Your router will, usually, always turn in the same direction, but an upcut versus downcut router can make a significant difference to the end results of your CNC operation. If you use the incorrect bit for the job, this difference can result in poor feeding or even in visible damage to the piece being worked upon.
The speed of your feed is important to understand. The CNC machine will naturally have to have the path modified if you use a cut that moves in the direction of the blade’s movement. If the blade is fed too quickly, it can make a less effective cut than if there is a more relaxed feed speed. Changing the feed speed can also be useful depending on how much is to be removed to make the prevention of buildup easier.
The direction of the bit’s movement and the direction from which it originates both matter to the cut. Among the debatable aspects of an upcut versus downcut router is from which direction you are approaching the piece versus the spindle’s rotation. In every case, you want to avoid potentially unseating the piece from its restraints, and attempting to take off too much at a time can damage or even snap the bit regardless of its orientation.
When you use upward cutting bits, you will often find greater difficulty in chip ejection. Fighting gravity as well as the rigidity of the materials is of importance because of heat and premature wear on the bit. This influences the speed at which you would be advised to feed the piece as well as the speed of the spindle and how often you have to disconnect to ensure the bit is clear.
Visibility in the Completed Project
In many cases, the speed at which you have the bit approach the piece can result in murmuring, particularly for upward cuts from the bottom. For pieces that will be visible in the finished product, downward cuts are often superior for their ease of keeping a clean cut. However, adjusting the speed can compensate for this to an extent.