A short cut is not always the best way!

[planescott]

Hi, I just finished my control box for my machine and was running it for the first time (you know playing).

I use bobcad v20.6 + mach3 and I made a rectangle so I could watch the machine and see if it was doing what it was supposed to do. It did fine at 150ipm but when I went over 200ipm it was cutting off corners. Does anybody know what is this problem I am having. I am assuming it is something in mach3.

Scott
( I ain't sweeping those chips mister, unless you threaten to fire me )

[Bloy2004]

This is inherent in Mach's CV(constant velocity) mode. You have a speed limit when using CV mode. If you run at a higher Khz, say 33Khz of 75Khz, you can get higher CVspeeds without the "rounding". Otherwise you have a speed limit to where it will round-off less noticeably.
Your other option is to use "exact stop" where it will actually stop at your rectangle corners and then continue on.

CV operates on "look-ahead". as the tool approaches the corner, it begins to move in the upcoming direction before the current direction is complete. This helps keep the velocity constant. There will always be a rounding of some degree near that maximum speed threshold.

[ger21]

Originally Posted by Bloy2004 This is inherent in Mach's CV(constant velocity) mode. You have a speed limit when using CV mode. If you run at a higher Khz, say 33Khz of 75Khz, you can get higher CVspeeds without the "rounding".

I think I may be misunderstanding what your saying. Corner rounding is mostly dependant on acceleration. The faster your acceleration, the less rounding you'll get. As John said, when you get to a corner, to keep the speed constant, one axis has to accelerate while the other decelerates. So faster accel will minimize the rounding. But, as velocity increases, so will the accel times, which will lead to more rounding as speed increases.

There are 2 things that might help you other than Exact stop mode.

One, create your toolpaths with radiused corners. A 1/2" diameter tool traveling along a toolpath with a 1/4" radius corner will leave a square corner on your part. Here's a pic, with the outer line being the toolpath, and the inner line being the part. You can get the same effect using Mach3's cutter comp, G41 and G42, which will automatically do this for you.

The second thing you can do, is enable angular discrimination in Config>Logic, and change the CV angle on the settings page so it won't round corners of certain angles. I'm not sure exactly how this works, so some trial and error may be in order.

[Bloy2004]

Thanks Gerry for expanding on this...I gave a pretty simplistic answer later into the evening...didn't think it through thoroughly.


Bloy

[planescott]

Thanks guys, I will try these suggestions next week.

This week I am going to try to get my router mounted and table frame painted, limit switches and that should just about do it. It will be finished.

I had just finished my control box and had been doing a little testing and noticed that it was'nt waiting till it got to the corner before it started moving along a different axis.



Scott