Lathe control, can it be done?

Good morning. I have an older lathe (1962), a large DC motor controls the speed of the spindle. The speed controller is a dual potentiometer, which is no longer produced. I am thinking there should be a way to control the speed via a microcontroller such as an arduino.

First let me explain how the dual potentiometer works. It consists of a 10k pot and a 50k pot. As I understand things, each of these pots are only active for half the rotation. For example, the 50k pot is active and varies the resistance during the first half turn and does nothing the rest of the turn, whereas the 10k pot does nothing for the first half turn, then varies the resistance during the rest of the turn.

So, is it possible to develop a control which can take the place of the above two pots or perform their independent actions by turning one knob? My thought is through programming a controller could be tasked with varying the resistance of one circuit through the first half turn, then once a certain point is reached, it no longer varies the resistance of that circuit and after above point is reached it begins varying the resistance of the second circuit.

I would think this should be possible. Thoughts??

Hi @medic29,

Welcome to the forum!

Do you have any specs on the motor? Do you have a broken speed controller or a photo of one? How is the controller supposed to be wired to the motor?

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I find the concept of a DC motor controller with a control for each half of a revolution not very likely. In addition, why would the two pots be different value.

Is the current device working? Can you describe the results of turning each pot separately?
Can you see the wiring of the two pots?

Does the DC motor have any nameplate information?

So my Dad had an old Hendey Lathe, well older that 1962. I believe it had a 14" swing and a 1 1/2 hp motor.

Can you tell us the size of the lathe?

Hi, what specs are you looking for in regards to the motor. It is an old GE Kinamatic 5 hp motor. There isn't a speed controller as you are probably thinking of one; no circuit board with other electronics, etc. This was way before circuit boards were a thing. The speed controller is a dual potentiometer, it controls the armature voltage during the first half of turn, then the other pot controls the field voltage. The power going to the motor also involved thyratron tubes.

The pots are wired to a terminal block in the electronic power unit compartment of the lathe. I can share the schematics if that will help.

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The control of the lathe was revolutionary back in 1962; the only one controlled this way that I'm aware of. The electronics contained 3 thyratron tubes (one smaller which controlled the field and two larger ones which controlled the power sent to the motor - as I understand it). Unless you're older you may not understand the electronics; this is pre-circuit boards. Resistors, diodes and rectifiers, and vacuum tubes were the main electronics with various transformers of which this lathe has eight. :slight_smile:

The 50k pot controls the armature voltage of the motor through the first half of the turn of that pot, then it does essentially nothing. The 10k pot does nothing during the first half turn, then it varies the voltage of the field during the second half. The pots have different values because they are controlling two different things which in the end affects the speed of the spindle.

The armature voltage is around 251vdc and the field voltage is roughly 34vac.

Again, this was state-of-the-art at the time it was created and was manufactured from the early 40s until the early 80s. The drive systems did change in the later units and became more updated using more electronics I believe.

But I believe you are trying to think in reference to current DC motor controllers; whereas the entire electronic power unit here is the motor controller.

I'm not asking anyone to try to change how the motor is controlled; rather I'm trying to see if there is a more up-to-date way to perform the functions of the dual potentiometer? Nothing more.

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This is a precision tool lathe. 10" swing 5hp GE kinamatic motor.

May I suggest you replace the TWO pots with two individual pots that you can control individually. There are rotation stops you can use to limit the rotation of individual pots. I presume there is a knob of some sort on the pots.

Yes, the lathe is currently working. I'm in the process of doing some troubleshooting. It wasn't working properly, but we have it running. It isn't perfect yet, but pretty good. I'm still short about 500 RPM of the max 4000 RPM. I also have to troubleshoot some of the relays which control some of the levers which allow me to engage the back gears, change the threading speeds, engage the cross feed and apron. I found one of the transformers was bad and two of the other transformers were 400v and I only had 240v coming into the lathe. Since it is near impossible to find replacement transformers to replace the two which drive two of the thyratron tubes, I just added a step up transformer (220v to 440v) and they now work correctly. This has been an interesting project to say the least. Way more than I originally expected. The speed of my first lathe was controlled by flat belt pulleys and the motor speed was constant, this is a bit different. That being said, there are very few who actually understand how this drive system works and unfortunately those folks are dying off.

Hi Paul, yes, the dual pot can be replaced by two individual pots; this is what I did to determine if my issue was the dual pot or something else. The dual pot setup is essentially one pot sitting on top of the other with the control shaft going through both and the carbon or fiber disc attached to the central shaft. If you google dual potentiometer you will be able to see a picture.

Again, I'm trying to find a way to control the function of these two pots by turning one knob, which makes it easier when trying to adjust the speed of the spindle. I could have two knobs and have to adjust one, then the other to get the proper spindle speed, but I prefer not to have to do that. This is where I was thinking a micro-controller would come into play. :slight_smile:

In my lifetime, I have seen and disassembled many dual potentiometers. I have boxes of old pots and many are dual. I have never seen one as you describe it. Has to be a custom design. You have not mentioned the manufacturer, is it stamped somewhere?

Regarding controlling the pots.

If the pots are both near a ground, or at least a voltage you can reference an Arduino to, you might look into a digipot.

Something like a "TPL0102 Two 256-Taps Digital Potentiometers". The likely issue is the digipots have limited voltage capability which may work in place of your pots.

If a digipot does not work, see if the voltage across the current pots is constant and DC. If it is you can feed voltage into what would have been the POT wiper. With a high voltage opamp this could be an option.

Hi Paul, the pot was manufactured by Allen Bradley and from what I was told it was specific for this lathe and not a common off the shelf production item.

How does this affect the question I have?

@JohnRob , see my post #5 for voltages

Have you checked any of the "home shop machinist" sites to see if anyone has that part laying around?

No one seems to have suggested this yet.

What about using two servos driven by your microcontroller to simultaneously adjust the two lathe pots? This is a very mechanical approach, but at least has the virtue of simplicity.


This is the pot


I am thinking I have not been clear..... I'm trying to see if there is an UPDATED way to electronically replace what these pots are doing via a microcontroller.

NOT -- replace the current setup with the same type of setup. Yes, I currently have a dual pot which is working the way I described. It is over 55 years old. These parts have a limited lifetime or life expectancy.

If the task a dual analog pot cannot be replaced utilizing a micro-controller and other parts, then okay, I just find that hard to believe that something they were able to do over 60 years ago, we can't do today.

I have thought of something like that which is a possibility.

I understand, but you must think of replacing WHAT the pots are controlling, not just the resistors. Is your device varying some voltage, or as I suspect, varying the AC frequency? 1961-1962, I worked in a roofing manufacturing company that used a room full of thyratron controlled variable frequency power for the speed of their 200 ft. long manufacturing line. I only glanced into the room one time. there were controls to adjust the speed of each of 3-4 sections.
Thyratrons were replaced by SCR controls and then even those were obsoleted.

@JohnRob , see my post #5 for voltages

The voltages I was referring to are the voltages on the pot.

Unless I'm missing something the plan would be to disconnect the old pots (or maybe use a switch so you could go back to them in an instant) and replace them with something that is Arduino controlled.

To do this we need to know the voltages at the pot (all three terminals) relative to ground.