# Mini-lathe speed setting without control knob



## dml66 (Oct 26, 2022)

Lathe is a HF 7x10.

I need to turn a bunch of parts at the same speed setting, lets say 200 rpm.

What I'd like is to turn the lathe on and have it run at 200 rpm without having to get close every time with the speed knob.

So, on=200 rpm, off=0 rpm.

Any tips or tricks to making this work as outlined?

I think I could place an SPST switch inline with the speed control knob on/off to fool the controller so it "thinks" the knob is off to start with but, I may be wrong. 

Thank you.


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## markba633csi (Oct 26, 2022)

Depends on how repeatable you need it to be- putting a sharpie mark on the dial would be a simple way.  Or, if you want more precision you could install one of the inexpensive aftermarket tachs which would give a direct speed readout
-Mark
You could also plug the lathe into a power strip and turn it on and off that way and leave the speed control in one position- however,
some lathe controllers need the user to turn the control to zero at each start up so that may not work for you


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## Mitch Alsup (Oct 26, 2022)

Simple, put an On-Off switch in front of the speed control but after the contactors (if any).

{Although your typical speed controller changes speed a little bit with temperature and definitely with load...}


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## dml66 (Oct 26, 2022)

markba633csi said:


> Depends on how close to the speed you need to be- putting a sharpie mark on the dial would be a simple way.  Or, if you want more precision you could install one of the inexpensive aftermarket tachs which would give a direct speed readout
> -Mark


Thanks. The machine has a tach already which was installed by the previous owner.

What I'm most after is not having to think about the speed; just on and off. If the speed varies +/- 10 rpm or so each time, that's fine. I have enough other turning operations I need to focus on, I don't want spindle speed to be one of them.


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## dml66 (Oct 26, 2022)

Mitch Alsup said:


> Simple, put an On-Off switch in front of the speed control but after the contactors (if any).
> 
> {Although your typical speed controller changes speed a little bit with temperature and definitely with load...}


That sounds like it would be just what I need, I'll examine the switch further to see if I can make it work as outlined.

Absolute speed is not critical, 200 +/- 10 rpm is fine.


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## markba633csi (Oct 26, 2022)

If your lathe is not one that requires you to turn the control to zero at each start up, then you are in luck. If not, then you would need to
go into the controller and try to defeat the "Nanny" circuit.  Possibly you could put a switch to open the upper leg of the speed control pot or some similar trick


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## dml66 (Oct 26, 2022)

markba633csi said:


> If your lathe is one of those that doesn't require you to turn the control to zero at each start up, then you are in luck. If not, then you would need to
> go into the controller and try to defeat the "Nanny" circuit.  Possibly you could put a pushbutton to open the upper leg of the speed control pot or some similar trick


Mine has the nanny . I'm so used to my VFD-driven drill press; turn it on, set the speed, turn it off, turn it on, it's at the same speed .


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## markba633csi (Oct 26, 2022)

I was thinking you could add a switch in the + dc line to the speed pot, then opening the switch would drop the control voltage to zero, fooling
the controller into thinking the knob was turned to zero.  I don't know if that would do what you wanted since the spindle may still be turning slowly with the pot at minimum-  (note: the pot might be some other value than 5K ohms)


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## dml66 (Oct 26, 2022)

The speed control pot has an on/off position, its the dark area between 0 and 1. If it's clicked on and the on-off toggle is switched to on, no-go, no matter what the speed setting is. If it's clicked off and the on-off toggle is switched to on, the speed control works.


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## markba633csi (Oct 27, 2022)

OK so the question then would be: If the on/off switch was somehow separated from the pot, could the pot be left at some preset speed and immediately turn on when that switch was enabled?  We don't know.  You might have to take the wires from THAT switch out to a separate switch and see.
It seems that the "nanny" isn't the act of turning the pot to zero, but instead, activating the pot's on/off switch. That's my guess anyhow
If that experiment worked, then you could wire the power switch (178) to take the place of the pot's switch.  In theory. 
If it was my machine I'd certainly try it-


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## tq60 (Oct 27, 2022)

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.

Sent from my SM-G781V using Tapatalk


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## RJSakowski (Oct 27, 2022)

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.


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## dml66 (Oct 27, 2022)

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.


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## tq60 (Oct 27, 2022)

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.
Sent from my SM-G781V using Tapatalk


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## dml66 (Oct 27, 2022)

tq60 said:


> That does not sound right.
> 
> Turning on should close or connect the 2 terminals, 120 volts to where?
> 
> ...



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.


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## dml66 (Oct 28, 2022)

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.


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## markba633csi (Oct 28, 2022)

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


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## dml66 (Oct 28, 2022)

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


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.


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## markba633csi (Oct 28, 2022)

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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