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I've built an SD card box a few years ago. Now I am trying to reproduce it in Lego Digital Designer.

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Closed

However when I tried to reproduce the closed version, LDD refuses to snap it in place.

A

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In real-life, my box just snap very nicely. Why am I not allowed to do that in LDD?

Here is the WIP .lxf file

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3 Answers 3

up vote 16 down vote accepted

I believe that the issue here stems from the fact that Technic bricks and System bricks are not 100% compatible. Here's a slide from a great presentation which explains the details:

enter image description here

This difference is barely noticable when building, especially in your scenario because the hinge adds some additional play, but I believe that LDD won't allow these to connect because they are actually off by .12mm.

I tried to take a few pictures to demonstrate what I mean, but they didn't turn out very well. Here's a System brick (tan) connected to a Technic brick (blue). You can just barely see the .12mm difference at the top:

enter image description here

The difference becomes more obvious when more bricks are added so that the error is allowed to compound:

enter image description here

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Seems legit (seeing the headlight brick example in the presentation slides). Just one question: is .12mm equal to 3/10LDU? That seems odd (I'm still a beginner in technical LEGO stuff) –  Alvin Wong Apr 29 '13 at 5:04
    
You are correct that .12mm is the same as .3 LDU, and this does seem like a strange offset. My guess is that this decision was made in order to create the absolute minimum difference between these parts. I'm guessing that most children who build with these parts have never noticed this minor inconsistency. –  jncraton Apr 29 '13 at 13:36
    
Great answer. I never knew about that offset. Thanks! –  Peter Cassetta May 2 '13 at 18:04
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Interesting... I just tried this myself (with real bricks) and the two parts definitely snap together nicely. No stress or anything. I'm wondering if the issue is the 1x2 plate. Maybe LDD can't deal with a hinged part having to snap into two places at once. What about if you remove the bottom 1x2 yellow plate? Would it snap in then? In that case perhaps first attach the 1x2 to the healight bricks, then attach the hinge-plate to the hinge, then rotate it towards the plate and see if it would snap on the 1x2 plate.

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LDD uses exact numbers, where as in real life ABS and polycarbonate have some give or flexibility that a computer program can't factor in. There are many documents about LEGO part stress that explain what a "legal" vs "illegal" build is. Any illegal build will not work in LDD. I break the rules all the time too, but LDD is looking at it from a pure math sense, which you can't cheat. Try and create a curved wall made of stressed 1x2 in LDD. You can do it in real life, but not in the program.

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