installed new spindle As I picked up the spindle pulley from the top of the headstock, where I had placed it, on top of the square black spindle cover, I noticed two tiny silver magnets on the cover plate. I left them there. While giving the lathe a test run, the digital readout showed a third of what I expected. A clue that something was wrong! Pulled the spindle pulley and found there were matching holes in the pulley for the magnets. After reassembly I found the same discrepancy in the digital readout vs. anticipated top speed. The next question...Do the magnets have to be placed such that they are aligned with the same polarity as they pass the sensor? Followed by....Does it matter which pole aims which direction as long as all are the same, or not? And last....How does one guarantee polarity uniformity?
Terry - Yup, you're on the right track. The displaced rare-earth button magnets are the problem. The sensor is being triggered only once per revolution, so "reads" only one-third the actual speed. They are only a light press fit in the spindle pulley, so can fall out when it's removed and placed face-down on the work bench. Polarity matters, so if you reinstall them backwards, you'll still read only one-third of actual rpm. Use the magnet that is still in place to determine correct orientation by holding each of the displaced magnets up to it. BUT, to avoid pulling it out, use a toothpick or similar to hold it in place. Identical poles will repel, opposite attract, as you know. With the magnets back in place, you should be good to go. You may have to adjust positioning of the sensor, since the pulley has been moved.
David, is there any reason why a person shouldn't put a single drop of thin CA at the junction of the super magnets and the pulley? That is, is there really ever a time when you might need to remove them? Just wondering.
Brice, I'm not aware of any harm in gluing them in, once it's certain they're oriented correctly. There's no reason they'd ever need to be removed. If it were ever necessary to replace the pulley, the button magnets come already installed. I'd only bother to do it, though, if the spindle pulley is out for some other reason.
Polarity was the problem! Removed the two offending magnets, reversed them and BINGO, digital tach was on the money! Didn't need to remove the pulley either. Used a large screwdriver. Placed the flat on a magnet and it lifted right out, same for number two, the third (the one that didn't self eject when the pulley was out) didn't come out, so I arranged the two offenders so polarity matched it and everything worked fine. Now on to the next issue.
