I'm sure you're all tired of reading about threads after my posts on
standoffs,
standoffs redux, and
screws, but this is the home stretch!
Designing the case part 6: Implementing PEMSERTs
In
Part 5 I talked about what PEMSERTs are, gave a basic overview of how to select the right one, and went through some of the issues I ran into with the standoffs but I haven't explained how to actually implement them in the case design. This update addresses that omission!
The main reference source for this update is Penn Engineering's "
The Self-clinching Fastener Handbook", in particular page 12.
And rather than talking about standoffs
again, I'll walk through implementing the self-clinching nuts instead.
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Selecting The Right PEMSERT II: The Sequelning
I'll briefly go over this process again because it's important to know which particular PEMSERT will be used because we need to know its specifications to incorporate it into the design.
This time I need a threaded nut so I'll go to the
relevant page on the PEMNET site. Then I just use the same process of elimination I talked about for the standoffs. I don't need Floating, Blind, Miniature, Locking, Right-angle, Spinning Flare Nut, or Hard Panel for this application. Flush Nuts look interesting but they require a minimum of 1.5mm sheet thickness and in
Part 3 I had decided on 1.29mm for STX160.0 so they won't work.
That just leaves the Standard Profile nuts (and they're called "Standard Profile" for a reason, 90% of the time you need a PEMSERT nut you'll be using one of these). So now it's off to look at
the catalog and pick out the exact one I need.
It looks intimidating but if you know what thread size and material you want it's pretty easy to narrow down which one you need. I want to use M3x0.5 screws throughout the case and I don't plan on anodizing so I need M3 nuts in steel.
That just leaves determining the correct
Shank Code and like I said earlier, the sheet thickness is 1.29mm. So checking the column for Recommended Minimum Sheet Thickness that means I need a Shank Code of "1" since the next size up is 1.4mm. For nuts you usually want the maximum shank length for the thickness of sheet you're using to get the most thread length possible.
And in case it isn't clear from the drawing, the shank (dimension A) is what ends up within the sheet. The other half (dimension T) is what sticks out from the backside of the sheet after the nut is installed.
Putting that all into the
Part Number Designation guide at the top gives me a PEMNET part # of
S-M3-1ZI. From the parts page on the website I can download the 3D file for use in the CAD model.
It's a simplified model so it's not much to look at but it has all the important dimensions that are needed.
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Moar specifications
So I have the right part selected and have downloaded the 3D model of it, now what? To begin implementing the PEMSERT into the case we actually need to go back to the spec sheet and double-check two things:
Hole Size in Sheet
Hole Size in Sheet is exactly what it sounds like, it's the diameter of the hole in the sheet that the PEMSERT is to be installed into. So the M3x0.5 thread size for this series calls for a hole 4.22mm in diameter.
Getting this right is important, if the hole is too small the PEMSERT won't fit but if it's too big then the PEMSERT will be loose or can't be installed at all. If you model the wrong size hole the manufacturer will often catch it when they prep the CAD file for manufacturing, but it's better to get it right the first time
Minimum Distance Hole Centerline (C/L) to Edge
The following illustrations are from Page 12 of the
The Self-clinching Fastener Handbook.
Minimum Distance Hole Centerline (C/L) to Edge is the minimum recommended distance between the centerline of the hole to the edge of the sheet. For the part we're looking at this value is 4.8mm.
Sounds simple enough right? The problem is that in SFF case designs you'll often run into situation where the PEMSERT has to be located near multiple edges, which has special considerations:
So this could be resolved by moving two of the edges further away from the hole if possible. If that's not possible, then using the
Miniature nuts may be one solution since they have slightly smaller C/L to edge distance requirements. If in doubt, ask the manufacturer what they recommend.
Another problem is when the PEMSERT has to be close to a bend:
This gets a bit confusing because Penn Engineering says that the Min. Distance Hole C/L to Edge spec is measure to the outside of the bend radius. But that really only works on thin sheets with tight bend radiuses, with 2mm sheet and the same M3 nut we see that a 4.8mm distance measured that way can't work:
The only way this is possible is if you teleport the PEMSERT there
So instead I'd recommend measuring from the bend line:
This would be the same as measuring from the outside edge and adding Material Thickness + Bend Radius to the Min. Distance Hole C/L to Edge number.
If the PEMSERT can't be moved further away from the bend, then using one of the Miniature/Micro/Close-to-Edge alternative parts is one solution and not bending the sheet in the area around the fastener is another. You're probably wondering what I mean by that second workaround, but I'll come back to it next update.
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Making holes and avoiding bends
Ok, enough theory, let's actually implement some PEMSERTs! I'll start with the nuts on the sides that the top panel screws since they're pretty straightforward.
So on the bottom side flange of the inner part, I make some holes with the necessary 4.22mm diameter and position them away from the bend.
I'm a little under the 4.8mm from the bend line but it's because I wanted a nice 7.0mm distance from the outside edge. Like I said above though, the Min. Distance Hole C/L to Edge is supposed to be measured from the outside edge so this is fine.
Next I position the CAD model of the PEMSERT to check how it looks:
Note how the nut sticks out a bit from the backside of the sheet. This is important to take into account because it can cause interference with other components. Also, this is a nut so be sure to consider the length of the screw that will be used in combination with it. For example, if I move that standoff so it's inline with the nut and scoot it a little closer to the bend:
The nut itself clears the standoff in this arrangement with room to spare, but a M3 x 6mm flat head screw does not.
Using a shorter screw would resolve this situation, but if possible I would recommend against doing that when a simple repositioning would fix the problem. You want to minimize the number of unique screw variations used and try to only use readily available and common sizes. This makes sourcing the screws easier and cheaper, both for the manufacturer for production and for the end user for replacement in case they lose some.
BTW, remember in
Part 5b where it looks like there's still a bit of room to move the video card up but I said I'd explain why I didn't do that in a later post? This is why:
There isn't room to push the nut out to the side so instead I have to leave a bit of a gap between the card and the top of the frame so there's room for that nut.
Even then I'm pushing it as close as I can so there's space left between the back of the card and the motherboard so I'll have to use a particularly short screw in this one spot.
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I know I said last post that this would be the last one regarding threads but this has gone long (as usual) so I'll punt how to put PEMSERTs close to a bend to the next update. It's less about threads and more about bend reliefs, flat patterns, and such anyway so I hope ya'll don't mind
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