Hi all,

I wanted to try some simple object with a moving part, and so I designed a cube with an embedded sphere.
The difference of radius between the sphere and the spherical hole of the cube is 0.5mm (the diameter of the sphere is 2cm).
I ordered two prints of the same model with different materials.
I received my first prints, and here are the results:

• the one in Tranparent Detail material is perfect: the sphere is freely moving inside the cube (and I can spend a lot of time playing with that )
  
• The one made of White Strong & Flexible material instead is just one solid piece, with no moving part .
  

I have seen topics about wall thickness, but what about "void" thickness? I mean what is the minimum amount of space you should have between 2 objects to be sure they will be disconnected ?

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From my experience, minimum gap thickness (for WS&F) really depends on two things: surface area (between contact surfaces) and print direction.

Here are some very small test models:


In this picture, the gaps are 2.0, 1.8, 1.6, 1.4, 1.2, 1.0, 0.8, 0.6 and 0.4mm.
In WS&F, only the 0.4mm gap was (at least partially) fused. All the other gaps (0.6mm and up) printed just fine. I didn't have a 0.5mm gap, so I don't know if would have been fused or not.

Also, the uploaded model was 3mm (H) x 11mm x 21.8mm. The measured dimensions of the printed model are 3.45mm (H) x 11.11mm x 21.90mm. So, the width and the depth are only about 0.1mm larger but the height (in the vertical print direction) is 0.45mm larger.

And, that's why print direction plays a part in minimum gap thickness. The "resolution" of the printer in a horizontal direction is better than its resolution in the vertical direction.

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Hi Magic.

I apologize for hijacking your thread here, but I thought it would be a good opportunity to share one more test print for moving parts.

I have one more print that is similar to the prints in the last post with a few changes.



In this model, the axles have diameters of 5mm and the gaps are 0.75, 0.7, 0.65, 0.6, 0.55, 0.5, 0.45, 0.4, 0.35, 0.3, and 0.25mm.

I built this model in the shape of an "L" so that it would be shorter than it was wide or deep. (I was hoping Shapeways would stage it so the shortest dimension was along the vertical axis and the axels would be "standing up" when it was printed)

Unfortunately, the model was still printed with the axels "laying down" and all but the 0.75mm gaps were fused.

The cylindrical holes that the axles pass through were all 20mm long.

Anyways, I hope this shows that the minimum "void thickness" really depends on a number of factors. So far, I haven't run into any problems where 0.75mm was too small of a gap (although it makes a pretty wobbly axle.)

-David

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Thank you very much for your answers.
From what I undestand, with White Strong and Flexible material, 0.5mm clearance can work most of the time but can be fused in the worst case (depending on printing direction, friction surface, no extra air gap etc.).
In my example, as the moving object is a sphere, I guess I am always in the worst case .
Instead, 0.75mm should be always fine.

PS: I double-checked and the WSF sphere seems really fused into the cube but I am sorry, I am not cruel enough to cut my very first creation, even if it is not perfect

[Updated on: Wed, 25 March 2009 19:00 UTC]