Runaway mini mill...

[voltsandbolts]

Symptom:
I plug in my Mini Mill (2X) and with the speed control pot in the off position I release the E-stop button and the mill immediately goes to full speed.

First noted:
When I completed milling a part and turned the control pot down to off with no results, it just kept running.

Fault?
Is it the pot or is there a deeper issue here?

Thanks for any info, if I can find a schematic I should be able to track down the fault.

Rob.

[LeeWay]

I'd suspect the pot. The sweeper may be damaged or burned off and not making contact anymore.

[Al_The_Man]

Disconnect the pot and you can tell right away, if you can identify the pot wiper connection (centre), only this one need disconnecting.
Otherwise remove all three.
Al.

[voltsandbolts]

Thanks for the input LeeWay, and Al.

[Jay C]

1 other thing, when you turned it to off, did it click? Can you turn it past the stop? Just suggesting that the knob may be at fault.

FWIW,
Jay

[digits]

I hope you guys aren't suggesting he disconnects the pot and fires it up to see if it still misbehaves? This is rather cheap mains voltage circuitry, and I wouldn't like to guess what'll happen if he fires it up with the pot in there at all.

By all means, remove the potentiometer, check its resistance with an Ohmmeter and replace it if necessary, but don't fire it up without anything in there!

Oh, and BTW, there may be something more seriously wrong with the control board - if it's anything like my X-1, the board should fault if started at anything other than zero speed on the pot.

[LeeWay]

Good point Digits.
It may be something like that wrong in the board itself and not even seeing the pot.
One other simple thing to check, though if Digits is right, it ain't this, is the knob itself loose on the pot? There is a tiny slotted set screw in the knob and it may be loose causing the pot to stay full on.

[Al_The_Man]

Originally Posted by digits I hope you guys aren't suggesting he disconnects the pot and fires it up to see if it still misbehaves? .

I don't see why not, All the drives I have ever used whether SCR mains fired, PWM or VFD will not do a thing with the pot disconnected.
Al

[Adamj12b]

It wont hurt anything disconnecting the pot from the controller and turning it on. I had did this a few time when I was trying to obtain control of the spindle on my X2 with Mach 3. With the pot disconnected from the controller the spindle will go full speed. The speed control works by outputting a 12 volt reference voltage on one pin, with the pot all the way to the off side, the wiper feeds the full reference voltage into the pin it is connect to (2 I think but dont quote me) and the last pin of the pot is connected to ground. When the pot is turned to increase the speed of the motor, it reduces the voltage going back to the controller giving it a value of where to run the motor. When there is no pot connected there is no voltage going back to the controller so it is just like having it all the way up.

On second thought though, now that I think of it, the switch carries a 120v ac signal to the controller, which if it is present when the power is turned on to the mill(e-stop opened) the mill not turn, until you return the pot all the way to zero. With some simple wire cutting and jumping you can bypass this, allowing the mill to be started at any speed.

Ok, so now that I have gotten way off track, it sounds to me, and from experience, that our controller might be bad, or there is a few lose connections. I hope this is of some help. Keep us updated.

[digits]

Originally Posted by Al_The_Man I don't see why not, All the drives I have ever used whether SCR mains fired, PWM or VFD will not do a thing with the pot disconnected. Al

Unless you know precisely what circuit the X2 uses, I don't see how you can say for sure that removing it is safe.

Even though these drives typically use a 0-10v or so signal for the pot, there is absolutely no reason why they can't be using 100-110v (relative to mains earth) as their voltage range (it's still obviously 10v) and I really wouldn't want to advise anyone to take apart anything that's likely to expose them to near mains voltages.

I don't know how the X-2 controller is wired, but if I had to answer the question 'What's the worst that could happen?' it would probably be something like you could get electrocuted and then it could catch fire while you lie quivvering on the floor. Saftey costs nothing IMHO.

[LeeWay]

They are wired a little differently than the run of the mill DC motor and drive like you see on treadmills and the like.
Some of the principles are the same, but the wiring and features on the X2 board are different as is the pot itself.

I didn't read it outloud, but when I was looking for boards to run my treadmill motor drive off computer, I noticed that the ones for Minimills and lathes were a little different. I didn't look to see what the difference was. Might be worth a look and to stroll over to CNC4PC or Homann designs and take a peak at the controllers specific to those.

[Al_The_Man]

Originally Posted by digits , but if I had to answer the question 'What's the worst that could happen?' it would probably be something like you could get electrocuted and then it could catch fire while you lie quivvering on the floor. Saftey costs nothing IMHO.

As the poster indicated he would like access to a schematic to trouble shoot implies a certain amount of respect and capability when servicing mains powered equipment and I assume by posting in this forum he intends doing his own trouble shooting, otherwise the prudent thing to do would be to call a service technician and forget about posting here.
By disconnecting the pot, I did not infer that he do this with the power on.
By looking at the Mini-Mill site, it would appear that this has a 110v DC motor, and the controller appears to be very similar to the SCR mains fired controller as fitted to Treadmills etc, they have the very same feature of not energizing the motor if the pot is at maximum at power up.
The only thing I find strange and contrary to all the drives I have come across is that the drive runs with no analogue input, this is contrary to the industry norm.
Al.