what kind of sheet-metal screw do I need for wood to 1/8" steel?

Question

I am trying to attach pieces of 2-by wood to the frame tubes of a conex shipping container.   The steel is about 1/8" thick.   I probably need screws at least 1/4" diameter to get the strength I need (100+ lbs shear).

Found this fairly comprehensive article about all the different types of sheet-metal screws:

http://www.fastenermart.com/understanding-tapping-screws.html

... and they say you need "thread cutting" (aka. "Type F") instead of "thread forming" for "thicker and harder" materials.

My question is: would 1/8" thick steel (I'm assume it's mild steel in a ubiquitous thing like a shipping container, and it wasn't that hard to drill through) be consider a "thicker" material, or will an ordinary #14 sheet-metal screw suffice ?

An ancillary question: If ordinary sheet-metal screw is unsuitable as I expect, will a "self drilling" type such as:

https://www.fmwfasteners.com/collections/hex-washer-head-self-drilling-screws-zinc-plated/products/14-14-x-2-1-2-hex-washer-head-self-drilling-screw-zinc-plated

... work ?   I would likely drill a pilot hole anyhow, and use these as thread-cutting type.   The self-drilling seems easier to find than thread-cutting.

StayOnTarget · Answer

I'd consider 1/8" in the "thicker" category.

Thinner would be like steel ductwork, thick enough to receive & hold screws, but thin enough to use an ordinary sheet metal which just has a pointy threaded tip (looks similar to a wood screw).

I have recently drilled wood into steel on a few different projects which was about 1/8" thick and used screws like this:

Not pushing any particular brand but this type did work well for my application. The key element of the screw is the tip which is similar to a drill bit. The flutes of that drilling tip have to be at least as long as the metal is deep.

A tip is to make sure your screwdriver bit fits the screw head really well. Since it takes some time for the screw to bite (compared to screwing into wood) you need a good solid connection to your driver. If the wood you're attaching is thick enough that's not likely to be a major problem, however.

what kind of sheet-metal screw do I need for wood to 1/8" steel?

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