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-   -   Strength of tapped bolts versus piloted sheet metal screws (http://www.sailnet.com/forums/gear-maintenance/51008-strength-tapped-bolts-versus-piloted-sheet-metal-screws.html)

GBurton 01-30-2009 11:48 PM

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?

trantor12020 01-31-2009 02:45 AM

pilot drilled sheet metal screw has large & deeper thread to bite into the fiberglass so my money is on sheet metal screw.

kaluvic 01-31-2009 12:31 PM

Me too...but the difference my be in the number of threads..the bolt may have more.
I still think the screew wins.

tommays 01-31-2009 01:04 PM

Maybe :D

BUT NOTHING holds anything all that strong in glass which is why most stuff gets through bolted

newpbs 01-31-2009 01:13 PM

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

sailingdog 01-31-2009 01:17 PM

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.

AllThumbs 01-31-2009 02:05 PM

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.

Eric

Omatako 01-31-2009 02:24 PM

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.

GBurton 01-31-2009 02:48 PM

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 :)

timebandit 01-31-2009 04:35 PM

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 (Post 441337)
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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