# Lathe continues in forward even after selecting reverse



## ddmunroe (Jun 17, 2014)

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


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## John Hasler (Jun 17, 2014)

ddmunroe said:


> 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.
> ...



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.


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## JimDawson (Jun 17, 2014)

+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.


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## GarageGuy (Jun 18, 2014)

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


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## JimDawson (Jun 18, 2014)

GarageGuy said:


> 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.


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## Marco Bernardini (Jun 18, 2014)

JimDawson said:


> 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".


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## Tony Wells (Jun 18, 2014)

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.


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## ddmunroe (Jun 18, 2014)

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


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## fastback (Jun 18, 2014)

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


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## den-den (Jun 18, 2014)

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.


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## John Hasler (Jun 18, 2014)

JimDawson said:


> 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.



I don't see that this circuit is going to work.  When you throw the switch from forward over to reverse it appears to me that you end up with both windings energized and the capacitor out of the circuit.  This will result in the motor continuing to run in the same direction while drawing more current.




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



I think this circuit will work, but it isn't the same as the other.

The motor is special at least in that it has a double-throw centrifugal switch.


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## ddmunroe (Jun 18, 2014)

I've used the term "instant reverse required during threading. This is being differently interpreted to how I meaning it in actual use.

This method of using your lathe to cut threads is carried out all the time. (does anyone else know this is common practise ?)
BUT
It really is a matter of some coasting to where you want to reverse, "then reversing. 
In this case most of the inertial loads are reduced therefore not breaching any structural limits of lathe parts.

Lots of comments have been made about damaging the machine. I'm starting to understand that you have the picture of some one willy nilly smashing their machine in reverse at any speed thinking this is ok.
Describing "it's like banging a car in reverse while moving forward is totally off topic :banghead:
So really what's needed if you have a capacitor motor is a method of circuit disconnection at the precise point stopping, then reversing.
This has been described previously by the astute.
In future I will be slowly gravitating toward VFD, brushless motors and controllers. Look forward to sharing my results.
dd


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## UncleRuss (Jun 19, 2014)

Mr. Hasler,

Very good point in your last post.  Yes the motor will continue to run in the same direction if switched too rapidly, not allowing the centrifugal switch to close.  Also good eye on picking up the fact a two pole centrifugal switch is being represented in the second drawing and that being the magic of the second diagram.  Going to have to slow down as I missed both points in reading the first time through.

May I ask what is the matter with machining a groove at the end of where the thread is to stop as is the old practice.  Or as I do quite often is to stop the lathe anywhere near the end of the thread and using a die by hand to finish off.  That is of course if you have the proper die.  I rather like the looks of the thread finishing in a groove.  Looks more professional to me.

Instant reverse is not very good for most anything I know of
Carry on Gentlemen, always more then one way to skin a cat.  Now pile up that _*SWARF!*_


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