R Word Versus Directional Vectors In Circular Motions

Almost all current model CNCs allow you to specify an arc size in circular motions with a very convenient "R word." Here is a tool path (based on cutter centerline motions) for Figure 1 using the R word as allowed by many machining center controls:

O0001 (Program number)
N005 G54 G90 S500 M03 (Select coordinate system, abs mode, start spindle)
N010 G00 X6.6 Y4.25 (Move to start point)
N015 G43 H01 Z-0.25 (Rapid to work surface)
N020 G01 X0.75 F5.0 (Mill top surface)
N025 G03 X-0.25 Y3.25 R0.75 (Mill upper left radius)
N030 G01 Y-0.75 (Mill left side)
N035 G03 X0.75 Y-0.25 R0.75 (Mill lower left radius)
N040 G01 X5.25 (Mill lower surface)
N045 G03 X6.25 Y0.75 R0.75 (Mill lower right radius)
N040 G01 Y3.25 (Mill right side)
N045 G03 X5.25 Y4.25 R0.75 (Mill upper right radius)
N050 G00 Z0.1 (Rapid out)
N055 G91 G28 Z0 (Move to Z zero return position)
N060 M30 (End of program)

In each circular movement, notice how simple the R word makes it to specify the tool path radius (0.25 workpiece radius plus 0.5 tool radius). But as with any convenience feature, there is one potential accuracy-related problem you must be aware of when you use the R word compared to using directional vectors (I, J and K) to precisely specify arc center.

Most CNCs are extremely forgiving when programs are written using the R word and will not generate an alarm even if the R word is incorrectly specified. Most will simply do their best to form an arc, regardless of whether the arc is correct. In almost all cases, this will result in a scrapped workpiece since the setup person will not be able to catch this kind of mistake during the program's verification. The motions will look good during a dry run.

If, for example, in the circular motion forming the upper left hand arc (block N025) the programmer incorrectly specifies the R word as R0.95 instead of R0.75, the motion will likely result as what is shown in Figure 2.

On the other hand, most controls treat directional vectors quite differently. If there is even the smallest amount of discrepancy in the arc center specification, an alarm will be sounded. From the standpoint of making your programs more fail-safe, directional vectors are better than using the convenient R word.