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 Post subject: Homemade Scarfing Tool
PostPosted: Wed Jan 05, 2011 11:15 pm 
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Location: Ballwin, MO
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.

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PostPosted: Wed Jan 05, 2011 11:22 pm 
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A few more pictures:
Here is the planer mounted to the scarfing guide.
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This diagram shows cross section of the tool in action.
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scarf diagram.jpg
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And, this is a picture of the scarf I was able to produce in 3/8" material on my second attempt.
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So, if you're interested in the John Henry guide, you may be able to craft one yourself. A more durable model would require better material than MDF.

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PostPosted: Sat Jan 08, 2011 1:57 pm 
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Location: Raleigh, North Carolina
Thanks for posting that Bob. I'm one of the Zip builders here and once it warms up and my job gets less crazy I need to do some more fine fairing and get ready for the ply so I've been researching scarfing methods.

The photos I've attached are of another interesting possibility I have discovered in my 'googlin'.

Just curious why you didn't fabricate more like the 'original' - replacing the 'shoe' of your planer since it looks like you have one just like mine - a Makita - I think its a 1900 model? Seems that might actually be less work than routering out the profile of the planer to the correct depth.

My main question is how do you vary and control the depth of successive cuts. I'm thinking you place shims between your bottom angled guide and the 'base' - then use successively thinner shims until your bottom angled guide is 'tight' to the base for the final cut.

It's creative how you have the slots to vary the angle. This I assume means you would need various angled guides for different angles of scarffing. Nicely done.


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scarfing_sander_jig.jpg
scarfing_sander_jig.jpg [ 88.38 KiB | Viewed 2155 times ]

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PostPosted: Thu Mar 10, 2011 7:43 am 
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Sorry for the delay in responding. I haven't looked at this topic in a while.
That is an interesting option for scarfing. I also found another one that put a frame around a belt sander and a guide frame that gave you the desired angle. The West System scarf attachment for you circular saw is also an option. http://www.westmarine.com/1/1/17746-875-scarfer.html
Regarding your questions.
My guide does replace the original shoe on the planer. The reason I had to route out an area was to get the thickness of attachment equal to the thickness of the planer shoe so the blade lines up with the bottom. Since I used 1/2" MDF I had to get the part that attaches down to the right thickness. I think the John Henry is the correct thickness so it doesn't need any routing.
When cutting, I have the depth set at 3/32, which I think is the maximum. You have to make about 20 passes to get the scarf cut. With early cuts, the guide really doesn't do much, you just keep taking a little off with each pass. However, the guide with the angle becomes the stopping point when you get close. It prevents you from going too far. That is why it is critical the guide and base thickness be matched. Here is a picture after a few passes.
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Yes, you do require different guides for different scarf ratios.

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PostPosted: Thu Mar 10, 2011 7:55 am 
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Location: Owasso, Oklahoma
Nice work Bob...your scarfs are a heck-a-lot cleaner than mine. I think for my next project I may try freehand just to see what I can do.

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I told my wife we needed a three-car garage for my projects...she told me to ask her for permission next time before I buy a house.
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PostPosted: Thu Mar 10, 2011 9:34 pm 
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For the heck of it, I hand scarfed the 3/8" plywood that is laminated to the keel.
A scarf is not necessary for this piece, but I did it just to see what the process is like. I only did about a 2:1 ratio, but it was relatively easy to do. I put one piece on top of the other, set it back and planed both with a block plane. The plies help guide the uniformity. A course sanding block helps finish it.

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