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Author Topic: Different speeds on lines and arcs  (Read 7967 times)

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Different speeds on lines and arcs
« on: October 01, 2009, 05:22:45 AM »
Hi,
i have a problem. Mach does not maintain constant speed (feedrate)  when executing my gcode. Differences are rather significant, ranging from 1800 to 1400 mm/min (you may imagine the jerking). and appear when i cut corners using g02/g03. my machine is plotter for cutting glass (thus high speed) but i observe this on 3 different pc, including C2Quad 2,4Ghz.
Has any of you came across similar phenomenon?

Offline bowber

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Re: Different speeds on lines and arcs
« Reply #1 on: October 01, 2009, 05:31:24 AM »
Are your curves G02/G03 moves or a series of lines (g01)?

Steve

Offline Hood

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Re: Different speeds on lines and arcs
« Reply #2 on: October 01, 2009, 06:06:37 AM »
Are you using ExactStop mode (G61)? Would probably be better using Constant Velocity (G64) if you are.

Hood

Offline bowber

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Re: Different speeds on lines and arcs
« Reply #3 on: October 01, 2009, 06:26:51 AM »
Doh, I should open my eyes ???
So much for scim reading.

Steve

Offline Hood

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Re: Different speeds on lines and arcs
« Reply #4 on: October 01, 2009, 06:29:39 AM »
Doh, I should open my eyes ???
So much for scim reading.

Steve

No your post is very relevant, there is a good chance the arcs are made of short segments and that would just make things even worse if in G61.

Hood

Offline bowber

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Re: Different speeds on lines and arcs
« Reply #5 on: October 01, 2009, 06:41:21 AM »
that's why I said it but then lloking again he said he's tried it with G02/03 moves.

Aonther one is maybe power supply, does it supply enough to run 2 axis at once?

Steve
Re: Different speeds on lines and arcs
« Reply #6 on: October 01, 2009, 07:01:30 AM »
Thanks for replies,
The problem is i test it on my pc, unplugged from machine, and i observe "deviations" ;) ,what i observe is also reported by the owners of the machine, so it;s not machine issue. I suspect it is something with mach configuration or mach algorithm for generating motor steps. My gcode is composed virtually  only of arcs, but they are very short (i take curve made of straight lines and then round the corners). gcode is correct. We tried to "walk around" the problem setting different speeds for arcs and straight lines (which are rare, but then they are very long so machine jerking is not a problem, speed is constant on a line). It helped a little for diminishing differences between speed for line and speed for arc, but when my path is composed of only arcs ( ie 10 arcs one after another making halfcircle) the speed differs while cutting such a path. ( i hope you follow me )

Offline ger21

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Re: Different speeds on lines and arcs
« Reply #7 on: October 01, 2009, 07:07:28 AM »
It helped a little for diminishing differences between speed for line and speed for arc, but when my path is composed of only arcs ( ie 10 arcs one after another making halfcircle) the speed differs while cutting such a path. ( i hope you follow me )
Why are you using 10 arcs instead of one? Have you tried it with one? I think that even in CVmode, the single arc will cut faster than the 10. What is the radius of the arcs?
Gerry

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Offline Hood

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Re: Different speeds on lines and arcs
« Reply #8 on: October 01, 2009, 07:10:57 AM »
Are you using G61 or G64?
If the moves are short and you are using G61 then at the end of each line the machine will stop then start the next. This means it has to decelerate then again accelerate for the next move and with the moves being short it is likely you will not be getting up to the programmed feedrate before its again having to slow down. If you use G64 then Mach will attempt to keep the velocity constant and will not slow to a stop between lines of code.


Hood
Re: Different speeds on lines and arcs
« Reply #9 on: October 01, 2009, 07:23:51 AM »
I use g64 ( it appears in the line with other codes in the line MILL->G15 G3 (...) G64).
i unfortunately must compose my arc from ie 10 arcs because i import PLT file and if there is big arc in the file it is made of straight lines (ie half circle is made from 20 lines) so in order to avoid stopping machine and turning knife in the air, i round corners between these short lines, so that i do not need to stop the machine and the resulting path is smoother.
Part of my file:
g90
g64
G01 X1775.295 Y-15.820
g17 G02 X3.923 Y-0.757 I-5.576 J-39.439 G91 A -5.749
G90
G01 X1782.358 Y-17.348
g17 G02 X3.835 Y-1.122 I-10.880 J-44.309 G91 A -5.020
G90
G01 X1789.157 Y-19.480
g17 G02 X3.716 Y-1.468 I-13.443 J-39.453 G91 A -5.494
G90
G01 X1795.702 Y-22.227
g17 G02 X3.562 Y-1.810 I-17.932 J-39.696 G91 A -5.258
G90
G01 X1801.910 Y-25.538
g17 G02 X3.384 Y-2.126 I-21.989 J-38.757 G91 A -5.140


when i type one big arc, the speed is constant for this arc, it is not a problem when speed differs for a line which is long ( either straight or in arc) but i only can generate little, short arcs and between these arcs speed differs most and couse machine jerking

thanks for your support :)
« Last Edit: October 01, 2009, 07:27:18 AM by macavity »