Monday, May 6, 2013

Joining Plywood - Jigsaw or Puzzle Joints

I apologise for my long gap between posts, life has been very full for awhile.

Jigsaw joints have become a popular method for joining 8ft plywood lengths into the long panels needed for building most boats. It is an easy process for those who are building from a kit but it is impractical for most people building from scratch. If building from plans or from patterns then you will have to make every cut by hand using a jigsaw and the accuracy necessary for a close-fitting jigsaw joint is just not possible to achieve.

For those who have not seen a jigsaw joint, the form is exactly the same as the interlocking curves of a jigsaw puzzle, except that the interlocking curves are laid out in a straight line rather than in a grid pattern. This forms a very accurate and secure junction between the two parts, unable to slide apart and needing no clamps, while the glue sets.

Jigsaw joint configuration.
The two parts have glue applied to the bonding surfaces, then they are laid on waxed paper or plastic to prevent adhesion to the surface below, aligned properly then a rubber mallet is used to force the fingers of the upper part down into those of the lower part. Care must, of course, be taken to ensure that the surfaces of the assembled joint are perfectly flush or there will be a permanent deflection at that point. The photos below, courtesy of Chesapeake Light Craft, show the joint before and after joining.

Applying glue to the bonding surfaces

Cleaning up the completed jigsaw joint
Tight joints as described above assume that you will assemble them on a large table or a workshop floor before moving the panel to the boat. This is OK for dinghies or for professional builders who have enough space and the staff to manhandle large panels from the shop to the boat and to position and clamp them accurately, all after applying glue to the framing of the hull and to the panel, trying to get it into exactly the right position first go so that it doesn't slide on that slippery glue and spread it everywhere except in the joint where it is needed, then clamp it accurately so that it can finally be fastened. You get the picture, of enormous potential for disaster.

For amateurs building bigger boats it is just not practical to assemble the panels before installation. Panels 8ft long are very convenient for one person to handle, without having to call on the wife, kids and neighbours to help with the task. I built the structures and interior of my Didi 38 "Black Cat" entirely single-handed. There was not a single piece of timber in that boat that I was not able to carry and set up without any assistance. But I made scarph joints by hand and I glued them in position.

Building with jigsaw joints and gluing the joints in position requires a slightly different approach. It is not practical to hammer a tight joint into place with a rubber mallet. The stringers and other supporting timbers are springy and will bounce away when hit, making it difficult to get the fingers properly flush. Another factor is that of longitudinal alignment accuracy. If the first piece is slightly out of longitudinal alignment on the framing then the accuracy of the jigsaw joint will force the other pieces to continue that misalignment. An error of 1mm on the first of 4 panels will grow to an error of 4mm by the end of the 4th piece.

The way to get around this is to machine in some tolerance into the joint. When the jigsaw joint curves are added to the CAD drawing of the panel, the curves are used to break the panels into the pieces that will fit onto 8ft sheets of plywood. Then the jigsaw curves must be offset into each panel by 50% of the tolerance that is decided upon. This means that each piece is minutely shorter than the designed length and the fingers of the joints are minutely slimmer. The result is a joint that can be easily pushed into place without hammering and which also has a small amount of wiggle room to allow for correction of minor installation misalignments.

This more loose joint may need a temporary doubler on the back between stringers so that small screws can be used to pull the fingers flush if the panel is curved at that point. Don't forget to use a waxed paper or plastic bond-breaker to prevent adhesion of the doubler to the panel.

After the glue has set you can use a belt sander to clean up the surface for a nice finish and a joint that will disappear in the painted boat.

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