Results 1 to 9 of 9
  1. #1
    Join Date
    Oct 2019
    Location
    Montreal
    Age
    43
    Posts
    3

    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.

  2. #2
    Join Date
    Feb 2003
    Location
    used to live in Sydney, now it's Canada
    Age
    64
    Posts
    10,769

    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 Canada

    ian

  3. #3
    Join Date
    Aug 2008
    Location
    Melbourne
    Age
    29
    Posts
    4,924

    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.

  4. #4
    Join Date
    Feb 2007
    Location
    blue mountains
    Posts
    4,029

    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

  5. #5
    Join Date
    Apr 2006
    Location
    Hobart
    Posts
    2,325

    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

  6. #6
    Join Date
    Jun 1999
    Location
    Westleigh, Sydney
    Age
    73
    Posts
    8,836

    Default

    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.
    Visit my website
    Website
    Facebook

  7. #7
    Join Date
    Oct 2019
    Location
    Montreal
    Age
    43
    Posts
    3

    Default

    Thanks for the quick replies guys. I'll go with thinner strips even though it's painful with such expensive woods! )

  8. #8
    Join Date
    Apr 2006
    Location
    Hobart
    Posts
    2,325

    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

  9. #9
    Join Date
    Oct 2019
    Location
    Montreal
    Age
    43
    Posts
    3

    Default

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

Similar Threads

  1. "Find the Kick-Back Piece" game
    By RedShirtGuy in forum SAFETY
    Replies: 8
    Last Post: 25th Nov 2014, 11:27 PM
  2. New Project: Spring driven "Vienna" clock.
    By Walesey in forum CLOCKS
    Replies: 9
    Last Post: 19th Nov 2014, 03:35 PM
  3. Some more mixed woods and acrylics "+" !
    By gawdelpus in forum WOODTURNING - PEN TURNING
    Replies: 8
    Last Post: 4th Nov 2012, 06:55 PM
  4. Olive timber from my "neck of the woods"!
    By robutacion in forum SWAP, BARTER, SUPPORT A WORTHY CAUSE
    Replies: 7
    Last Post: 19th May 2009, 10:21 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •