I am trying to figure out why constant velocity is not working in mach 3, and a mach technician told me that because my g code file is producing arcs in the corners instead of just a 90 degree angle, it is not doing CV. This happens when using the outside offset feature of sheetcam to compensate for plasma kerf width, because it offsets the cut to the outside, which means the plasma cutter makes a small arc while hugging the shape in the corners, but this evidently cancels any potential to use CV and it will stay on that path entirely, instead of allowing it to cut the corner by my set tolerance to keep the speed more consistent.  Any way around this while still using outside offset (to produce true sized parts) and Constant Velocity (to prevent total slowdown in corners).  I am ok with some corner rounding on most parts, as just a little bit will allow much better movement and closer CV speeds to achieve less dross and heating in corners, especially on thin sheet.  Also on decorative cutting, where corner sharpness does not matter at all.

Brian

I've just set a small table up and after many dozens of questions, the best results by far have been found by turning off all the cv dialogue in mach, just leave the Use CV mode on or put it in the g-code. Then tune the machine to have as high an acceleration setting as possible without losing steps and that's it.

I'm getting great corners, THC is working ok, I do use loops on some outside corners if I want really sharp corners.

I can get sharp corners, thats not what im trying to achieve.  It seems that I have the opposite problem of most (trying to eliminate rounding), although i have some strong motors and can accel/deccel fast.  I am trying to fine tune my machine and may end up only using a small amount of cv rounding on light gauge decorative type work, as any slowing builds the heat up fast on thin stuff, and also introduces extra dross.  I guess for a different type of explanation, i am trying to go from a family car slowing for turns at an intersection to a formula 1 car that slightly cuts the corner and can keep the speed up substantially.  When i lower the accel way down, i would expect alot of rounding to be done in an attempt to keep the speed up, but it does not, it slows for the corner and then speeds up when it leaves.  I have a few things to try, but i burned out a fuse on my drive (bad connection on motor wire i think) so i have to wait till my fuses arrive to try.

Les from sheetcam said "CV should work on arcs but try this: In your cut operation turn on 'triangle' in the 'loop sharp corners' box. Set the loop size and angle threshold sliders fully to the left. Corners will now be sharp angles rather than being rounded."

This is because a mach 3 tech said that my post was showing tiny arcs in the corners (because of the outside offset) which was preventing the CV from working, but that is yet to be proven.

I will try that, and I also updated to the latest Mach 3 version and see if i can round some corners, then i will adjust the settings so that it is just ever so slightly and see what it is like.   I am also going to try to get it setup so that it kicks out THC on corners to prevent dive and I am also going to play with settings for smaller holes, so i can eliminate some of the plasma drift that happens when moving fast on small holes and creates a tapered hole.

I find 70% speed reduction works for me on awkward small holes! but that's relative to my machine I guess.

I am still having difficulty grasping why you would want to induce corner rounding, I spent considerable time and effort to get rid of it getting it was easy - just poorly design a machine and try to use it

When you say slowing accel helped a bit, did you go way low?

I find that by using a reasonable speed (my machine is not very fast) and fine-cut consumables on my little torch, I can get good results on 1mm sheet

I'm not sure who this mysterious "mach tech" is or what you were told - maybe there was a misunderstanding somewhere. If CV couldn't do arcs it would be as much use a as a chocolate tea pot. 

If you're saying you get slowdown on corners etc. then you either have the feed for arcs set lower somehow in SCam, you're turning CV off for arcs somewhere in SCam, your machine doesn't have the acceleration required to maintain your chosen feedrate OR - you have some CV crippling option in effect - e.g. CV distance or feedrate or whatever. You should have NO CV crippling options set in Mach.

I'm not sure who this mysterious "mach tech" is or what you were told - maybe there was a misunderstanding somewhere. If CV couldn't do arcs it would be as much use a as a chocolate tea pot. 

If you're saying you get slowdown on corners etc. then you either have the feed for arcs set lower somehow in SCam, you're turning CV off for arcs somewhere in SCam, your machine doesn't have the acceleration required to maintain your chosen feedrate OR - you have some CV crippling option in effect - e.g. CV distance or feedrate or whatever. You should have NO CV crippling options set in Mach.

I am with Stirling on this one 

Well all that fooling around, and i think it was just a bug in my install/version of mach.  The mach 3 tech was Scott Nichols, scottn@machsupport.com, and then asked for Les's help with what he was talking about with sheetcam, and in the end, you are right, it doesn't matter it works either way the post is made with sharp corners or arcs.......  That is after i updated to the latest version of mach.   FYI, I believe the old version that i had a problem with was 3.042.040, now i am using most recent, .063 i think?

I was able to cut a 3" star at 500 ipm with .05 tolerance and it was perfect, only very minor inside corner rounding, and hardly any dross or hear build up during the corner, .1 and it was borderline for what i want, 672 ipm and 1.0 for tolerance and it was massively rounded even at 75 accel.  Just what i wanted it to do though, so i can see the results and find what is ideal for me.  Thanks guys for your help!

Right now my machine is set at 850 ipm velocity and 75 accel, haven't lost a step.  I also haven't pushed it faster to see if i can get more, because i think this will be plenty.  Yes, all i am using is the option on the main mach screen to type the tolerance or distance, cant remember what it says, and it works perect now.

Brian