Threading table on my lathe is wrong or I'm an idiot.

strantor

strantor

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Logan 9" lathe circa 1950s or 1960s.

I set it up for threading 4.5 TPI according to the table/chart plate on the QCGB and it was cutting 4.0 TPI. Double checked, triple checked, scratched head and backside, everything is correct except the pitch it's cutting. So I tried cutting 5TPI according to the chart and got the 4.5 TPI that I was looking for. I didn't try any other pitches to see how far the error goes.

Does anyone else have this lathe and could verify that the chart is wrong or right? I'm not sure how I could be doing it wrong.
 
I don't know about your Logan, but with my South Bend 13" I have to change out a gear to go below a certain thread per inch using the QCGB. Does your thread chart show this as well? A picture of the chart on your gearbox would help...

Ted
 
This is the chart on my 11" Logan...

20171120_185222.jpg

I don't know how different yours would be, so this chart may not help...

The first thing I would do is count the teeth on your stud gear and see if it matches what is on your chart... on my lathe the stud gear should have 72 teeth.

Would it be possible that someone has put a non-standard stud gear on your lathe? Maybe in an attempt to turn metric threads? Stranger things have happened... also, if you can see up under the QCGB, check to make sure the handle is engaging the correct gear on the end of the lead screw. Something may be installed backwards, causing the next gear over to be engaged when the handle is in the correct sector.

I don't know if that is even possible... I'll have to drag my gearbox out of storage and see how it is designed.

-Bear
 
strantor said:
Logan 9" lathe circa 1950s or 1960s.

I set it up for threading 4.5 TPI according to the table/chart plate on the QCGB and it was cutting 4.0 TPI. Double checked, triple checked, scratched head and backside, everything is correct except the pitch it's cutting. So I tried cutting 5TPI according to the chart and got the 4.5 TPI that I was looking for. I didn't try any other pitches to see how far the error goes.

Does anyone else have this lathe and could verify that the chart is wrong or right? I'm not sure how I could be doing it wrong.
Click to expand...
You're probably using the wrong stud gear.
 
Did you replace the 36 tooth stud gear with the 72 tooth?
Though I would expect to get 9 TPI instead of 4.5 if the 36 was in place.

Damn that's a nice looking plate!
 
682bear said:
This is the chart on my 11" Logan...

View attachment 446351

I don't know how different yours would be, so this chart may not help...

The first thing I would do is count the teeth on your stud gear and see if it matches what is on your chart... on my lathe the stud gear should have 72 teeth.

Would it be possible that someone has put a non-standard stud gear on your lathe? Maybe in an attempt to turn metric threads? Stranger things have happened... also, if you can see up under the QCGB, check to make sure the handle is engaging the correct gear on the end of the lead screw. Something may be installed backwards, causing the next gear over to be engaged when the handle is in the correct sector.

I don't know if that is even possible... I'll have to drag my gearbox out of storage and see how it is designed.

-Bear
Click to expand...
Yes, you need to swap the stud gear 36 with the 72. This is the same way my South Bend is. It's near the banjo arm gearing on the end of the lathe. Pop the cover off and take a look.

Ted
 
If every pitch is off by about the same amount, then the stud gear is off by that amount (~1/8). If I have my head screwed on right this morning (not guaranteed) that would make your stud gear a 40t.
 
If the stud gear is wrong, then the pitch of every thread will be off by the ratio wrong gear to the correct gear. If the measurement was correct and the 4.5tpi setting produces a 4 tpi thread, the ratio will be 4.5/4 or 1,125. Setting for a 5 tpi thread as described will produce a 5/1,125 or 4,444 tpi thread. If the required stud gear was 72 t, the actual gear would have to be 72x1.125 or 81 tpi to produce the result described.

More likely, the gear selector rail is off by one gear. Perhaps, a P.O. did some work on the gear box and reassembled it incorrectly.
 
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