Reversing switch

[Downunder Bob]

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.

When threading imperial threads on an imperial machine it's quite common and normal to disengage the half nuts as long as you don't disengage the reversing tumbler. Depending on the pitch being cut you can disengage the half nuts and re-engage according to the threading chart for that particular machine. as often as required..You just have to re-engage at the correct position on the threading dial, and it's all good.

 

[darkzero]

When threading imperial threads on an imperial machine it's quite common and normal to disengage the half nuts as long as you don't disengage the reversing tumbler. Depending on the pitch being cut you can disengage the half nuts and re-engage according to the threading chart for that particular machine. as often as required..You just have to re-engage at the correct position on the threading dial, and it's all good.

Yes, using the half nut is how I thread even for metric threads (imperial leadscrew). With metric threads and an imperial leadscrew (or vice versa) you can disengage the half nut. Difference is you have to reverse the spindle then reengage the half nut exactly where you disegaged on the threading dial to bring the carriage back.

What I was saying is to never disengage the leadscrew. Reversing leadscrew direction, like the OP was asking, disengages the leadscrew & will loose timing.

 

[RJSakowski]

To summarize:
Two reasons for reversing lead screw; left hand threading and power feed from left to right.

Reversing the spindle can have multiple applications. The ability to wind back when metric threading with an inch lead screw is a big one. Years ago, when I was making multiple miniature parts, I had two tools mounted on the tool post. I could perform one set of operations from the front side with one tool and reverse the spindle and complete another set of operations from the backside with the second tool. All this without losing registration.

I should inject that this was done on an Atlas/Craftsman 6" lathe with screw on chuck. I tend to tighten my chucks fairly tight. In forty plus years of using that lathe, I never had a chuck loosen. Well, once when my 602 was still on the shipping skid. I was playing around with it, getting familiar with the machine and had just screwed the chuck on hand tight. I flipped the switch to reverse and to my surprise, the chuck spun off. I have a ding in the ways as a visual reminder of the experience (fortunately close enough that the tailstock will never travel there). I was surprised because I had never experienced this before.