Programming manual
486
CNC 8070
15.
STATEMENTS AND INSTRUCTIONS
Programming statements
(SOFT V02.0X)
456
15.1.19 High speed machining
Nowadays, lots of parts are designed using CAD-CAM systems. This
type of information is later post-processed to generate a CNC
program, usually made up of a large number of very short blocks of
several mm or just a few tenths of a micron.
In this type of parts, the CNC must be capable to analyze a large
number of points in advance so it can generate a continuous path that
goes through (or near) the points of the program while keeping (the
best way possible) the programmed feedrate and the restrictions of
maximum acceleration, jerk, etc of each axis and of the path.
The command to execute programs made up of lots of small blocks,
typical of high speed machining, is carried out with a single instruction
#HSC ON. The parameter of this instruction is the maximum contour
error permitted. From this instruction on, the CNC modifies the
geometry through intelligent algorithms for eliminating unnecessary
points and automatically generating splines and polynomial
transitions between blocks. This way, the contour is traveled at a
variable feedrate according to the curvature and the parameters
(programmed acceleration and feedrate) but without going beyond the
set error limits.
Chordal error setting
As mentioned earlier, the error caused by the CNC between the
programmed part and the resulting part is never greater than the
programmed value. On the other hand, the CAM system also
generates an error when processing the original part and converting
the paths into a CNC program. The resulting error may be the sum of
the two; therefore, the desired maximum error must be spread
between both processes.
Selecting a large chordal error when generating the program and a
small chordal error when executing it, the execution is slower and of
lower quality. In this case ridges will appear because the CNC
perfectly follows the CAM generated polyhedron. It is recommended
to post-process at the CAM with a smaller error than the one used for
high speed cutting HSC (between 10% and 20%). For example, for a
maximum error of 50 microns, we could post-process with an error of
5 or 10 microns and program the rest in the HSC command. This larger
margin for the CNC lets you modify the profile while respecting the
dynamics of each axis without causing undesired effects like ridges.
Finally, since the CNC works with an accuracy of nanometers, better
results may be obtained if the coordinate have 4 or 5 decimals than
if they only have 2 or 3. This has no negative effect because the block
processing time does not change noticeably. The slight increase in the
size of the programs does not represent a problem for storing them
thanks to the high capacity hard disk or for transmitting them thanks
to Ethernet.
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