# Mini Mill Stopped Working



## Gary_in_Ohio (Dec 24, 2014)

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?


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## canadianhorsepower (Dec 24, 2014)

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


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## Nerdz (Dec 24, 2014)

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).


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## MCRIPPPer (Dec 24, 2014)

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.


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## Justmental (Dec 24, 2014)

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.


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## Gary_in_Ohio (Dec 29, 2014)

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.


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## /// (Dec 29, 2014)

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.


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## Naiveambition (Dec 30, 2014)

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.


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## chucketn (Dec 30, 2014)

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


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## Gary_in_Ohio (Jan 23, 2015)

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?


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## chucketn (Jan 24, 2015)

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


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## Gary_in_Ohio (Jan 24, 2015)

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.


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## /// (Jan 25, 2015)

Gary_in_Ohio said:


> 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).



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.


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## DJP (Jan 31, 2015)

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.


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## DJP (Jan 31, 2015)

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.


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## Gary_in_Ohio (Feb 1, 2015)

DJP said:


> 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.


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## Clinton Egbert (Jul 21, 2019)

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


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## Cogsy (Jul 22, 2019)

Clinton Egbert said:


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


The site works fine for me - just click on the 'Service Detail' tab and his address and email show up too: http://www.olduhfguy.com/


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## Clinton Egbert (Jul 22, 2019)

Cogsy said:


> The site works fine for me - just click on the 'Service Detail' tab and his address and email show up too: http://www.olduhfguy.com/


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


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## Cogsy (Jul 23, 2019)

Clinton Egbert said:


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


Your link was missing the 'f' in UHF and I wondered if that was your issue.


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## Rocket Man (Jul 23, 2019)

Chinese parts are top quality they manufacture parts for American companies according to American specifications.  When American factories receive a shipment of Chinese parts American quality control tests them before they are sold or used.   Buy a new replacement part ohm your part so you know what value you need make sure it has a switch and stem the length you need that part will come in 75 different sizes, options, shaft length, etc.  I buy most of my electronic parts on Ebay but there are several American companies selling parts that all come from China.  Parts buy China parts it takes 2 to 4 weeks to arrive 50 parts of 99 cents free shipping.  American resale companies buy Chinese parts and resell them at a higher price mailed to your door in 3 to 5 days plus you pay $7 to $10 extra for postage.   Go to the Electronic forum there is a list of Electronic parts sellers in America.  You can also ask for help some one can find the part for you in 60 seconds then provide you with the link to buy it.

https://www.electro-tech-online.com/


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## Clinton Egbert (Jul 23, 2019)

Cogsy said:


> Your link was missing the 'f' in UHF and I wondered if that was your issue.


Ya, I noticed that!!


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## trlvn (Jul 25, 2019)

nm


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## peter2uat (Jul 30, 2019)

I'm wondering - the pic shows a 4K7 pot and not a 50K one - not being an electronics expert I suppose you would get a somewhat different response from the mill than before? I opened up mine which has those troubles with starting too and found the same 4K7 pot , so I will try the same steps you have taken.  Just waiting for some cooler weather to go shopping for the parts. In the meantime I inserted a 15A/125V (10A/240V) DPDT switch into the motor-line and now I have the option to run my mill counterclockwise - might come in handy somedays.


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