This is exactly why I asked this question. I thought I had my answer,but now I am not so sure anymore. I have three pulleys in there,the drive stack,idler and spindle stack. I have the belt on the pulleys that will produce 1300rpm max currently and vary the speed via external potentiometer on my vfd. Where can I put the magnet if there isn't a suitable spot on the spindle to give me the precise spindle speed at any given time? If pictures will help,I can take some tomorrow,but from what I know, there is no place to mount the magnet other that on the spindle pulley.
I can’t speak for bear. Can only assume he thinks the magnet is going on the motor stack or idler stack? He will need to jump in and clarify. I am 100% confident that putting the magnet on the spindle stack will give you spindle RPM. The back gears just control the speed of that spindle stack. Larger pulley slower and smaller pulley faster. Think of it this way, I have my lathe magnet mounted on a collar on my lathe spindle. It does not matter what gear ratio I apply to my head stock gears. And it made no difference what size collar was on my spindle or if I mounted directly to the spindle. The magnet reads 1 revolution each time past the sensor whether geared to 43 RPM or 2000 RPM. If I put the magnet on the motor pulley it would read 1800 RPM for the motor no matter what gear the lathe was in or what speed the spindle went.
Your spindle pulley stack should be keyed to your spindle shaft so the stack and shaft have to be going the same RPM.
So since the OP plans to mount the magnet to the spindle pulley stack that is keyed to the spindle shaft. The RPM reading will be the RPM of the actual spindle correct? This is what he is trying to get a consensus on.
