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Many computer cases have these holes / cut-outs on the frame. What are they for?

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Ramhound
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VAN
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4 Answers4

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As far as I know these serve 2 purposes:

  1. The small lip in the middle of each shape is usually slightly bend outwards so it touches the lid/side-cover when that is on. This provides a number of electrical contact points all around the cover with improves the "Faraday-cage" effect of the case as a whole. This in turn should help preventing the computer generating electronic noise to other equipment near it. (The 6 normal retaining catches are not enough to do this: Many covers are slightly bend/warped because they are of very thin, cheap metal and don't sit flush to the entire frame because of that.)
  2. The other reason, which sounds a bit dubious to me to be honest, is that the little lips help to dampen vibrations in the frame and/or cover caused by the moving parts (cooling fans, hard-drives).
Tonny
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  • Although all the lips point upwards in the image, I have seen some cases where the lips point upwards on one edge and downwards on the other. Any reason for that? Or is that simply because the edges are manufactured as single pieces and then cut and turned? – VAN Sep 27 '16 at 21:00
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    *"generating electronic noise"* -- The proper phrasing is "to control EMI emissions". Unless you can cite credible sources, IMO you should omit #2. The fingers have only an electrical function, not a mechanical one. – sawdust Sep 27 '16 at 21:17
  • The parts of the case also need to be electrically connected so they all stay grounded, so an electrical fault goes to ground rather than electrocuting you. It's not a high-risk given how most of the components are low-voltage, though. – BlueRaja - Danny Pflughoeft Sep 27 '16 at 22:39
  • Another point against the electrical grounding usage is that there is no need for so many pins / fingers / lips to make sure contact exists between the case and the lid, far fewer would have been enough. The dampener usage seems more credible. – VAN Sep 27 '16 at 22:49
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    Any long gap in the chassis will allow EMI out. You need to break up the long gaps. That's what the little conductive fingers do. – Dave Sep 27 '16 at 23:07
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    Similar holes can be seen on the 3.5" devices holder. Do they perform same/similar functions? – VL-80 Sep 28 '16 at 00:23
  • @sawdust they need some level of springiness to them to make sure they don't themselves rattle as the case vibrates. Of course as you say this isn't a primary function because if they weren't need for EMI they wouldn't be present to rattle. – Chris H Sep 28 '16 at 11:58
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    They don't "dampen vibrations". They prevent rattle by removing slack from imperfect latches. The final effect is similar, though. And it's pretty much same logic as for EMI - if there would be no gap, there would be neither room for rattle nor gap for the EMI to escape. – Agent_L Sep 28 '16 at 14:44
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    @Agent_L I think yours is the most comprehensive answer summing up all the answers. The fingers / lips reduce the ill effects caused by the gap between frame and case lids. Can you please add it as an answer instead of a comment. – VAN Sep 28 '16 at 15:44
  • I don't think rephrasing or summing up other answers counts as a legitimate answer. – Agent_L Sep 28 '16 at 15:45
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    The filling gaps explanation which addresses both EMI leak and cage lid rattle problems still doesn't explain why similar holes are present in the hard drive cage that VL-80 pointed out. @VL-80 May be you could start a new question and link back to this question as well. I uploaded a new annotation here: http://i.stack.imgur.com/srIyR.jpg in case you need one :-) – VAN Sep 28 '16 at 16:19
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    @ChrisH *"hey need some level of springiness to them to make sure they don't themselves rattle"* -- No the "springiness"* is solely to ensure electrical contact. I've provided a link in a comment above that shows other styles of EMI gaskets. Nobody has provided anything other than guesses (disguised as fact) for this alleged rattle/vibration function. – sawdust Sep 28 '16 at 18:24
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    @sawdust: On the other hand, you've not provided anything but unverified assertions for the loudly repeated claim that there is no such function! It's clear from your link that they provide an electrical function, but that doesn't mean they can't have ancillary use. Note that I'm not personally claiming they do or they don't. – Lightness Races in Orbit Sep 29 '16 at 11:57
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    To add to this answer, the "hole" or "cutout" isn't the important part -- it's the "finger" of metal that's left within the hole or cutout that actually performs the function. Cutting or punching these like so is simply a quick and economical way to get there. – Doktor J Sep 29 '16 at 19:18
  • @VAN I'm guessing it's a matter of how the case is constructed. If the frame is a single sheet of steel, it's probably more economical for them to just take one long sheet and repeatedly punch in the same direction, notch at the corners, fold the edges, then fold the panel around the frame. In a case as shown in your picture the panels are made separately, then riveted onto the frame, so they can be put on whichever way the manufacturer desires and at that point it's just down to aesthetic preference. – Doktor J Sep 29 '16 at 19:23
  • @LightnessRacesinOrbit -- I was challenged to provide substantiation to my comments on grounding/EMI. I did. I challenge the numerous "vibration/rattle"claimants to do the same. They haven't posted anything so far. IMO if the fingers provide any "rattle/vigration" relief, then it's incidental and unintended. See the OP's own answer. **There's no mention of such a mechanical function in the patent for these fingers.** – sawdust Sep 29 '16 at 20:52
  • @DoktorJ I too think so. In fact, the chassis patent itself states that prior art was to attach few contacts to the perimeter to improved metal-to-metal contact and this design is a more economical one in that in can easily be automated during manufacturing. – VAN Sep 30 '16 at 06:16
  • The requirement of the tags is two fold. Firstly, the case itself must fit together snugly, otherwise you will have unwanted rattles. Further to that, it must protect against electromagnetic radiation, thus is must have a well grounded case with a metal structure around which blocks radiation of concern. If the tags weren't metal, they would be replaced with rubber bungs to prevent noise which would weaken it to EMR. If they weren't there to hold the side of the case in position, they would be replaced by magnets/screws/something. – XtrmJosh Sep 30 '16 at 13:19
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The spring fingers are definitely for reducing gaps and to provide better grounding, both of which reduce EMI emissions. The relevant patents covering chassis and peripheral cages.

Editing to add some references that support the damping idea as well. Although they are not for the chassis per se, one patent covering media drives mentions vibration damping and a second patent for a hard disk frame in fact mentions both the uses.

VAN
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    Congrats on finding a definitive answer (to your own question). Note that *"reducing gaps"* has nothing to do with reducing rattles or vibration. – sawdust Sep 29 '16 at 20:56
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    @sawdust Thanks to you actually, I went looking for the EMI answer and landed on the patents. I don't get why the EMI point is still being discussed even after posting these links. Now, the dampening argument may be valid, but, like you have been saying, a reference to that in this specific context would be good. I haven't found any so far. – VAN Sep 30 '16 at 06:10
  • @sawdust that edit should now make all of us happy :-) – VAN Oct 01 '16 at 04:46
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    In support of this answer - I actually sent e-mail to one of the major computer cases manufacturer and asked them. The answer I got was `They are called “EMI clip”. It’s a structure to lower the EMI emissions to a safe level obligated by US government.` – VL-80 Oct 03 '16 at 14:19
  • @VAN I got most of the points, but you actually substantiated with references. Good job! Will give you my points in the form of the bounty tomorrow (takes 24 hours before I can grant it). – Tonny Oct 26 '16 at 15:26
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The "holes" aren't very important, but the little fingers inside press outward against the side, lessening vibration and noise. Another way to do it is with little insulating pads glued between the frame and the side, but that adds several construction steps and cost.

Christopher Hostage
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    Any references that explain this in more detail? – VAN Sep 27 '16 at 22:06
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    @VAN I don't have a reference, but this is a standard mechanical engineering design to supress vibrations. If the case tries to vibrate, there will be relative movement across the fingers, and the friction between them as they move absorbs energy. A "rigid" joint (e.g. several bolts along the edge that prevent and relative movement), or no contact at all along the edge would be much less effective at removing energy that could make the casing vibrate. – alephzero Sep 28 '16 at 00:03
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These tabs are designed to every so slightly push the side panel that you slide on the side of the case off of itself to prevent vibration noises and to hold the panel more securely in place by providing tension.

bcm27
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