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Saturday, January 16, 2016

3 Scarfing Techniques and Jigs

If you are going to build a boat, you'll probably have to scarf wood. There are a number of ways to 'lengthen' wood.

Here are three ways to scarf or taper that I've used.

Plywood Scarf

I used this when I needed to make two 4' (1220mm)by 16' (4877mm) plywood sheets when I built Ross Lillistone's Flint.

The general idea is that we put an 8:1 wedge shaped slope on the end of two sheets, turn the top sheet over, then glue (typically epoxy) the two wedges together, resulting in a 4' by 15'10" (4826mm) sheet. Note the loss of 2", due to the overlap in the scarf.

For Flint, I needed two sheets approximately 16'. This first picture shows the stack of 4 sheets of 1/4 inch Ocume, each sheet stepped back two inches (1/4 inch times 8 is 2 inches resulting in an 8:1 scarf. In this picture, about half of the wood has been removed to make the 'wedges').
Stack of 4 sheets, each offset by 2"

Once all four sheets were perfectly aligned, each sheet stepped back 2" (51mm) from it's neighbor below, I drove two screws through the stack to ensure they did not move. I aligned the edge of the bottom sheet with the edge of the supporting bench. I also drew a line across the top sheet exactly 2" back from the edge of that sheet to use as a guide for making the 'wedge' on the top sheet.

Using a sharp (SHARP!) plane, create, on all four sheets, the wedges. The plywood layers provide a perfect guide to make sure the wedges are straight in all directions. I continued to plane and sand until all four wedges ended in a 'feather' edge, shown in the second picture below.

Final view after planning and sanding


Note the chip out in the red outline. This was because the plane became dull and I didn't sharpen it. Now I was ready to create the two full-sized sheets I needed.

Glueing up the full sized planks (4' by 15'10") involved the following steps:

1) I used a 2' by 4' bench to support the forthcoming joint and two supports for the ends of the plywood so they wouldn't sag.

2) Placed a piece of (minimum) 12" (305mm) wide wax paper under the forthcoming joint (center of the bench).

3) Laid one sheet on the supports, wedge slope up, wedge centered on the wax paper. I screwed this sheet down so that it would not slip.

4) Laid the matching sheet (slope down) on top so that the two slopes exactly matched. Ensured that the sides of the two sheets make a perfectly straight line for the full 15'10". Marked the edges of the two sheets at the center of the wedge where they overlapped to act as an alignment guide in preparation for the next step.

5) Slid the top sheet back a couple of inches and slathered epoxy on the bottom sheet slope. Then slid the top sheet back so the alignment guide was accurate. Verified that the sides of the two sheets made a straight line. I then screwed the second bottom sheet to the underlying bench so that it wouldn't move.

6) Repeated steps 2) through 5) for the second two sheets (I used clamps rather than screws to hold these in place).

7) Put a third layer of wax paper over the joint and weighed down the two joints with concrete blocks and bricks. Let epoxy set for at least 24 hours.

That's it, I now had two sheets 4' wide and 15'10" long.

Scarfing Long Narrow Wood

The first picture below shows a jig made of plywood with a 1/2" by 3/8" (13mm by 9.5mm) runner to fit in the table saw groove. Screwed to the top is a 3/4" by 1 1/2" (19mm by 38mm) fence set at 8:1 angle.
Jig for cutting 8:1 scarfs
Jig with stock clamped in place
















This jig will produce a slope of 8:1 on any long piece of wood that is clamped to the fence as shown in the second picture. Make absolutely sure the blade is perfectly vertical. I always check this with a square because the supplied tilt index on the table saw is not accurate enough. Also make sure the front clamp is far enough back that the saw blade clears it. Ask me how I learned this...


Frame to be used for tapering stock at any angle
The picture at the left shows a frame that can be used for any angle, such as cutting a taper in a spar.

The cross braces should be as low (and thin) as possible, yet still enable two clamps to be used to hold the stock without hitting the table. The limiting factor in using this frame is the height of the table saw blade compared to the thickness of the stock plus the height of the frame.





Frame with stock clamped in place

What other scarfing techniques/jigs have you used? Include your ideas in the Comments below or write to me at TomOarCruising@gmail.com and I'll include your ideas in a future blog.

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