I've got a 1940's era South Bend workshop 9"x42" lathe, which has a big GE instant reversing motor (which seems to be common on these).

My question, being not familiar with these, can you slam right from forward to reverse? I don't think its covered in my old SBL books.

My understanding is it going into brake mode until stopping, then reverses?

Aynone know for sure?

Don't know for sure but I've heard yes.

No real reason to, unless it's to stop the spindle faster.

I've also heard to be careful of your chuck spinning off if you do it.

I've heard that as well, about the chuck unscrewing itself.

I've never had a reason to do it, just wondering. Was thinking it would be nice for tapping things and backing off the tap.

For tapping you may look into making a crank handle.

I made one. Fits into the headstock hole (Far left) and I tighten it like a tailpipe expander.

Then you can tap by hand and thread externally up to a shoulder w/o worrying about a crash.

I don't know anything about an "instant reversing motor," but what you refer to is generally known as plugging. HUGE debates over whether this is ok or not. Search some of the machinist forums for "plugging."

IMO - based upon limited experience - it's fine. I don't do it 'cause I'm more worried about taxing my phase converters than anything else, plus the only reason I'd need to at home is power-tapping on my mill w no Tapmatic. We did it on an old Southbend all the time at school, and my instructor used to work at the local railroad where they had a small army of SB's, where they did it on all of these machines. I simply didn't have the patience to wait for it to slow down, and my instructor encouraged me to reverse it to stop it, so I did!

Electric motors take a ton of abuse. In the end I feel it's one of those things that in theory seems horrible, in practice motors still last for decades.

It's a good way to fawk up the threads on your headstock. Ask me how I know.

Use at your own risk. My experiences were not good.

BTW-wasn't the 9 X 42 really, really, really rare? I've got three "Depression Busters".

There is no need to plug... not for me anyway. My machine has a footbrake, and i can stock the thing on a dime once it hits neutral.

ONLY do this at LOW rpm period. IF your chuch is a screw on type I would make sure it's secure once you stop it after every time you reverse it. I have had a 8" chuck fly out of a CNC lathe (not my fault) and almost hit me it the face if it would have took the right bounce. Would have killed me. Was spinning @ 3500 rpm. Scarred me so god damn bad my legs were shaking

I was in the middle of changing the chuck and the lathe. This cnc lathe had a bad glitch, where the spindle would come on at the last set rpm in the program when the chuck was in the closed posistion. We all knew this so we would program the slowest rpm possible at the end of every program we ran on that lathe. It was random to boot. Sometimes at the end of the program it would just run at that rpm instead of stopping. My boss had just ran a job on it but forgot to program the slowest rpm possible. He was running parts @3500 rpm. The way the chuck bolted to the spindle hub, it would seet in the hub after being bolted up for a while. easiest way to "pop" the chuck off once the bolts were removed was to hit the foot peddle closed then open real quick and it would pop it off into your hands.

I'm talking about a 60/70+ LB chuck. This particular day my boss said change the chuck and bolt up the collet holder and setup for another job. The spindle was not coasting, it was stopped. I jogged it a few times starting and stopping, with the chuck closed, was stopping fine. So I get ready to catch it, hit the foot peddle to close and just before I hit open again it went wide fucking open, I was in a funny posistion and hit the peddle again! it popped off right in my hands in a second. Had no time to react. Shreaded my left palm and left arm, not horrible bad thank god but took meat off. It didn't hit the tailstock ways but fell into the machine. Had it hit the ways and bounced the direction it was rotation I would not be typing this. I never ran that lathe again or set it up. Told my boss I would not unless he got it fixed. He never did. What I'm saying is it's NOT fun to have a chuck flying at you.

It's a good way to fawk up the threads on your headstock. Ask me how I know.

Use at your own risk. My experiences were not good.

BTW-wasn't the 9 X 42 really, really, really rare? I've got three "Depression Busters".

Not sure about the rarity, I love the tag on it that proclaims "This machine conforms to the orders of the wartime production board". I found a guy selling a whole garage woth of stuff (moving out of our nicely economically depressed state), its in real nice condition for $400, with boxes of tooling, steady rest, 3 and 4 jaw chucks and drive plates, and a workbench. Couldn't pass that up.

I haven't ever tapped anything with it, I'd only be doing that on low speed operation anyway, or use my mill/drill for it.

Just mainly a curiosity thing. Hard to find info on the way the instant reversing motors work. Somehow different than a regular reversable motor.

run alot of machines, tapped alot of holes, reversed alot of electric motors and one thing is pretty constant over that time....

going slow.

while the electric motors themselves never seem to mind it is usually another part of the machine that takes the abuse, a key on a shaft, a spline shaft, a belt, a lathe chuck that's threaded etc. etc. can suddenly or over time just FAIL.

If I can't let a motor fade into a gradual almost stop..going very slow now... then engage the reverse after a couple revolutions then I will usually try a different approach.

Inertia is a bitch. :eek:

I can't say I have used many lathes with chucks that thread on, but that seems like a bloody death waiting to happen. Not sure why anyone would even make one besides a purpose built machine.:shaking:

I can only think of a few reasons why you need to instantly reverse a lathe chuck.

1)Tapping a hole.
up to about 7/16 the top 2-3 threads of most spiral POINTED taps (which is a good choice btw) is not any good really anyway. No reason to "bury" the tap since your are probably just widening up the starting top of the thread anyway. After 7/16 many taps have a smaller shank the diam. and generally taps with a smaller shank will have good threads the entire length.

I will also add that taps..almost all but specialty taps just do not enjoy suddenly going backwards. You have a long chip in front of that flute, usually the backside of flutes get stuff in there from the cutting edge behind it. I will always just take 2-5 more minutes to bury a tap into blind hole by HAND in lieu of digging out a broken tap, or part of tap, and taking 1/2 hour trying to save a part I have hours into already. Most "bottoming taps" are 4 flute taps..I don't like "power tapping" any 4 flute unless it's like a 3/4 inch..and a 3/4 inch tap suddenly bottoming out into a blind hole will usually break something besides your tap.

2)Chasing/cutting a thread
An old school trick I learned from a really smart guy was to reverse cut threads. Basically never disengage feed on the machine. Not all machines can do this, but most can. It comes in handy on inaccurate lathes that never seem to find the same spot twice. But you also have to know a bunch of things about a good thread, how to measure the fawk out of it, having a good set-up with your tool, cross slide etc. etc.

Again when coming to the end of a thread (the shoulder) I would always let the machine slow down a few turns before I went backwards. If I need to cut it right to end then you need to move fast and be set up to do so.

Most everyone who I have witnessed quickly reverse the motor on even a standard few horse bridgeport mill has never had to tear the head apart to fix it. Big difference between .5 seconds and 1 second over time.