HEIDENHAIN iTNC 530 (340 422) ISO programming - Manual (Page 204)

204 7 Programming: Miscellaneous Functions
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Feed rate at circular arcs: M109/M110/M111
Standard behavior
The TNC applies the programmed feed rate to the path of the tool
center.
Behavior at circular arcs with M109
The TNC adjusts the feed rate for circular arcs at inside and outside
contours so that the feed rate at the tool cutting edge remains
constant.
Behavior at circular arcs with M110
The TNC keeps the feed rate constant for circular arcs at inside
contours only. At outside contours, the feed rate is not adjusted.
Effect
M109 and M110 become effective at the start of block.
To cancel M109 and M110, enter M111.
M110 is also effective for the inside machining of circular
arcs using contour cycles. If you define M109 or M110
before calling a machining cycle, the adjusted feed rate is
also effective for circular arcs within machining cycles. The
initial state is restored after finishing or aborting a
machining cycle.