Re: Coarse or Fine Threads

At that thickness, sheet metal screws are coarse. A bolt, would of course be captured on the other side with a fender washer and a fiberlock nut.

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The operative words were "drill and tap".

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FINE threads are always stronger .. .. ..
The Wolf wrote:

-- I AM NOT PARANOID .. .. .. but EVERYONE thinks I am !! !! !!
<<<__ Bob __>>>
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On 9/11/03 11:43 PM, in article snipped-for-privacy@att.net, "<<<___ Bob

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Several threads in the metal. thin= fine Thicker = coarse. 3/16? go 3 thread minimum.
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How big is the bolt? I always heard the rule "3 full threads" when it came to how many threads in a tapped hole. Up to about 3/8-18 I would say course is OK in 3/16" steel.
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On 9/12/03 1:01 AM, in article snipped-for-privacy@mb-m26.aol.com,

So would 3/8-24 be stronger or weaker?
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opined:

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
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Didn't he say 3/16 steel? How strong does this need to be? I think you would part the bolt before you pulled a 3/8-18 stove bolt out of 3/16" steel. YMMV by the grade of bolt.
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"The Wolf" writes:

The knee jerk answer would be fine thread; however, a couple of questions:
What kind of 3/16" material?
What size fastener?
What is the load that will be held?
OK, that's 3 questions<G>.
--
Lew

S/A: Challenge, The Bullet Proof Boat, (Under Construction in the Southland)
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I think you are on the relevant points.
It seems the harder the material, the better fine threads will work for you. Soft materials usually seem to have course threads.
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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.
Ian
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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