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wades_block.stl buggy in git repo?

Posted by dawa 
wades_block.stl buggy in git repo?
January 22, 2015 03:11PM
I cloned the mendel90 github repo today, in order to print a new wades_block.stl.
I opened it in blender, and found that the hole for the hobbed bolt is covered by a thin layer, at two places.
A friend printed a new one for me, and we found it when the print was finished. He had opened it in netfabb, and saw the same thing as I did.

I have noticed when I play with openscad, that there are often thin walls like that when doing differences.
For instance, when I describe a tube as the difference of two cylinders of the same height but different radius, there is a thin lid and bottom on the result.
Sometimes these thin things survive the rendering and even the slicing. (right, or have I dreamed it? well I am *almost* sure) This is strange I think.
I usually subtract a 0.02 mm higher cylinder translated [0,0,-0.01] to work around this. But I am not very experienced in this ...
Maybe the stl error is caused by something similar?
The openscad oddity has been present for at least a year I think. Is it a bug or a feature? (haven't bothered to ask anybody until now--maybe I should report it)

Re: wades_block.stl buggy in git repo?
January 22, 2015 04:34PM
It isn't an error, it is an intentional one layer thick support membrane because you can't print a hole in mid air. I remove them with a penknife.

Several other parts have them as well, for example the nut traps in the X ends.

Re: wades_block.stl buggy in git repo?
January 22, 2015 05:47PM
Ok, I see, that makes sense!

The printer that printed my part has two nozzles, and has one of them for support that is soluble in caustic soda.
So in that case it could have printed it without the extra membrane :-)

Re: wades_block.stl buggy in git repo?
January 24, 2015 05:14AM
In the examples you gave from the Mendel90 parts, the membranes are intentional and are a result of designing within the limitations 3D printing - many people don't realise that 3D printing isn't yet at the stage where we can grab a 3D mesh from anywhere and expect to print it without modification or tweaking. Until dissolvable support becomes a standard feature, we have to think about how our layers are laid down and the effects of gravity on those layers.

The workaround you gave for making a tube is a slightly different case and is a consequence of CSG modelling. You'll notice that a lot of us use a CSG 'fudge factor' at the top of our scripts that allow us to nudge objects just far enough to eliminate the scenario where faces from different objects lie on the same plane. e.g.:

ff = 0.05;	// CSG fudge factor

Tube(radius=10, thick=2, height=50);

module Tube(radius, thick, height)
		// outer shell
		cylinder(r=radius, h=height);

		// inner cavity
		translate([0, 0, -ff/2])
			cylinder(r=radius-thick, h=height+ff);
Re: wades_block.stl buggy in git repo?
January 24, 2015 08:26AM
Yes, I see. Thanks a lot!
Great to know about the fudge factor :-)

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