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

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    Those are contact fingers for the sole purpose of ensuring that the mating panel is electrically grounded.
    – sawdust
    Commented Sep 27, 2016 at 21:10
  • 17
    They are speed holes. i.imgur.com/icegw4k.png
    – CptEric
    Commented Sep 30, 2016 at 7:06

4 Answers 4

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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).
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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
    Commented Sep 27, 2016 at 21:17
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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
    Commented Sep 27, 2016 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
    Commented Sep 28, 2016 at 0:23
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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
    Commented Sep 28, 2016 at 14:44
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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
    Commented Sep 28, 2016 at 18:24
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+100

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.

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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
    Commented Sep 29, 2016 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
    Commented Sep 30, 2016 at 6:10
  • @sawdust that edit should now make all of us happy :-)
    – VAN
    Commented Oct 1, 2016 at 4:46
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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
    Commented Oct 3, 2016 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
    Commented Oct 26, 2016 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.

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    Any references that explain this in more detail?
    – VAN
    Commented Sep 27, 2016 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
    Commented Sep 28, 2016 at 0:03
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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.

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  • 11
    So Harley-Davidson owners push these buggers inward to get back the proper sound.
    – Kaz
    Commented Sep 29, 2016 at 0:46
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    But, do Harley riders' PCs leak oil?
    – Sensii Miller
    Commented Sep 29, 2016 at 20:37

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