corners getting radiused at higher speed on plasma table

[hemi43]

If I try to cut thin material on my plasma table, any inside sharp 90 degree corners end up getting a large radius ( around 3/8 radius), except where the cut starts and stops.
If I cut any ouside 90 degree corners, the corner comes out perfect.
Any ideas?
I've increased my acceleration rate to 40, but the problem persists.
Dan

[stirling]

Hi hemi43

CV will *attempt* to keep your actual feed close to commanded feed by rounding or blending. The higher your feed, the more rounding you get, the higher your accel the less rounding you get. The outside corners are probably square(er) because your CAM will most likely have introduced an arc at each corner with a diameter the same size as your kerf width so unlike an inside corner, the outside path is more CV friendly. (the outside arc is like in milling where the *correct* way to do a corner is to "roll" the tool round it). Your start and end is likely ok because starts and stops are effectively exact stop mode (as opposed to CV).

You either have to do one or more of these until you're happy. Increase your accel (if you can), lower your feed, tweak CV params (see CV angle, CV distance, CV feed etc.), use exact stop (not good with plasma) or live with it I'm afraid. This is why the often forgotten quest for good accel is every bit as (if not more) important as the more poular quest for speed.

[hemi43]

Thanks Stirling;
I know for a fact that my CAM is not putting an arc on the outside corners, and that's why I'm confused about this.
There is such a huge difference in radius size between inside and ouside corners at the same speed. I think I will have to learn to cut at slower speeds, but I was hoping not too.
Dan

[stirling]

if you want to post your g-code I'll have a look at it.

[rrc1962]

You can either turn on "stop CV on angles" and enter 89 degrees....or turn on "CV distance" and set it to something like .01".  I agree in that if you're not getting rounding on outside contours, your CAM is probably rolling the tool around the corner.  What CAM are you using?  Post some code of a square with an outside offset.  

[stirling]

You can either turn on "stop CV on angles" and enter 89 degrees....or turn on "CV distance" and set it to something like .01".

Well you could but you might as well select exact stop of you're going to hit the proverbial CV nut with a sledge hammer. 

[rrc1962]

Although we try not to use any of the CV crutches, when running at 250IPM+, they work better than exact stop.  Running a plasma in exact stop at those speeds just causes the machine to shake.  The "proper" fix would be to (1) increase acceleration until the problem goes away or (2) slow the job way down. 

Option 1 isn't possible in most cases as it would take accelerations that the machine is not capable of.  Most plasma machines (even the high end industrial machines) are not build for speed and acceleration.  When you have a gantry weighing a few hundred pounds traveling at 300IMP, it takes a lot to slow it down.  Short of rebuilding the machine so you can achieve triple digit acceleration, the CV setting are the next best thing.

[BR549]

(;-) True industrial machines ARE built for accel AND speed. Ya can't have high speed and not high accel or else you get what the OP is describing.

You can't idle around at 100IPM cutting sheet goods and make money or even get good quality cuts with high power cutters.

One trick is to CHANGE your post to switch back and forth between exact stop and CV ONLY when it is cutting areas that are crutial to a sharp square inside corner.

Ya got to HAUL the mail so to speak, (;-) TP

[stirling]

Although we try not to use any of the CV crutches,

Curious description of tuning parameters

when running at 250IPM+, they work better than exact stop.

250 ipm - you think that's fast?

Running a plasma in exact stop at those speeds just causes the machine to shake.

And what you suggested IS effectively exact stop for anything over 90 degrees.

The "proper" fix would be to (1) increase acceleration until the problem goes away or (2) slow the job way down.  

Which is what I said I believe

Option 1 isn't possible in most cases as it would take accelerations that the machine is not capable of.  Most plasma machines (even the high end industrial machines) are not build for speed and acceleration.

When you have a gantry weighing a few hundred pounds traveling at 300IMP, it takes a lot to slow it down.

300 ipm you think that's fast?

Short of rebuilding the machine so you can achieve triple digit acceleration, the CV setting are the next best thing.

Which is more or less what I said.

[rrc1962]

When you have a gantry weighing a few hundred pounds traveling at 300IMP, it takes a lot to slow it down.

300 ipm you think that's fast?

It is when you have a gantry weighing a few hundred pounds and you built the machine using NEMA 23 motors, which is VERY common.