Surface Quality, Accuracy, Speed - Siemens SINUMERIK 840D Manual

1.4
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Basic information
1.4

Surface quality, accuracy, speed

Special attention must be paid to the CAD -> CAM -> (post processor) -> CNC process chain
when machining three-dimensional geometries, e.g. free-form surfaces. CAM systems generate
NC programs for free-form surface machining. The CAM system receives the workpiece geome-
try from a CAD system. The CNC machine has to process the NC data generated and convert it
into axis movements.
3
G1
G1
© Siemens AG All rights reserved. SINUMERIK, Manual, 5-axis machining
In CAD systems, surfaces
are constructed (free-form surfaces).
For example, in order to be able to mill an
1
entire surface - or for collision checking - the
CAM system generally converts the CAD free-
form surface into a polyhedron.
This means that the smooth design surface is
approximated by a number of individual small
2
planes
2
This produces deviations from the original
free-form surface.
The CAM programmer overlays this polyhe-
dron with tool paths. From these, the post pro-
cessor generates NC blocks within the
specified error tolerances. These usually com-
G1
prise many short straight line elements, G1 X
Y Z.
3
1
G1
Thus, the machining result is no longer a free-
form surface, but a polyhedron. The small
planes of the polyhedron can be visibly
mapped on the surface.
This can result in undesirable remachining.
of higher orders
1
.