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Brian Dixon's Plywood Scarf Clamping Jig

Components in the drawing (from top to bottom):
A. Side view of clamping jig
B. End view of clamping jig
C. End view of 48”x4”x3/4” piece of plywood used for clamping
D. End view of 48”x4”x1/4” piece of high-density foam, also used for clamping
E. Scarfed plywood
F. Side view of table (left), and side view of another table or sawhorse (right)
G. 48” 2x4 cedar stud

Building the Jig

0) Check the flatness of the table... it’ll bow up or down a little.

1) Look for a nearly straight 2x4 stud that has a bit of bow in it edgewise to be used for straightening (flattening) the table. I used cedar, but that’s because that is what I had that met my requirements.

2) Countersink and screw the the 2x4 stud to the end of the table, making sure the stud’s and the table’s curves are in opposite directions. Check the flatness of the table with a straight edge, e.g. 48” aluminum ruler. Try different 2x4’s until one works. My first one was close enough.

3) Dry fit the scarf as shown, making sure you pay attention to how well the scarf joint fits (fix it if necessary). Mark a horizontal line across the center of the scarf joint, and make large X marks across the top of the joint. These lines will make it easy to line things up later on when your wood is all slippery from epoxy and hard to line up because the stuff is squeezing out all over the place.

4) Place the piece of high-density foam on top of the joint as shown. The foam will help distribute the clamping load more evenly.

5) Build, try out, and fine-tune the clamping jig (pictured in side view above the scarfing diagram). I just took a piece of 1”x4” (actually 3/4”x3 1/2”) clear, vertical grain (CVG) fir and made the jig as follows (mine only took about 30 minutes to make...don’t let the long instructions fool you):

A) With the 1x4 laying flat on a table, partially nail in a thin finishing nail into the very edge of the wood, in the exact center. Partially nail in 2 more finishing nails near the ends of the wood, about 1/4” from the edge, as shown by the arrows above. The nails should be near vertical and stick up about 3/4” or so. These are guides for the next step, using a batten to draw a fair curve.
B) Using something like an aluminum ruler as a batten, bend it around the nails that you placed in the wood (outside of the center nail, inside of the end nails), and draw a fair curve along the batten. The line should just touch the edge in the center area, and should intersect the ends at about 1/4” below the edge of the wood. Remove the batten and the nails.
C) Plane the wood down to the curve you drew in step b).
D) Now place the 48”x4”x3/4” piece of plywood on top of the high density foam and the scarf joint, and then clamp the clamping jig on top as shown in the figure. The curved side goes down, and you should only clamp hard enough to just barely squeeze the foam in the end areas.
E) Going by ‘feel’ for how much clamping pressure there is (not too hard!), and by looking at the high-density foam, figure out if the clamping jig is pressing too hard, or more in the center section, or more towards the ends. You don’t want it to clamp too hard (epoxy is just as strong without all of it forced out of the joint!). You want even clamping pressure in both the center and end areas of the joint. It’s better to start out too stiff, then to lighten things up a bit at a time. Here’s how I reduced my clamping pressure:
a) If the jig clamps the outer areas more, plane a slightly sharper curve into the jig and try it again.
b) If the jig clamps the center area more, screw the jig to a straight piece of wood and shave a little of the straight side to thin the jig a bit. You have to screw the jig to another piece of wood because it’s too hard to follow the fence on the saw with the curved side of the jig. Try the jig again.
c) If the overall pressure is too high, shave some wood off the straight side as in step ii) above, then try it again, paying attention to how the jig pressure in the middle compares to the pressure in the outer areas.

Using the Jig

0) Making sure you use wax paper everywhere it is appropriate, disassemble the scarf joint and wet-out the planed surfaces with epoxy. Repeat every few minutes until the wood just won’t absorb any more.

1) Let this epoxy green-up a little (about an hour). It needs to be sticky to the touch (little stringies follow your finger when you touch it and pull your finger off). This step ensures that no matter how much clamping pressure you use, you cannot squeeze out all of the epoxy, or starve the joint. (Thanks to Al Gunther of Al’s 26 in WoodenBoat magazine for his suggestions and experience). In fact, if your scarf is really well fit (REALLY well fit), you can skip using a thickened epoxy as your glue and just glue up the joint at this point... just put a fresh coat of clear epoxy on the planed surfaces and skip the following step).

2) Thicken up your epoxy with either microfibers or Cab-O-Sil (colloidal silica) to what West System calls the ‘catsup’ thickness or slightly thicker. Spread evenly (thinly) on both planed surfaces and use a notched trowel (homemade, wood or plastic, etc) to comb lines into the epoxy. Note: I use Cab-O-Sil for everything in the boat including the scarf joints. West System agreed with this approach too (except for building spars and other extremely high stress structures). It’s much smoother and easier to work with than microfibers. If you want, you can blend a little wood flour (from your sander’s dust collector) with it to make the color more wood-like. This works quite well and doesn’t require much wood flour.

3) Using the lines you drew previously, reassemble the the scarf joint and the full clamping jig (high-density foam, clamping board, clamping jig as shown above).

4) Clamp the joint until the ends of the clamping jig just start to compress the high-density foam.

5) Let the joint ‘rest’ for about 10 minutes to give the epoxy a chance to squeeze out.

6) Now you need to scrape off the excess epoxy above and below the joint. Note that this is the reason for the over-hanging joint (over the edge of the table) as shown in the drawing. You can use a sharpened mixing stick, a plastic putty knife, or whatever for this step. I cut out a special tool from the flat side of a plastic 1-gallon milk jug. I liked how the milk jug plastic was thin (lifts the epoxy better without smearing it all over) and how it followed the wood nicely (just the right amount of flex). By making a special effort at this point to clean things up very nicely (leaving a flush, nice flat surface behind), your post-cure cleanup will be almost nothing...a major key to making good looking scarf joints.

7) Wait about another 20 minutes and check the joint for more squeezin’s. Sometimes tiny little beads of epoxy will continue to come out for awhile. Just keep scraping them off as in step 6 above.

Well, that’s about it. The keys to this technique are the overhanging edge of the scarf joint that allows you to clean up the top and bottom surfaces ahead of time, e.g. before the epoxy cures, the tuned clamping jig (easy to do), and the careful setup of everything in advance so that you are clamping to the the table near the center of the joint, while still allowing enough wood to hang out so you can clean the joints up on the bottom side while everything is still clamped.

Good luck!

Brian <>

High-Density Foam:
A closed-cell 1/4" thick foam called F-Cell, in a beautiful charcoal grey. It's a "cross-linked polyethelene foam with some vinyl added", according to the expert at The Foam Man in Corvallis (541-754-9378 ). It came in a roll that was 6 inches wide, and I had it cut to length (2 ea 48" pieces).

If you can't get the F-Cell, then he said to ask for Sentinel XPE closed-cell foam, 1/4" thick. You may have to do some cutting to get it to what you want.

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