# Why CNC Machines Slow Down on the Curves

Actual feed rates are slower than the programmed rate when the cutter has to trace a curve. Pros in the know explain why. June 28, 2005

Question
I am wondering, why is the machine that I am using for circular cutting not reaching 40m/min? I have been trying to cut parts at 40m/min, but for some reason when I cut a circle 48" in diameter or any other diameter, the machine never exceeds 30m/min. The smaller the diameter of the circle, the slower the machine actually travels. When I rout a rectangular part, the machine reaches 40m/min, although it does decelerate on corners, but that’s normal. Any help would be appreciated.

Forum Responses
(CNC Forum)
From contributor J:
What most likely happening is that when you’re cutting a circle it is interpolating. There is probably a machine parameter or limitation that causes the rate to be 30m/min.

From contributor T:
Most of the newer machines slow down for arcs in order to cut true arcs. The smaller the arc radius, the slower the machine needs to run in order to cut it properly. If the machine ran wide open during arc cutting, the arcs would actually look like ellipses.

From contributor M:
Contributor T is correct, most machines limit the feed speed when rounding corners, and the more acute the radius is, the slower the machine will go. Most machines also slow down prior to making acute angled turns (such as a 90 degree corner), and this is known as axis deceleration. This however, happens so quickly it is barely perceptible. The configuration of the machine parameters, axis drives, and in some cases the machine programs, all influence this feature.

From the original questioner:
I know that machines decelerate and accelerate on 90-degree corners. My question is though, why does the machine not reach programmed feed when cutting a 30", 36", 48", etc. diameter circle?

From contributor H:
It depends on the max acceleration set for the X and Y axis out on the machine. In fact, when you machine along a circle, the acceleration for each axis involved is proportional to the feed, and inversely proportional to the square radius.