After a bit of a detour where I talked about
sheet thickness/bend radius and then
parts allowance, now it's time for:
Designing the case Part 4: Vent cutouts and frickin' laser beams!
While there are SFF cases with
no vents at all, they're typically low-power ruggedized machines intended for commercial and industrial applications. Pretty much any computer case intended for decently-specced hardware is going to require vent holes in some form or fashion for air to move through the chassis and cool the hardware (unless it's an open frame design like
Inwin's D-Frame).
So what does something as mundane as vent holes have to do with something as cool as lasers you ask? Well, to understand that I'll need to go over the different ways of cutting those vents out of the sheet
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Cutting sheet metal
SheetMetal.Me has a good overview of the different options from tin snips to CNC laser cutting machines but we'll just focus on the industrial methods since they're what would be used for a small-medium production run.
Plasma Cutting
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This process works by using electricity to heat a stream of compressed gas until it turns into plasma which is then blown through a nozzle towards the metal sheet. The sheet is grounded back to the plasma cutter's power supply so it makes an electrical arc. The heat from this melts the metal in the path of the stream, which is then blown away creating a cut.
Not as fast or precise for cutting thin sheet metal so it's not typically used by precision sheet metal shops.
Water jet cutting
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Did you know it's possible to cut metal, glass, granite with water? It's true! Water jet cutting works by pumping a mix of water and abrasives under very high pressure through a small nozzle, creating a highly erosive stream that can cut through most materials.
It's precise, creates clean cuts, and doesn't generate heat that could warp thin sheet metal, what's not to love? The downside is that water jet cutters have high operating costs (the nozzles are synthetic diamond/sapphire/ruby!) and cut slowly so they're typically only used when the other methods aren't otherwise suitable.
Punch Press
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Punches shapes out of the sheet, like a paper hole punch on steroids. It's a great method for when lots of holes of the same shape are required (like vent patterns). Also called Numerical Control Turret (NCT) or turret press punch, where the "turret" part of the name comes from the rotating cylindrical
Automatic Tool Changer (ATC) used to hold extra punch tooling that can be swapped automatically as needed to punch different shapes/features. Visible in the above picture with the numbered labels denoting the different tool stations.
The shape of the hole is limited by the available tooling though. Also, a punch press can't be used on it's own, it has to be used in combination with one of the other cutting methods to cut out the other profiles of the part. This can be done either by transferring the part from one machine to the other or by using combination laser/punch machines that combine both systems into one machine so both cutting and punching can be done in one step.
Since it cuts the holes via mechanical shearing there isn't much heat generated unlike lasers/plasma, but since there is a lot of mechanical force applied to the sheet warping can still occur with dense vent patterns in thin sheets. NCASE has run into this issue with the M1 since the top and side panels have no bends to stiffen the sheet and there are a large number of holes that have to be punched.
Other methods
These methods are not typically used for sheet metal enclosure production so I'll just briefly mention them so I can spend some extra time talking about the frickin' laser beams:
CNC milling,
photochemical etching,
stamping (only used at high production volumes), and CNC routers (like a really big Dremel).
Laser cutting
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Lasers! How do they work? This is what Wikipedia says: "A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation." I'm no physicist so I'll invoke Clarke's third law and say "magic" and focus on the applications instead, of which there are many: messing with cats, livening up concerts, mounting to the head of
sharks mutated sea bass, what else... oh yeah, precisely cutting sheet metal
Laser cutters are precise, versatile, and fast* so they're the workhorse of many sheet metal shops. It's very likely the parts of your sheet metal SFF case will be cut partially, or entirely, with a laser.
*There's a caveat to that speed. Lasers cut fast once they've started the cut but the initial pierce through the sheet takes a bit of time.
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Vents vs Lasers
And the sheet piercing limitation is how vent cutouts and lasers are related! A vent pattern consisting of many small holes like the
NCASE M1 pictured in the
previous update would be impractical to cut with a laser since the laser would have to stop and pierce the sheet, make just the small cut needed for the vent, then reposition, and stop and pierce again. In comparison, a vent pattern like the
NFC Systems S4 Mini with fewer but larger holes is more suitable for cutting with a laser.
So being aware of what equipment your manufacturer has is important when considering the vent pattern for the case. I would only recommend using vent patterns consisting of large numbers of small holes if they have a CNC press punch. Water jet cutting is just too expensive to use for more than prototypes or very small runs unless it's just required by the material choice or warpage concerns.
Heat warpage is another issue with laser and plasma cutting since they work by heating up the metal until it melts. The cut itself is very narrow but the heat still transfers to the surrounding sheet (the
Heat-Affected Zone) and depending on the material type/thickness, number of cuts, and the geometry of the part this can cause the sheet to warp slightly distorting the part. This would be another reason to avoid cutting out dense vent patterns with a laser.
Image source
There is
much more to laser cutting (assist gas, CO2 vs fiber, fixed vs flying head, beam focus, power, etc.) so for brevity I'll just go over one more consideration.
Which do you think is easier to cut with a laser: steel or aluminum? Aluminum seems correct, it's softer right? But counterintuitively aluminum is actually
more difficult for lasers to cut through than steel! The reason for this is that molten aluminum is highly reflective and reflects much of the laser power which reduces efficiency, so this is why I mentioned in the
post on sheet thickness that most manufacturers can't effectively laser cut more than 3-4mm aluminum.
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Other vent considerations
Aesthetics
Depending on the design goals and intended audience, how the vents looks may or may not matter. If it does matter then the best recommendation I have is experimentation.
What kind of pattern looks best will heavily depend on the design: placement (where on the case the vent is located), surface area (how much of the panel/frame the vent takes up), airflow needed (ratio of open vent vs sheet material), other elements of the design (angles, themes, etc.), material, and so forth.
Experimenting with different designs in the CAD program just costs time so I'd try several different patterns in the model and get feedback. Here
@K888D is experimenting with 3 different vent patterns on the different sides of his
LZ7 case (a great example of a non-sheet metal case BTW). But sometimes it's hard to tell if the pattern will look good without just seeing it in person so that's what prototypes are for.
Airflow
Very related to aesthetics is how obstructive the vent pattern is to airflow. A vent pattern with a bigger ratio of vent hole to surrounding material should allow for better airflow. Taken to the extreme though a single hole taking up the entire side of the case would provide great airflow but wouldn't exactly look nice (IMO but I think most people would agree
) but on the other hand a single 2mm slot at the front edge of the side panel would be very unobtrusive but cooling will suffer.
FWIW, Intel recommends a Free Area Ratio (total vent open area / vent area) of at least 53% in their
Thermally Advantaged Small Chassis (TASC) Design Guide. Here's their example vent pattern:
So doing the FAR calculation: (537 vent holes x π•2mm•2) / (130mm x 100mm) = 52%
Not sure why their example doesn't meet their recommendation but I did the calculation twice and came up with 52% *shrugs*
Also, I wouldn't necessarily recommend implementing this exact vent pattern here. As I've mentioned, this isn't suited to laser cutting, but even for a punch press extending this pattern to a large area would result in a lot of time spent punching holes.
This may require experimentation either with production prototypes or physical mockups to figure out the best compromise between aesthetics, cooling, manufacturability, and the other considerations I'll go over.
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I have a few more points to go over but I want to show what I came up with for STX160.0 now to illustrate the next consideration:
So picking up from
Part 2, the next thing I did after roughing out the enclosure halves was start figuring out the vent pattern I wanted. Normally this is something I would do later on in the process but in this case the vents are an important aesthetic consideration since I want the end result to plausibly pass as an ATX PSU at first glance.
I had a few design goals in mind for the top vent in particular:
- Good airflow (it's the video card's only intake)
- Visually interesting
- Instant recognizable as a PSU vent (to help sell the deception)
- Manufacturable with laser cutting
After looking at lots of pictures of power supplies for inspiration, I ended up going with offset concentric circles reminiscent of
higher-end EVGA units with the addition of 4 holes in the center fan hub section like
FSP units. Overall I think it strikes a good balance between the different design goals, hopefully it looks nice in person too
For the other vents I went with a simple obround pattern. While a
tight hex pattern is more common these days for better airflow I wanted something suitable for laser cutting and the simple slot pattern is also used on cheap/old PSUs (in keeping with the theme) so win/win.
Structural integrity
After all my talk about how lasers aren't good at cutting lots of small holes, you're probably thinking that a single row of taller cutouts for the side vents or half circle instead of quarter circle cutouts on top would be better because it'd have fewer cuts and less airflow obstruction right?
True, but it would also leave a long and thin unsupported strip of metal between the cutouts. Since this is only 1.29mm aluminum those thin strips would have quite a bit of flex to them as
@FCase unfortunately found with the rear vents on his/her
Chameleon prototype
So aside from how they'll be cut, another big consideration with vent patterns is maintaining structural integrity of the sheet. This means avoiding long, unsupported strips of metal and making sure there's enough material between vent holes to prevent excessive flex. How much material to leave is going to depend on the material type and thickness, you could get away with much narrower vent spacing with thick steel than thin aluminum for example.
What I do when designing vents is imagining pressing my finger against the middle of the vent pattern and trying to visualize how much it would flex. Some deflection is inevitable with the thicknesses of metal typically used for SFF cases, so it's a matter of limiting it to an acceptable amount, not eliminating it entirely.
A
Finite Element Analysis (FEA) simulation could help but if you have the software and know-how to do that then you probably aren't in need of this build log for guidance
Safety
This is pretty straightforward, are the vent holes big enough that children could stick their fingers through it and get hurt by a fan?
Another test some people use is: could someone accidentally drop a coin into the case? I think this may be going too far though because where does it end? What if someone drops a handful of metal BB's? So up to you.
EMI
Things like power supplies and video cards emit
Electro Magnetic Interference (EMI). Complying with the regulations for this isn't normally an issue for small-run indie SFF cases but it's something to keep in mind.
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I think that covers everything, but let me know if I missed some aspect of vent design!
Next post I'll go over standoffs for screwing motherboard to and other pressed-in hardware, but before I end this update let me just leave this here:
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