Potentiometer not reading right rpm on 3520

[Bob Bergstrom]

For a couple of weeks now my potentiometer is not reaching the higher rpm on either. It maxes out around 7 to 8 hundred rpm. It doesn't matter which speed range it is in. I have unplugged it, blown the dust out of both the headstock and electronics in the back. Is there a pickup sensor or reset button I could try. Final answer will be to call Powermatic. It is still under the 5 year warranty. Thanks for any additional ideas.

[Tim Rinehart]

I'd completely unbolt the RPM display and pickup (not really a potentiometer) and make sure everything clean and undamaged on back side. Check all connections good, and even pull them off and put back on one at a time.

[Dennis Ford]

Is the lathe not getting up to speed or is the display reading lower than the actual speed?

[Mike Peace]

Probably a loose set screw on I believe part 64 in the manual's parts diagram. Maybe that is what Tim is calling the pickup?

[Bob Bergstrom]

I pulled the pickup out as far as I could and blew out the dust. If I put it in loosely I can get a true reading. If I tighten the screws it reads about 1/2 of the actual speed (650 when it is wide open on the slower pulley). It seeHSBC the pickup misses every other revolution.

[Jeff Myroup]

Bob we can take a look at it next week. The mag pick-ups I use at work have a set clearance to work properly. I am sure we can figure it out

[Russell Neyman]

Sometimes blowing the dust out isn't enough, because the fine wood dust (really pesky stuff!) bakes on. You might also try to scrape it with an exacto blade.

[Ken Fitzgerald]

One of the most common problems is the block on the shaft ( item #46 on the parts listing) isn't in the correct place to trigger the proximity sensor. The set screw loosens and the block moves. The block should align so that the square portion aligns with the proximity sensor and the "lock" pin should insert properly in the hole. It's the 4 corners of the block that trigger the proximity sensor. The triggering causes pulses to be generated which are counted and become the RPM reading.

[Faust M. Ruggiero]

I'm glad I have a 3520. We've developed quite a group of experienced mechanics on this forum.
faust

[Bob Bergstrom]

Ken thanks for the info. I moved the square block to the left about 3/16" of an inch and it worked, but when I tighten down the cover it started dropping down on the counter from 3250 to about 2,000. I"ll play with that adjustment some more when I get a chance. As Faust said It is good to have such a resource here on the Creek. Thanks again and I will update the beginning of next week.