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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.

  • I use 14-1 self tappers all the time on shipping containers to mount equipment on the walls including transformers, service panels and disconnects. The only problem with self tappers is the tip protruding on the outside. To solve the protruding tip "safety" problem I use a thin "razor" cut off disk for metal and cut flush with the metal. Using self tappers in this way will support a very heavy load even if the containers are moved. – Ed Beal Mar 7 '17 at 3:07
  • Sounds like you're attaching to the sheet metal (14ga ?) of the container walls ? I'm trying to avoid penetrating the "envelope", instead attaching wood to the square-section tubes that run along the top edge. Then I can nail shelves etc to that and to the wooden floor. So I'm asking about those top rails, which appear to be made of 1/8" steel. – RustyShackleford Mar 7 '17 at 5:03
  • The nomemclature is confusing me. You mention "self tappers" but that's not actually a term that appears in my link above (which is by far the most detailed thing I've read about these screws, but still doesn't really answer my question). Sounds like you're talking about what they call "thread cutting" or "self drilling". – RustyShackleford Mar 7 '17 at 5:07
  • Seems to me like I need a "thread cutting" screw. They seem to have finer threads (1/4-20 is typical), whereas the "self drilling" type seems to have the much coarser threads of regular thread-forming screws. So I tend think the latter wouldn't provide enough threads in metal that's only 1/8" thick, yet the metal is too thick to be warped into a thread (which is what I figure "thread forming" must mean. – RustyShackleford Mar 7 '17 at 5:34
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    I would also suggest, if you don't plan to remove them fairly soon, is a thread lock compound also. No matter what size thread you go with, it will lock it to the container. Blue is the lowest strength, if you plan to someday remove them, use Red if you don't plan to possibly ever remove them. – Jeff Cates Mar 7 '17 at 21:49

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