Smithy CNC Mills - Manual (Page 19)

SmithyCNC Programmer’s Reference Manual:G Codes
SmithyCNC Programmer’s Reference Manual: G-Codes 2-4
must be used. The R number is the radius. A positive radius indicates that the
arc turns through 180 degrees or less, while a negative radius indicates a turn
of 180 degrees to 359.999 degrees. If the arc is helical, the value of the end
point of the arc on the coordinate axis parallel to the axis of the helix is also
specified.
It is an error if:
* both of the axis words for the axes of the selected
plane are omitted
* the end point of the arc is the same as the current point.
It is not good practice to program radius format arcs that are nearly full circles
or are semicircles (or nearly semicircles) because a small change in the location
of the end point will produce a much larger change in the location of the center
of the circle (and, hence, the middle of the arc). The magnification effect is
large enough that rounding error in a number can produce out-of-tolerance cuts.
Nearly full circles are outrageously bad, semicircles (and nearly so) are only very
bad. Other size arcs (in the range tiny to 165 degrees or 195 to 345 degrees)
are OK.
Here is an example of a radius format command to mill
an arc: G17 G2 x 10 y 15 r 20 z 5.
That means to make a clockwise (as viewed from the positive Z-axis) circular or
helical arc whose axis is parallel to the Z-axis, ending where X=10, Y=15, and
Z=5, with a radius of 20. If the starting value of Z is 5, this is an arc of a circle
parallel to the XY-plane; otherwise it is a helical arc.
2.3.2 Center format arcs
In the center format, the coordinates of the end point of the arc in the selected
plane are specified along with the offsets of the center of the arc from the cur-
rent location. In this format, it is OK if the end point of the arc is the same as
the current point. It is an error if:
* When the arc is projected on the selected plane, the distance from the current
point to the center differs from the distance from the end point to the center by
more than 0.0002 inch (if inches are being used) or 0.002 millimeter (if millime-
ters are being used).
When the XY-plane is selected, program G2 X- Y- Z- A- B- C- I- J- (or use G3