# Screw on lathe chucks -- threading in reverse



## Dorn (Feb 18, 2019)

I've often seen the recommendation to do single point threading with the tool upside down and running the machine in reverse.  For those of us with screw on chucks ( SB, Logan, Atlas, etc.) this is generally considered a no no because of the risk of the chuck unthreading.  One solution is to use a separate tool holder on the back of the cross slide to hold an upside down tool.  While that is in my long term plans I just implemented an easy solution that allows one to use a screw on chuck on the reverse direction. 

I made a T shaped fixture.  The cross bar of the T is sized to fit inside my chuck and is long enough so that it presses against the back plate.  The base of the T extends into the spindle and has the same threads as my collets on its bottom.  So I can use my collet drawbar to pull the T against the back plate of the chuck.  Because the threads on the collet drawbar and the threads on the spindle nose have different TPI, there is no danger of the chuck unthreading when I run it in reverse.  Of course this means that I can't run a long bar through the spindle but I think this will be good enough for me 90% of the time.  The other 10% of the time (when I have a long bar to thread at one end) I'll either set up the fixed steady or thread in the forward direction.

This also allows me to use the cutoff tool in the preferred upside down position with the lathe running in reverse.  Or, at least it will when I finish making a cutoff tool holder that fits my QCTP and holds the cutoff tool high enough.


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## markba633csi (Feb 18, 2019)

Good idea!
Mark


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## RJSakowski (Feb 18, 2019)

The major advantage of threading in reverse is to be able to thread away from a shoulder.  Simply mounting an upside down threading tool on the back side of the lathe will still thread towards the headstock.  

The may be some advantage regarding reduction of chatter but aside from that possibility, no reason to not thread in the conventional manner.
Personally, I have tried the upside down threading from the rear and not seen any advantage over conventional threading.

Using the drawbar through the tailstock to secure the chuck will work although it should be tightenrd securely.  The finer threads on the drawbar won't close as quickly as the coarser threads on the spindle open, resulting in the chuck loosening.  If I were using this approach, I would make a custom drawbar, with LocTite on the Tee nut and double nutting or a castillated nut aand pin to secure the drawbar at the outboard end.


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## Dorn (Feb 18, 2019)

RJSakowski said:


> ...
> Using the drawbar through the tailstock to secure the chuck will work although it should be tightened securely.  The finer threads on the drawbar won't close as quickly as the coarser threads on the spindle open, resulting in the chuck loosening.  If I were using this approach, I would make a custom drawbar, with LocTite on the Tee nut and double nutting or a castillated nut and pin to secure the drawbar at the outboard end.



RJ  
 I don't understand your objection.  The way I thought about it was that since the spindle nose is 8 TPI and the collet threads are 26 TPI, if the chuck were to slip it would be trying to move out along the spindle nose faster than the base of the T would be unscrewing in the back.  That is just going to make the T insert grab the back plate of the chuck even harder.  Am I missing something?


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## RJSakowski (Feb 18, 2019)

Dorn said:


> RJ
> I don't understand your objection.  The way I thought about it was that since the spindle nose is 8 TPI and the collet threads are 26 TPI, if the chuck were to slip it would be trying to move out along the spindle nose faster than the base of the T would be unscrewing in the back.  That is just going to make the T insert grab the back plate of the chuck even harder.  Am I missing something?


You are correct.  The chuck will be trying to loosen but the drawbar will be tightening, assuming the Tee nut isn't spinning in the chuck. My error.


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## macardoso (Feb 18, 2019)

Maybe I am a crazy one here, but I tighten my chucks onto my threaded spindle with a 6' 2x4 clamped in the jaws. I thread in reverse often and have never had a concern about the chuck working loose. I haven't done any crazy aggressive threading however.


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## hman (Feb 19, 2019)

macardoso said:


> Maybe I am a crazy one here, but I tighten my chucks onto my threaded spindle with a 6' 2x4 clamped in the jaws. I thread in reverse often and have never had a concern about the chuck working loose. I haven't done any crazy aggressive threading however.


That's definitely a way to get leverage.  But you should also consider what's "resisting"the leverage.  In other words, how is the spindle locked?  The "conventional" way to do this is to engage the back gear without disengaging the direct drive pin.  Doing this puts a lot of stress on just one or two teeth of the back gear system - a very nasty place to have a failure.

If you use a different method of locking the spindle, then there's probably no problem.


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## macardoso (Feb 19, 2019)

hman, thats' a really good point. I do use the backgear and pin method to lock the spindle. When I got the lathe, this was the only way I could remove its very stuck chuck. I've just been doing it ever since. I'll try to come up with another way, however I do want my chuck to be on there tightly so I prefer this over hand tightening


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## mmcmdl (Feb 19, 2019)

Put the lathe in neutral and smack your chuck with a soft hammer . Never use a pry bar . Cut OD threads torwards the chuck , if your afraid to thread ID threads to a shoulder , thread out .


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