Slic3r filling in nut traps

I am trying to print out ABS parts to replace the PLA parts that my original Mendel90 parts kit is made of. While printing out the X idler bracket and X motor bracket, I found that Slic3r is filling in the traps for the z-screw nuts as if those traps were not even there. Cura does not have that problem, but I am having problems with dimensional accuracy in Cura. Why would Slic3r be filling in something that is supposed to be there, while Cura is not?

You may have a non-manifold STL that Cura is handling more gracefully. Slic3r has a tendency to repeat the previous layer when encountering a non-manifold condition. Look for a note about a number of errors. If there is one, running it through Netfabb or another program with the ability to repair broken STLs could help.

I agree that the STL probably needs repair. I am puzzled as to why Cura should produce dimensional inaccuracies - AFAIAA it should create exactly the same perimeter positions as Slic3r.

Dave

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dmould
I agree that the STL probably needs repair. I am puzzled as to why Cura should produce dimensional inaccuracies - AFAIAA it should create exactly the same perimeter positions as Slic3r.

Dave

The new version of Slic3r has an X/Y compensation setting that helps to adjust for things like undersized holes. I have not found a similar feature in Cura that does not involve resizing the entire part. So with Slic3r I have been able to get my other nut traps to come out on size (not counting the filled-in one), while with Cura I get undersized holes. I have my X/Y comp set at -0.15 in Slic3r.

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Rich K.

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dmould
I agree that the STL probably needs repair. I am puzzled as to why Cura should produce dimensional inaccuracies - AFAIAA it should create exactly the same perimeter positions as Slic3r.

Dave

The new version of Slic3r has an X/Y compensation setting that helps to adjust for things like undersized holes. I have not found a similar feature in Cura that does not involve resizing the entire part. So with Slic3r I have been able to get my other nut traps to come out on size (not counting the filled-in one), while with Cura I get undersized holes. I have my X/Y comp set at -0.15 in Slic3r.

OK, I've not tried that feature. Most of my prints are my own design so I simply make small holes a bit oversize so they print correctly. I often design nut traps to print out a little undersize, then pull a heated nut into the trap with a bolt. The nut melts its way in and after it cools it stays put, making assembly of the unit a bit easier and ensuring the nut is a perfect fit. Put a very long bolt through the hole, screw a nut onto the protruding end and heat with a hot air gun (not too hot and shield the printed part from the air), then pull the bolt to draw the hot nut into the trap, ensuring the nut is positioned correctly to the orientation of the trap.

Dave

The new Mendel90 x-ends have the z-screw nut traps at the top instead of the bottom. If you think about how the plastic is laid down (in particular the order of perimeters and infill) then you'll realise that you cannot have a floating nut-trap. Nophead gets around this by putting a single-layer membrane at the base of the nut trap on which to build the rest of it - this is easy to remove afterwards.

Unfortunately, I remember reading that OpenSCAD gets the surface normals the wrong way round when exporting what is essentially an enclosed cavity, and Slic3r doesn't understand what this is so it just fills it in. Not very helpful, but that's the reason why.

Nophead uses Skeinforge to slice these models which is more resilient than Slic3r and has better dimensional accuracy than Cura.

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QuackingPlums
The new Mendel90 x-ends have the z-screw nut traps at the top instead of the bottom. If you think about how the plastic is laid down (in particular the order of perimeters and infill) then you'll realise that you cannot have a floating nut-trap. Nophead gets around this by putting a single-layer membrane at the base of the nut trap on which to build the rest of it - this is easy to remove afterwards.

Unfortunately, I remember reading that OpenSCAD gets the surface normals the wrong way round when exporting what is essentially an enclosed cavity, and Slic3r doesn't understand what this is so it just fills it in. Not very helpful, but that's the reason why.

Nophead uses Skeinforge to slice these models which is more resilient than Slic3r and has better dimensional accuracy than Cura.

OpenScad has recently released a new version that I believe fixes that bug (but I have not tested it). It was fairly easy to workaround - just put a small (0.01mm) hole in the design from the outside into the cavity (e.g. through the membrane) and the STL will be correct - albeit with a small hole that Slic3r should ignore (being too small to print).

Dave

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dmould
OpenScad has recently released a new version that I believe fixes that bug (but I have not tested it). It was fairly easy to workaround - just put a small (0.01mm) hole in the design from the outside into the cavity (e.g. through the membrane) and the STL will be correct - albeit with a small hole that Slic3r should ignore (being too small to print).

Dave

Thanks Dave, I was about to go check on the status of that myself. Did the small hole trick actually work? I'd read about it but I was also waiting on something else from Slic3r before upgrading both parts of my workflow. Maybe now is the time to start producing my own spares, given that the printer has been going for over two years now.

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QuackingPlums

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dmould
OpenScad has recently released a new version that I believe fixes that bug (but I have not tested it). It was fairly easy to workaround - just put a small (0.01mm) hole in the design from the outside into the cavity (e.g. through the membrane) and the STL will be correct - albeit with a small hole that Slic3r should ignore (being too small to print).

Dave

Thanks Dave, I was about to go check on the status of that myself. Did the small hole trick actually work? I'd read about it but I was also waiting on something else from Slic3r before upgrading both parts of my workflow. Maybe now is the time to start producing my own spares, given that the printer has been going for over two years now.

Yes, I used the small hole trick several times. I have not however made a design with an enclosed cavity since upgrading my OpenScad, so I cannot verify whether that bug is fixed - but the upgrade is worthwhile IMO if for nothing other than the ability to display tick marks (rulers) on the axis - and the ability to change variable values without using the "assign" statement is also very handy, and there are a few other new good features as well.

Dave

If non-manifold vertices is the issue, you can import the STL into Blender and press Ctrl+Shift+Alt+M to select all non manifold vertices, then do a little bit of work to fix it up.

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dmould

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QuackingPlums
The new Mendel90 x-ends have the z-screw nut traps at the top instead of the bottom. If you think about how the plastic is laid down (in particular the order of perimeters and infill) then you'll realise that you cannot have a floating nut-trap. Nophead gets around this by putting a single-layer membrane at the base of the nut trap on which to build the rest of it - this is easy to remove afterwards.

Unfortunately, I remember reading that OpenSCAD gets the surface normals the wrong way round when exporting what is essentially an enclosed cavity, and Slic3r doesn't understand what this is so it just fills it in. Not very helpful, but that's the reason why.

Nophead uses Skeinforge to slice these models which is more resilient than Slic3r and has better dimensional accuracy than Cura.

OpenScad has recently released a new version that I believe fixes that bug (but I have not tested it). It was fairly easy to workaround - just put a small (0.01mm) hole in the design from the outside into the cavity (e.g. through the membrane) and the STL will be correct - albeit with a small hole that Slic3r should ignore (being too small to print).

Dave

That would be wonderful if I knew how to use OpenSCAD. I haven't found ANY free CAD software that is user-friendly enough for me to figure out yet, though, including 123D Design, Freecad, NetFabb Basic, Librecad, or Inkscape. The lack of a decent GUI that shows me what I am doing as I do it, and the use of Python scripts, puts OpenSCAD beyond me. I'm one of those people who needs an actual instructor looking over my shoulder teaching me how to use software if it isn't intuitive and user-friendly right off the bat (which is also why I only use Cura and Slic3r...)

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Rich K.
That would be wonderful if I knew how to use OpenSCAD. I haven't found ANY free CAD software that is user-friendly enough for me to figure out yet, though, including 123D Design, Freecad, NetFabb Basic, Librecad, or Inkscape. The lack of a decent GUI that shows me what I am doing as I do it, and the use of Python scripts, puts OpenSCAD beyond me. I'm one of those people who needs an actual instructor looking over my shoulder teaching me how to use software if it isn't intuitive and user-friendly right off the bat (which is also why I only use Cura and Slic3r...)

OpenScad does not use Python, it has its own language. It is not all that difficult to learn if you take it in stages. Start with very simple shapes - a cube, then a small cube on top of a bigger cube, then a cube with a hole through it etc. There are examples and tutorials on the Web. You can hit "F5" or "F6" after each command or set of commands you type to see the effect. Once you get used to it you may find, as I did, that it is a lot easier, quicker and far more versatile than using a GUI input. Yes, there is a learning curve that is steeper than many applications, but it is perfectly do-able (without a live instructor), and is well worth the effort.

Download and install OpenScad, fire it up and type the line "cube ([10,15,20]);" Do not include the quotes, but type everything between them. Hit "return" after the semicolon and then press F6. You should see a cube of dimensions 10 X 15 X 20 mm You will have just created your first OpenScad design. You can export it to an STL file ("File" > "Export"), slice it and print the cube. I'm pretty certain that within 2 minutes you will be able to design and print cubes of any size. You can then learn the syntax of the other basic shapes, then how to move and rotate the shapes to a specific position and orientation and how to subtract shapes rather than add them to create holes and slices etc. Each step is no more difficult to learn than the cube example, and you'll be surprised how quickly you can achieve quite intricate designs.

Dave

Edited 1 time(s). Last edit at 03/27/2015 10:01AM by dmould.

I use Designspark Mechanical for creating geometry. It's a free cut down version of Spaceclaim available from RS components and I've not found anything easier and faster. It definitely has limitations like the lack of a text tool (you have to create letters in Sketchup and import them) and limited input formats, but I like it. It doesn't always produce manifold STL files and I sometimes have to use Netfab to fix them.

I'm pushing to get the full version at work (we can get academic pricing, which is really good). There's a 3D print preparation module included and you can convert meshes to solid geometry.