Reversing switch

[DavidR8]

My South Bend came with a reversing switch for the motor.
And there’s reverse for the lead screw.

In what circumstances would I reverse the motor instead of use the reverse gear?

Obviously there’s a risk of spinning the chuck off.



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[francist]

Threading where you don’t want to disengage half nuts (metric, eg). You can wind back by hand but reversing the motor is obviously faster.
Also some prefer a rear mount threading or boring tool for better visibility on internal bores, so spindle needs to run in reverse.

There’s probably others reasons I’m missing, something with a tool post grinder maybe?

No, you don’t want to accidentally spin the chuck off...

-frank

 

[Downunder Bob]

No, you definitely don't want to spin the chuck off. I have seen some lathes with screw on chucks that have been modified to prevent it from spinning off. You might want to look into that. there may be some members on here who have done that modification.

 

[MrWhoopee]

Backing out after tapping a hole comes to mind, as well as cutting a right hand thread from the inside out and tapping a left hand thread. The risk of unscrewing the chuck is not nearly as great as it might seem. These low horsepower belt drive lathes don't usually accelerate fast enough.

 

[T Bredehoft]

Drill and tap a hole into the side of the chuck to intersect with the threads on the spindle. Put a soft brass rod in the hole and run a set screw down on it. Don't forget about it when removing the chuck.

 

[MrWhoopee]

Drill and tap a hole into the side of the chuck to intersect with the threads on the spindle. Put a soft brass rod in the hole and run a set screw down on it. Don't forget about it when removing the chuck.

The brass rod needs to be retained/attached in some manner to prevent loss when the chuck is removed.

 

[pontiac428]

There’s probably others reasons I’m missing, something with a tool post grinder maybe?

Yes, this is a necessity when doing external grinding with a tool post grinder.

 

[ErichKeane]

The brass rod needs to be retained/attached in some manner to prevent loss when the chuck is removed.

On my Logan lathe, I just made the set-screw out of brass. Its a pretty simple project (since you have a lathe!) and made it so I didn't have to worry about losing the little brass plug.

As for the original question: While threading, you cannot reverse the lead screw, otherwise the spindle-to-leadscrew relationship is lost. There are some occasions (metric threading in particular, though there are others*) where you cannot disengage the lead screw, since the thread dial doesn't have the correct resolution/relationship to the thread. In those cases, people typically will just reverse the motor to come out of the cut.

I also have used reverse to do boring into a blind hole (cutting on the opposite side from you makes seeing what you're doing WAY easier!) as well as internal threading. Finally, power-reverse is REALLY nice when removing a tap or die from a hole.


**Another interesting case, my lathe has both 1/2 and 1/4 thread sizes. The thread dial only works in the 1/2 TPI cases, not the 1/4.

 

[Bi11Hudson]

Lots of good answers on why one would reverse the lathe. But I don't see a good answer for why you would reverse the leadscrew.

The use of the leadscrew reversing lever is to reverse the relationship between the spindle and the leadscrew. With the spindle turning counter-clockwise(normal forward) the forward drive of the leadscrew cuts from right to left(toward the chuck), Reversing the leadscrew will cut from left to right(away from the chuck) That's why my first modification to an Asian 9X19 was a way to reverse the feed. See photos at http://www.hudsontelcom.com/9X20Gear.html The intent was to gear down the speed of the spindle. But there are good photos of the reversing mechanism.

There are several reasons for doing this, the most prominant being cutting a left hand thread. Starting the thread at the chuck end with the chuck turning CCW and the threading tool cutting away from the chuck, the thread will come out backwards. Left hand, or reversed.

Knurling is another use, for me. With the leadscrew running out, there is no way to crash the tooling into the chuck. Saves running the lathe in reverse. I do have a pin to "lock" the screw-on chuck, but this way prevents an accidental loosening. And is much faster set-up time.

Reversing the entire machine is a wholly different issue. Running in reverse means the tooling must be upside down or on the backside of the cross-slide. In my case, my machine is very loose and I cut small threads. 24 TPI is large for me, 40 and smaller is the norm. The easiest way to maintain registration of the thread is to back the tool out and reverse the lathe. Then run the tool back in and set depth of the next cut. Essentially the same as if cutting a metric thread. It's a pain in the wazoo to thread this way but beats rebuilding a 70~ year old machine that I don't have the time, or money, to do.

I could go on with this answer but won't. It should explain what you are asking well enough.

Bill Hudson​

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[darkzero]

In what circumstances would I reverse the motor instead of use the reverse gear?

I use reverse for other reasons not mentioned but I don't have a threaded spindle.

In regards to threading like already mentioned. If not using the threading dial & disengage/engage the half nuts to make the next pass, you'll need to reverse the spindle to bring the carriage back for the next pass. If you reverse the leadscrew & run the spindle forward to bring the carriage back you will loose the thread timing.

Don't ever disengage the leadscrew when threading regardless if you are using the halfnuts or not until you are finished threading. I was never taught this & found out the hard way. I did & lot of cussing that day & some tools may have got thrown around but I learned a valuable lesson & have never done it again.