Pem Nut vs Tapped in Aluminum Sheet?
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  1. #1
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    Default Pem Nut vs Tapped in Aluminum Sheet?

    I'm designing an aluminum bracket for an electronics enclosure. The material is 10 gauge (2.588 mm) 6061 T6 aluminum. Each piece will be fastened with 9 stainless M3 screws. We'll have the brackets fabricated by a local sheet metal house in quantities of about 100 per order.

    What are the pros and cons of using Pem Nuts versus tapping directly into the aluminum?
    Which is cheaper?
    Which is stronger?
    If I do use Pem nuts, are aluminum (CLA) or Zinc plated steel (CLS) Pem better in aluminum sheet?

    Thanks.

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    In thin sheet, PEM nuts will definitely be stronger. Also more consistent in thread quality. You'll have to ask your particular sheet metal house about the relative cost of tapping vs (nut+insertion). I'd be inclined to use the CLS nuts, for no specially good reason.

    By the way, who is your sheet metal house? Triax Metal Products and Parks Metal Products are local to me (Beaverton area west of Portland), and I've used them both successfully for sheet metal parts. But I am always on the lookout for more suppliers.

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    2.588mm is five threads deep. The threads should be acceptably strong if made with a form tap.

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    Is there a specific requirement to use stainless screws? Stainless steel and aluminum make a galvanic couple, normally zinc plated steel would be used in such places

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    Quote Originally Posted by Mark Rand View Post
    2.588mm is five threads deep. The threads should be acceptably strong if made with a form tap.
    Buttt...if someone chamfers/deburr/first threads the hole both sides, that reduces to less than 4...

    Personally for production, i'd install pemserts every time

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    A long time ago i heard a saying that 3 threads are as good as 30, it has served me well over the years. I would tap them. As far as galvanic corrosion goes, if it stays dry, usually less of a problem.

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    If I was making a 100 of a bracket with 10 tapped holes, it would cost a few bucks

    With 10 pems, it would double the cost.

    If you are not loading the bracket high enough to strip the threads the PEMS are not worth it


    Galvanic couple is not an issue with ss/aluminum outside of salt water, or unless it was an aluminum screw in a stainless plate

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    Oh, and sheet metal houses have some cool tricks create more thread depth in sheet, if you can tolerate a dimple on the back

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    It's important to keep in mind the application is an electrical enclosure. I'd oversize the holes, maybe even egg shape them a little, then tap half of them using a 1/2" impact wrench with lots of air pressure and self-tapping screws. The other half, I'd leave rolling around loose in the bottom of the enclosure.

    The last thing you want to do is provide electrical equipment that has sound mechanical design and manufacture. No one will know what to do with it, never having seen it before.

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    I’d go w/ Zi plated steel PEM’s (S’s) unless weight is an issue, in which case I’d use the aluminum CLA’s. If I recall, CLS’s are 300 series stainless, not my first choice in aluminum.
    It depends though, is it a one time ass’y, *never* to be undone again *and* cheaper to tap and break edges on both sides? Then tap it, or drill/edge break and use thread cutting screws. If occasional disassembly is a possibility, them PEMs.

    I assume this is a flat part, as it’s 6061-T6? If bent, make the bend radii LARGE, or consider 5052-H32 instead. I’m told 6061-T4 (vs. -T6) is bendable/formable w/o cracking too, but have never used it.

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    I'm guessing that this is one off, so the costs won't be bad either way and threading is indeed fine if you weren't planning to mount and demount the parts a lot. If this is a production thing, 10 GA would be quite thick for a bracket to hold electronics. I could see using something that thick for DIN rails and larger electrical components inside of a NEMA box for example, but generally anything you can hold with M3 or 6-32 screws could be mounted to more like 18Ga Al or 20Ga cold rolled steel. Go look at desktop computers or servers, it's all like 22Ga steel that's formed and galvanized. Go see what specialized electrical enclosure companies suggest. To this end, if you are mounting circuit boards and the like, think about using female thread PEM studs. You need to stand off the board anyway, so integrating the standoff and panel minimizes fastening hardware. My experience with sheet metal is your first attempt at a design, even after you've been doing it a while is always like 2 gauges thicker than it needed to be!

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    Quote Originally Posted by PocoLoco View Post
    A long time ago i heard a saying that 3 threads are as good as 30. . .
    That only holds up if the male and female threads are of the same material. Most people don't have a feel for things like that, and 3 aluminum threads mating with a steel screw will die a squishy death sooner than later.

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