Mini-lathe speed setting without control knob

Easy....

First, if separate power switch no work needed there if it is built into the speed control then justa but more work needed.

My description can be used for both, describing just the speed control, if power also needed then just do same for the switch on the speed pot.

Locate a place where a toggle switch can be installed.

SPDP for speed, 2P2P for speed and power.

Also locate another pot of approximately save value as speed control.

You will need to do some voltage measurements to confirm things, but we will ASSUME the speed control is connected between a supply voltage and ground with the center "wiper" providing the signal to the controller.

If this is true, then locate a spot to mount second pot, a multi-turn pot would be better as it is less likely to drift.

Connect a wire from each end of existing pot to new one so they are parallel.

STOP

Test lathe to make sure still operates normally then unplug and wait minute.

Disconnect the wire from center of existing pot to center terminal of switch, a new wire from outside terminals of switch to the center of each pot.

Now you can select existing that you can adjust on fly or second one.

The second one is your fixed speed, pot can be hidden or otherwise.

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Another possibility would be to replace the pot with one which has a push/pull for on/off. This would remove the requirement of rotating to the off position. If possible, get a pot that has a double throw switch so you can wire it for pus for off rather than push for one. Then it will also function like a E stop in an emergency.
 
Below is a picture showing the back of the speed control. The two terminals circled in red are the on/off switch. When the switch is in the OFF position the contacts are closed, 120v appears on both terminals with the lighted main on/off switch turned on. Once the speed control is rotated to the ON position, 120v is no longer available at terminal 5.

lathe switch.jpg
 
That does not sound right.

Turning on should close or connect the 2 terminals, 120 volts to where?

Pull the plug and measure with ohm meter.

We had one of these long ago and forgot the fine details.

If the switch on knob stops motor then the main power switch may be that, main power then the pot is STOP then speed.

You should be able to turn the knob to some place and just flip the switch for it to run.

If you do something like we described above you would need to use a 2P2P switch and route the wire on the back of the pot through the other side of the switch so when you use your preset pot the switch on the back of the pot is not connected.
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Last edited:
tq60 said:
That does not sound right.

Turning on should close or connect the 2 terminals, 120 volts to where?

Pull the plug and measure with ohm meter.

We had one of these long ago and forgot the fine details.

If the switch on knob stops motor then the main power switch may be that, main power then the pot is STOP then speed.

You should be able to turn the knob to some place and just flip the switch for it to run.

If you do something like we described above you would need to use a 2P2P switch and route the wire on the back of the pot through the other side of the switch so when you use your preset pot the switch on the back of the pot is not connected.
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Click to expand...

120v is measured from each speed control switch's terminal, referenced to neutral.

I did disconnect the lathe from power and measured the resistance across the two terminals; it shows closed circuit when off, open circuit when on.
 
OK, figured it out this morning, amazing what a cup of joe can do.

Terminal 5 on the speed control switch goes to the controller board. When the lathe is powered up with the speed switch off (closed circuit), that applies 120v to the controller board terminal which in turn activates a latch circuit. I suspect the latch is one of the relays on the board, I can hear it click. Once it's latched it doesn't matter if the speed control switch is on, off or even disconnected from controller board, the speed control will start, stop and run the lathe.

So, the solution in this case is either a switch that, when on, applies 120v to the latch circuit, or just hard wire the latch to 120v. I'll probably wire it with a switch in case I miss the nanny.
 
So essentially you can just short the two terminals together? It never needs to see an open circuit?
Certainly a simple fix if that's the case- just put a clip lead like Barney from Mission Impossible LOL
 
markba633csi said:
So essentially you can just short the two terminals together? It never needs to see an open circuit?
Certainly a simple fix if that's the case- just put a clip lead like Barney from Mission Impossible LOL
Click to expand...
Yup, I shorted the two terminals for now, works just as I'd hoped it would :). I figure it doesn't hurt to leave 120v on the latch circuit since that's the state it's mostly in anyway, when the lathe is off.

My only other concern is the duty cycle of the main on/off switch, it's going to see a lot more use than it did before.
 
Well if it wears out just short that one too and use a power strip! ;)
If one short is good two must be better ha
 
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