CNC: Grooving on CNC lathes – why you need a dwell at the bottom
The grooving insert moving into the groove is actually moving along a spiral path. When it reaches the final destination diameter (the ID of the groove) it has cut the ID at only one point, not along the whole circumference. If it is withdrawn out of the groove immediately, the root of the groove is a spiral instead of a circle in the cross section. E.g., if the feed rate is 0.1 mm/rev, there is a diameter difference of 0.1 at the bottom.
To get a circular cross section at the bottom, you have to add a dwell of at least 1 revolution after the tool reaches the bottom of the groove before withdrawing it.
Action point
If the cross section is critical and MUST be circular, add a dwell of at least 1 revolution after the tool reaches the bottom of the groove before withdrawing it. The cross section is critical when the groove is going to hold a piston ring, circlip groove, etc. Most times the cross section at the bottom is not critical, and a spiral is OK (e.g., if the groove is at a shoulder on a shaft, or at the end of a thread).
Pics. and text source: CADEM NCyclopedia multimedia CNC training software.
