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### Author Topic: The Laser Project.  (Read 855130 times)

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#### Tweakie.CNC

• 8,021
• Super Kitty
##### Re: The Laser Project.
« Reply #170 on: August 13, 2010, 02:57:28 AM »
Hey Guys,

I wonder if you could help me out here.
I have just laser cut two 50mm diameter discs from 6mm acrylic - one has visible vertical striations one does not. The parameters (power, federate, air assist etc.etc.) are the same for both discs, the only difference is the GCode (disc 1 has been cut using a predetermined number of straight lines, whereas disc 2 has been cut using circular interpolation [GCode for each attached]).

Now this might sound like a daft question but “What causes the vertical striations to be made ?”.

Is it the fact that the cutting head stops and starts ?.
Is it the sudden change of direction ?.
Is it acceleration / deceleration of the cutting head ?.
Etc.

With circular interpolation the step size (or length of each flat that emulates the circle) is, obviously, a lot smaller but just how small is it ?. The highest (theoretical) resolution I can achieve on my machine is 0.00625mm – are the circular interpolation steps smaller than this ?.

Grateful for any comments or info on this one, thanks,

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

#### Dan13

• 1,244
##### Re: The Laser Project.
« Reply #171 on: August 13, 2010, 03:11:22 AM »
Hi Tweakie,

Not sure about your question... I would get the same results if I milled the circles on my mill, once with circular interpolation and once with linear approximation. You just see the exact trajectory the machine was commanded to travel - if it's composed of small lines, then this is what you'd see.

Not sure this was the answer you were looking for, but then I may not be understanding the question

Dan

#### gittt2000

• 3
##### Re: The Laser Project.
« Reply #172 on: August 13, 2010, 06:27:05 AM »
> “What causes the vertical striations to be made ?”.

You did . If you tell the machine to cut lots of straight lines that is is exactly what it does.

Richard
UK

#### Sam

• 988
##### Re: The Laser Project.
« Reply #173 on: August 13, 2010, 08:50:22 AM »
If I had to take a guess at it, I would say that it's because the first piece is not really that circular. If you zoom in on the circle, it looks like a drunk person on the road. It weaves left and right. At the cutting speeds that your using, I doubt that accell/decell, or anything-other than lack of resolution-would be the factor (that is, of course, with a sober circle).
"CONFIDENCE: it's the feeling you experience before you fully understand the situation."

#### Tweakie.CNC

• 8,021
• Super Kitty
##### Re: The Laser Project.
« Reply #174 on: August 13, 2010, 11:36:41 AM »
Thanks Guys for your input, It is appreciated.

Dan and Richard,
You are, of course both quite right but...
The surface of the cut should appear to be a series of small flats (as you say a milling cutter path would produce) and there is really no reason why it should be striated - just a series of smooth flats. Unfortunately that's not quite what it looks like - for some reason.

Sam,
That's exactly what they look like - they weave left and right. They also get a whole lot worse if I increase the feedrate and subsequent laser power setting.

I did read, somewhere on the net, that they were caused by a reaction between the air-assist and the material vaporization but this doesn't hold true, otherwise both discs would look the same.

Tweakie.

« Last Edit: August 13, 2010, 11:42:13 AM by Tweakie.CNC »
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

#### budman68

• 2,360
##### Re: The Laser Project.
« Reply #175 on: August 13, 2010, 11:39:42 AM »
Could they indeed be straight lines but since you may have an overmelting factor (I'm grasping here  ) maybe it actually creates the waviness?

Dave
----------------------------------------------------------------------
Just because I'm a Global Moderator, don't assume that I know anything !

Dave->

#### Dan13

• 1,244
##### Re: The Laser Project.
« Reply #176 on: August 13, 2010, 11:53:03 AM »
OK, if it's not a sequence of straight lines to reflect the G-code and the striations are more pronounced than they should be if it were the G-code, then my guess is that the machine has some dwell between each line segment and the laser melts the material leaving those marks. Could you see if cutting in "Exact Stop" mode makes them more pronounced? I think that could prove the above point...

Also, may be playing with the Lookahead value will show some difference.

Dan

#### Sam

• 988
##### Re: The Laser Project.
« Reply #177 on: August 14, 2010, 12:55:43 AM »
Hmmmm, maybe I'm missing the real question your asking. I interpreted it to be... "why does part 1 not look like part 2?"
Upon loading up the code for part 1, and zooming in, its clear that it is not a circle made up of equal length lines. When I think of a faceted circle, or arc, I think of an octagon or hexagon, or whatever. Equal length lines with a consistent angle between the lines. I think if the first circle had been faceted uniformly, and to a high enough resolution (or subdivision), it would have matched the second one in appearance. As it is now, the striations are there because, well...there programmed in.
"CONFIDENCE: it's the feeling you experience before you fully understand the situation."

#### Tweakie.CNC

• 8,021
• Super Kitty
##### Re: The Laser Project.
« Reply #178 on: August 14, 2010, 08:19:33 AM »
Thanks for the magnification of the cutting path Sam - that looks pretty much like the striations.

So Guys, correct me if I am wrong, but would it be right to say that the striations are being caused by the lack of resolution in the exported arc ?.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

#### Tweakie.CNC

• 8,021
• Super Kitty
##### Re: The Laser Project.
« Reply #179 on: August 14, 2010, 08:23:36 AM »
Hey look, an acrylic footprint (much more eco-friendly than carbon)

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.