# Speed going variable? Loss of torque?



## zeeprogrammer (Dec 16, 2009)

I have a 7x12 mini-lathe, import...the kind you can buy through several vendors but made at one factory. Cummins (ToolsNow) 5278.

A few days ago I noticed the spindle speed would suddenly vary. Usually up and down while I changed the pot, but even when I left the pot alone.

My initial thought was a dirty pot.

Today I was turning .005 at a decent speed and the motor stalled. That doesn't seem like much of a cut to do that. (I don't recall actual speed...but it wasn't so slow that I would think it was that.)

Any thoughts? Just a dirty pot? Or is the control board going?

As a side note, it's been pretty dry here and I build up a nice static charge when I go to the machines. (I have lino tiles on the basement floor). I've been getting zapped quite a bit. In fact, today, the lathe was on, direction was set forward, and pot was set to 0 speed...i.e. off. I zapped it, the motor suddenly started and then stalled.

Fooey...still have the flywheel to do. Maybe I'll just make an insert and use the flywheel off the Launch Engine.


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## Cedge (Dec 16, 2009)

Carl
Had that problem myself from time to time. Adjusted the motor brushes and cured it. The first time i figured I was in for replacing the controller, but the guys at micromark solved the riddle in about 2 seconds. 

You can adjust the torque and the low end speed by making adjustments to the pots inthe control box. I think instructions are still on minilathe.com 

Steve


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## zeeprogrammer (Dec 16, 2009)

Thanks Steve.

Adjusted the motor brushes? Or replaced?
I'll check out mini-lathe.com too. Thanks.

Hm...one other guy mentioned that possibility too. Maybe he DOES know what he's talking about...sometimes anyway. ;D


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## black85vette (Dec 16, 2009)

I think the lathe and mill motors are similar. First just tighten the caps holding the brushes in. It is critical that they be tight. It's an easy first check


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## Cedge (Dec 16, 2009)

Tighten them.... I eventually replaced them but they didn't really need it. A small PITA, that.

Steve


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## bearcar1 (Dec 17, 2009)

Now I don't know about the brush holders being loose, that certainly could be a suspect, but if that does not eliminate the problem, my vote would be a cold soldered joint on the motor speed control board. This would require the opening of the box/panel that houses the board and carefully and deliberately moving the mounted components/wires, looking for the trouble to clear or to occur again. I have had this happen to two of the controllers on my Sherline units. It was hair pulling doing any kind of work until I diagnosed and re-soldered the bad joints.

BC1
Jim


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## zeeprogrammer (Dec 17, 2009)

I tightened down as much as I could. Seemed to improve things but it's still hit-n-miss as I slowly turn the pot.

Is it interesting that this problem seemed to pop up as the air around here got dry enough to produce a lot of static?

Is it worthwhile to take the brushes out and clean them?

Could this explain why I'm having such problems cutting? I've been at this long enough now (if 10 months is long enough) that I know about rubbing, cutter too high or too low, speed and feed...but I can't seem to get a decent finish. I had more luck with my old carbide tools that came with the lathe. The HSS are not working well. Carbide inserts seem okay. Facing seems to go well. I've tried a half dozen or so cutters.

Your help is much appreciate...thanks.


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## Twmaster (Dec 17, 2009)

I had the red version of that from Harbor Freight. My speed controller blew up a week out of warranty... 

If it is your controller www.repaircontrol.com can fix it for about $55. There is a troubleshooting guide here: (PDF) http://www.littlemachineshop.com/Reference/DriveTroubleshooting.pdf

LMS should stock the brushes if you need them. I have a good motor if you need it Zee....


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## zeeprogrammer (Dec 17, 2009)

Thanks very much Mike.
According to the PDF, the brushes are still the most likely.
I'll try to keep people posted with what I find.

Thanks again veryone.


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## black85vette (Dec 17, 2009)

Zee, if you decide it might be the control board let me know. I had to replace the mosfets and rectifier on my 7x10. It was not difficult.


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## zeeprogrammer (Dec 17, 2009)

Thanks black85vette.

I got some brushes ordered for overnight. A day without my lathe? No way!!!

The most difficult part of replacing mosfets and rectifiers (or any electronic part) is finding and getting the part. A soldering iron, and companion solder sucker ;D, are probably the two tools I'm most familiar with.

Well I'm also familiar with those tiny pry bars...er...screwdrivers.

And groveling...wait...that's a skill.


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## black85vette (Dec 17, 2009)

zeeprogrammer  said:
			
		

> I got some brushes ordered for overnight. A day without my lathe? No way!!!
> 
> The most difficult part of replacing mosfets and rectifiers (or any electronic part) is finding and getting the part. A soldering iron, and companion solder sucker ;D, are probably the two tools I'm most familiar with.



LMS has the mosfets but not the rectifiers. I was able to cross reference the rectifier and bought 2 so I would have a spare.  If you need to order one let me know.

Since you already have new brushes on the way you might try something. I had some success on DC motor brushes. Sometimes they get "glazed". Take some 600 grit emery paper and rough up the end of the brush. Then put it back and let it run without a load for a few minutes to let it seat a little bit. This is another easy to-do but does not work every time.


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## zeeprogrammer (Dec 17, 2009)

Thanks. Yeah I was thinking that went it came time...I would take the old brushes, inspect them, see if I could do anything to improve them and try them again.

Still, I'm happy to have spare brushes on hand and they fit the mill as well.


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## black85vette (Dec 17, 2009)

Just in case you ever need it, I looked up my order. Got the parts from Mouser Electronics.

512-GBPC1210W 12A Bridge Rectifier $2.45ea 
A little bigger than the orignal but fits fine.

511-STW20NK50Z N-Ch 500 Volt 17 Amp $3.60ea
Mosfet for motor control board. Replace in pairs.


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## zeeprogrammer (Jan 1, 2010)

Bit of an update...

I replaced the brushes and thought there was some improvement but still not happy. I thought I'd go with the potentiometer next.

I didn't have a chance to do much with the lathe until today. But before replacing the pot I thought I'd run the lathe some more. Hm...seems better still. Perhaps the brushes have seated better.

I think I'll wait and see.

Couple of questions though...for any of you that have a similar 7x mini-lathe...

There seems to be a delay between rotating the pot and a change in spindle speed. No problem when you change the pot slowly...but a 'quicker' twist and the response is slower. I don't know why I hadn't noticed that before...maybe I hadn't done it. Do any of you have the same response? I wouldn't be surprised by it...just want to ensure it's normal.

With the spindle running very very slow, I slowly applied some pressure to the chuck (I hope I don't get a gasp here). I could feel the torque build up and then it stalled. Again, not sure if this is normal or not. How much pressure? I have no idea.

Thanks all.


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## black85vette (Jan 1, 2010)

zeeprogrammer  said:
			
		

> There seems to be a delay between rotating the pot and a change in spindle speed. No problem when you change the pot slowly...but a 'quicker' twist and the response is slower. I don't know why I hadn't noticed that before...maybe I hadn't done it. Do any of you have the same response? I wouldn't be surprised by it...just want to ensure it's normal.
> 
> With the spindle running very very slow, I slowly applied some pressure to the chuck (I hope I don't get a gasp here). I could feel the torque build up and then it stalled. Again, not sure if this is normal or not. How much pressure? I have no idea.



Zee; both my lathe and the mill build up speed and not instantly. I suspect it is a design of the motor control and not a bad idea on a small machine. Rather than applying full torque all at once and maybe shearing off a gear. They apply it gradually.  I also think this is what you are experiencing at low speed and holding on to the chuck.  On a DC motor you are not going to get a lot of torque at lower speeds.  Again a good idea in case something jams or is stuck the motor will stall at start up rather tear up the machine.

On the brushes, I think I would turn it on mid range and let it run for 20 or 30 minutes. Brushes do need to seat to fit and run well.


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## John Rudd (Jan 1, 2010)

I think there may be something wrong with the motor control board ( but I could be wrong..)

The torque is determined by the back emf of the motor...Theoretically it should remain fairly constant throughout the speed range...

I use a a 20mm milling cutter in my minimill at low speeds without any probs..


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## black85vette (Jan 1, 2010)

John Rudd  said:
			
		

> The torque is determined by the back emf of the motor...Theoretically it should remain faily constant throughout the speed range...



On my mill there is a pot labeled "torque".  Could it just be an adjustment? I have not messed with it.


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## gmac (Jan 1, 2010)

Zee;
These may be of some help if you get into the control board;

http://www.varmintal.com/alath.htm#Slowing
(scroll down to the title "Slowing Down The Speed")

http://www.dcmotorcontroller.net/

I'm watching this thread - don't own a 7x12 but do own an X2....

Good luck
Garry (a newb following in your wake.)


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## John Rudd (Jan 1, 2010)

black85vette  said:
			
		

> On my mill there is a pot labeled "torque".  Could it just be an adjustment? I have not messed with it.



Yes its an adjustable setting, If you want to 'play' with it, just make sure you mark the original position of the pot so you have a datum


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## Kermit (Jan 1, 2010)

Pulse width modulation hits the motor with the FULL voltage, so you develop FULL field strength. At low motor speeds the time in which the motor is getting full power becomes a shorter duration than the time during which the motor is getting no power. An unpowered motor when driven by mechanical means becomes a generator! 

The magnetic fields built up by the motor currents grow and collapse much slower than the electricity moves, so the next motor drive pulse comes in and keeps things going in the same direction, but not before the motor has slowed itself down, by generating reversed electricity. Enough inertia remains in the spinning mass to keep things chugging along at a reduced speed.

The back EMF could be causing your troubles, the currents being felt at the brushes are not steady and the extra surges are creating more arcing and sparking on the brushes and commutator. This(low speed operation) would be much harder on the mechanical parts just mentioned because of that.  With out special protection devices these pulses can destroy transistor junctions and other electronic things as well. Result is extra 'stress' on those high power silicon junction devices. Meaning a higher number of units manufactured will have power problems somewhere down the road. Building them with better electronic protection equals increased manufacturing cost. (not gonna happen at the rock bottom prices chinese wholesalers are seeing) Just put Mosfet devices in which have much larger margins of 'withstand' voltage. Cross your fingers and ship them. Many will do fine, but lots more will fail than we consumers think should.  

Blackvette has the solution. Repair and replacement of parts will get you going again. Zee, periodic maintenance including removal of brushes and cleaning of motor parts you can reach will extend the life of those brushes. 

For the rest of us, we have to cross our fingers as well, and hope the stresses of operation are falling on 'healthy' silicon and mechanical devices that can stand the abuse. ;D 

I'm not a wiseass, but I play one on the internet,  
Kermit


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## Noitoen (Jan 1, 2010)

You could add an RC network and a Varistor to help with the high voltage spikes. A high speed freewheel diode across the motor (before the reversal switch) also helps protect the control mosfets.


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## zeeprogrammer (Jan 1, 2010)

Thanks Rick. Yeah I'm thinking it just hasn't run in enough. We'll see how it goes over the next few days.

I wouldn't think I should have to adjust the torque pot. On the other hand...I've never tested the system to that extent...maybe it wasn't right in the first place. I doubt that though.

John, yeah I would expect the torque to build if I hold the chuck. And always good to remind people to mark position before moving anything...not just pots.

Kermit...he he he he...riiiigght. ;D

Noitoen...hm...not for me. I'd need to see it have been done or know a whole lot more about the circuit before I go playing with it.

Thanks all.


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