Mini Mill Stopped Working

My Harbor Freight 44991 Mini Mill stooped working on me. I was milling slots in 6061 and the head just slowed down and stopped. Now it will not start up again. When this happened, I'd just finished a cut and was lowering the head for the next pass.

The mill is about 6 months old. I've been using 6-8 hours per week since I got it. Mostly milling or drilling into AL.

Here's what I've seen/checked so far:

• The green power light comes on
• The amber fault light is not on
• The fuse is good
• I tested the potentiometer with an ohm meter. The amount of resistance varies as I move the knob. But I've never tested one of these before so I don't know if it's changing by the correct amount.
• I looked at the controller board. Everything looks good. No capacitors or resisters appear blown or swelled.
• All the connections appear good. No wires are loose, broken or frayed.

The only issue I've ever noticed before (other than busting gears when I forget to lock the head in place) is that yesterday I turned it off but the head continued to slowly turn. I turned it back on and off and then the head stopped. I used it for several hours last night and this happened twice. I used it for less than a half hour this morning and this did not occur.

Any ideas on what I can check next?

 

it's not waranty after 6 month. To my opinion your power transistor is burned.
That would be the one attached to the heat sink

good luck

 

One issue with checking transistors is they are hard to check in circuit. You could measure across the base and emitter, and then across the collector and emitter to see if you get a drop, but it will all be a little guess work. Its a little harder with a FET. All these steps require you to have the pin out of the component in question...which if they are Chinese then good luck! If you wanted to replace the component, you would have to desolder it and then test it with a meter. I would say its the control board with 100% certainty though. You checked all the obvious stuff.

If you want to go further you would need a scope to rule out the possibility of the motor, to check to see if you get a waveform on the output. A Regular meter will *not* show the correct value (simple RMS wont work because square waves are technically a bunch of sine waves added together-unless you know what to expect).

 

if it died when moving the head up and down it could be a wire going from the controller to the motor got broken.
my mini mill and mini lathe (both brushless versions) spin on there own sometimes because the potentiometers are so cheap they wear out. if i fiddle with the pot they stop turning.



if you have the brushed dc motor, you can simply check the output voltage right at the board while you adjust the speed knob. it should be about 90V i think. even though it puts out PWM at a constant voltage, the multimeter should read out a varying voltage. if oyu have voltage at the board and not at the motor, you have a bad wire. also when brushed motors stop running, it tends to be the brushes losing contact. unscrew the brush cap and clean up the brushes/ get new ones.

 

there is a big white resistor on the main board on the back of the mill. have the same problem with mine.
seems to be a common fault.

 

It was not because I was moving the head. I was only moving it about 0.050" at the time.

I ohm tested the resistors and they were good.

I sent the board off to olduhguy.com. He said "You had a broken toroid coil (factory problem)." I should get it back in a few days. I'll let you know if that fixes my problem. I think it will. That coil felt loose when I handled the board. But not being an electronics guy, I didn't know what it was supposed to be like.

Thanks for the help. Hopefully this tread will help someone with the same problem in the future.

 

Ah, sounds like a dry solder joint. Probably made worse by vibration. When you get it back, it wouldn't hurt to squeeze some hot melt glue around the toroid to glue it to the board so it can't vibrate loose again, if it hasn't already been done.

 

May or may not help. I have the harbor freight 7x10 lathe and have experienced the movement you described after shutting off. On the lathe it is a rotary switch and if I put some extra pressure to turn off, it will stop. Same as yours it is intermittent and does not happen all the time Ive thought of replacing, but seems to stop if I hit the off switch, so not really an issue I'm not sure which style of switch the mills have , but could possibly be just the switch.

 

Gary, I had the exact same problem with the toroid. I re-soldered it an it ran for a while. Eventually one of the MosFets gave out. Even though I ordered the replacements, I substituted a treadmill motor and controller. It was such an improvement, I left it on and never looked back.
Now my 7x lathe has issues blowing fuses so it's next for the conversion.

Chuck

 

Well it ran fine for three weeks. I noticed that it would shut off under a load a lot easier than before the repair. Tonight ran fine for a while then it started to shut off while making small cuts into 3/16" x 1/2" aluminum.

Assuming it's the toroid coil again, does anyone know the specs on this part so I can get a replacement?

People have mentioned replacing the controller board with one for a treadmill. Any ideas on where to get one and what to look for? What is involved it replacing the board? Is the wiring the same?

 

I used a MC-60 controller salvaged from a Freecycle treadmill. I have another running my sand muller for my foundry. As the Freecycle treadmills seem to have dried up here, I found an MC-60 on E-bay for $35. Haven't tested it yet, but it will replace the controller on my 7x lathe when the weather warms up. All you need to get the MC-60 functioning is an ac power cord with crimp on bayonet connectors, a 5K ohm linear pot, some wire, and switches to turn off the ac and stop the motor. If you need/want to add reverse, easily done with a double pole, double throw, center off toggle switch.

Chuck

 

I removed the board. It turns out that it was not the toroid coil. It was the two white things next to it (sorry, I don't know their proper name). One leg on each was broken. I was able to melt the solder, push out the broken pin, insert the broken end and resolder it. I gave it a quick test and so far so good.

I'm debating removing the controller box and attaching it to the wall in order to avoid the vibration. Had I done this when I first got the mill, I don't think I would have every had a problem in the first place.

If this thing breaks again, I'm getting a treadmill motor controller.

 

They're resistors, 0.33 ohms, 5 watt wirewound ceramic.
If the repaired leads don't hold up, these cost about 50 cents each at any decent electronics store.

 

My mini mill was acting strangely with intermittent start problems. At times I would activate the start knob three times before the machine would power up. I found poor wiring connections at the circuit board which I repaired with solder but the problem persisted. Last night I removed the pot (speed control) and found that the start switch attached to the pot was the problem. This pot looked like a cost reduced design and not robust enough for repeated cycling, so I replaced the it with a 50K pot for speed control and a separate switch for starting. The machine works again and by separating the speed and on/off functions it should last longer before these parts ever wear out.

While testing the machine with the pot removed I found that two 20K resistors in series can replace the pot speed control to provide 1/2 speed and the on/off wiring can be connected with an alligator test lead to start the machine. If the pot ever failures during an important job there is a simple way to make the mill work again to finish the job.

 

One further update.... I was curious about why the pot/switch failed so I opened it up. The plastic disc holding the wiper and the cam for the on/off switch was brittle and broken. Too bad as the rest of the switch looked like new.

 

All too typical of Chinese manufacturing. They use cheap materials for one small part that fails and makes the whole assembly useless.

 

How do you get hold of olduhguy.com,
Anyone have a phone number, can't get thru on internet,

 

Thanks, it worked with the address you gave me...??