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  1. #1
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    Default "Spring back" with 2 exotic woods

    Hello,

    (First post here, hopefully in the right sub-forum!)

    I'm soon going to start working on a katana project. I plan on making the sheath and handle fit as seamlessly as possible (no tsuba). The design I currently have in mind looks like this (see picture). Essentially, it's 1 strip (about 3/8") of bloodwood surrounded by 3 strips of slightly thinner ebony on either side. (The actual design is more complicated, but this simple one illustrates the glue-up challenge.)


    IMG_2070.jpg

    I'll make a jig for the curve (based on the blade I already have) and glue all those strips together.

    Here's my question: how much of a spring back do I have to allow for? I know it varies with wood species, but I've never done that with those two exotic woods.

    Any tips?

    Thanks.

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  3. #2
    Join Date
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    back in Alberta for a while
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    Default

    some comments

    "best practice" is to do the glue up using an ODD number of strips and to use strips of consistent thickness. Perhaps rip the bloodwood into two strips giving you 5 strips to laminate.
    spring back decreases with an increasing number of strips -- 5 strips will spring back much less than 3, perhaps even so little that you won't notice.
    regards from Alberta, Canada

    ian

  4. #3
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    Default

    +1 for more, thinner strips.

    Glue choice is another factor, each has its pros/cons:
    PVA will tend to creep over time and can leave you with a ridge at each glue line, but it's easy to work with and dries clear.
    Polyurethane won't creep and will leave a thin glue line similar to PVA, but it expands as it sets so you'll need to make sure you have plenty of even pressure all the way along or you might get areas between the clamps that push out and leave a foamy glue line.
    Epoxy also won't creep, but you'll have a darker, slightly thicker glue line. Given the colours you're working with, this might not be much of an issue at all.

  5. #4
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    Default

    Like the others have said the thinner the strips are the better. Use epoxy but not the quick setting stuff as it will go off before you can get it all clamped up. There will still be a slight bit of springback as there is always some but by using thin lams it should not be much.
    Regards
    John

  6. #5
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    Default

    Bonjour Olivier

    Thicker wood will spring back more than thinner wood so if you make your laminations thinner you will reduce or virtually eliminate springback.

    When the laminate grain runs in opposite directions (eg plywood) then it is very important to have an odd number of laminations. However, when all the grain is alighned in the same direction, as with your sheath, I just laminate to the required thickness and never bother about the number of laminates. It just does not seem to matter. [I am aware this view is different from Ian and Elan's whose opinions I greatly respect.]

    To glue up the laminations you will need a very accurate "bending form" - any imperfections in the form will be transferred to and magnified in the laminate remain highly visible. I like to have a minimum of 150mm excess material at each end of a glue up to ensure smooth curves.

    Because of the creep issue my preferred glue is slow setting epoxy. And more layers of thinner laminates (say 2 - 3 mm) will facilitate smoother curves and minimal springback. No steaming should be necessary.


    Cheers

    Graeme

  7. #6
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    As I read your post, you will have an odd number of strips - 1 bloodwood and 6 ebony (3 either side), so no problems there. I've never used those timbers, so I don't know what they are like to bend, but if the grain is nice & straight they should be OK. Those thicknesses should be fine to bend as long as you have a strong form and plenty of clamps, and spring back should be negligible. An approximate formula for springback (reduction in chord depth) is y=x/n^2, where y is the reduction in chord depth, x is the chord depth and n is the number of laminations. I've used this and it seems to be about right.
    Definitely use epoxy, and if you use something like Techniglue you can tint it, although that probably won't be necessary with those timbers. Just be prepared to work quickly once you've started work, and good luck. There's another recent thread about bending that has some good advice about preventing the strips from moving while clamping up, and I'd suggest you read it.
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  8. #7
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    Thanks for the quick replies guys. I'll go with thinner strips even though it's painful with such expensive woods! )

  9. #8
    Join Date
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    Default Good Videos

    Hi Olivier

    FineWoodworking has just sent me some links that are very relevant to your katana. "One seeing tbetter than 100 hearing" and all that.

    Introduction: Garden Chair - FineWoodworking

    I commend Michael Fortune's technique; the following episodes are particularly relevant to your project:
    • Episode 1 - Bending Forms,
    • Episode 2 - Resawing Thin Plies,
    • Episode 3 - Bent Lamination Glue-Up.


    Hope this helps - lots of experience in his advice. You are right about use of expensive timbers - my estimate is t5hat with thin laminations wastage usually exceeds 70% - ouch. But its worth it!


    Cheers

    Graeme

  10. #9
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    Oct 2019
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    Default

    Thanks for the link! I'll have a look before starting my project.

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