Lathe continues in forward even after selecting reverse

[ddmunroe]

Hi guys,

My switching works well but ..
While my lathe is still turning slowly forward, when I click it into reverse and it still rotates forward.
The reason I did this or want to do this is to quickly reverse when threading close to a shoulder.
One time it kept going and panic set in even at slowest speed selected 65 rpm "and was coasting down
It will reverse as long as your not over "say 20 rpm

Question: Why doesn't it reverse immediately and 2 how can I tweak it to reverse immediately.

Thank's for any input
dd

 

[John Hasler]

Hi guys,

My switching works well but ..
While my lathe is still turning slowly forward, when I click it into reverse and it still rotates forward.
The reason I did this or want to do this is to quickly reverse when threading close to a shoulder.
One time it kept going and panic set in even at slowest speed selected 65 rpm "and was coasting down
It will reverse as long as your not over "say 20 rpm

Question: Why doesn't it reverse immediately and 2 how can I tweak it to reverse immediately.

Thank's for any input
dd

That's how capacitor-start single-phase induction motors work. The phase relationship of the current in the start and run windings is what determines in what direction the motor starts but once it is running at more than about 20 rpm the centrifugal switch disconnects the start winding.

 

[JimDawson]

+1 what John said. This is one limitation of single phase motors.

You might be better to put a manual foot brake on the lathe with a switch that would also cut motor power. Maybe a motorcycle brake attached to the motor or the spindle.

The other option would be to install a 3 phase motor and a VFD, this would give you nearly instant reverse.

EDIT: It looks like there is a way to do this with a single phase motor. This also may be a special instant reversing motor diagram, I'm not sure.




This diagram looks to be the same electrically, but is a bit more clear and easier to read.

 

[GarageGuy]

Even if your motor could reverse instantly while still moving, that could possibly damage your machine. With all the gears and heavy chucks, there is a lot of rotating mass in a lathe. Instantly reversing it could break teeth off your gears, or spin your chuck off the spindle.

GG

 

[JimDawson]

Even if your motor could reverse instantly while still moving, that could possibly damage your machine. With all the gears and heavy chucks, there is a lot of rotating mass in a lathe. Instantly reversing it could break teeth off your gears, or spin your chuck off the spindle.

GG

You raise a really good point. I hadn't thought of that. Now I like the brake idea even better. Even though I can instant reverse my lathe, I have never actually done it, not sure what would happen. I have a D1-4 chuck so it won't spin off, but it sure could tear up gears.

Thank you for pointing this out.

 

[Marco Bernardini]

You raise a really good point. I hadn't thought of that. Now I like the brake idea even better. Even though I can instant reverse my lathe, I have never actually done it, not sure what would happen. I have a D1-4 chuck so it won't spin off, but it sure could tear up gears.

Thank you for pointing this out.

You can try to put your car in reverse while still moving… then please tell us what the garage will tell you!
:roflmao:
Instant reverse is used just on boats, since they don't have brakes, but even that is not actually "instant".

 

[Tony Wells]

Even though it is actually a rapid decel/accel cycle, larger machining centers are able to do this for a rigid tapping cycle. But then, those are not simple induction motors.

I have a bp clone that is a 2hp single phase, and have that issue while power tapping. If I don't wait until the speed is slow, naturally, it just picks back up to speed in the same direction. Only once in a long while will I break a tap, but it is a risk.

For a lathe, a foot brake with a power cutout switch would seem to be the best solution, but probably not the easiest to implement. You'll find yourself accustomed to using it quickly.

 

[ddmunroe]

You won't tear anything up
If you have a screw on chuck maybe it could unscrew but it would have to be loose to begin with.

Because I'm implying threading speeds ... these are low.
Job shops that use manual lathes do not disconnect the half nuts and watch a dial line up etc.
They instant reverse as this is the most productive method of threading, approx 200-300 rpm depending on length of job. Albeit they used a brake in conjuction at the critical timing stop. Was mentioned in previous post
I have first hand experience as I was one on the floor that did it but my lathe has a totally different setup (capacitors etc).
Due care is always taken if the part is a large diameter you don't instant reverse.
I'm talking up to 2 inch medium length
Anything over 12 inch at 200-300 rpm In.. Reverse will make a large lathe jump around the floor.
Pretty much have gotten my answer about why my lathe continues when lever switched in reverse
T Y)
dd

 

[fastback]

While 3 phase motors are capable of instant reverse it really is not something that should be done. It is one of the best ways to destroy a motor never mind the drive train on the lathe. I know a lot of professionals that so this, but in most cases they don't own the machine and really don't care about the machine. Or maybe they just don't know any better.

Paul

 

[den-den]

If you have a VFD, it can be OK to switch from forward to reverse once the acceleration / deceleration parameters are set to suit your equipment. It will not be "instant" reverse as the VFD will decelerate the motor by reducing the frequency down to zero and then increasing the frequency in reverse. Mine is set up for a 5 second ramp 0 - 60 hz (when threading at 12 hz it takes one second). If I added a braking resistor, could reduce the time by half or more.