I think that it doesn't matter how strong the threads are, in that thin of
steel, the weak link is the steel. It will not hold much, you realize. The
poster that said a sheet metal screw is the best bet is probably right.
Jim in NC
Generally, coarse threads are stronger. To get full strength, the material
thickness needs to be about equal to the screw root dia (tap drill size),
assuming the material strength is approx equal the the screw material
strength. For a 3/8 tap, the drill size is about 9/32 or 5/16; therefore
your 3/16 material is going to be weaker than the screw, meaning the screw
will strip the thread in the 3/16 stock before it breaks.
The key to a strong joint here is tight threads - make sure when you tap the
holes the tap is sharp, straight and most important, clean it out often -
3/4 turn forward, 1/2 turn back. Pull it all the way out every 2 or 3
times through this process and clean it - then lubricate (don't need
machinists lubricants - 3 in 1 oil is fine for this size thread).
I don't think you'll see much difference in fine or coarse threads - I
believe 3/8 coarse thread is 16 TPI, this will give 3 full threads, fine
thread is probably 22 or 24 TPI, giving about 4 threads - what you may gain
in having the extra thread, you'll loose in the weaker thread - a wash.
PS - Trivia question - what's the difference between a screw and a bolt?
Answer - A bolt is inserted through a piece of material and has a nut put on
the other side. A screw (wood, sheetmetal, machine etc) is screwed directly
into the piece of material. Wood and sheetmetal screws are generally self
tapping, machine screws need a tapped hole.
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