# Change gears



## DavidR8 (Dec 27, 2019)

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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## cathead (Dec 27, 2019)

Reversing gear likely?


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## mikey (Dec 27, 2019)

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.


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## SLK001 (Dec 27, 2019)

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.


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## benmychree (Dec 27, 2019)

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.


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## DavidR8 (Dec 27, 2019)

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.


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## benmychree (Dec 27, 2019)

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.


Yes, as I outlined above.


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## ErichKeane (Dec 27, 2019)

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.


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.


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## kb58 (Dec 27, 2019)

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.


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## Aaron_W (Dec 27, 2019)

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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## SLK001 (Dec 27, 2019)

If you look at the picture of your QCGB, you will see a column "STUD GEAR", which will have the value of 20 or 40.  The picture above has the 40T stud gear currently driving the gear box.  The 20T stud gear is right beside it, resting in its normal storage position.  To use the 20T gear, just swap the positions of the two gears.


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## mickri (Dec 27, 2019)

And by changing the bottom two gears with the right gears you can cut metric threads on your new to you lathe.  Halligan142 has a video on this for a south bend lathe.  




Being able to cut metric threads is handy.


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## DavidR8 (Dec 28, 2019)

mickri said:


> And by changing the bottom two gears with the right gears you can cut metric threads on your new to you lathe.  Halligan142 has a video on this for a south bend lathe.
> 
> 
> 
> ...


That was great! Thanks @mickr!


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## Downunder Bob (Dec 28, 2019)

mikey said:


> 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.
> [/QUOT
> ...


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## ErichKeane (Dec 28, 2019)

Downunder Bob said:


> It is my understanding that any lathe can be converted from imperial to metric threads, you just need the correct gears to change the ratio between the spindle and the leadscrew. Most larger machines use the  127 x 120 T wheels. Many smaller machine use the 63 x 60 T wheel set. other combinations also exist, but they are all approximations, close but not 100% only the 127 x 120 T set will give you perfect conversion It only matters if you are making a long threaded shaft like a lead screw, for normal nuts and bolts the difference is so small as to not matter.


Close, the perfect set of change gears for metric is 127/100, however those tend to be too large for smaller lathes(like the 10" lathes).  They don't tend to fit!  However, you can use a 47/37  set that actually gets you REALLY close, apparently it is within 0.02%of perfect.​​HOWEVER it DOES mean you cannot use the threading dial and have to leave the lathe on the leadscrew at all times.  You have to stop the whole lathe at the end of the thread, and reverse the lathe (from the motor!) in order to keep from diengaging the leadscrew.​


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## SLK001 (Dec 28, 2019)

ErichKeane said:


> Close, the perfect set of change gears for metric is 127/100...​


​
Actually both are correct.  The perfect set will always have a 127T gear - it doesn't matter if the secondary gear is 120T or 100T (or even other values).  The gear train down the line will determine the final metric thread.  I have the 127/100 set that fits just fine in my later version 10L.  The earlier versions (up to ~1950) had to have a different banjo and gear cover.  This is the metric set for the early 10Ls:




For the later 10Ls, you just need the 127/100 combo gear and the set of loose spur gears.


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