3D Milling Motion Jerky - Now Solved.

[Eclipze]

Having done a bit of 3D milling, I was becoming increasingly frustrated with the jerky motion. If I cut a 2D circle profile, I would get a smooth cut at the desired feedrate. But when it came to more arbitrary shapes where splines and non-linear 3D surfaces were present, the motion would slow right down and become jerky. Not only did this increase the job time by at least 3 times, but it also caused slight bouncing with the gantry and subsequently more noticeable machining marks in the surfaces.

Increase the motor acceleration increased speed, but also increased the magnitude of the shuddering. Some materials don't machine very well going so slow either, and I was become more concerned about the prolonged effects... fatigue/wear on the CNC, as well as steppers getting quite hot from longer machining cycles and the constant speed changes (using more power).

Took a while, but I solved it.

Mach3 has two motion control modes, Constant Velocity and Exact Stop. This is set under the Config --> General Config. I did have it on constant velocity, but it was acting like exact stop mode. In the Setting tab (Alt-6), there is an option for "CV Feedrate", which was enabled. I turned this off and the difference is more than apparent. I could turn it on and off while air cutting and it's a BIG difference. With it "OFF"... very smooth motion on 3D surfaces and 2D spline based shapes. The indicated feedrate remains at the set 1000mm/min, which I could also verify while just running the gcode as a simulation (offline mode). Turn back on the "CV feedrate" and during 3D corners it would erratically slow right down to below 300mm/min and shudder it's way around curves, then accelerate to 1000mm/min on the linear straight segments.

Chapter 10 of the manual only mentions that the Constant Velocity mode, when enable, produces smoother motion, although can round the corners. I assume it's not of great significance if you ensure the CAM software generating the gcode does so with a small step size (large gcode file).

I would recommend other Mach3 owners consider trying this, as it should improve the cut finish and reduce fatigue/wear on the CNC.

I do not know how the CV Distance and CV Feedrate effects the Constant Velocity mode. I couldn't find it in the manual and unsure what effect in tolerance that this CV Feedrate has. I tried other values for CV Distance and found smaller numbers were better, but turning off was still the smoothest.

If anyone can shed some light on the subject, I'd prefer to understand what these are and if turning off CV Feedrate is appropriate.

[ger21]

http://www.machsupport.com/docs/Mach3_CVSettings_v2.pdf

[Eclipze]

Just read it. Still quite lost on these variables.

"CV Feedrate – This will CV a move BUT at the federate that you ask for."
I don't understand how this relates to a value. The example reads like the value imposes a limit for the feedrate under CV control. I tried a CV feedrate of +0.01 (original default was +1) and it improved the smoothness, but not eliminate it. This certainly isn't limiting the feedrate, as it's far faster than this. The document doesn't mention what happens if you turn this parameter off.

I'm assuming CV Distance is the "CV Dist Tolerance" they talk about in the article.
"CV Dist Tolerance_____Units.. - This value affects the amount of rounding at the corners."
I have the CV Distance on the default of +180 and running at 1000mm/min.
"This is the distance from the end of the line that it is cutting to where the arc starts rounding… So it is the distance from the intersection of the CV arc to “true” end of the move (if it was done with Exact stop) "
How does this value actually relate to the actual geometry of the profile followed? It doesn't make much sense, as with a value of 180 it would never get to the end of the line to begin the rounding arc... unless the two points were a massive 180mm apart.

[elmerfud]

interesting, i would have thought that was baud rate or line look ahead.
thanks for sharing.

[Claude Boudreau]

Is the value of cv feedrate in the setting page suppose to be the same as in your motor tunning ? My motor were running weird until I enter the same value.