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How to mount boards in enclosure
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Please be positive and constructive with your questions and comments.

How to mount boards in enclosure

by jag_man653 on Tue Aug 11, 2020 2:45 pm

My Arduino project requires a 1/2 size Perma-proto board and a relay module. That module is on a small board with screw connectors. I've ordered the SparkFun PRT-13838 aluminum enclosure which is just right in dimensions. Now I'm trying to decide how to attach the boards to the enclosure base.

The option I've been considering is using 1/8" Nylon standoffs and M3-0.5 machine screws which will require drilling & tapping. No problem with that. There's a problem with screw length though. 7mm would be just fine since it would give full thread engagement in the 1/16" thick base. Unfortunately, if this screw exist at all it will be hard to find since McMaster-Carr shows 6mm and 8mm but no 7mm. With the 8mm I'd have to grind off the ends to avoid 1mm protrusion. I'm equipped to do that too. But there's another problem in that mounting holes in the relay board are perilously close to the screw connector blocks, so screw head size is an issue.

At this point I can't help but wonder if there isn't a better way. What other methods should I consider?

jag_man653
 
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Re: How to mount boards in enclosure

by adafruit_support_mike on Tue Aug 11, 2020 11:50 pm

Ultimately, anything that works is 'right'. The concept of 'best' only has meaning in the context of deciding how well a set of options serve a specific set of design constraints.

For tight spaces, it's sometimes useful to solder nuts, screws, or threaded rod directly to a PCB. Brass hardware solders easily, and zinc-coated steel will accept solder with a hot iron and plenty of flux. Flat-headed screws sit nicely on a through-hole and make low-profile connection points. Nuts created a strong threaded socket on a PCB surface.

Another option is to tap the PCB directly. A 0.1" through hole will take a 4-40 / 3mm-0.5 thread and FR4 is stronger than a nylon nut. A 1/16" PCB will provide about three turns of thread engagement. 90% of the force in a screw connection is taken up across the first two turns, and the forces for mounting connections are maybe 1% of a screw's nominal working load (an SAE grade-2 4-40 is rated for 200 pounds).

You can use a hot needle (or a 1/32" / #65 / 0.8mm drill) to make a hole along the axis of a nylon screw, solder a piece of pin header into a through-hole, then press the long part of the pin into the screw. It can simply be used to mount the screw, but you can also run the screw into a base plate and use the pin as a press-fit connection.

Also consider gluing spacers, standoffs, or angle brackets anywhere that might be useful. Cyanoacrylates and polyurethane glues (Gorilla Glue) make strong connections, and give you the option to put holes/surfaces where you want them.

Approach your design by deciding where it would be effective and convenient to make connections, then use whatever materials and techniques are necessary to put a connection there.

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Re: How to mount boards in enclosure

by jag_man653 on Wed Aug 12, 2020 1:38 pm

adafruit_support_mike wrote:Another option is to tap the PCB directly. A 0.1" through hole will take a 4-40 / 3mm-0.5 thread and FR4 is stronger than a nylon nut. A 1/16" PCB will provide about three turns of thread engagement. 90% of the force in a screw connection is taken up across the first two turns, and the forces for mounting connections are maybe 1% of a screw's nominal working load (an SAE grade-2 4-40 is rated for 200 pounds).


Thanks for this idea. I discovered the mounting holes on both boards tap nicely to 6-32. I'll drill the base for corresponding 6-32 clearance and countersink the outside so flat-head screws can come up from the bottom. I'm assuming there are 1/8" nylon stand-offs that 6-32 will pass through.

jag_man653
 
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Joined: Tue Aug 11, 2020 1:13 pm

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