Help needed: Bevelled single lap joint connection (epoxy/wood)

Discussion in 'Boat Design' started by jhan, Aug 23, 2013.

  1. jhan
    Joined: Jul 2013
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    jhan Junior Member

    [​IMG]


    I need help,as you can glue joints only once, better do it good is the idea.
    The picture shows what I am about to do: bevelled single lap joint connection.

    I can cut the wood so it makes a tightfit connection, but that´s not what is the strongest connection for epoxy / wood (15 mm okume) I have been told.

    Should I cut in a degree that it leaves 1 or 2 mm space for the epoxy on 1 side?
    This would mean bevelled cutting, for example 43 degrees instead of 45 degrees each?

    I hope you can understan what I mean. Englisch is not my native language, and that´s really a handicap when it comes to complicated technical issues
     
  2. michael pierzga
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    michael pierzga Senior Member

    Hard to understand what you ask.

    For a strong scarph jopint use best craftmansship and produce a tight joint. A PERFECT JOINT.

    When you glue the joint... first prime with epoxy...let stand for a ten minute...inspect, perhaps prime again..then thicken the epoxy, coat both surfaces...align and clamp.

    DO NOT USE HIGH CLAMPING PRESSURE. Gentle pressure...you dont want to squeze all the epoxy out. Epoxy has good gap filling abilty.

    Google plywood scarf joint and you will see it demonstrated.

    Remeber..Do not use high clamping pressure. Firm pressure only.

    For many panel scarfs I only use nails for clamping pressure..

    Allow the epoxy to cure a minimium of 24 hours...48 hrs even better ..before you unclamp and manhandle the panel
     
  3. jhan
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    jhan Junior Member

    Thanks Michael

    You do understand, should I make perfect joint.
     
  4. jhan
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    jhan Junior Member

    I have planned use aerosil with the glue -after priming with only epoxy- what mix should I use for this job?
     
  5. michael pierzga
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    michael pierzga Senior Member

    Yes... a perfect joint. For many pieces of timber I use a router and cut the bevels. These bevels are machined perfect..... a near invisible hairline joint when mating the two pieces dry. DRY !!!

    You must only remember to use low clamping pressure when glueing the joint. Don't squeeze all the epoxy out of the joint. Gentle clamping...just enough to hold the two pieces stationary without squeezing all the epoxy out of the joint

    Always best to make practice scarfs on scrap wood...glue them up, then break them.

    Scarfs are easy and strong.

    The big mistakes are a dry joint..all epoxy squeezed out, wood not primed and manhandling the glued piece before full cure of the epoxy.

    This is a copy of the Guogeon brothers boatbuilding book..very good info on wood and epoxy inside

    http://www.westsystem.com/ss/assets/HowTo-Publications/GougeonBook 061205.pdf

    I think the link works..give it a click

    http://www.westsystem.com/ss/filler-selection-guide/
     
  6. jhan
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    jhan Junior Member

    Perfect Peter, very good info indeed. This is really helping. I can move on with the project
     
  7. messabout
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    messabout Senior Member

    In case you have not been advised, you need to make the length of the bevel about eight times the thickness of the joined parts. In this case, with 15mm ply, the bevel would need to be about 120 mm long. The beveled surface ( the hypothenuse of the triangle) needs careful atention so that it is a perfectly straight surface.
     
  8. jhan
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    jhan Junior Member

    You people deliver priceless info, where would I be without internet.

    @ Messabout: that could be my next question, if I still dared to ask. Very good you bring it up. 8 X thickness, is much, do not know if that works on my saw bench. I will make some test pieces to see what I can do.
    As a beginner there are many problems you have to overcome, and the more you know, the more of them you have to face.
     

  9. michael pierzga
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    michael pierzga Senior Member

    Scarfs are different ratios depending on the required strength. 4x1, 8x1. 12x1.

    Scarfing wastes wood. the shortest scarf, that gives the required strength, is the best.

    8x1 is the commonly accepted ratio for epoxy glued plywood.

    What are your scarfs for ? Plywood, structural members, mast ?
     
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