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 Post subject: Center cap design?
PostPosted: Wed Jun 19, 2013 3:56 am 
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I was working on a puzzle, and ran into an issue making center caps.

I've done some searching, and many links led to Grigr's model but I can't download that for some reason.

So, my basic cap design is rods extending from the cap into the center piece, simple enough.

The problem is tolerances on these rods. Should I leave 0 tolerance or add 'negative tolerance' by making them a little bigger than they should be?

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 Post subject: Re: Center cap design?
PostPosted: Wed Jun 19, 2013 4:47 am 
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Can you provide a picture?

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 Post subject: Re: Center cap design?
PostPosted: Wed Jun 19, 2013 4:57 am 
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Here it is.

Image

On a side note, for some reason, whenever I need to fillet things like megaminx edges, I end up needing to split the part into 4 equal parts, fillet one and mirror them back in then combine them. What's going on? This never happened before.

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 Post subject: Re: Center cap design?
PostPosted: Wed Jun 19, 2013 6:47 am 
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Wow, that looks mighty complicated. I just did this:
Attachment:
File comment: Cap
Centre Cap.png
Centre Cap.png [ 102.34 KiB | Viewed 1624 times ]
The are two bumps in the centre piece and four depressions in the cap so that the cap can be rotated 90 degrees if needed during assembly. The flexibility of the WSF material allows the parts to clip together. I can't help you with your Solidworks problem as I don't use it :(

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 Post subject: Re: Center cap design?
PostPosted: Wed Jun 19, 2013 7:12 am 
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Gus's design is far superior. The rods design requires very precise tolerances which 3D printing does not have. If you add 0 or negative tolerance it will not go together. If you add any tolerance it will be loose.
The design Gus proposes uses the flexibility of the material and does not depend on getting close tolerances or a tight fit.

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 Post subject: Re: Center cap design?
PostPosted: Wed Jun 19, 2013 7:41 am 
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I just tried to make sure that the positions of the bumps and depressions was such that there should always be a "pulling together" force on the parts, even allowing for the manufacturing tolerances.

I also thought about doing it like this:
Attachment:
File comment: Cap 2
Centre Cap.png
Centre Cap.png [ 41.88 KiB | Viewed 1598 times ]
But as I wanted to get this design in WSF polished, the polishing process would almost certainly have broken off the stalks.

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 Post subject: Re: Center cap design?
PostPosted: Tue Jul 16, 2013 2:30 pm 
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Gus wrote:
I just tried to make sure that the positions of the bumps and depressions was such that there should always be a "pulling together" force on the parts, even allowing for the manufacturing tolerances.

I also thought about doing it like this:
Attachment:
Centre Cap.png
But as I wanted to get this design in WSF polished, the polishing process would almost certainly have broken off the stalks.


Sorry for the bump - but how big are those bumps? Is it just a half cylinder? I want to copy it for my designs...

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 Post subject: Re: Center cap design?
PostPosted: Tue Jul 16, 2013 3:18 pm 
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Yes, they are just a "half cylinder" shape. All you have to do is to ensure that there is always a "pull together" force when the bumps engage with the depressions even if they do not exactly align due to manufacturing tolerances. The bumps/depressions are about 0.4mm, which gives a nice click when engaged but they can be pulled apart if necessary.

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 Post subject: Re: Center cap design?
PostPosted: Tue Jul 16, 2013 6:45 pm 
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Gus wrote:
Yes, they are just a "half cylinder" shape. All you have to do is to ensure that there is always a "pull together" force when the bumps engage with the depressions even if they do not exactly align due to manufacturing tolerances. The bumps/depressions are about 0.4mm, which gives a nice click when engaged but they can be pulled apart if necessary.


Thanks - I had guessed about .3-.5mm, but nice to know what works...

What exactly do you mean by "pull together" - just that one part pushes into the other?

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 Post subject: Re: Center cap design?
PostPosted: Tue Jul 16, 2013 8:33 pm 
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I just had an interesting idea. The problem with caps is that you can cheat by putting the cap back in the right orientation, if the puzzle requires such. So, rather than using the same polygon as the cap shape, use another one.

For instance, use a pentagonal hole for a square cap. So it only fits flush with the other pieces in one orientation, cheat proof!

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 Post subject: Re: Center cap design?
PostPosted: Tue Jul 16, 2013 9:09 pm 
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for large caps clips are great. I use 0.1mm tolerances on every side and that will friction fit average sized caps.

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 Post subject: Re: Center cap design?
PostPosted: Tue Jul 16, 2013 9:43 pm 
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cubedude76 wrote:
for large caps clips are great. I use 0.1mm tolerances on every side and that will friction fit average sized caps.


Do you use a positive or negative tolerance?

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 Post subject: Re: Center cap design?
PostPosted: Tue Jul 16, 2013 11:21 pm 
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I take away material

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 Post subject: Re: Center cap design?
PostPosted: Wed Jul 17, 2013 2:38 am 
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jabeck wrote:
What exactly do you mean by "pull together" - just that one part pushes into the other?
Please refer to my very exaggerated diagram.
Attachment:
File comment: Cap Lock
Cap Lock.pdf [20.84 KiB]
Downloaded 58 times
As the cap is pushed into the cubie, the side walls deform slightly. When the bump begins to slide into the depression, there is a force which pulls the cap towards the cubie due to the deformation of the wall and the shape of the bump/depression. You should try to design the position of the bump/depression so that there is always some residual force (A) which is left in the deformed wall which is trying to pull (B) the bump and depression into exact alignment. This keeps the cap nice and tight. If you exactly align the bump/depression, then, due to the manufacturing tolerances, although the two parts will snap together, the cap might still be "loose" and move about slightly.

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 Post subject: Re: Center cap design?
PostPosted: Wed Jul 17, 2013 9:10 am 
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Gus wrote:
jabeck wrote:
What exactly do you mean by "pull together" - just that one part pushes into the other?
Please refer to my very exaggerated diagram.
Attachment:
Cap Lock.pdf
As the cap is pushed into the cubie, the side walls deform slightly. When the bump begins to slide into the depression, there is a force which pulls the cap towards the cubie due to the deformation of the wall and the shape of the bump/depression. You should try to design the position of the bump/depression so that there is always some residual force (A) which is left in the deformed wall which is trying to pull (B) the bump and depression into exact alignment. This keeps the cap nice and tight. If you exactly align the bump/depression, then, due to the manufacturing tolerances, although the two parts will snap together, the cap might still be "loose" and move about slightly.


Brilliant! Thanks.

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