After having researched scarfing tools and jigs, and having a Makita planer, I made an attempt to replicate the John Henry scarfing tool. This tool and its use is fully described in this forum: http://forum.woodenboat.com/showthread.php?110517-John-Henry...-Easy-scarfing-and-glueup
If you check out that forum you will understand how it works and is set up.
Since this was only needed for a few scarfs, I didn't want to invest a lot of money and it didn't need to be industrial strength. So, I took a piece of 1/2" MDF and routed out the outline of the planer fixed base and other areas to allow the planer to sit flat.
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It is important that the depth of the cutout where you attach the planer is such that the bottom of the base aligns with the cutter head. It took a little tweaking to get it right. If you cut too deep, you can shim it up. Before you start this, make a test piece by setting the depth at maximum and plane a couple of inches on the edge of a 1x2 scrap. This can then be used to test the alignment of the base.
I then cut a guide bar out of maple. It has to match the depth of the base (what you attach the plywood to) and be angled to match the scarf ratio (e.g. 8:1 or 12:1) For a 12:1 scarf, the angle is 5 degrees, for 8:1, the angle is 7.2 degrees. Also note that because the planer blade is only 3.25" wide, you limited in how much of a scarf angle you can produce in different thickness material. Fortunately, a 12:1 scarf in 1/4" material is 3". The guide bar must be positioned so the cutter head just reaches or slightly overlaps the guide. Here is the base with my guide bar. This bar is for scarfing 3/8" material. It has an 8:1 ratio. It also requires a 1/2" base.
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Finally, I used a small dowel, with some ability to adjust, that supports the tool. This piece rests on top of the material being scarfed.
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To use the guide, you need a flat table or sheet of 3/4"plywood on sawhorses, etc. To this you attach or clamp the base. The base must be exactly the thickness you designed into the guide piece. I used 1/2" MDF as the base since it is dimensionally very accurate. Plywood, as many of you know, it not the thickness it is claimed to be. So measure carefully. Now, you align the plywood you want to scarf with the edge of the base material. You can clamp it or nail/screw it down. It must not move. I had a little clamp slippage and nailed a few brads through the plywood into the base to hold it. Make sure the clamps and nails are not in the way of the guide as you slide it along.
Now, to scarf you hold the guide bar against the base and move across the piece. Put a piece of scrap that is the same thickness as your work piece next to it at the end. This will support the dowel as you reach the end and prevent a snipe. It will take multiple passes as the planer can only take a little at a time.
With some tweaking and practice you should be able to make very decent scarfs. Next post will have a few more pictures.. I guess 3 is the limit.
Completed Malahini (launched 6/24/2012)http://bobsboatbuild.blogspot.com/