Siemens Sinumerik 840D sl Programming Manual page 263

The contour point is approached/exited with constant curvature. There is no jump in
acceleration at the contour point. The path from the start point to the contour point is
interpolated as a polynomial.
Approach/retraction with KONTT
The contour point is approached/exited with constant tangent. A jump in the acceleration can
occur at the contour point. The path from the start point to the contour point is interpolated as
a polynomial.
Difference between KONTC and KONTT
The figure below shows the differences in approach/retraction behavior between KONTT and
KONTC. A circle with a radius of 20 mm about the center point at X0 Y-40 is compensated with
a tool with an external radius of 20 mm. The tool center point therefore moves along a circular
path with radius 40 mm. The end point of the approach blocks is at X40 Y30. The transition
between the circular block and the retraction block is at the zero point. Due to the extended
continuity of curvature associated with KONTC, the retraction block first executes a movement
with a negative Y component. This will often be undesired. This response does not occur with
the KONTT retraction block. However, with this block, an acceleration step change occurs at
the block transition.
If the KONTT or KONTC block is the approach block rather than the retraction block, the contour
is exactly the same, but it is machined in the opposite direction.
Fundamentals
Programming Manual, 01/2015, 6FC5398-1BP40-5BA2
11.2 Approaching and leaving contour (NORM, KONT, KONTC, KONTT)
Tool radius compensation
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