear "in" and put my

3 position gear selecter in the "center". The result was much more than 10 tpi and more that I could count. So I set it for 5 tpi and tried again. This time I got about 15 tpi. I have my Bull gear pin "out" and my back gear "in" and my flat belt on the middle pully.

What am I doing wrong? Could I have metric gears? I bought the lathe used and dont know its history.

Any ideas on what to try? Thanks Steve

Reply to
gtslabs
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Steve-

Sounds like you might be using the power carriage feed instead of the half nuts. On my SB heavy 10, which like yours has the single lever gear box, the power feed is one third the speed of threading. In other words, the setting for 5 tpi corresponds to a longitudinal feed (with the power feed clutch engaged instead of the half nuts) of .0669 inches per revolution, or just about 15 tpi. If you don't have one already, you ought to pick up a copy of the SB publication "How to Run a Lathe". Originals and reprints are readily available on ebay and elsewhere for modest cost.

Hope this helps,

-Bill Fill Olympia, WA

Reply to
Bill Fill

Bill is correct of course, but be certain that your gear selection is also correct on the left side of your lathe. You will find a stud gear selection guide on the front of your quick change gearbox. Make certain you are using the correct stud gear which is mounted on the drive side of the gearbox. The large gear is a 40 tooth and the small gear is a 20 tooth. The gears are mounted together. The choice is made by inverting the gears. Also make certain that you have an inch lead screw as opposed to a metric pitch. I believe the inch screw is 8 TPI. A good clue is that the thread indicator has 4 major divisions, if the leadscrew is inch. Steve

Reply to
Steve Lusardi

See what Bill commented above.

Are you using the half nuts to engage the leadscrew?

If so, then somebody may have put a non-standard gear in the train.

Jim

Reply to
jim rozen

I had the same confusion with my SB 11", and, as Bill suggests, when I used the half nuts instead of the carriage feed, things got a little easier. It also took me a little bit to figure out that there is a stop built in and the half nut lever won't move and engage the half nuts unless I pull the cross feed "button" out. I don't know if your H10 is like this, but I figured I'd mention it.

Peter

Reply to
Peter Grey

It's a good point, and his 10L is like that. There's a lockout to prevent engaging the longitudinal feeds and the halfnuts at the same time. If one were able to do this (and some old machines lack such a lock-out) it has the effect of putting two power transmission paths in parallel, and because they have two different ratios, it will lock the driven gear and blow up the weakest link in the driveline.

Jim

Reply to
jim rozen

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