Why does my rotary job crawl (or race) compared to the feed rate I set?

Why it happens

The feed rate in the tool database is a surface speed in inches or millimeters per minute. A rotary axis does not move in inches. It moves in degrees, and controllers measure its speed in degrees per minute.

How fast the wood passes the bit depends on the radius: rotation speed times distance from the axis equals surface speed. The same rotation speed gives a slower cut on a thin part than on a thick one.

What happens to your feed number depends on the post-processor and controller:

• Some posts convert it for you. One machine maker's wrapping post writes a line that divides the feed by (diameter times 3.14159 divided by 360), turning it into a rotation rate based on the job diameter.
• Some controllers treat the number as degrees per minute. One controller maker warns that a feed of 10 can then take about 36 minutes for a single turn, and that people mistake this for a fault.
• Some controls work it out from Z. The official documentation notes that with Z zero on the cylinder axis, certain controls can calculate the right rotation speed from the tool's distance to the center. This is one more reason to use the axis for Z zero.

What to do

1. Read your machine maker's rotary notes to learn which of the three cases applies.
2. If nothing compensates, raise the feed as the diameter drops. One maker's rule: half the diameter needs double the feed. In their worked example a programmed 2000 mm/min gave about 1773 mm/min of real surface speed on 4 inch stock and only about 443 mm/min on 1 inch stock.
3. Set the feed for the starting diameter and use the controller's feed override as the part gets thinner.
4. Check the rotary axis maximum rate in the controller. A low cap limits every move that rotates.

Pro tips

• A toolpath uses the tool's one cutting feed rate for moves in both directions, so choose the raster direction with the slow axis in mind (see the entry on raster direction for rotary finishing).
• Do not copy another machine's multiplier. Test on scrap.