feed rate different - arcs and straights

Re: feed rate different - arcs and straights

Postby TReischl » Thu Jun 16, 2016 8:06 pm

Too many "nodes" (which translate into short linear motions on the machine) can definitely slow up a machine, whether it has a "modern" control or not. Every control has what is known as a "loop cycle" time. This is a loop that the control runs. The loop contains things like "has the operator pressed the e stop?", "has a limit switch been tripped?" etc, etc. The list is actually quite long. It takes X milliseconds to run the loop. If the machine motion is so short that it takes fewer milliseconds to read the commanded destination, then the machine does exactly nothing in terms of motion. To combat this, look ahead buffers are created. But if the motion is really sort, those buffers get emptied and the machine starts to slow down.

You can play around to cause the effect even with a straight line. I once did it with an industrial controller. Programmed .0002 moves in a straight line. Tens of thousands of them. Sure enough, things ran nice and smooth right up until the buffers empted. Then, slowwwww and chattering.

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Re: feed rate different - arcs and straights

Postby sseale » Fri Jun 17, 2016 8:25 am

Had a moment to run some quick air-cut tests with a vector which was curve fit with beziers, and also the original vector (many nodes) which had the
G64 path blending tolerance reduced to 0.01 from 0.001. The overall cut time for the reduced node version and the original was pretty much same at around 20s, but the original with P factor reduced to 0.01 took only 12s, and the feed velocity could be seen approaching the 100IPM setpoint on some sections of the ellipse with less curvature. End of the day, its a big reduction in time and likely fine for 'decorative' type machining. Will run some more tests in scrap pvc bits just for cut quality comparison.
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