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Fully threaded wood screw

Discussion in 'General DIY' started by pooksahib, 14 Sep 2020.

  1. pooksahib

    pooksahib

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    Hi. This is one of the wood screws that I want to replace with stainless steel versions. Using my old ruler, I think they're M8's. The screw minus the hex-head is 50cm. Everywhere I look, the potential replacements are only partially threaded with a smooth section at the top.
    Have fully threaded screws gone out of fashion?
    If I have to go with a half-thread version will I have problems getting it into the existing hole?
    Thank you.
    screw.jpg
     
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  3. JohnD

    JohnD

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    I think you mean it is 50mm long. Length of screws is measured by the length that penetrates the surface, so excludes the head, except in countersunk heads.

    Yes, they all seem to have a plain shank near the head.
    https://www.ebay.co.uk/sch/i.html?_...osacat=0&_odkw=stainless+coach+screw+m8+-bolt

    unless you are fixing something very thin, such as a metal plate, to a piece of wood, there is no purpose in a thread near the head.

    What is the thickness of the thing you are fixing? Is it one piece of wood fixed to another?

    If the upper piece is a very hard material, you can put a short pilot hole in it, but for ordinary softwood, the screw will force itself in.
     
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  4. Notch7

    Notch7

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  5. matlob

    matlob

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  6. JohnD

    JohnD

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  7. pooksahib

    pooksahib

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    Yes, 50mm it is. Typo... My satellite dish bracket rusted away so I'm fitting a new dish with new bracket, both aluminium. The house is timber clad, there's a square block of wood behind a section of the cladding which I asked the builder to put in specifically to hold the dish. The new aluminium bracket is 5mm thick so I reckon I want as much thread as possible. Preferably stainless steel. I'll keep looking. Thanks, lads.
     
  8. pooksahib

    pooksahib

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  9. JohnD

    JohnD

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    the end of the screw that penetrates deep into your wooden block does the work to pull it tight. The shaft near the head is just a dowl.

    If it was the other way round, you'd use a short screw.

    BTW lube the stainless, and isolate it from the alloy with a plastic washer or something to prevent corrosion.
     
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  11. pooksahib

    pooksahib

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    Hi JohnD, and thanks for the tips. What's the benefit of the lube (and are you talking, say, general purpose oil or something like silicone grease?)?
    If the screw is stainless and the bracket aluminium where would the corrosion occur?
    Also, what do you mean by 'the other way round'?
     
    Last edited: 15 Sep 2020
  12. conny

    conny

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    You can counter bore the wooden block to accept the smooth shank but that defeats the object to be honest.
    Just drill pilot holes to make it easier for the threads to bit in and they will draw the smooth shank into the wood giving a nice tight fitting. The lubricant is to cut down friction when tightening the screws into the wood. When you tighten screws into wood they actually get warm, or even hot depending on the speed you drive them in, this is caused by friction between the screw and the wood. It can also form a barrier between the screw and any moisture. Silicone grease is better than oil, I've even used a bit of cooking lard when I didn't have 'proper' grease to use.
     
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  13. StephenOak

    StephenOak

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    At the junction between them. See https://en.wikipedia.org/wiki/Galvanic_corrosion for some background.

    John said that the part of the screw furthest from the head does most of the work and the part nearest the head does little or no work.

    The 'other way round' would be the part of the screw furthest from the head doing little work and the part nearest the head doing most of the
    work. If that was the case you would always use a short screw.
     
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  14. big-all

    big-all

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    i think john d will be referring to the fact the end at the head only has the purpose off pulling the the work together
    the grip will be the total off the thread along the full length not in the surface timber so not complicated but very simple as in not a complicated equation
    in general the fixing in the timber will be around 1.5 times the surface thickness so a 20mm plank will need around a 50mm screw that works best if no thread grips in the surface to avoid "jacking" where the screw entering the lower timber pushes the surface away before gripping
    this can often simply be archived by drilling a hole in the surface timber 1mm greater than the screw [shank size]
     
    Last edited: 16 Sep 2020
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  15. scbk

    scbk

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  16. pooksahib

    pooksahib

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