Speed issue on Powermatic 3520 lathe

[Acharya Kumarswami]

I've been using our Powermatic 3520 lathe for 8 years or so. I have always had it set to the high speed range.
I was able with that setting to slow down to as low as 250 or so. Recently that changed. Now the slowest that it will spin at the high range is 700rpm or so.
Anyone have knowledge about this area on the PM? Also the red reset button on the control box on the back of the head does not seem to do anything now.

There are no instructions in the manual about that box, so it's a bit of a mystery to me.

Thanks.

[Dennis Ford]

The speed problem sounds like a bad potentiometer to me.

[Joe Kaufman]

If you have the Delta VFD-S series inverter

1. With the stop button pushed, watch the display on the inverter while you slowly twist the speed potentiometer full range. The value displayed is the speed reference in Hz and should range from 1.1 to 135 or so. If your speed pot is bad, the value will probably stop about 29Hz / 700rpm as you turn the pot from max to minimum. Mine ranges from 1.1 to 136. The drive software will stop motor operation at 5.1 Hz. on mine. If the value doesn't change uniformly, change the pot.

2. Try exercising the pot full range at least 10 times and see if that improves the situation.

[Neal DaMommio]

If it is a potentiometer issue you could try DeoxIT to see if that helps. DeoxIT is used to clean the pots on electronic equipment as well as other things of that nature.

[Acharya Kumarswami]

thank you. this is helpful. what do you mean by exercising the pot full range? (Frankly, I'm actually not clear where this part is located.)

[Acharya Kumarswami]

thank you. this is helpful. what do you mean by exercising the pot full range? (Frankly, I'm actually not clear where this part is located.)

in case my original post wasn't clear, Previously, on the high range, "I was able with that setting to slow down to as low as 250 or so. Recently that changed. Now the slowest that it will spin at the high range is 700rpm or so." Instead, the speed jumps immediately to about 700. I can run it faster than that, as normal. But It won't spin slower than 700rpm.

[Joe Kaufman]

Sorry I wasn't clear. The pot refers to the speed control on the front of the machine. Turn it from the minimum to the maximum range at least 10 times. I would first check the drive display on the back of the headstock as I stated in the first response. Start with the speed control fully clockwise/max speed and slowly rotate counterclockwise to the minimum speed. The display should read 135 or so at max to 1.2 on so at minimum. The speed reference is about 29Hz at what should be approximately 700 rpm in the high speed range. If the reference signal doesn't go below the 29 and down to 1.2 then most likely it is the speed potentiometer on the front of the headstock.

[Bert Delisle]

It sounds like the potentiometer has gone bad. When you look for a replacement don't go cheap, the good ones (military scones) cost about $8.00 and the cheap ones about $1.50. I had a potentiometer fail once while turning, it went from turning speed to max instantly. I excersized the potentiometer as others have suggested and it worked for a while but the failure mode to high speed prompted me to get a good new one pronto. Another woodturner had a similar incident when he thought his unit was at minimum when starting a large off balance piece. It went to full speed and when it flew off the lathe it broke the tail stock live center morse taper #2 shaft!! Very lucky that no one was injured as he was demonstrating how to turn very large pieces, start slow.... potentiometer fail..... and all hell breaks loose.
Equipment components can give forewarning of impending failure, the trick is to recognize it and take appropriate maintenance action. Stay safe and have fun.

[Mike Goetzke]

All the talk about "exercising" the pot made me think. On the 3520B I have I've seen some set the speed and use the on/off when they need to stop the lathe and others turn the speed dial down to stop. I have gotten into the latter habit. Seems dialing speed up/down may keep the pot clean? Or wear it out faster?


Mike

[Bill Boehme]

This doesn't sound like a defective potentiometer issue to me. I would be more inclined to check the programmed parameters. There is a possibility that could have somehow been reset to factory default values rather than the settings that Powermatic uses. If you have a multimeter checking out the resistance of the potentiometer is easy, You can get a decent multimeter for a fairly low cost.

[Brice Rogers]

Bill, it sounds like the little wires in the pot have a break in them. I have a Griz G-0766 with a similar controller and mine acted exactly the same. Sometimes Occam's razor applies: the simplest solution is often the correct one. But your suggestion to check the resistance of the pot is a good one. I suspect that checking it end-to-end will show an "open" . On the Griz, it takes about 5-10 minutes to open it up to check. Probably best to check it "out of the circuit".

[Bill Boehme]

Bill, it sounds like the little wires in the pot have a break in them. I have a Griz G-0766 with a similar controller and mine acted exactly the same. Sometimes Occam's razor applies: the simplest solution is often the correct one. But your suggestion to check the resistance of the pot is a good one. I suspect that checking it end-to-end will show an "open" . On the Griz, it takes about 5-10 minutes to open it up to check. Probably best to check it "out of the circuit".

Brice, you are thinking about a wirewound pot which is mostly suited for set and forget type of service or in power applications. That is basically ancient technology. If a wirewound pot were used the characteristic indication of a break in the wirewound element would be minimum speed or full speed depending upon which side of the break the wiper is on. The type of pot used in this application would be either a carbon film type or a conductive plastic type. By far the best would be the conductive plastic because of low noise, ruggedness, and very long life.

My suggested check of the pot would be an end to end test first followed by checking the wiper to one end over the full range of rotation. Then check wiper to the other end. The pot is typically 10K so the wiper should go from near 0 to 10K. The only way to check it is out of circuit. It's fairly easy to disconnect the wires at the inverter if there isn't a connector in the harness or at the pot itself.

[Brice Rogers]

Bill, does the Powermatic 3520 lathe use a conductive polymer? My G0766 uses a wirewound and when I replace the pot, I would like to upgrade to a better type of pot.

[Acharya Kumarswami]

This doesn't sound like a defective potentiometer issue to me. I would be more inclined to check the programmed parameters. There is a possibility that could have somehow been reset to factory default values rather than the settings that Powermatic uses. If you have a multimeter checking out the resistance of the potentiometer is easy, You can get a decent multimeter for a fairly low cost.

I am pursuing the replacement of the Pot. But I was also wondering about the settings on the control box on the back of the lathe. I do think they got changed inadvertently. Does anyone know what the settings should properly look like?

[Dwight Rutherford]

Check out Doc Greens website. If he doesn’t have programming parameters for your Powermatic listed. Bet he can help.
http://www.docgreenwoodturner.com/articles.html