Stepover, stepdown, scallop and stock to leave

Four settings decide how much each pass cuts and how close the result comes to the model. They appear in almost every operation.

Stepover: How far the cutter moves sideways between neighbouring passes. A smaller stepover takes more passes, takes longer, and leaves a smoother surface.
Stepdown: How deep each roughing layer goes. A larger stepdown removes material faster and loads the tool more. It must stay within what the tool can cut: its flute length, its strength and the material.
Scallop: The small ridge a round-ended cutter leaves between two neighbouring passes. Its height depends on the tool’s radius and the stepover. Some finishing operations let you set the scallop height you will accept and work out the stepover from it.
Stock to leave: Material an operation leaves on the part on purpose, so that a later operation can finish it. Roughing leaves some; finishing leaves none.

How they work together

Figure 1. Three passes of a ball end mill, seen in section. The stepover is the distance between neighbouring passes; the ridges left between them are the scallops.

Halving the stepover roughly doubles the number of finishing passes and the time, and makes the scallops much smaller.
A larger ball (a larger tool radius) leaves smaller scallops at the same stepover, but cannot reach into tight curves.
Roughing with stock to leave of a few tenths of a millimetre gives the finishing tool an even, light cut.

The program’s default tolerance, how far a toolpath may stray from the model, is 0.01 mm. You can change it in each operation.