Parts designed in CAD software are composed of features, including Extruded Cuts, Fillets, Chamfers, and Holes. A CNC milling machine creates these features using machining operations like Face, 2D Contour, 2D Pocket, and various Drilling operations. Knowing which machining operation to use to make which feature is sometimes obvious. For example, the slots are created using a Slot Mill pocketing operation, the large extruded cut using 2D Pocket, and the Chamfer using Chamfer milling.

However, sometimes these decisions are not so obvious. For example, the hole through the part center could be created using Drill, 2D Contour, 2D Pocket or Circular Pocket milling. You may wonder, is the large flat (where the holes begin) a 2D Contour or 2D Pocket? Furthermore, which features on this part should be machined from the Top and which from the bottom?

The operations the CNC programmer chooses and their sequence depends on a bewildering number of factors, including feature size, tool used, capabilities of the machine, feature tolerance and how the part is gripped. .

To begin with, in most cases you want to first machine the side of a 2D part that has the most features; finishing as much of the part as possible with the first CNC setup. This is often the Front view of a part designed in CAD software.