2 questions (filing in a hollow space in slicer, and weirdness in Blender)
 

2 questions (filing in a hollow space in slicer, and weirdness in Blender)  

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scott.a.mayo
(@scott-a-mayo)
Active Member

I need help understanding how to change some of slicer's decisions. I have a cylinder in my model, and it prints fine, but when the part is installed there's some lateral force on the cylinder as it's pushed into place. It's prone to snapping, just where it attaches to another part. I think it would be fine if I could make that part of the cylinder solid instead of hollow, but I don't see a way to do that. is there one?

And, in Blender, I came across something really odd. I understand what the terms union and difference mean. I use both in my model and they worked fine - until I added (union) a sphere to the top of a cylinder/cube union. I definitely chose Union, but it did a Difference operation. To make sure, I exported to slicer and it also showed that a Difference had occurred. When I switched the modifier to Difference in blender... it did the desired Union, and printed properly. I can provide screen shots on request. Anyone seen this? Did I hit a weird bug, or is there some strange side effect of something I did that changes these operations in this bizarre way?

Any help appreciated. 

Posted : 31/08/2020 6:54 pm
Neophyl
(@neophyl)
Noble Member

 

Check your normal directions for the blender objects.  If your object is 'inside out' then the effects are reversed.

For example the basic cube that Blender starts with.  Go into edit mode, select one of the side faces and pull it through the cube so its out the other side.  It will look normal.  Unless you go into the view options and turn on view normals.  Outsides of objects are blue and insides are red (by default).

As for slicer and your breaking parts.  Theres a few things you can do.  You could add a modifier mesh to increase the  number of perimeters in that area (perimeters add more strength than infill), or you could put a small cylinder modifier inside and set it to a different perimeter count, that way you can force a hollow volume inside with its own walls.  You could also model in  a void in the stl file to do the same thing with more control as placing objects is much easier in Blender.

If you post your project (File>Save project) from Slicer we can take a look.  

This post was modified 2 months ago by Neophyl
Posted : 31/08/2020 8:12 pm
scott.a.mayo
(@scott-a-mayo)
Active Member

@neophyl I finally found the option to turn on face coloring and... I have 3 UV spheres that are inside out. They were spheres I added recently.

How is that even possible? I did not hand-draw these - I shift-a added a mesh and selected UV sphere. I would never have imagined they'd be generated inside out, or that I could do anything to one to turn it inside out. 

I don't think I've have ever figured that out - it didn't occur to me until now that normals would even be used when generating polygon intersections. Thanks once again.

I think I have a print that will work now, but for future reference, how do I turn the sphere inside out?

Posted : 31/08/2020 9:11 pm
Neophyl
(@neophyl)
Noble Member

I'm not sure really.  Not with a sphere.  I just know enough that when you have weirdness happening with booleans then its always a good thing to inspect the normals as a first check.

Posted : 31/08/2020 9:40 pm
Diem
 diem
(@diem)
Eminent Member

@scott-a-mayo

As @neophyl suggests, the easiest way to add internal reinforcing is to insert narrow voids into the part where you want extra internal perimiters.  Sometimes this is not enough, parts are weakest between layers and if a narrow part is printed vertically it is weakest when subject to a lateral force.

As it is extruded plastic is stretched along the path of the prnt head and molecules are aligned along the strand; this is the strongest dimension, so:

If possible print the part so that the narrowest part is horizontal on the bed.  This is tricky with a long cylinder so there are two things to try:

* If you are desiging the whole construction try to avoid cylinders that will only print well vertically.  If possible use faceted rods and print them horizontally.

* If you are making spares for a pre-existing device then you might be able to make a cylindrical outer part with a hollow, square centre and print a seperate square sectioned strengthener, horizontally, to insert as reinforcing.

 

Hth,

Posted : 01/09/2020 5:32 pm
Neophyl
(@neophyl)
Noble Member

Or my favourite depending on the size of any ‘posts’ is to size them to fit metal rods and then do holes in both sides so that the metal rod/pin is the strength part. Bicycle spokes are usually 2mm and make excellent reinforcing media. You can buy them really cheap and then trim them to length with a big set of cutters or a hacksaw blade. Great for hinge pins too 😊

Posted : 01/09/2020 6:13 pm
scott.a.mayo
(@scott-a-mayo)
Active Member

@neophyl That's brilliant. Overkill for my current project (I did manage to print a shape I could use and install it) but something I will think about for future projects. 

Posted : 01/09/2020 7:36 pm
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