Whenever you enter a value into an edit box you can use any units you like. The units you use will be converted into your 'preferred units' as soon as you leave the edit box.
For example if your default units are millimetres but you are using a 0.25 inch cutter then just type in 0.25 in. The units are not case sensitive so 0.25IN 0.25 inch or 0.25 Inch would all work. If you type in a value without any units SheetCam assumes you are using your preferred units.
As soon as you leave the edit box it will be converted to 6.35mm

Metric: um, mm, cm, m
Inch: mil, thou, in, inch, ", feet, foot, '
degrees, deg, radians, rad
Metric: mm/min, m/min, mm/sec, m/sec
Inch: IPM, inch/min,
IPS, inch/sec