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Failed! Zero and Two (of Five)

Discussion in 'Woodturning Discussion Forum' started by Chris Edwards, Oct 15, 2020.

  1. Chris Edwards

    Chris Edwards

    Joined:
    Mar 1, 2020
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    Location (City & State):
    Lebanon, TN
    So to practice my spindle skills, I'm making these simple things called Nostipenne's for the wife. They are used for hand winding wool into balls.

    In the picture, the Zebrawood one gives you an idea of what I'm trying to achieve. It is about 12" long.

    Now I'm trying to get a bit fancy and put a Celtic Knot in the center. I've done some Maple with a Walnut knot and the reverse, Walnut with a Maple knot.

    I started out squaring 15" pieces of wood to about 1.25" square. I then made one pass through my table saw at 60 degrees for the knot insert. I didn't cut the full height of the thickness, so cut to about 1 1/8". I then planed and drum sanded the insert pieces for a very snug fit into the saw slot. Glued this in with TiteBond III. After the glue was dry, I trimmed off the excess of the insert. I then repeated, glue insert and trim on the other three faces.

    These were then left overnight.

    So far of the 5 that I have made, two have failed, while turning at the glue joint, one while in the sanding phase and the other, in the picture, while turning.

    These are about 3/4" in diameter at the point of (glue) failure.

    I'm thinking of switching out the glue process for epoxy (T88) as I really can't clamp the piece as I put each insert in, so if there is a slight void, the epoxy will fill this better.

    [​IMG]
     
  2. Don Wattenhofer

    Don Wattenhofer

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    I did several Keltic knots many years ago and of course I read about the idea of cutting partially and forcing the contrasting piece into the slot as you have described, but soon dropped the idea due the the poor quality of the joint.
    The method I developed uses full cuts and the contrast pieces cut from a blank of the same dimension the thickness of a saw cut.
    The glue up is done in a right angle clamp block with polly sheeting between the block and the piece. The first piece is clamped into the fixture with the long point opposite the clamp block, then all surfaces are coated with titebond and the other pieces put in place. The two sides are pulled together by clamping the ends with enough side force on first piece such that the joint will not slide out of alignment.
    The other suggestion would be to put the detail on the handle end and maybe make the angle 30 degrees.
    101_0752.JPG
     
  3. GRJensen

    GRJensen

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    Location (City & State):
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    I did some celtic knot peppermills a few years ago using a process very much like the one Don Wattenhofer described above. I had several failed attempts to use the 'cut part way through' technique before I built a jig and set of cauls. Made for much stronger glue joints.
    DSCN0239.JPG
     
  4. john lucas

    john lucas AAW Forum Expert

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    Location (City & State):
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    It's and end grain glue joint. Use epoxy. If you use PVA glue size the joint by coating it with thinned PVA let it sit for a minute or so and the put full strength PVA on. I did an end grain to end grain test a few years back and was blown away by how weak the joints are. I glued 3/4" square by 5" long pieces of maple together end grain to end grain using a wide variety of glues. Then I put one end in a vice and put a 2lb weigh on the other piece right next to the joint. slid the weight out until the joint broke. Polyeurethane, CA and straight PVA glue all broke as you slid the weight out the 5" piece. A sized PVA joint held well enough I had to clamp another piece to the wood and slide it out another 5 or so inches. Epoxy ( I tried 3 brands and all seemed to work the same) held up to about 14 to 16".
    When I was preparing my article on Celtic knots I used CA for the pen blanks and epoxy for the wine stoppers and Christmas ornaments.
     
    Emiliano Achaval and Tim Connell like this.
  5. Richard Coers

    Richard Coers

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    Location (City & State):
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    3 things I see. Some burn marks on the end grain of the maple. PVA won't stick to burnt wood. I can also see some gaps in the glue line, something else that PVA doesn't like. And 3rd, I see some spiral chatter marks on the maple. So you are getting some wood flex and that is over stressing the glue strength in that kind of joint you are making. Change to a sharp saw blade or touch it up on a disc sander, make a fixture so you can clamp the joint, and use a spindle steady until your skew technique improves.
     
    Tim Connell likes this.
  6. Don Wattenhofer

    Don Wattenhofer

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    Good call I didn't look close at the photo those open joints definitely would not hold up to the stress of turning or use plus they don't look like quality joinery.
     
  7. Don Wattenhofer

    Don Wattenhofer

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    Did you glue the cauls on so that you could clamp perpendicular the the joint?
     

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