# Freshened up my Mini Lathe



## Cedge (Oct 16, 2008)

My little Micormark lathe is now three years old and it's had many a mod to it and many more cuts made on it. I'd recently noticed it had developed a couple of annoying tendencies, but it still went to right to work..... when asked nicely. 

The motor had developed a "skip" in certain RPM ranges which varied the spindle speed as much as plus or minus 40 rpm. The other problem was a noticeable loss of torque, especially when taking anything more than a medium cut and cutting off was becoming a challenges to keep the lathe running without hitting overload and shutting down. Like I said... operable, but annoying none the less. 

The prevailing wisdom on the "skip" is to tighten the motor brushes, but this was not solving the problem. The brushes are still good and the springs are still good as well. I began to suspect the control board was developing a glitch and misfiring the square waves that push the motor. More of a gut instinct than any true troubleshooting.

I popped out on the net and searched for ways to increase the torque. I found several mechanical approaches, but I was drilling dry holes for information about adjusting the control circuit for anything but its lowest speed. I finally gave up on the search and decided to go out there and make it right or make it worse, but one or the other was going to happen. 

I used a wooden stick from a cotton swab which I trimmed to work as a screw driver to begin making test adjustments. (less chance of accidental magic smoke release) After a couple of false starts, I soon began to get clues as to what each of the three pots were adjusting. One adjusts low end speed.. ( now set for 20 RPM) Another adjusted both torque and speed while a third adjusted only torque. These last two had to be played against each other to find the sweet spot. 

It was easy to test the torque by placing my hand on the chuck and attempting to stop it. Too low and it stopped quite easily and too high took it into the lands of intermittence. It took about 10 minutes to find and verify the sweet spot.

I then loaded a notoriously tough piece of 1.5 inch aluminum and tried a few turning cuts. I stopped pushing it at .060 depth of cut, without the machine shutting down. Next I loaded up the cut off tool and slowly worked up to a fairly aggressive cut rate before the lathe stalled and shut down. Checking the cut off tool revealed it was not as sharp as it could be, so I swapped it for a T shaped blade I recently acquired. 

Cutting off is not a huge problem here, but that T blade and the extra torque made it some kind of sweet. It never bogged down during the rest of the cut. Hot roll steel gave similar results as long s the cut was well lubricated 

The little 7 inch lathe now performs "better than new", even while driving a 5 inch chuck under an aggressive hand. You wouldn't believe the depth cuts it made using the diamond tool... I'm chuffed!!.....LOL 

Steve


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## Brass_Machine (Oct 17, 2008)

Thanks for the info Steve... I may go down and 'tune' my mini lathe.

Eric


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