How to internally strengthen a post going into a platform in CAD/STL
 
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SeattleDavid
(@seattledavid)
Estimable Member
How to internally strengthen a post going into a platform in CAD/STL

I have a design which basically consists of a post going into a platform/stand. The post is subject to force which will stress the point of intersection between the platform, and the post (as indicated in the drawing below.)

I would like to know if there is a technique to strengthen this intersection, preferably by thickening the walls at this point, or by adding some sort of pattern top the design.

There are a number of limitations which preclude several of the more obvious answers:

  1. A sleeve slides over the post, so nothing may protrude out of the post, although there may be slots, grooves, or indentations into the post.
  2. The deliverable is an STL file, so the solution cannot involve slicer-based solutions, such as changing in fill density on certain laters or in a certain region. The STL file may be sliced for numerous (unknown) printers. So the sloution needs to be eliminated in the CAD design so it appears in the STL file.

What I am hoping is that by adding some sort of pattern, I can cause slicers (specifically prusa slicer) to thinking up the amount of plastic surrounding the post-to-platform joinery.

Here are a few things that don't work:

  1. Widening the base of the post. Not only does this interfere with the sleeve that slides down the post, but it actually doesn't thicken the plastic infill.
  2. Extending the post down into the platform...Prusa Slicer sees this as superfluous and just eliminates it and turns it into standard infill.
  3. Adding negative (indented) gussets doesn't work.

Any thoughts?

Best Answer by Neophyl:

As you have ruled out the use of a fillet to avoid a stress riser along those edges the only one that occurs is as follows - add some hollow voids into the model itself.  As these are proper hollows the slicer should add perimeters around them as usual.  They need to be big enough not to fall foul of the slice close radius but otherwise dont need to be that big.  That should work with any/most slicers.  

Make sure the voids extend slightly below the surface so that perimeter/shells go lower than that interior you have pictured.

This topic was modified 8 months ago by SeattleDavid
Posted : 14/04/2021 7:18 pm
Neophyl
(@neophyl)
Famed Member
RE: How to internally strengthen a post going into a platform in CAD/STL

As you have ruled out the use of a fillet to avoid a stress riser along those edges the only one that occurs is as follows - add some hollow voids into the model itself.  As these are proper hollows the slicer should add perimeters around them as usual.  They need to be big enough not to fall foul of the slice close radius but otherwise dont need to be that big.  That should work with any/most slicers.  

Make sure the voids extend slightly below the surface so that perimeter/shells go lower than that interior you have pictured.

Posted : 14/04/2021 7:40 pm
Baklin
(@baklin)
Reputable Member
RE: How to internally strengthen a post going into a platform in CAD/STL

I have really no idea if this works for you and if Prusa does this for strength. But I noticed this when modifying the Z-axis top pieces for my MK3s.

 

Download the files from the Prusa site and slice the z-axis-top.stl. You will see when you look at the internal layers that they made small gaps in the parts. This will add extra perimeters inside the part and add strength.

 

Posted : 14/04/2021 7:41 pm
Baklin
(@baklin)
Reputable Member
RE: How to internally strengthen a post going into a platform in CAD/STL

I see neophyl posted the same as I was typing.

Posted : 14/04/2021 7:44 pm
SeattleDavid
(@seattledavid)
Estimable Member
Topic starter answered:
RE: How to internally strengthen a post going into a platform in CAD/STL
Posted by: @neophyl

add some hollow voids into the model itself.  As these are proper hollows the slicer should add perimeters around them as usual.  They need to be big enough not to fall foul of the slice close radius but otherwise dont need to be that big. 

Make sure the voids extend slightly below the surface so that perimeter/shells go lower than that interior you have pictured.

This is a simple, great answer! Yes, it should work. Thank you.

Posted : 14/04/2021 7:47 pm
Neophyl
(@neophyl)
Famed Member
RE: How to internally strengthen a post going into a platform in CAD/STL

Thanks for the pictures Baklin.  They illustrate what I was describing better than I did 🙂  The shape of the void is basically determined by wherever you want the extra perimeters to go so can be simple of complex.  I don't know if its a common technique but its been around a long time and I've used it in the past when I've needed to.

Posted : 14/04/2021 7:49 pm
SeattleDavid
(@seattledavid)
Estimable Member
Topic starter answered:
RE: How to internally strengthen a post going into a platform in CAD/STL

This solution has worked perfectly! Simple, elegant, easily described, easily implemented, easily documented.

Thank you so much. I am certain I will use this technique again.

Posted : 14/04/2021 7:54 pm
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