A few basic standards

Driving in a few wood screws tonight made me remember a few things memorized. Are they correct or good standards if not? No idea of the standards other than possibly "Backstage Handbook" on the tie line.

-80% of the holding power of a wood screw is in the tip of the screw. (Thru drilling or extra depth pilot holes don’t help in holding power, you want to at least finish driving the screw by digging it into the wood.)
-Sheer strength of a drywall screw is 120#. (Suspect this is in reference to a #6 drywall screw.) So any plate that is in sheer - with load on the screw instead of lumber to lumber should have at least 100% more screws for load rating than expected load say per step.
-Breaking strength of a 5/16" Lag bolt, or anything Grade 2 is about 200# in general.
-90# SWL for 1/16" GAC wire rope.
-280# SWL for 1/8" GAC wire rope.
-40# SWL for tie line - static line.


-14 TPI blade for anything 16ga or thicker up to 3/8" in mild steel, but marginally up to only 3/16" in aluminum - all with brushed on cutting fluid for say up to 10" length of cut. 10TPI after for corded type hand tools for thicker excluding band saw. 18TPI for anything down to say 20ga including EMT conduit, and 20-24TPI for less thickness in cutting. (Been around longer than I in the industry someone’s use a 14 TPI Sawzall on Din Rail.... Yea, he went back to his hack saw. Really old timer...)

I'm sure everyone has a different opinion, but generally if I'm worried about the holding power of a fastener I don't use a screw. And generally I try to design things so that load is supported by the strength of the material, rather than the shear strength of the fastener. (Why screw a platform leg to the inside face of the stile when you can use a compression leg under the stile?)

I'd rather go by the manufacturers specs when dealing with wire rope than a general rule. Breaking strength can vary and by extension the SWL will vary. Not to mention your safety factor. I've seen them spec'd from 5:1 to 10:1 depending on the application. So the 1/8" Cable from Peak Trading that I usually buy that has a MBS of 2000# that wouldn't pass muster if your "standard" SWL is 10:1 (200#), but would pass if it was 7:1 (285#) or less.

Have fun with the TPI. As long as it cuts through I'm happy. If I'm going to be cutting the same thing all day I might care a little bit more, but I try to go as coarse as possible without burning up the blade after 2" of cutting.

I don't think I would ever rely on drywall screws for holding much. I think that the people that use them to attache legs to platforms are - politely - more than a half bubble out of plumb.

BillConnerASTC said:

I don't think I would ever rely on drywall screws for holding much. I think that the people that use them to attache legs to platforms are - politely - more than a half bubble out of plumb.

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Seen this done, I thought it was insane (why you don't let a Cabinet Maker be your Master Carp!)

RileyChilds said:

Seen this done, I thought it was insane (why you don't let a Cabinet Maker be your Master Carp!)

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Actually we had a master carpenter who was a cabinet maker, he was very apt with building and I know that when the use was appropriate he would use the correct hardware. So lumping all cabinet makers bad at building anything but cabinets is a disservice

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DuckJordan said:

Actually we had a master carpenter who was a cabinet maker, he was very apt with building and I know that when the use was appropriate he would use the correct hardware. So lumping all cabinet makers bad at building anything but cabinets is a disservice

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Not speaking for Riley (who in the small world category I went to a Brewers game with when he was much younger than he is now) but probably to suggest a drywaller than a cabinet maker.

I'm going to go with the idea of, "It doesn't matter what you build it with as long as you build it well." I find many appropriate moments to use multi-purpose screws when attaching legs. If you design a platform with the screw being the most likely point of failure then you need to rethink your structure. Bolts may not even help if you don't understand loads, compression, lateral loads, and sheer forces. I see nothing wrong in using three well placed #10 deck screws per leg to hold a platform at 8" off the deck. I am more amazed by those who use dry-wall screws to attach rigging hardware. I spent 3 years berating a TD to finally convince him of the errors of his way and I finally took over rigging all the flying pieces to stop him. If anyone knows who is teaching this technique to use screws on rigging hardware, please make them stop.

I think we have all read a lot into these general questions about screw concepts. My example was just an example, perhaps a poor choice given the above.

In this case I was driving in some screws from ½" 5-ply AC plywood into oak drawer glide rails that had already been glued and pneumatically narrow crown stapled from the opposing side. These are tool drawers for a cabinet and even if not heavy persay (given 1" - 1.1/2" high x 11.1/2 wide x 17" deep drawers.) I wanted extra strength to the rails from the plywood weaker side because, I always overkill what I build and hate when stuff falls apart even years later..

-Always fasten from the weaker material to the more dense, thicker or harder material. (Added rule?)

The 80% of holding power question came up in thought while doing this with the rest easy memorized factors to follow in post. On this cabinet I specifically made my pilot holes to a depth just short of the screw’s taper tip. This as opposed to just doing long enough or thru pilot holes in making it easier in getting the screw thru the oak.

Given I was on center for the 7/16" x 7/16" rails and had already glued and stapled them, even if in this case into 1-10 year old oak scrap lumber, it wasn’t going to split as a factor. Was just extra mechanical strength.

I've taken to using deck screws with a T25 head. The local box stores sell them by the pound and I find my students are more adept at driving a torx over the more standard phillips. They are also self-tapping which makes life simpler. Although I can't find any shear data for them, a cursory glance makes me believe they're a bit more robust than their drywall screw brethren. In any event (and I think this point has been made before) screws function as helical clamps and a good deal of the holding power is in the frictional coefficient between the two fastened pieces. Anything permanent (like platform framing) gets screws and wood glue. I often will use recycled drywall screws as temporary fasteners in joints that need to stay put until a stronger fastener can be applied.