Mini-lathe speed setting without control knob

[dml66]

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.

 

[markba633csi]

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

 

[Mitch Alsup]

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...}

 

[dml66]

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.

 

[dml66]

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.

 

[markba633csi]

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

 

[dml66]

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 .

 

[markba633csi]

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)

 

[dml66]

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.

 

[markba633csi]

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-