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Best way to join this wood

Discussion in 'Wood / Woodwork / Carpentry' started by DanGB, 14 Oct 2019.

  1. DanGB

    DanGB

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    Hi all,

    I am recreating an old railway signal arm to replace a rotten one.

    Due to the size:
    Length 1700mm, width 250mm, thickness 18/19mm, I need to use two pieces of cedar. Joined at the 19mm edge (1700mm long)

    What would be the best way to join these together, taking into account the forces that it will be put under. As its a signal arm it goes up and down, and when it drops it bounces a bit, so a bit more stress perhaps. See image attached for context.

    Would just a butt glue joint be ok, or is it advisable to do something else, like dowels? I don't have a biscuit jointer, but plenty of other workshop tools.

    And is there a best glue people recommend for this application? It will be painted all over afterwards.

    Many thanks!
     

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  2. johnny2007

    johnny2007

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    You don't need a biscuit joiner, you can make a tongue and groove with a table saw if you have one.
    Do you have a table saw?
    Why not buy a 250 width piece of wood?
    Are you recycling? (Nothing wrong with it, just curious)
     
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  3. DanGB

    DanGB

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    Hi and thanks.

    I do have a table saw yes.

    Also, I wasnt able to source any 250mm wide cedar. Max seems to be 200-220mm from about 15 places I tried.

    I keep reading about how wood glues are super strong, so hopefully the join will be good, but any help to make it stronger to make it last would be helpful maybe? Or is it even worth it or is a basic glue join ok?
     
  4. johnny2007

    johnny2007

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    If only one side is for show, you could use metal plates on the underside, recessed in the wood.
    Don't you love chiselling?
     
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  5. DanGB

    DanGB

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    Actually I have seen a metal plate on the rear of some signal arms. As you can see on one end, the 'pivot' end is encased with the metal bracket, so the metal plate would be good for the other end.
     
  6. johnny2007

    johnny2007

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    Something like this.
    This is the inside of a sliding wardrobe door, not extremely neat because nobody would ever look at it (1000mm doors).
    Hopefully it's clear.
    Your of course won't be an angle, but straight plates.
     

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  7. JobAndKnock

    JobAndKnock

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    If you are using glue it needs to be fully weatherproof - I'd suggest a D4 modified PVA such as this:

    D4 PVA Glue 001_01.jpg

    This is NOT a PU (polyurethane glue), so don't confuse the two. Your joints will need to be nice and flat (ideally "shot" with a jointer plane and no sign of daylight when one piece is laid on the other) and you'll need to ensure that the two parts are cramped together and left alone for several hours whilst the glue sets to ensure the best joint. Because cedar is oily I'd also suggest wiping the mating surfaces with a clean cotton rag dampened with isopropyl alcohol just before glueing up (to remove the oils at the surface and make a better glue bond). Finally for extra strength hardwood dowels (10mm) would be a better solution than biscuits for strengthening the joint
     
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  8. johnny2007

    johnny2007

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    Dowels... of course
     
  9. JobAndKnock

    JobAndKnock

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    Yep, old-fashioned, but cheap as chips and potentially extremely strong - which is what you need in this application. I'd have suggested Dominos, but have you seen the price of those machines? :eek:
     
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  10. johnny2007

    johnny2007

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    As we're here, why do you think that dowels make a stronger joint than biscuits?
    I never found any difference in my many projects
     
  11. DanGB

    DanGB

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    Thanks, I have one of these, so sounds like it will do the job.

    https://www.screwfix.com/p/titan-ttb579pln-204mm-electric-planer-thicknesser-230v/15774
     
  12. JobAndKnock

    JobAndKnock

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    In some cases, yes. I think this is one of them. Basically biscuits only extend 10 or 12mm into the wood but only4mm thick and for that they require a largish pocket to be cut out. That makes for a long join which will be less stable/strong when, say, a wind blows against the flat face of the signal than would 10mm dowels inserted 50 to 70mm into each of the edges of the timber on similar drilling centres. You could also set the dowels on closer centres - biscuits really need to be on a minimum of 80mm centres. Finally biscuits are made from compressed beech. Whilst this is perfectly suitable for interior use, where it remains dry, for exterior use, where the MC of the timber will end up at 16 to 20% quite quickly, it will be disasterous as the beech will blacken, rot and swell over a relatively short period (3 to 5 years) and cause the joint to open or fail. This is not a problem you'll have with oak dowels (avoid beech like the plague in this instance)

    BTW there are loads of hardwood dowels to be found on ebay - just remember to taper the ends (sandpaper, block of wood) and put a couple of saw cuts down the full length of the sides to relieve glue pressure

    Yes, but add a 12mm plywood or MDF fence plate (3 to 4in/75 to 100mm deep) onto the edge guide to make it easier to maintain a dead square cut and check it for square with the sole of the planer because not all fences are square and some shimming may be required
     
    Last edited: 15 Oct 2019
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  13. DanGB

    DanGB

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    Thank you @JobAndKnock you are always very helpful on here!
     
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