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The quality of the surface finish on a work piece depends on a number of factors: the material, the spindle speed and the cutting tool. The surface finish is also affected by the fact that the tool moves in discrete steps of 1/72mm: move, pause, move, pause etc. The shorter the pause the more steps there are a minute and, therefore, the higher the feed rate in mm/min.

If the tool steps left after every two complete revolutions of the billet, it will cut for the first revolution and do nothing for the second: it will just rub on the work piece. This may reduce the tool life.

If the feed rate is higher so that the tool steps left after half a revolution of the work piece, the remaining half circumference of the material is not cut. The result is a series of parallel grooves on alternating halves of the circumference.

                    

If the tool steps left at the end of every complete revolution, the surface finish will be a series of parallel grooves round the circumference linked by short axial grooves where the tool tip has moved left. To work like this at a spindle speed of 2400 rpm, there needs to be 2400 steps a minute. This is a feed rate of 33mm/min (2400/72).

To sum up: the higher the feed rate, the more the path cut on the work piece starts to look like a screw thread, a helix. Also, it is only the bottom of a groove which is at the correct diameter.

Finishing Cut Directions
Feed Rates