Hello Guest it is March 01, 2021, 03:16:33 PM

Author Topic: To many Nodes?  (Read 4446 times)

0 Members and 1 Guest are viewing this topic.

To many Nodes?
« on: August 23, 2008, 01:09:32 AM »
Hi all,

I have a technical question regarding number of nodes vs resolution of machine.

When I  export a Dxf file from my cad program, process it with my Cam program and then run the file om my machine,
it produces very jerky movement when cutting curves and arcs. I can fix the problem by exporting using a lower resolution on the dxf export.

What is happening here?
 
Is it because the distance between nodes in th gcode is smaller than the resolution of my machine? Is it an acceleration /de-acceleration setting problem in Mach3?

It would be nice to understand what is going on for future reference.


Bruce

Offline jimpinder

*
  •  1,232 1,232
  • Wakefield, West Yorks, UK
    • View Profile
Re: To many Nodes?
« Reply #1 on: August 23, 2008, 03:59:08 AM »
The majority of CAD CAM programs do NOT produce curves and arcs, but actually produce a vast number of short straight lines to produce the arc. You can see why, because looking at an arc, unless it was reqular shape to which there is a mathmatical formula (e.g. a circle, or elipse etc.) it would be impossible to mathematically predict where the curve would go next.

GCode requires a seperate line for each of these short straight lines.

If you have your machine on Exact Stop (Config/General Config - 2nd Column), then the machine does that - all axis stop after each line - which accurately produces the shape written. It does however produce what appears to be jerky movement - since some of the lines will only be millimetres, or fractions of an inch long.

If you downgrade the resolution, the CAM produces fewer lines, resulting in fewer stops, producing less jerky movement.

You can change your machine to Constant Velocity and here, Mach 3 calculates the next line, and, as it is slowing at the end of the prrevious line, it also accelerates into the next line, so the machine does not stop.

The disadvantage (or advantage of this) is that it also smooths out corners - i,e, the points disapear - so if describing an arc it can actually help to produce a smoother curve. Because the machine does not stop, it is less jerky.

If you are, however, in the same program also cutting sharp corners, you need to change back to exact stop to do so, since constant velocity rounds corners, whether cutting inside or outside the shape.




Not me driving the engine - I'm better looking.

Offline Hood

*
  •  25,838 25,838
  • Carnoustie, Scotland
    • View Profile
Re: To many Nodes?
« Reply #2 on: August 23, 2008, 08:37:09 AM »
Jim,
 if you use a CAM programme it will arc round your sharp corners at the radius of the tool thus preventing the CV problems and giving you a beautifully sharp corner, you want to try it some time ;) will give you a few lessons next time you are up in the land of your forefathers ;D


Hood

Offline ger21

*
  • *
  •  6,289 6,289
    • View Profile
    • The CNC Woodworker
Re: To many Nodes?
« Reply #3 on: August 23, 2008, 09:00:03 AM »
Jim,
 if you use a CAM programme it will arc round your sharp corners at the radius of the tool thus preventing the CV problems and giving you a beautifully sharp corner

G41/G42 will do the same thing. Give you a toolpath with a radius corner that cuts a square corner. :)
Gerry

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

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

Offline Hood

*
  •  25,838 25,838
  • Carnoustie, Scotland
    • View Profile
Re: To many Nodes?
« Reply #4 on: August 23, 2008, 09:02:29 AM »
Jim,
 if you use a CAM programme it will arc round your sharp corners at the radius of the tool thus preventing the CV problems and giving you a beautifully sharp corner
Yes, no doubt about that, there is a difference though, one way you have to think the other you dont. I am better at not thinking ;D

Hood
G41/G42 will do the same thing. Give you a toolpath with a radius corner that cuts a square corner. :)

Offline jimpinder

*
  •  1,232 1,232
  • Wakefield, West Yorks, UK
    • View Profile
Re: To many Nodes?
« Reply #5 on: August 24, 2008, 04:27:12 AM »
That may be, but Bruce was, I am sure, talking about "jerky movement" - which we have discussed many times.

A CAM program may arc aroung sharp corners ( I remain to be convinced) but I am sure that Bruces program, where it is describing an arc as many short straight lines, does not, and as we know, if you have exact stop, the machine will stop at each line.

I was asking Bruce if he was using "Exact Stop" or "Constant Velocity" and trying to explain the difference.

On the CAM program and sharp corners - how does the program know if my machine is in Exact Stop or Constant Velocity. If two lines of GCode follow one another, and describe two cuts at right angles to one another (or any other angle for that matter) Constant Velocity will round the corner. The way it works, it must do. I appreciate that by writing additional code you can go past the angle, and then return so that it does not round the corner. I have not delved enough into tool diameter compensation to comment on that, I am still taking lessons from Graham Waterworth
Not me driving the engine - I'm better looking.

Offline Hood

*
  •  25,838 25,838
  • Carnoustie, Scotland
    • View Profile
Re: To many Nodes?
« Reply #6 on: August 24, 2008, 05:48:56 AM »
Jim I was teasing ;)

But however CAM is much more economical than you seem to think, you said
Quote
  I appreciate that by writing additional code you can go past the angle, and then return so that it does not round the corner.

it doesnt do that, what it does is calc where the cutter would be leaving the part because of the radius of the cutter then it would arc the cutter round the corner and thus actually travelling exactly the same distance (x and y) that you would if you had 4 lines of code to go in a rectangle the CAM would have a four more. The thing with these extra 4 lines is it makes things better as speeds are kept up, no sharp direction changes, no cutting of air etc etc. Pictures speak louder so have a look and see what I mean. Also here is the code.
G1 X57.653 F100
G3 X60.653 Y0. R3.
G1 Y34.843
G3 X57.653 Y37.843 R3.
G1 X0.
G3 X-3. Y34.843 R3.
G1 Y0.
G3 X0. Y-3. R3.
G1 X14.413


Hood

Offline jimpinder

*
  •  1,232 1,232
  • Wakefield, West Yorks, UK
    • View Profile
Re: To many Nodes?
« Reply #7 on: August 24, 2008, 12:08:22 PM »
Yes - my apologies - I never thought of that

Bruce - I hope we have not gone too far off the subject.
« Last Edit: August 24, 2008, 12:10:28 PM by jimpinder »
Not me driving the engine - I'm better looking.
Re: To many Nodes?
« Reply #8 on: August 25, 2008, 03:21:38 AM »
Thankyou for help.
I have checked the config and it has been running in constant velocity mode. I have my XYZ axies Accelaeration set at 600mm/sec2 which is very "crisp". Starts and stops are nice.  As i have said earlier, the problem is not evident when I draw the CAD curves and arcs etc. at low res. The  curves are pretty dam good anyway.

Thanks again

Bruce