Your drill press is fast and you love it, but for some reason you need to slow it down and drill more precise. I am going to tell you the best and the cheapest way to do this. Sure, the easiest solution is to buy a new motor which is less strong than your current one, but not everyone can afford that. Lower rpm motors are made, but for that money you can buy two more drill presses.
So let’s see the other options.
The pulleys represent a ratio, hence, it doesn’t matter if you can accurately measure the pitch diameter. You just need to measure at the same point on each pulley. From there, you multiply your motor speed by the ratio. For example, 1750rpm motor x 4/5 (4″ pulley diameter on motor, 5″ pulley diameter on spindle) = 1400rpm. Another way to do it is to use a tachometer on the spindle itself. Record each rpm (or look it up in the speed chart on the drill) for each pulley selection. Spindle rpm / motor rpm = pulley ratio (as a decimal–not a fraction).
If you have a center freewheeling pulley you can get a pulley with a larger and smaller pulley on it. You might only need one new fan belt. It depends on how big a pulley you get. But that would probably be the best way to slow it down. You may have trouble using it in all the other speeds though. Because you may not be able to get the center pulley to swing away from the walls of the enclosure enough.
If you can figure out a way to mount another motor off to the side, you could put a small pulley on it and run a belt to the top of your current motor pulley. Then your current motor would become a jackshaft when you need lower speeds. The second motor would not have to be very big with all that reduction, maybe 1/2 HP. Looks like you could make a slick mount that clamps on the center column, way up high and you could swing it to tighten the belt.
I will also mention that the belt changing drill presses work very well because when you want a slow speed because you’re using a large bit, the torque is greatly increased. When using a high speed for a small bit you lose the large amount of torque, but don’t care because it’s not needed.
I hope this article was helpful for you and that you will of course be careful when doing any of these things.