# $10 Add-on Lathe Reverse Switch



## TwelveFoot (Dec 30, 2015)

Looking around a little, wondering if I can add a switch to reverse my lathe. It's as simple as switching the two wires to the motor, right?

>This switch< does just that; but due to some woeful marketing failure it's advertised as the middle switch when you have three switches running one light.

If anyone knows of a reason why these two should not be wed, speak now or forever hold your peace.



Wikipedia said:


> A "4-way" (intermediate) switch is a purpose built double pole, double throw (DPDT) switch, internally wired in manufacture to reverse the connections between the input and output and having only four external terminals.


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## TwelveFoot (Dec 30, 2015)

And while we're on the subject.
How much of a gimmick is reverse on a lathe?
Does anything keep the chuck from unscrewing?


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## Alchymist (Jan 2, 2011)

Electrically it will work, depends on the size of the motor you are reversing. Most DPDT light switches are not motor rated.

4 lathes in the shop, none reverse. Not much need except in special situations.


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## TwelveFoot (Dec 30, 2015)

2hp, 110v. According to the I.D. tag the lathe draws 12 amps, that whole wall of the shop runs off a 20a breaker, so no problem in that regard. Does "not motor rated" just mean they don't phrase it that way?


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## Alchymist (Jan 2, 2011)

TwelveFoot said:


> 2hp, 110v. According to the I.D. tag the lathe draws 12 amps, that whole wall of the shop runs off a 20a breaker, so no problem in that regard. Does "not motor rated" just mean they don't phrase it that way?


No. Switches for use in motor circuits are rated for the horsepower they are designed for. Here's an example:

http://www.delcity.net/store/Heavy!Duty-Motor!rated-Toggle-Switches/p_789066


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## TwelveFoot (Dec 30, 2015)

Not trying to say you don't know what you're talking about, but that page doesn't say anything about horsepower. Just "motor rated, heavy duty". And horsepower and amps are basically just different measurements of the same thing.

Amps x volts = watts
Watts are a measure of work done, as is horsepower.

So for that switch I started with 20A x 110V = 2200W / 745 = 2.95HP


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## Alchymist (Jan 2, 2011)

TwelveFoot said:


> Not trying to say you don't know what you're talking about, but that page doesn't say anything about horsepower. Just "motor rated, heavy duty". And horsepower and amps are basically just different measurements of the same thing.
> 
> Amps x volts = watts
> Watts are a measure of work done, as is horsepower.
> ...


Not quite, try this link:
http://www.carlingtech.com/amp-hp-volts

or do a google search on "motor rated switch".

And HP and amps are not the "same thing" Lots of other factors involved.


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## TwelveFoot (Dec 30, 2015)

Alchymist said:


> And HP and amps are not the "same thing" Lots of other factors involved.


Ah well, it was worth a shot. :shifty: I did say "basically".
I did search "motor rated switch" earlier, everything just looked like product listings. But *now* we're getting somewhere. Per your link:


> Switches that are going to be subjected to high-inrush inductive loads, such as an AC motor, will often be rated in horsepower in addition to volts and amps. This rating reflects the amount of current the switch contacts can handle at the moment the device is turned on. An AC motor will draw up to eight times its running current when first turned on or when held stationary while energized (stalled rotor).


My lathe is variable speed, the way I've got it worked out in my head it's going to be set up: wall -> lathe -> existing power switch -> speed control -> DPDT reverse switch -> motor. The addition of the DPDT being the only change.

The lathe has a safety that keeps it from starting from the power switch, it must be started by turning the speed up from 0. So there shouldn't be any significant power surge. The DPDT is never going to be switched while powered.

Thanks for your time, hope I'm not being a bother.


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## tewitt1949 (Nov 26, 2013)

I may have missed something but I don't think it is just that easy as just adding a switch to reverse the motor.

If that is the case, you can just unplug it and turn the plug around/over and plug it back in. Some motors are reversible but you will have to get into the motor and extend wires to reach the switch.


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## TwelveFoot (Dec 30, 2015)

tewitt1949 said:


> I may have missed something but I don't think it is just that easy as just adding a switch to reverse the motor.
> 
> If that is the case, you can just unplug it and turn the plug around/over and plug it back in.


Not just any switch, a DPDT switch







There's a circuit board for the speed control, flipping the plug would probably fry that. Which is why I'd be adding the switch past the board, just before the motor.



tewitt1949 said:


> Some motors are reversible but you will have to get into the motor and extend wires to reach the switch.


I'll have to get into the body of the lathe, but not into the motor itself. There's 6" of wires coming out of the motor (enough to reach the board), so I can just unhook those and move them to the new switch. I'll just need another 6" of wires to go from the board to the switch.


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## Bill Boehme (Feb 9, 2014)

Since your motor runs on 120 vac that says that your system is not a VFD driving a three phase AC induction motor. So, do you know if the motor is a DC motor or a universal motor? Also, is the control unit outputting DC or chopped AC? The likeliest case is that your motor is a universal type that has both a wound rotor and wound field. If that is the case, you will need to get into the internal windings so that you will be able to change the phase relationship between the rotor and field windings ... In other words you are in essence reversing either the rotor leads or the field leads, but not both. As already mentioned, the DPDT switch is the right switching configuration, but the wrong service rating. Motor switches have special requirements ... on startup there is a large inrush current, but even more significant is that a very fast acting large air gap snap action switch is needed when the switch is turned off. The reason is that the inductive load of the motor has to dissipate a lot of stored energy from the collapsing magnetic field. If you use an ordinary switch, that energy creates a very high voltage that arcs across the switch contacts and either burns them up or welds them together.

My advice would be to not attempt this modification unless you know what you are doing.


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## TwelveFoot (Dec 30, 2015)

Bill Boehme said:


> So, do you know if the motor is a DC motor or a universal motor? Also, is the control unit outputting DC or chopped AC? My advice would be to not attempt this modification unless you know what you are doing.


I was waiting for you to come through Bill. Concerns noted, project pushed back pending more research.



Bill Boehme said:


> As already mentioned, the DPDT switch is the right switching configuration, but the wrong service rating. Motor switches have special requirements ... on startup there is a large inrush current, but even more significant is that a very fast acting large air gap snap action switch is needed when the switch is turned off.


I think I've got that covered?


TwelveFoot said:


> The lathe has a safety that keeps it from starting from the power switch, it must be started by turning the speed up from 0. So there shouldn't be any significant power surge. The DPDT is never going to be switched while powered.


Thanks for putting up with me. :smile:


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