Threading puzzle with Logan quick-change

[Jim Davis]

I have a Logan 11" with quick -change gearing. It always did a great job cutting threads. Then, I swapped the 24 and 48-tooth stud gears to cut 6 tpi. Well, it didn't match up with the thread I was trying to recut, but I figured the piece I was working on might have gotten stretched in use.

I found a different usable part and changed the lathe gears back to what they had been. But it still doesn't cut the selected thread pitch. When 8 tpi is selected, it cuts exactly 10 tpi. When 16 tpi is selected, it cuts exactly 20 tpi.

I have the 24 tooth stud gear driving the idler, 80 tooth on the screw, and 8 tpi selected with the change gears.

How can this be? What have I done?

Jim Davis

[ctwo]

I will write a formula to illustrate the problem:

First, your 80T screw gear produces 10 TPI, but you want something (x) to produce 8 TPI

80/10 = x/8

8 = x/8

x = 8 * 8

I think the screw gear should be a 64T

My 10" Logan has a 72T on the "screw" (or input to the QCGB), but I also had an extra 64T and 48T that came with the lathe (used).

[dly31]

I think ctwo has it right. Did you by any chance mix up the idler and screw gears?

[Lazz]

Looking at a Logan 820 gear change chart

The three gears to do a 8tpi are
32 stud
72 idler
32 screw