Strength of tapped bolts versus piloted sheet metal screws - SailNet Community

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Old 01-31-2009
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Strength of tapped bolts versus piloted sheet metal screws

OK for all you machinist/engineering types out there here is a question:
Is a drilled and tapped hole (bolt) into fiberglass more resistant to pullout than a correctly pilot drilled sheet metal screw of the same diameter?

Last edited by GBurton; 01-31-2009 at 11:45 AM. Reason: Clarify
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Old 01-31-2009
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pilot drilled sheet metal screw has large & deeper thread to bite into the fiberglass so my money is on sheet metal screw.
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Me too...but the difference my be in the number of threads..the bolt may have more.
I still think the screew wins.
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BUT NOTHING holds anything all that strong in glass which is why most stuff gets through bolted
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The question does not seem to be worth a lot of thought, IMHO. If you are concerned about the holding power of a particular fastener use a backing plate and through bolt the assembly.

I would always choose a tapped hole in steel over a self threaded hole.

Paul
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Old 01-31-2009
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Fiberglass is too brittle to take screws well. If the screws get highly loaded, the threads of the screw will just tear through the fiberglass.
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My thoughts is sheet metal screw unless the fiberglass is quite thick, If it's thicker than the diameter of the screw I might be inclined to vote for the tapped bolt.

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Old 01-31-2009
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Glass fibre never holds a fastener tightly for long and will wear away with even minute movement and a fine thread will leave very little "meat" for it to be strong in GRP (OK in metal). So a fine thread on a bolt will let go first, a course thread on a bolt will let go next, a course self drilling screw will let go third and a through-bolt won't let go at all.

I know what my choice would be.

BTW, if it's a light-usage fitting in reasonably thin GRP, the better fastener IMO would be a pop rivet of suitable size, as long as the end of the rivet goes all the way out the other side of the skin.

But still, a doubler plate on the other side would be my choice.
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Yes - of course a backing plate and through bolted is the way to go, but not possible in all applications. I'm leaning toward the pilot drilled SS sheet metal screw......thanks for the help.

P.S. Sailingdog - couldn't stay away could you
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Something I use--

After setting the screw remove it and put a drop of super glue in the hole and allow 24 hrs. before reinserting.

Quote:
Originally Posted by GBurton View Post
Yes - of course a backing plate and through bolted is the way to go, but not possible in all applications. I'm leaning toward the pilot drilled SS sheet metal screw......thanks for the help.

P.S. Sailingdog - couldn't stay away could you
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