hi, i still not quite understand about this conversion.
example i have 3,800m/min
does it mean that i have 3800 x 2.5 = 1520rpm?
I assume this figure is the surface speed of a tire, wheel, pulley or something like that. A wheel of diameter d has a circumference pi*d. If d is in meters, then the rotational speed is 3800/(pi*d) revolutions per minute.
I don't know what the 2.5 represents in your question, but it appears you divided rather than multiplied. At any rate, the relationship depends on the diameter of the wheel 3800 m/min would only be 1520 rpm if d = 0.7958 m.
In equipment like a band saw, you may have a pulley drive arrangement where the rpm is measured on one pulley and the band speed is determined by surface speed of another pulley, in which case you have to include terms representing the diameter ratios of all pulleys, gears, etc in the drive train.