R Curve
What does “R5” mean on a CNC drawing?
It indicates a 5 mm radius fillet or arc at that corner or edge feature.
Why can't an end mill cut a true sharp inside corner?
Because the tool is cylindrical, the cutter diameter leaves an internal radius equal to the tool path geometry; no corner can be perfectly square.
Why do programmers prefer I/J arc offsets over R for arcs over 180 degrees?
I/J defines the arc center explicitly, avoiding the ambiguity of R-format, which some controls interpret as the shorter arc unless a negative R is used.
In CNC machining, an R-curve denotes a radius or rounded curve defining an arc, fillet, or corner radius on a part. The “R” value on a drawing specifies the radius of a circular arc; in drilling cycles it can also indicate the retract plane. End mills inherently produce internal corner radii, so the design dimension and manufacturing constraint are linked.
On the shop floor, an R-curve is far more than a decorative contour. When a programmer encounters an internal radius in a pocket or slot, the first decision is cutter diameter: the end mill must be smaller than the specified radius or the corner cannot be generated. If the pocket is deep, that smaller cutter is forced to stick out farther, which increases deflection and chatter, so the wall finish suffers and the actual radius often opens up beyond tolerance. The CAM system typically creates the arc using G02/G03 motion, and the programmer must choose between R-format and I/J center offsets. For arcs beyond 180 degrees, I/J is the safer choice because controls can misinterpret a simple R as the shorter sweep. Ultimately, the specified R dictates cycle time, tool wear, and fit-up; a generous radius cuts cleaner and faster.
Undersized internal radius: The drawing calls for a corner smaller than the cutter can physically leave, so the tool leaves a cusp, forces a smaller end mill, and invites chatter and premature wear.
R-format ambiguity beyond 180 degrees: On arcs greater than a semicircle, a positive R can make the control choose the wrong sweep, gouging the endpoint or producing a faceted curve instead of the intended radius.
Depth-to-radius mismatch: A tight radius in a deep pocket forces excessive stick-out, causing cutter deflection, wall taper, and a finished corner radius that drifts out of tolerance.