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Author Topic: G2/G3 codes and CV.  (Read 11307 times)

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Offline wyama

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G2/G3 codes and CV.
« on: July 24, 2008, 06:43:20 PM »
I can't seem to get constant velocity when doing spirals or thread milling.  I've reduced a facing operation to test the CV mode and when I change the init at the top from G61 to G64 I see the CV light on the Mach screen turn on/off accordingly.

The G1 code cuts a straight line and it seems to "bounce" or turn around without really ramping down and then back up.  That's seems very harsh as far as direction change worst case yet G2 code or G3 coding seems to ramp down and stop for each line.

I can't seem to get it to be continuous.  I've tried all the settings on the general config page in Mach and on the settings screen for CV feedrate.

Nothing seems to make a difference.

(magc_shave 7-13-2008)
G90 G64
 M6  T1 (TOOL CHANGE endmill0500:reg) (MSG, LOAD TOOL # 1) G43 H1
G0 G54 X0.075 Y0.05
S1909
M01
 Z-0.1
G1   Z-0.19  F2.9
  Y-0.05  Z-0.21 
  Y0.   F5.7
G3 X0. Y0.075  R0.075 F2.9
G3 X-0.075 Y0.  R0.075
G0 Z0.10
M05 M09
M30

Any thoughts?

Wayne

Offline ger21

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Re: G2/G3 codes and CV.
« Reply #1 on: July 24, 2008, 08:43:37 PM »


I can't seem to get it to be continuous.  I've tried all the settings on the general config page in Mach and on the settings screen for CV feedrate.



Turn all the CV options OFF.
Gerry

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Offline wyama

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Re: G2/G3 codes and CV.
« Reply #2 on: July 24, 2008, 09:27:23 PM »
I seem to have tried every combination.  I just double checked and turned off all CV options in the general configuration and still nothing.

It would be nice to resolve as thread milling takes forever with all the starts and stops.

Wayne

vmax549

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Re: G2/G3 codes and CV.
« Reply #3 on: July 24, 2008, 09:54:21 PM »
HI WAYNE, DOes it happen all the time or just sometimes. WHat version of Mach are you running?


If it does it again try two things

1  Try a Regen toolpath to see IF it stops doing it.

2  Try a total COLD rebooting of windows and restart MACH to see IF it stops doing it.

IF you have a complete piece of code that shows it it will help.

Please let us know, (;-) TP
« Last Edit: July 24, 2008, 10:05:46 PM by vmax549 »

Offline ger21

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Re: G2/G3 codes and CV.
« Reply #4 on: July 24, 2008, 09:57:44 PM »
You need to turn off both options on the settings page as well as all the settings on the General Config page.
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html

Offline wyama

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Re: G2/G3 codes and CV.
« Reply #5 on: July 24, 2008, 11:48:49 PM »
HI WAYNE, DOes it happen all the time or just sometimes. WHat version of Mach are you running?


If it does it again try two things

1  Try a Regen toolpath to see IF it stops doing it.

2  Try a total COLD rebooting of windows and restart MACH to see IF it stops doing it.

IF you have a complete piece of code that shows it it will help.

Please let us know, (;-) TP

The machine has been doing this since since I acquired it a couple months ago.  I have a Syril SX3.  The code posted was stripped down from a finishing operation I had run on some heat sinks.

It appears on all jobs small or large do the same thing.  I have turned off all CV setting in the general config and on the settings page and it still ramps down and then back up.

I do notice that the when it ramps down and stops or nearly stops it has overshot into the next command.  ie.  Coming back to 0,0 before the first arc it comes to rest part way onto the arc.

If I single step line for line each line ends as specified in the Gcode.

Before posting I have basically spent all day playing with all the different settings, finding threads associated or related here, on CNCzone and the mach yahoo forum.

Today I also downloaded and installed the latest mach locked version 2.62.  I believe I was running 2.4xx.  Both versions behave the same.

If I change G64 in the 2nd line to G61 the CV light in Mach goes out, but, the behavior is very similar.

I tried creating a sample thread milling from within Mach and resulting code also start/stopped for every G2 command.  Just wanted to check if it was something FeatureCam is generating vs other code.  I even tried changing the G2/G3 config from X, y, radius to X,Y I,J format.

The feed rate is slow.  I would expect issues with high feed rates.   ???  :o

I had to reboot the machine after installing the latest version.  I have rebooted the laptop many times previous and for good or bad it's very stable and repeatable.  The more I use the machine the more annoying it gets and I believe it's the start/stopping that is causing burrs in the thread mill operation and of course takes 3X longer than it should due to the large quantities of stops.  It's like going 50MPH starting and stopping every 10 feet.

Maybe, something here that will lead to a clue or resolution.   ;)

Wayne

Offline jimpinder

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Re: G2/G3 codes and CV.
« Reply #6 on: July 25, 2008, 01:11:30 AM »
Now I have read the post, I'm not sure what you are after - whether you want Constant Velocity on or off.

With Constant Velocity ON as a line is coming to the end of a move, the next line is calculated, and is started before the old line is finished - and the net output combined to give a position that suits both (or suits neither). This leads to rounding of corners etc. I tried this on my lathe/mill and on an acute angle, the table was stopping nearly 30thou short of the line end.
Exact Stop is, as described, an exact stop, where the line is completed and the axis stops BEFORE starting the next line. In many applications, particularly complicated patterns where arcs are made up from many, many little lines, this gives a jerky movement. It is, however, exactly accurate.

There are several moves where constant velocity does not function - and G2 and G3 is one - becasue the axis co-ordinates for these moves are continually generated in Mach3 as the move progresses. I could see also where having constant velocity on would ruin a cut, and I would have thought that threading is one of them, since accuracy is called for.

If you want to make your lathe/mill smoother, then ramp down the acceleration on the motors. You can keep the max speed, but cut back on the gas - and make for a smoother run.

I have not adjusted my CV - I have left it at the default values. You may have adjusted it so that it is almost like exact stop.

I think the problam is with your basic motor acceleration.



« Last Edit: July 25, 2008, 01:15:56 AM by jimpinder »
Not me driving the engine - I'm better looking.

Offline wyama

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Re: G2/G3 codes and CV.
« Reply #7 on: July 25, 2008, 01:28:33 AM »
In two cases it would be preferable if the head continued in a constant fashion.

The partial code above is a spiral cut.  Each G2 command cuts 90 degrees or a partial circle.  The next command continues the circle so it would seem if CV is working there would be a smooth transition from one to the next since it is cutting a circle to begin with.

Thread milling does the same thing.  The commands are broken up into partial radius command each approximately 90 degrees or 1/4 of the circle/spiral.

If there are 20 threads with 4 gcode lines per turn then there are some 20 X 4 or 80 Gcode lines.  At the feed rate I'm going at the machine is spending 60% of the time either stopped, slowing down or speeding up.  What a real waste of time.

Since each command is a partial circle there should be no angle break or difference from one line to the next as it is part of a drawn circle.  It's not like it's making a 90 degree corner turn.  i can't see why there would be any rounding or distortion caused by two adjacent lines of code.  It's not like traveling in the Y direction and then traveling in the X direction on the next command.

It's frustrating to know there is CV to account for this and not being able to get to it.  Not only is it doubling or tripling the machine time it's not very efficient.

I have CV enabled, but, it's not working as expected.

Wayne

Offline Hood

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Re: G2/G3 codes and CV.
« Reply #8 on: July 25, 2008, 01:34:04 AM »
CV stays on when I run your code here, could you attach your xml and I will run with that and see what happens.
Hood

Offline wyama

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Re: G2/G3 codes and CV.
« Reply #9 on: July 25, 2008, 01:52:22 AM »
Here's my mach3 xml file.

Wayne