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I'm new to the electronic scene and lately I been purchasing a lot of project boxes for the stuff I make, I've noticed in about 75% of the boxes I purchase they come with small square shaped holes on both the base and the lid, I assume these are for some kind of mounting or for pillars. I've looked online and on here and I can't find any details.

Below is an example of one of the boxes I recently purchased of eBay.

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What is the technical term for these and what do they do?

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Just added another picture with more detail. I've purchased 4 boxes, 4 of them have those square holes, all from different sellers, so I imagine there must be a reason. –  bybe Aug 20 at 15:56
    
The squares don't seem to be in any practical angle either. –  bybe Aug 20 at 16:01
    
It would be nice to have an answer explaining why the inner hole is square. –  Chris Stratton Aug 20 at 16:08
    
The square hole is probably designed to give the plastic shavings somewhere to go so it doesn't stress the post. –  Max Yaffe Aug 20 at 17:47
    
@bybe The various rotations are because a square core pin is used that is loose in the mold. They obviously don't care about the angle for the intended application. –  Spehro Pefhany Aug 20 at 18:14

4 Answers 4

These are called bosses. A boss is a post with a hole that can accept a screw. If the boss is tall in relation to the diameter, it should be reinforced with gussets (those shapes on the side that help keep the boss from snapping off at the base).

enter image description here

Sometimes the posts have molded-in or ultrasonically pressed in brass inserts that provide a thread.

Edit: Given the detailed photos the OP added, I'm thinking the square holes are intended to take square inserts, as mentioned above. I'm not 100% certain of this, as most are round with teeth around the outside. Making them square would have some advantages, but does not seem to be common outside of India (though perhaps that Chinese company is Indian owned).

http://www.brasspartfittings.com/Brass-inserts/Brass-Square-Inserts.htm

For low cost, thread forming screws are used. Note that they are not the same as self-tapping screws used for metal. The thread form is quite different (much skinnier with sharper flutes) to cut threads into the plastic.

http://www.trfastenings.com/pages/Pan+CR+Z+PR30

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The square hole is probably designed to give the plastic shavings somewhere to go so it doesn't stress the post. –  Max Yaffe Aug 20 at 17:44
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My boss could use some gussets to keep from snapping ;-) –  Tut Aug 20 at 18:15

It seems obvious that square holes will have a somewhat wider tolerance for a range of screw diameters, which is handy for hobbyists who have an assortment of screws laying around.

The screw has to forge a thread for itself, and that means displacing plastic. If the hole is round, then it works best when the screw has a fairly precise diameter in relation to that of the hole. The inner diameter of the screw has to be narrower than the hole, because the inside of the thread (the "valley") provides space for the plastic displaced by the "crest" during thread formation. If the plastic has nowhere to go, it is increasingly difficult to turn the screw. The post could crack from the pressure. Yet, the screw cannot be so thin that the outer thread diameter doesn't provide a solid grip; it will then easily strip the newly formed thread when tightened.

If the hole is square, then the screw thread's inner diameter can be exactly as wide as the the sides of that square, yet it will still fit without undue difficulty; the displaced plastic can flow toward the nearby empty corners of the square. Yet, the screw can still be as narrow as what would be acceptable for a round hole of a diameter that can be inscribed in the same square.

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EDIT: As the OP pointed out he really meant SQUARE holes, so this answer isn't a match anymore. Which brings up kind of a meta question: Is it considered good practice to remove an answer after it has been voted on but becomes somewhat irrelevant for the question?


I hesitated to write an answer for a pretty obvious thing but here you go. The hole with the red circle is indeed a PCB mounting hole, similar to this:

enter image description here enter image description here

In most cases the holes are threaded (is this the right word?) and you can put a screw in there. The "pins" like those in the middle of your example casing either support the screw holes to hold the PCB in place or even snap into pillars coming the top of the casing. Providing lots of mounting options is a big plus for a PCB casing, because you have more design choices when designing the PCB around it.

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usually they are not threaded/tapped but you tend to use self-tapping screws which will tap into the plastic very easily –  JonRB Aug 20 at 14:27

You can mount circuit boards on top of these holes, using short, self-tapping screws. I'm not a native speaker, but you might call them mounting posts (or something alike).

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