if you have calibrated your machine (place a bit with a fine point in it, and move the machine an amount, measure the distance the machine moved. calibrate the stepper to move the amount requested). there are several post in this forum on the exact procedure to do this, some with spreadsheet with the math equations built in, and you are sill off then check your bit diameter, just because you bought a 5.5 mm bit doesn't mean that the bit is exactly 5.5 mm. The best way to measure a bit when you need precise cuts is to make a cut with the bit in scrap material and then measure the width of the cut, rarely will it be exactly what the bit was listed as, sometimes more sometimes less. Take that measurement (width of cut) and place it in as bit diameter. (5.415 mm for example) instead of 5.5 mm. hope this makes sense.