The most common mistake made when designating a shaft or bushing length is using the wrong reference point for the measurement. All shaft and bushing lengths are measured from the mounting surface or FMS. The mounting surface is the flat area at the base of the bushing that would normally contact the rear surface of a mounting panel. Both shaft and bushing lengths should be measure from this point.
A common error occurs when the shaft length is measured from the end of the bushing. Since bushing can come in different lengths, you could end up with the wrong shaft length.
For example if you specified a 3/8" long shaft and a 3/8" long bushing, based on industry standard using the FMS model, the shaft and bushing would be the same length – the shaft would not extend past the end of the bushing. If you were planning on using a knob on this design it simply would not work. The correct designation for the shaft length in this example would be ¾" FMS.
