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I am designing a box and lid (for a home-made micro computer). Using Autodesk Inventor (Student Version), I've created my box which looks something like this:

I plan on 3D printing (ABS Plastic) the box and lid in the end.

enter image description here

And I am going to create a rectangular lid to encapsulate the contents. I know accuracy will be important to the printing, so I want to make sure I have the sizing right. If it helps you answer my question, the dimensions of the top of the box are 3in by 4in and I want my lid and its outer lip to be around 0.1in thick. I would like the seal to be pretty tight (won't fall off easily) but if it's too small I risk breaking the ABS Plastic when putting on or taking off the lid.

My question, however, is what should the difference in size between the outer rim of the box's top and the inner rim of the lid be?

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    $\begingroup$ What will hold the lid on the box? Is it going to be a "snap" connection? Will there be glue? Will you rely on gravity? How willing are you to "make it big" and then trim it by hand? $\endgroup$ – hazzey Dec 2 '15 at 3:13
  • $\begingroup$ @hazzey Sounds like like he wants an interference fit? $\endgroup$ – OnStrike Dec 2 '15 at 8:09
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To make the lid fit on reliably with as tight as possible, the only difference you need in the box and lid dimensions is the resolution of the 3-D printer. You need to account for manufacturing inaccuracies and inconsistencies. Rapid prototyping is very reliable and not subject to the same issues as machining like tool wear and fixturing issues, but no part will ever be 100% to the design. This sort of information should be available from whoever is printing the parts for you.

You don't want a full perimeter interference fit, as the corners of the box will be pretty stiff and won't compress easily, and a full perimeter interference isn't necessary to lock it on.

However, you should look at some method of securing the lid besides it being a snug fit. There are a couple simple ways to do this, especially when 3-D printing the parts. Some kind of snap fit will prevent you from adding extra parts to the design, but will add extra stress to the printed parts. This shouldn't be a big deal, because you don't need a lot of interference, but you may want to give yourself some extra strength and bump up to 1/4" thick walls.

Another possibility is adding holes to the corners of the case to allow the lid to be screwed on. If you make the holes about the size of the shank of the screw, they should automatically thread the hole, and you won't need to tighten it too much to keep the top closed if it's just sitting on your desk.

If you want to get even more complex, you could probably even make a hinged top, buy a standard door hinge at your nearest hardware store, and use a snap fit on the other side to lock it down.

Play around with some of these ideas in Inventor and see what looks good. 3-D printing is a powerful tool that allows you to create some complex geometry, and while a couple of rectangles is really easy, if you experiment with the CAD software to see what you can design, the 3-D printer can probably do a lot of that stuff without much extra effort.

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You seem to want an a slight interference fit so the top will snugly seat into the housing. Depending on your wall thickness and size you could probably oversize the "male" portion of the cap like .002-.003" compared to the "female" dimension.

That being said, and understanding that 3D printing generally cannot hold those tolerances I would try an alternative fastening method. Unless you have some time and material to play with to keep tweaking the 3D file for shrinkage and reprint until it fits how you want.

Possibly consider a snap fit like so(upper right and lower left examples): snap fit examples

You could probably get away with only one "tapered beam" and have the top rotate downward along the edge opposite of the snap fit downward and snap into the case. Think tv remote battery cover. You would have better allowances for fit and would be a nice detail for your project. Make sure you have a detail to be able to remove the top like a little slit for a screwdriver or coin to pry it open.

You could always use screws as well

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