Sherline (Microkinetics) motor problem

[tr4252]

I have a Sherline/Microkinetics benchtop 3 axis vertical mill, and it's giving me problems lately with programs which have run OK in the past. I'm cutting out parts from 5/16" 6061 aluminum with an 1/8" 4 flute end mill; 2000 RPM, feed rate about 1" per minute, depth of cut is .015" per pass. Slotting, in other words.

I've noticed the machine "lurch" occasionally when traveling in the Y axis. It seems to happen at the start of the move, as far as I can tell. In checking the machine, the screw turns without binding or undue friction, backlash is about .004", gib is not set too tight. I found that when pushing the table by hand (when the machine is at rest), there is axial play in the motor shaft itself, around .050". The shaft normally stays in place, apparently held in its position by the magnetic force in the motor, and will return to its normal position when I stop pushing against it. The X axis motor does not do this. Coincidentally, I'm seeing the tool path deviate occasionally on the Y axis, and scrapping parts.

I must admit, I don't have a clue, and am no expert in these matters. The OEM hasn't been able to advise me, though I just noticed the movement of the motor shaft and hadn't mentioned it to them when I spoke with their support person.

I'd appreciate any advice I can get.

Tom

[Jeff-Birt]

Steppers motors typically use some sort of spring washer to take up and axial play. You usually have to press on the shaft moderately hard to get it to move. I'm not sure what the stepper couplers on the Sherline are like but on the Taig you can manage to start compressing the spring washer when mounting the motor if the coupler is not properly adjusted. That will cause binding. It would also be that the spring washer is defective.

If you can swap the X and Y motors that would let you see if the problem followed the motor. If it does then you'll need to replace the motor.

[Fixittt]

Simple thrust bearings in between the stepper motor can and the coupler to the screw will keep the shafts from moving in the stepper. Worst case is if your stepper is double shafted you can put a thrust bearing on the outside shaft with a knob or something on the shaft to keep tension on the thrust bearing.

[tr4252]

Thanks. I ended up fabricating a bushing to fit between the shaft coupling and motor. It has about .002" end clearance, prevents the shaft from moving more than that amount, and has worked so far. The O.E.M. had advised me to pull the shaft out to the limit of its axial play and set the coupling to hold it there, but I couldn't see putting so much preload on the screw.

I did use a ball thrust bearing on the Z drive some months ago, which reduced a lot of friction there.

I should probably start looking for a more appropriate machine; I'm demanding a lot of this little entry-level unit.

Thank you for the advice Jeff-Birt and Fixitt.

Tom