Siemens SINUMERIK 828D Function Manual page 170

B1: Continuous­path Mode, Exact Stop, LookAhead
3.3 Continuous-path mode
Comparison between G642 and G645
When rounding with G642, the only block transitions rounded are those which form a corner, i.e. the velocity of at
least one axis jumps. However, if a block transition is tangential, but there is a jump in the curvature, no rounding
block is inserted with G642. If this block transition is traversed with finite velocity, the axes experience some
degree of jump in acceleration which (with the jerk limit activated!) may not exceed the parameterized limit
(MD32432 $MA_PATH_TRANS_JERK_LIM). Depending on the level of the limit, the path velocity at the block
transition may be greatly reduced as a result. This constraint is avoided by using G645 because the rounding
movement is defined here in such a way that no jumps occur in acceleration.
Parameter assignment
The following machine data indicates the maximum permissible path deviation for each axis during rounding with
G645:
MD33120 $MA_PATH_TRANS_POS_TOL
This value is only of relevance to tangential block transitions with variable acceleration. When angular, non-
tangential block transitions are rounded, (as with G642) the tolerance from
MD33100 $MA_COMPRESS_POS_TOL becomes effective.
See also
Free-form surface mode: Basic functions Free-form surface mode: Basic functions [Page 193]
Basic Functions
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Function Manual, 09/2011, 6FC5397-0BP40-2BA0