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Author Topic: Relationship between acceleration and CV.  (Read 2191 times)

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Relationship between acceleration and CV.
« on: October 11, 2010, 08:58:52 AM »
Hi All...

I was wondering if someone could set me straight on this.  I'm having some trouble getting arcs to cut smooth.  For conversation sake, lets say we have two kinds of arcs.  (1) consistent radius arcs where you have one arc with a consistent radius from start to finish and (2) variable radius arcs where the arc is made up of small line or arc segments.  Variable radius arcs are causing the problem.

This is on a plasma machine and speed is set to 500IPM with an acceleration of 40.  Steps per inch are 1712. on X and Y.  Yesterday I was cutting a file with lots of variable radius arcs from 1/8" steel at 230IPM and noticed that I had a lot of jerky motion in the arcs.  The longer consistent radius arcs were smooth and cut at full speed, but the variable radius arcs were jerky and cut much slower.  CV is on and none of the CV helpers are checked in general config.  It seems that this is amplified the higher the acceleration is, almost as if the higher acceleration begins to override CV where it begins to act as if it's in exact stop mode, especially at higher speeds.   I can reduce acceleration where arcs are smoother, but then at higher speeds corner rounding is excessive.  To reduce corner rounding I have to lower the speeds.  It also appears that I can reduce speed and arcs get better.  The issue seems to be high speeds AND high acceleration.

Here is little chunk of code from one of these variable radius arcs.  This is just a sweeping curve and uses a bunch of small arc commands to complete the curve.

N0710 G03 X6.6918 Y7.5278 I0.1871 J-0.0141 F170.0
N0720 X6.6919 Y7.5145 I0.2151 J-0.0051
N0730 X6.6928 Y7.5007 I0.2417 J0.0092
N0740 X6.6945 Y7.4866 I0.2618 J0.0250
N0750 X6.7043 Y7.4444 I0.2762 J0.0415
N0760 X6.7089 Y7.4310 I0.2518 J0.0791
N0770 X6.7141 Y7.4182 I0.2274 J0.0851
N0780 X6.7198 Y7.4062 I0.1975 J0.0869
N0790 X6.7247 Y7.3974 I0.1673 J0.0863
N0800 X6.7304 Y7.3882 I0.1516 J0.0888
N0810 X6.7386 Y7.3771 I0.1374 J0.0923
N0820 X6.7695 Y7.3476 I0.1239 J0.0993
N0830 X6.7776 Y7.3421 I0.0989 J0.1368
N0840 X6.7861 Y7.3371 I0.0958 J0.1502
N0850 X6.7950 Y7.3324 I0.0929 J0.1651
N0860 X6.8074 Y7.3268 I0.0923 J0.1866
N0870 X6.8204 Y7.3218 I0.0899 J0.2162
N0880 X6.8341 Y7.3174 I0.0874 J0.2517
N0890 X6.8485 Y7.3137 I0.0847 J0.2943
N0900 X6.8634 Y7.3105 I0.0816 J0.3453
N0910 X6.8785 Y7.3079 I0.0777 J0.4066

This is another sweeping curve that uses a bunch of line segments to complete the arc.

N1630 G01 X7.7780 Y6.6682 F170
N1640 X7.7775 Y6.6666
N1650 X7.7691 Y6.6421
N1660 X7.7683 Y6.6400
N1670 X7.7529 Y6.6040
N1680 X7.7517 Y6.6016
N1690 X7.7394 Y6.5788
N1700 X7.7381 Y6.5766
N1710 X7.7242 Y6.5554
N1720 X7.7224 Y6.5529
N1730 X7.7069 Y6.5338
N1740 X7.7047 Y6.5314
N1750 X7.6920 Y6.5188
N1760 X7.6900 Y6.5170
N1770 X7.6810 Y6.5096
N1780 X7.6801 Y6.5088

There is some logic in Sheetcam to determine when to use arcs and when to convert those to linear moves.

Basically I'm wondering if there is something in this code that could be changed to smooth things out at higher speeds or if I'm just up against the physics of the machine and/or software.  Anything you guys can suggest would be greatly appreciated.

Thanks in advance.

Re: Relationship between acceleration and CV.
« Reply #1 on: October 12, 2010, 09:59:54 AM »
I changed look ahead distance from 20 to 300 and it helped.