Why does endmill leave a circle on carving?

[mhotchin (Michael Hotchin)]

The problem is almost certainly that the mill is a ‘fishtail’ mill, and the circles occur where it plunges and retracts without moving.

Problems with 'Fishtail' endmills

Not all endmills create a flat bottom when plunged, for example the Carbide #251. In general, you can assume an endmill is creating a flat bottom *only once it has travelled at least 1/2 its own width". Edit: As Tod says below, the bit needs to move its own width. My current project is pocketing around letters, so the toolpaths are not regular shapes. In several places the toolpath is just a plunge with little to no travel, and in those areas the bottom is raised in the center because the #2…