Why Rotary Feed Depends on Diameter
A 4th-axis rotary table doesn't move in linear distance, it moves in degrees, so a programmed surface feed rate in inches or millimeters per minute has to be converted based on how far the part's surface actually travels per revolution. That travel distance is the circumference, which grows directly with diameter, so the same surface feed rate calls for a much slower rotary speed on a large-diameter part than on a small one.
Recalculating Along Tapered or Stepped Parts
Many rotary 4th-axis jobs involve parts whose effective cutting diameter changes as you move along the part, such as tapered shafts, stepped pulleys, or contoured cams, and the correct rotary feed rate at each diameter is different. Recomputing this conversion at each significant diameter change keeps the actual surface speed consistent rather than letting it drift as the cutter works its way along a varying-diameter part.
From Rotary Feed to G-Code
Once you have the rotary feed in degrees per minute, most controllers expect it entered directly as the feed rate for the rotary axis word in the program, while the rev/min figure is useful for cross-checking against a CAM post processor's output or a machine's rated rotary axis speed limit.