Change gears

DavidR8

DavidR8

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The thread cutting section of “How to run a lathe” gives me the impression that there are gears to be swapped out even if there’s a quick change gearbox.
Specifically the spur gear.
Am I correct or misunderstanding what’s being described.

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No lathe can cut every thread there is. The QCGB will cut most of the common threads but not all. If you must cut a thread that the installed gearing cannot cut then the only way to cut it is to change the gearing to enable you to cut it. These older American lathes came with gear sets to cut almost all common Imperial threads and if you do not have that set then you should go find it. They usually will not cut Metric threads.

More modern lathes can cut both Imperial and Metric threads plus modular and diametral pitches. All require change gears despite having a QCGB.
 
What is circled is just an idler gear. Look at the front of your QCGB - that will give you all the threads that you can cut right out of the box. There is another gear on your gear box feed that can be used to change gear ratios.
 
The gears to be changed are the ones on the change box at the bottom right, the one circled could not be changed because it is not adjustable in it's mesh with the reversing (tumbler) gears.
Simply put, one could set the QC for the number of teeth on the change box input, and replace it with a gear the number of teeth of the desired thread; say if you wanted 27 threads, and the gear on the input was 32, you would set the QC for 32, and replace the 32 tooth gear with a 27 tooth gear. I made a set of change gears for my 19" Regal Leblond, and can cut nearly anything, including metric, diametral pitches and nearly any special pitches and modules.
 
Ahh Ok I think I get it now.
So there may be occasions where the supplied gears on the banjo are not correct for the desired thread. In that case I would need to swap the banjo gears for the appropriate ones.
 
DavidR8 said:
Ahh Ok I think I get it now.
So there may be occasions where the supplied gears on the banjo are not correct for the desired thread. In that case I would need to swap the banjo gears for the appropriate ones.
Click to expand...
Yes, as I outlined above.
 
DavidR8 said:
Ahh Ok I think I get it now.
So there may be occasions where the supplied gears on the banjo are not correct for the desired thread. In that case I would need to swap the banjo gears for the appropriate ones.
Click to expand...
Not the Banjo, that is just transfering power. You swap the two on the lower right between eachother (front one is just for storage!) to get to the other settings on your gearbox. See the setting 'stud gear' on your threading chart: https://www.practicalmachinist.com/...everything-do-my-9c-615-9a-conversion-339579/

I know thats not the lathe you have, but check out the 40 and 20 on the LHS of the chart. The 40 tooth stud gear in 'A' gets the top row. The 20 tooth in 'A' gets the 2nd row (and all the ones below).

Show a picture of your threading chart and we can likely show you exactly what it means.
 
Depending why you're asking, what you have, and what you want/need to do, there's always the option of an Electronic Lead Screw.
 
Also keep in mind you still have a mini-lathe that can cut metric threads. Just one of many reasons having a second lathe can be handy.
 
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