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Need help with basic understanding
« on: February 19, 2011, 11:11:28 PM »
So I recently completed my home made laser cutter.

Ive been generating gcode in various software, however sometimes the software will output the Y coordinates in the - values, and sometimes it won't. My table at the moment is setup to work in + coordinates, however why is there all this variation? Is fixing the -'s as simple as just removing all the - symbols from the gcode? Which brings me onto my next question.

Sometimes the output is upside down, mirrored, backfliped, frontflipped and whatever other orientation you can think of. Usually I can fix the mirroring by switching the axis around (Making X Y etc), though that is tedious, and can lead to my table smashing into the limit switches if I forget to switch it back when I do something else.

I think what I'm not understanding is how the coordinate system on a XY table actually works, and how software produces the g-code with coordinates. For example, if I draw a 10cm diameter circle in Adobe Illustrator, export it as a DXF, import it into Lazycam, then into gcode and into mach3, will the circle at the end actually be 10cm diameter?  Any help is appreciated :)

Also, how do you calculate how many steps per mm the machine is? I am using 200step/rev steppers, connected to These pulleys: http://ausxmods.com.au/t5-pulleys/15t5-27-6.35mm-bore-timing-belt-pulley
With this belt: http://ausxmods.com.au/t5-timing-belts/t5-16mm-wide-open-timing-belt
Running at 1/8 microstepping.

Cheers,
Dan
« Last Edit: February 19, 2011, 11:21:54 PM by Things »

Offline ger21

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Re: Need help with basic understanding
« Reply #1 on: February 19, 2011, 11:31:39 PM »
Quote
For example, if I draw a 10cm diameter circle in Adobe Illustrator, export it as a DXF, import it into Lazycam, then into gcode and into mach3, will the circle at the end actually be 10cm diameter? 

Provided that Illustrator outputs the .dxf properly. From what I've read, Illustrator's .dxf export is not very good at all.

One thing to note. You need to use the same unit of measure throughout the process. From design to machine.
.dxf files typically won't be drawn with any specific unit. What I mean, is that your 10cm circle will really be 10 units. So, If you draw a 10cm circle, and I draw a 10 inch circle, and someone else draws a 10mm circle, all 3 .dxf files could be exactly the same.

You machine will most likely be set up in mm's, so, typically, you would draw the circle with a 100mm diameter. Some programs that export .dxf files have a tendency to assume your drawing in a specific unit, and will output in another unit, by actually scaling the drawing when exporting. You'll need to do some testing to make sure the .dxf's you export are the size you expect them to be.

Since my machine is set up in inches, for me to get a 10cm circle, I'd draw it with a diameter of 3.937 inches.


Steps/mm can be easily calculated. Tell us how it's driven, and the specifics of the components.
Or, use the calibrate function in Mach3. I think it's on the settings page.
Gerry

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

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html
Re: Need help with basic understanding
« Reply #2 on: February 19, 2011, 11:40:09 PM »
Hi, thanks for the reply.

Right after the modify button expired, I found the set steps per feature on the settings page. Seems my guess of 20 steps per mm was accurate.

I'll try a few softwares and see what results I get.

Do you happen to have an answer for my +/- coordinate problem?

Cheers,
Dan
Re: Need help with basic understanding
« Reply #3 on: February 19, 2011, 11:58:00 PM »
OK well, I drew up a 10x10mm box in google sketchup, exported it as a DXF using a free plugin, loaded it into LazyCam, then into Mach 3, and I got a 10x10mm square :D

One more question I have, what determines where the engraving/cutting actually starts? All the g code I have seems to have an offset from home it starts at, and I have to test it a few times on scrap before I know its in the right position.

Cheers,
Dan

Offline ger21

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Re: Need help with basic understanding
« Reply #4 on: February 20, 2011, 12:13:40 AM »
When you have a .dxf file, there is an origin, where X and Y = 0. Depending on where the part is drawn relative to the origin, you may have positive or negative values, or both.
Some CAM software will let you move the origin to wherever you want it. Some use it exactly as drawn.

Removing the -'s is not the answer.
As for the mirroring, backflipping and other stuff, you'll need to provide specific info for me to tell you why.
Gerry

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

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

Offline ger21

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Re: Need help with basic understanding
« Reply #5 on: February 20, 2011, 12:15:58 AM »
Just like the origin in the .dxf, the machine has a 0,0 position as well. This may or may not be the same as home.

You can set the machine's 0,0 position anywhere you want.
Gerry

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

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html
Re: Need help with basic understanding
« Reply #6 on: February 20, 2011, 12:34:26 AM »
OK, that makes sense :)

Just one more question, which is probably more mechanical related.

I drew this up in Sketchup:



Then ran it on the laser, and ended up with:



I have circled 2 points, where I'm not quite sure what happened. It seems almost like the machine has lost track of where it was and kept going, before realizing it was off track.

I don't think it's my machine skipping steps, because other stuff drawn after those points lined up OK, so seems more like a glitch.

EDIT: I just ran the exact same program again, and it glitched in exactly the same spots. To me, that doesn't seem like a machine problem (skipped steps etc)

Cheers
Dan
« Last Edit: February 20, 2011, 12:49:31 AM by Things »

Offline Tweakie.CNC

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Re: Need help with basic understanding
« Reply #7 on: February 20, 2011, 03:30:28 AM »
Dan,

The glitch top left is usually caused when the object vectors are not joined (LazyCam takes the shortest route to the next vector).

What did the whole thing look like on the LazyCam screen ?
Did it show these errors ?

Tweakie.
KEEP SAFE !
Re: Need help with basic understanding
« Reply #8 on: February 20, 2011, 04:06:12 AM »
Hi, this is how it looks in LazyCam straight from the DXF (No optimizations)



Cheers,
Dan
Re: Need help with basic understanding
« Reply #9 on: February 20, 2011, 05:44:11 AM »
I tried rotating the image 90 degrees, and that causes the triangle at the bottom to be on the left side of the vertical line and not partly thru it, though I still have the problem with the little box and the diagonal lines not meeting up.