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2. Scarfing plywood sheets - scarfing jig

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Marine and other plywood is sold in 4' x 8' (2.43m) or 5' x 10ft (3m) sheets so if your kayak is any longer, the plywood will have to be joined into longer pieces. The issue is how to connect it together so that the joint between the sheets will have the same or very close properties as the rest of the plywood. This means that the bond should not fail easily when the plywood is bent/flexed, the 'seam' should look visually clean, the joint has to assume curvature to the same degree when bent or twisted as the surrounding material and last but not least, the joinery must be easy to make in an average shop with common tools.
These requirements as well as working with 1/4"(6mm) or thinner plywood will limit the number of practical joint configurations. The examples below shows a few common types:
  Scarf joint

Advantages:
» large bonding / gluing surface
» skins as well as the core bond to each other across the boundary = continuity and strength
» degree of bending "curve of curvature, "fairness" or "second derivative of curve" very close to the    surrounding plywood
» allows small degree of misalignment between joined sheets without forming gaps
» same thickness as surrounding plywood
» relatively easy to do
Disadvantages:
» difficult to make accurately especially without specific tools and a jig


  Half lap joint

Advantages:
» degree of bending "curve of curvature, "fairness" or "second derivative of curve" relatively close to the    surrounding plywood
» same thickness as surrounding plywood
» tight joint if done well
Disadvantages:
» most difficult to make well without more complicated jig and setup
» only the core has large bonding surface but the stressed skins are weakly 'butt' joined
» doesn't tolerate misalignment or variation in lap thickness


  Butt joint with a block

Advantages:
» clean and tight joint line
» very easy to do without any jigs
» OK for no-twist, low-bend or flat panels in structurally non critical areas
Disadvantages:
» the degree of bending "curve of curvature" in tight curves or in twist may be very different than the    surrounding plywood resulting in an "unfair" surface with a sharp 'kink' or 'flat' section at the joint.
» strength of joint depends on the cohesion of surface skin within the plywood
» interruption in surface thickness - bad for fiberglassing the affected side
» heavier than other joint types
There are other joint types such as 'finger jointed' plywood such as the One Ocean Kayaks ScarfLOCK. The crossection of the plywood is 'butt' joined but the joint line itself assumes a serpentine shape or 'interlocking fingers' (as in dovetailing) to increase the glued area and to distribute the bending properties of the plywood. Such joints can only be made with precise CNC machinery. One Ocean Kayaks stitch and glue kits are made with this type of joinery. See the ScarfLOCK joining process.

Given the choices, I recommend the you to scarf all of your plywood (with a few exceptions) for it has the most good points namely excellent 'fairness' and negligible disadvantages.
The creation of "Quick&Cheap" scarfing Jig. As I pointed out earlier, this scarf joint is easy to do since the tooling jig can be made from scrap material (but nicely flat!) and put together in about 15 minutes.
My cost: $0.02 and a bandage for hot-glue-singed finger.
Once this thing is built, you can be scarfing consistently and accurately ad infinatum.

The size of the two 5/8"(16mm) MDO plywood pieces is 9" x 32" (23 x 81cm). These are hot+yellow glue stitched together with three blocks. And the acute angle between the plates is ??
90º - 7.12º =~ 82.9º I have cheated a bit so my angle is not the ideal 82.9º but 'just as good' 80.5º :)
Make the blocks (gussets) short so you can clamp the jig to a table.
Bring the saw blade (71/4" [14,4cm] diameter) tight against the corner and draw a line that defines the edge of the saw base - saw guide strip.Try to be as accurate as you can and make sure the guide strip is parallel to the edge of the jig along the entire length. The guide strip is made from 1/4" x 5/8" (6 x 1,6cm) hardwood.
Close-up of what it should look like. The angle on the saw blade is 90º.
Note the guide strip at the bottom of the picture. This supports the saw so that it cuts at the same height along the scarf. Clamp the plywood as close to the blade as possible (1/4 - 3/8" gap). It is vital that the clamping piece is very stiff and perfectly straight. I am using a piece of rock maple but an angle iron and such will work too.
The whole setup. The scarfed plywood is 24" (61cm) wide and 8 ft long. Note the blocks clamped to the edge of the table. These hold both plywood sheets together and even. The front edge (face) of the jig SHOULD BE perfectly perpendicular (90 degrees) to the edge of the table (blocks) and the sheets. If the sheets are not scarfed 'perpendicular', any error in alignment will double as you unfold and join the sheets together.

Important: The sheets are scarfed with the good faces together!!

I have 'taped' the blade guard in the open position to make things easier. Do at your own risk! The blade is captured spinning. Set the saw all the way to the right of the jig so that the plywood is not in the way - that is why the jig is 32" long not 24". Engage plywood and cut slowly at constant speed with the base of the saw nicely flat against the jig base and guide strip.

Notice the economically minded offset of the plywood sheets. This saves couple inches of 'length' overall.
Finito! This is as good as it gets. You can also cut the scarf with a router jig, planer jig or simply by a hand plane. All these options have some good points but the problems or difficulties far outweigh the benefits.

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Last page update: 29 October 2013