# When do you use the bull gear for reduction on lathe



## magu (Aug 23, 2019)

When in practice do people use the bull gear to gear down their lathe? Is it usually done to slow things down for the operators sake? For more torque?


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## jakes_66 (Aug 23, 2019)

I think you're referring to what most manufacturers call 'back gears'.  These gears usually sit 'in back' of the spindle and provide a deep speed reduction.

The reduced speed is commonly used when turning large diameter or out-of-balance parts where normal speeds are too fast.  The slower spindle speed brings the surface speed on bigger diameters within range.  I sometimes use back gears when threading larger diameters so I can disengage the cutting tool more consistently.


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## gradient (Aug 23, 2019)

jakes_66 said:


> I think you're referring to what most manufacturers call 'back gears'.  These gears usually sit 'in back' of the spindle and provide a deep speed reduction.
> 
> The reduced speed is commonly used when turning large diameter or out-of-balance parts where normal speeds are too fast.  The slower spindle speed brings the surface speed on bigger diameters within range.  I sometimes use back gears when threading larger diameters so I can disengage the cutting tool more consistently.



As mentioned above, used for threading to slow things down to enable better control of the half-nut to move the carriage.  Gets hairy when the cutting tool is moving toward the chuck at high speed.  I'm too old to react quickly anymore so the back gear use suits me just fine.


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## BGHansen (Aug 23, 2019)

As mentioned above, used to slow down the lathe.  Tom's Techniques has a nice video on picking RPM's for cutting.  Quick version for steel is start at 400 RPM divided by drill diameter (drilling operation) or work diameter (for turning).  So start with 1600 RPM if drilling with a 1/4" drill into steel ( 400 RPM /  0.25").  Or for turning, set the lathe to 200 RPM if turning a 2" steel round ( 400 RPM / 2" diameter).  I'd have to review Tom's video for other materials, recall using something like 1000 for aluminum and brass.  It all boils down to surface feet per minute at the edge of the cutter.

Another use (though debatable by many) is go to back gear to lock the spindle for screw-on chuck removal and install.  Atlas recommends this method in their manuals, probably other manufacturers too.

Bruce


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## Ulma Doctor (Aug 23, 2019)

when using the back gears,
the spindle is being driven through the back gears even though the belt is still driving the pulley.
the spindle is disconnected from the drive pulley during back gear use.

i bought and rebuilt an Atlas TH42
the last owner tried to switch to back gear drive on the fly.
the problem is you have to pull the drive pin from the spindle, he didn't
it got ugly, quickly

don't be that guy


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## magu (Aug 26, 2019)

Back Gear! that's the word, it was Friday morning, what can I say. Thanks for the input gentlemen, I have occasionally used the one one my old clausing to slow the speed down when threading, I just didn't know if there was some other function I was completely missing.


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## epanzella (Aug 26, 2019)

On my Logan when changing chucks I would engage the back gear and the bull gear lock pin at the same time.  This totally locks the spindle as it is in two speed ranges at once. This is not something you would do to remove a siezed chuck that you have to beat off with a hammer as you could break a tooth in the drive, but for everyday chuck changes it works fine.


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