TLDR: Can I mount PEM captive nuts in the lip of an aluminum chassis that is 0.040" thick, but PEM recommends swaging the shank into a minimum sheet material thickness of 0.056". Am I in danger of having the fasteners drop out because my material is 16 thou under the recommended thickness? Or should I consider shimming the lip with aluminum strips to build up the thickness?

I'm building a hobby project, so it's not a particularly critical application - not adhering to the manufacturer's specs would not cause major problems, rather at most it would be a minor inconvenience - nevertheless it would be good to know in advance whether I'm likely going to waste my time doing this.

I want to fasten an aluminum cover plate to the bottom of a Bud aluminum chassis (which has a 0.5" wide lip around all four sides of the opening to which I will screw the cover plate), but I want to be able to remove and re-fasten the cover plate multiple times.

The Bud chassis is part number AC-404 - the chassis dimensions are 10" x 5" x 3" - chassis spec is here: https://www.budind.com/pdf/hb401.pdf

This is not a precision application - it's just going to be the housing for a hand-held shortwave UV light box that I'm building that I hope to use for 'prospecting' for fluorescent minerals at night - I will need to put the cover on and take it off again while I'm building it (because I don't yet know if I have designed the electronics correctly yet, or even whether I have designed the mechanical 'guts' properly yet. Even after I've finished building it I will need the capability to (infrequently) pull off the cover to remove and replace a burned-out lamp, or maybe even to replace the UV filter if it gets scratched or broken or fogged.

I'm planning to put a 0.1" thick neoprene gasket between the lip of  the chassis and the bottom of the cover plate, so if I don't use a shim between the chassis lip and the cover plate, the extra 16 thou of PEM fastener would not be pressing directly against the cover plate.

I was originally thinking of drilling holes into the bottom cover plate and into the 'lip' of the chassis, and using self-tapping sheet-metal screws directly to do the job, and I may just end up using those. After all, the manufacturer of the chassis and cover plate sells them pre-drilled with four holes that accept #8 sheet metal screws (I'd like to use more than four holes to do the job, however, to get a tighter closure).

So I'm thinking of some possible alternatives, for example:

- #6 or #8 sheet metal screws into J-clips or U-clips (sometimes referred to as "speed nuts") - Harbor Freight sells an inexpensive kit ($6.00, or less with coupon) with an assortment of U-clips and matching screws, but I'm not sure of the dimensions of the clips and whether they will allow for the screw hole to fit within the width of the chassis lip, which is 0.50". It's also not clear what thickness of material those clips are designed to handle - the kit is here: https://www.harborfreight.com/170-piece-u-clip-and-screw-assortment-67588.html.

- or, possibly I can find similar speed nuts that take #6 or #8 machine screws

But what I'd prefer to use are PEM press-fit self-clinching nuts - while I have not used them in my own projects before, I have seen them used in high-quality off-the-shelf electronic equipment enclosures, and they seem to be well-suited for the task.

I can get a pack of 50 or 100 of them for something like 18 to 25 cents each off of eBay.

I would not be using the special tool that PEM sells for setting, rather I'd probably just jury rig something with a C-clamp and a couple small scraps of steel plate to act as anvils for making the press-fit.

The problem is that the thickness of the chassis lip is 0.040", while PEM recommends swaging the shank into a minimum sheet material thickness of 0.056". I will probably get #8-32 nuts, PEM part number CLS-832-2 (spec is here: https://catalog.pemnet.com/item/all-categories/self-clinching-nuts-types-s-ss-cls-clss-sp-unified/cls-832-2). For reference, the entire PEM fastener line description is here: https://www.pemnet.com/fastening_products/pdf/cldata.pdf

Also, I know just a little about the material hardness - PEM recommends a Rockwell Hardness Number B scale of 70 or less for inserting into aluminum, but the chassis does not list the hardness spec or aluminum alloy that is is built from, so I'm not sure if I need to be concerned about that.

Am I overthinking this? Probably.

Worst case I'd be out $20 for some fasteners that I can't use on this project, but might use on some future project, and some wasted time.

But if anyone here has actual experience using these and can shed some light on my question, I'd appreciate it.

Doubtful that they will not function.

Try a C-clamp and a ball bearing to set a couple of them

Ball bearing is an interesting idea, thanks.

I'm wondering whether an alternative might be 'nutserts' or 'threaded insert rivets' or 'rivnuts' - have not used those, and not sure if they are similar, or whether they require a much thicker panel than 0.040".

A 'rivnut' would leave a collar protruding out of the top side, but since the thickness of the material that I'm using is less than what PEM recommends, I'm likely to have something like that with the PEM nuts as well. Since I will be using neoprene gasket material, that is probably OK in my application.

I'm an admitted rivnut addict. They work in a wide variety of materials and thicknesses, are dirt cheap and look professional. The setting tools are pretty inexpensive as well and have a wide variety of applications.

I think setting the nuts you have with a ball will negate the thickness problem, but if you have room on the back side for rivnuts I'd probably use them if for no other reason cost.

After a bit more research it seems that I can get #8-32 stainless rivnuts that have a grip range of between 20 thou and 100 thou, which ought to work in this application.

I will see if I can find similar ones in aluminum - but I think that either aluminum or stainless would do the job.

Amazon for bunches of rivnut tool and nut assortment kits, then just buy more stainless nuts also from amazon at the same time.

Aluminum fasteners that are not hardened and surface treated suck donkey dick and are extremely prone to corrosion. Use stainless, use some form of anti-seize and call it a day

I ordered an assortment of stainless rivnuts - will either get a tool or will try setting them with a bolt.

Thanks for the suggestions...

Either one will work.

One is faster and much less likely to compromise the threads during installation.

The fastener of choice in the aircraft industry, where reliability matters, is nut plates riveted in place. I understand your project isn't as critical but nut plates are super reliable and so simple to install a caveman could do it...

I have seen references to special types of rivnuts that include a tab of sorts to prevent that, but those are not what I have (nor are they likely usable for ordinary DIY projects).

Would something like Loctite help to prevent that?

It may help, but not prevent. And once those rivnuts are spinning in an inaccessible place you're SOL.

I may be biased as I work in the aerospace industry. Rivet-on nutplates and solid rivets are what you seek. They're stupid simple, just drill holes, squeeze rivet in-place, be done. You often don't HAVE to have any special rivet squeezers; I've bucked MANY solid rivets using a hammer and punch.

TL:DR

I’m a design/mfg engineer. If you install then in accordance with manufacturer specifications, they will perform up to their rated specifications. Important things about PEM fastener inserts is material thickness, alloy, pilot hole tolerance, distance to edge, installation method, perpendicularity of reaction loads, and respecting the recommended maximum torque.

If you take shortcuts, there is a very real (almost for certain) chance they will not perform up to specification. They will work to some degree, but not 100%.

Just get your PEM stuff from McMaster.
There’s a bunch of fasteners rated for .040 sheet... even #8 hardware. Sound like a larger number of #4 or #6 screw holes would be appropriate though.
https://www.mcmaster.com/pem-style-nuts

Attached File

I hate Riv nuts.

I’m OK with PEM nuts. Clip nuts are good. Click bond is fantastic, but probably more money than your average hobbyist wants to spend.

I'm getting an education in captive nut hardware here, thanks...