Smithy CNC Mills - Manual (Page 97)

SmithyCNC Programmer’s Reference Manual: Tool File & Compensation 9-15
This method will also work at a concave corner on a tool path contour, if the
actual tool is oversized, but it will fail with a tool path contour if the tool is
undersized.
Other Items Where Cutter Radius Compensation is Performed.
The complete set of canonical functions includes functions which turn cutter
radius on and off, so that cutter radius compensation can be performed in the
controller executing the canonical functions. In the interpreter, however, these
commands are not used. Compensation is done by the interpreter and reflected
in the output commands, which continue to direct the motion of the center of
the cutter tip. This simplifies the job of the motion controller while making the
job of the interpreter a little harder.
Algorithms for Cutter Radius Compensation
The interpreter allows the entry and exit moves to be arcs. The behavior for the
intermediate moves is the same, except that some situations treated as errors in
the interpreter are not treated as errors in other machine controls.
Data for Cutter Radius Compensation
The interpreter machine model keeps three data items for cutter radius compen-
sation: the setting itself (right, left, or off), program_x, and program_y. The last
two represent the X and Y positions which are given in the NC code while com-
pensation is on. When compensation is off, these both are set to a very small-
number (10 -20 ) whose symbolic value (in a #define) is "unknown". The inter-
preter machine model uses the data items current_x and current_y to represent
the position of the center of the tool tip (in the currently active coordinate sys-
tem) at all times.
Jon Elson's Example
All further system-specific information refers to NIST's EMC program, but much
of it applies to most modern CNC controls. My method of checking these pro-
grams is to first select tool zero, which always has a diameter of zero, so offset
commands are essentially ignored. Then, I tape a sheet of paper to a piece of
material that sits level in my vise, as a sort of platen. I install a spring-loaded
pen in the spindle. This is a standard ballpoint pen refill cartridge made of
metal, in a 1/2" diameter steel housing. It has a spring that loads the pen
against the front, and a 'collet' at the front that allows the pen to retract against
the spring, but keeps it centered within a few thousandths of an inch. I run the