If I am cutting 18mm MDF with 6mm cutting bit (so 6mm pass), what can be the maximum speed rate of cutting and spindle speed of router?

The best way to determine the cutting speed is to determine this with respect to the edge quality you desire. This recommendation generally comes from the major end mill manufacturers. The feedrate (travel speed) should be determined by the chip load of the end mill, the number of flutes, material of the end mill (Solid Carbide, HSS - High Speed Steel, Cobalt, etc.) and the RPM of the spindle. You will need to see the manufacturer's specifications of the end mill to determine this information. Formula: Chip Load = (feed rate IPM)/(Spindle RPM x Number of Flutes) or Feed Rate IPM = Chip Load x Spindle RPM x Number of Flutes Once you find this information, apply this to the material at the full depth of the end mill. If you find that your edge condition is aberrant, pull back on either the speed, or the depth per pass. If the edge condition is acceptable, then push the speed higher until the edge condition starts to deteriorate in quality and then pull back to the accepted level. Using this method, you will be able to reduce the wear on the end mil which will be the most expensive consumable on the machine. Additional Information: