Glue & Screw

Discussion in 'Materials' started by riverrat373, May 5, 2016.

  1. riverrat373
    Joined: Apr 2016
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    riverrat373 Junior Member

    The Spira boat project that I am going to build is a plywood on frame boat. The plans call for stainless wood screws to fasten the plywood. Would stainless deck screws hold better than regular wood screws? :confused:
     
  2. Barry
    Joined: Mar 2002
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    Barry Senior Member

    What material are your frames made of?
     
  3. riverrat373
    Joined: Apr 2016
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    riverrat373 Junior Member

    They are going to be standard 2X4's.
     
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Stainless deck screws have an aggressive thread for fast inserting, often with a self taping slot cut in as well. The stainless is pretty low quality and they're not especially strong. This said they'll work, but a stainless sheet metal screw or machine screw will work better. There isn't a "regular" stainless wood screw unless you special order them. Maybe FastenAll carries real stainless wood screws, but everyone else carries sheet metal or machine screws.
     
  5. Barry
    Joined: Mar 2002
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    Barry Senior Member

    A machine screw has a parallel thread, either fine or coarse, that takes a nut. So they should not be used

    A Sheet metal screw has parallel threads all the way up to the top of the head. This makes it hard to pull the first layer down into contact with the lower layer UNLESS the first layer, the plywood, is either drilled to the larger thread diameter or the screw is turned hard enough to strip some of the plywood to allow the threads in the 2 x 4 to pull the plywood down to make solid contact. This will create voids in the plywood.

    Properly sized deck screws have a thin shank with a rolled thread. Ie the diameter of the threaded part is larger than the diameter of the smooth shank. When the screw is turned in, the threads can open the hole size up a bit larger than the shank and there will be voids in the ply. And the shank might not be tight to the plywood

    A wood screw, properly sized will have a cut thread where the shank diameter is as wide as the threaded diameter so the shank will be tight to the plywood and reduce the chance of voids in the plywood.
    To do the job properly, you would purchase a wood screw drill bit.
    This will be sized to whatever the plans call for.
    For example a #8 wood screw 1 1/2 inches long.
    This drill has a small pilot drill at the end then has a larger drill for the shank and then is countersunk for the head.
    So the 2 x 4 is piloted for the threaded section, the plywood is drilled for the shank and it is countersunk for the head.

    Note: you want to buy the type of wood screw bit that will allow you to countersink the screw head down below the surface of the 2 x 4, in case you want to fill this after before painting. You can get some of these bits that will just countersink the first layer enough so the head is flush with the plywood

    Use a water proof glue on the screw before putting the screw in

    Re the quality of stainless

    If you go into most bolt supply shops and ask for a stainless wood screw you will, as Par suggested, get the lowest grade of stainless. You need to specify 316 or 316L. You will pay more but this is not where you want to save a buck, or several, if you want to keep the boat for awhile.

    If you can, buy Robertson style heads as they have less chance of stripping the head and are easy to drive
     
  6. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    You might also like to check the data that found decent rolled self tappers were better in shear load than 'proper' wood screws. I think it's from the Gougeon tests way back, but still valid. So chose your poison wisely.

    Perhaps bizarrely, I had occassion to use the data tables to justify some specification on a UK railway carraige fit out!. Pretty hot, sometimes the HMRI...;)

    I'm with Barry on the stainless grade, a minimum of 304 but 316 is better and TEST it. There's a lot of rubbish which usually shows up with poorly forged heads which strip, and also reflect poor rolling and hence tensile strength. Once you find a good supplier you will see and feel the difference.
     
  7. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Agreed, but with a side note: the proper name of this stainless is "grade A4" in the world of fasteners.

    The "L" in the 316L indicates a low-carbon version of 316, which is actually important only in applications where welding is required. Low carbon content decreases the carbide and chromium precipitation in welded areas, and hence improves the corrosion resistance of the welded product.

    So, again, you should look for grade A4 fasteners.

    Cheers
     

  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Agreed, testing has proven that machine screws actually have better pull out resistance than most wood screws and exceed shear strains (as mentioned) by a huge amount, particularly over the bugle headed, very coarse deck screws, now so common.

    Barry you are correct in that you need a pilot and clearance hole (and countersink if necessary) for real wood screws, but these screws are now hard to find, as most are simply rolled. Real wood screws fair much better against machine threads, but still fall a little short, though compaired to a rolled thread, they're superior.

    Stainless screws are usually sheet metal, not wood screws, again unless you special order of find a supplier that carries them. Rolled threads are used, simply because of the many options this process opens up to the manufacture.
     
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