Yesterday my 3520 threw a 20in maple block off, breaking the shaft of the tool rest and fracturing the live center shaft. The extreme vibration was horrible to see. ,The speed selector had been set to zero just a minute earlier. How it came to its fastest speed is a mystery. That I was not injured is a miracle. The speed selector has no resistance whatsoever. This same thing occurred 5 years ago. I want to know if other 3520 owners have had this happen and how they fixed the problem??? Do your 3520 speed selectors have any resistance to movement??? Thank you brother turners. PfD
For the lathe to run at near maximum speed, a voltage of nearly 10 V DC must appear at the second control terminal inside the door of the inverter. If the lathe ran at nearly full speed when the speed control was turned all the way down (counterclockwise), the indication is that there is a problem with the speed control circuit, namely the pot.
This assumes there is no problem inside the inverter.
First thing to do is measure the voltage at the speed control terminal - the second terminal, green wire. See the photo in the article. You do not have to have the lathe running to measure this voltage.
If the voltage is nearly 10 V and does not vary with the setting of the speed control knob, the pot is probably open close to the ground side. Details on how to check it are given in the article.
The knob of the speed control should turn easily, like the volume control on a radio or the knob on a light dimmer.
>Do your 3520 speed selectors have any resistance to movement???
The first statement sounds like an electrical issue but the second sounds physical. I assume you mean this is only a physical issue, right?
This situation is horrifying - I'm glad you are still around to write about it!
If your speed selector potentiometer rotated by itself due to vibration from the chunk of wood, it certainly needs some mechanical resistance/damping. I checked the knobs on my Jet 1642 which I think are similar and while they turn freely, there is a very slight resistance to movement - I can't imagine them turning by themselves. However I don't often turn large unbalanced chunks.
If it turns too freely, few things come to mind that could help: a thick felt washer behind the knob; disassemble and pack the inside with dielectric grease; add a soft spring thrust washer behind the knob preferably bearing against a very thin flat washer next to the knob.
On the other hand, if the speed jumped instantly from to low to maximum without the knob turning, it could be an electrical issue at the control. I can imagine this might happen if the terminals shorted inside the control housing. I would check that the control is securely tightened on the housing, that there is no mechanical wobble in the shaft, and examine for any bare or frayed wires inside that might touch. If it were mine, I would disconnect the wires, clip a meter inside, and check the resistance through the entire rotational range. If you don't have this experience, perhaps someone nearby does - the check would onlt take a few minutes.
On the Jet, when the selector is set at zero, nothing turns you're at zero rpm. Is the Powermatic different? Are you saying it accelerated spontaneously, with nothing being touched? Or did you brush the knob and it accelerated? On the Oneway, it actually has shut down the VFD with a large mass, and when the vibration was bad.
Did the speed go to zero, assuming it was running, when you set the pot to zero?
When it happened 5yrs ago did it just fix itself or was there a repair?
Mechanical resistance in a pot is very low but it is detectable. I'm wondering if the knob or shaft is turning but not turning the slider inside the pot such that the control voltage is not changing.
If the pot loses the bottom connection which is likely a ground reference either internal to the pot or a broken wire externally it is conceivable that the full 10V got applied to the inverter since there was no voltage divider action due to the break.
The 'dial-a-speed' buttons do go bad after a while, at least the one on my 3520A did. Same with the off/on button. I haven't heard of this happening though. Mine just started being erratic, so I replaced it.
I'm glad you weren't hurt. I'm not sure what your asking about the zero resistance. As was mentioned by someone else are you talking about zero resistance on the force it takes to move the dial, (mine takes very light pressure but there is some resistance to turning) If your talking about zero resistance as an electrical problem that is different.
I have not heard of this happening with any other 3520's that I know of. We have quite a few in our club and I know of lots of others. Also don't remember hearing about it on the Forum's.
I don't have one, but I've used plenty of them. I think I know what you mean.
The control knob on the early models was way too easy to turn. A butterfly wing could change the setting. Some people dealt with the problem by taking the knob off and putting an O-ring on the shaft and then putting the knob back on. This prevents inadvertent change of speed.
The early 3520's had a lot of problems that have more or less been dealt with.
Sorry, one and all, I had no idea the word resistance could mean an electrical term in my description. The speed control knob is free to turn without input by me. Did I touch the knob? Not to my knowledge. It is apparent to me that I need a new switch. But I also want to warn others of this inherent danger to life and limb.
Sorry, I am still confused. You had a block of wood in the lathe, it was at zero rpm, not running, and it just accelerated on it's own with you not touching or being near the lathe at all?
A very common problem on the 3520 is leaving the speed knob set at a higher speed when turning something small. Then using the on off switch to turn the lathe off. When you turn it back on of course it goes to the last speed you left. I have done that on mine. I finally started using the speed knob to turn the lathe off so that I wouldn't repeat that.
"The speed selector had been set to zero just a minute earlier. How it came to its fastest speed is a mystery"
Something caused the speed to go to maximum even though the speed control potentiometer was set to zero.
I was involved in a lot of failure investigations during my former working life so I don't discount anecdotal information because however improbable it might seem, there are many instances where the improbable actually did occur and following that lead resulted in solving the problem. In this instance, if the ground side of the potentiometer became open circuit, that could have been the reason. Why would it go open circuit? That would require further investigation, but it could be a failure in the pot itself or it could be the wiring to the pot.
One of the lessons to be learned from this is to not only depend on the speed control to stop the lathe, but to actually use the stop switch. In fact, the stop switch should be the primary method of stopping the lathe. If you are in the middle of a project where the lathe is being momentarily stopped, but will resume turning at the same speed, leave the pot alone and save wear and tear on it. Let the inverter take care of ramping the speed up and down.
Potentiometers do wear out. They have a rotating wiper that rubs on a carbon or conductive plastic element so that the center tap voltage can be adjusted from zero to max (don't recall what it is on the PM inverter). I have seen the pot used in the PM and it is just an ordinary pot that isn't really made for heavy duty use where it is continually being rotated.
EDIT: I just noticed that Doc Green had already pointed to the cause of the problem.
The lack of physical resistance (I think that friction would be a better term in this situation) is hard to say what it means without tearing the pot apart. I doubt that it is zero, but I am sure that you knew something had suddenly and very noticeably changed in its behavior -- one moment it felt "normal" and after the incident, it was different. That is enough to suspect that the problem is within the pot itself. Potentiometers are not expensive (unless you buy them from the lathe manufacturer). Instead go to an electronics jobber such as Mouser. For a replacement, I recommend getting a conductive plastic type. They cost quite a bit more than the basic carbon type, but they will hod up far longer -- probably longer than you or the lathe. If you have deep pockets then you can spring for a MIL spec pot that will survive anything but a direct atomic blast.
Seems strange to me that everything worked fine for 5 years between incidents. My lathes see a fair amount of vibration in 5 years. Surely a ground issue would have reoccurred with more frequency. Part of my confusion is based on the lack of other reports like this. Just seems improbable.
It may be off-topic a bit, but I've touched the speed control knob by accident on several One-Way lathes and rotated to full speed by brushing my hand against the knob. In the past I've suggested placing a felt washer under the knob to give it some resistance to prevent this from happening.
Further to Bill's good instructions I ordered a replacement potentionometer yesterday and it will be here on Tuesday. The cost, including shipping, is less than $20 so I am looking forward to a safe-to-operate PM 3520 once again. The tech specifics for the part came right off the face of the one in place so ordering one was easy. If you have any doubt of the operation of your speed control/potentionmeter then I suggest you order a replacement ASAP. Philip
