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Router Scarfing Jig


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I promised to report on the success or failure of a scarfing jig that I built for joining 1/4" plywood sheets to build a B&B Spindrift. I built a fixture to mount my Porter Cable router to my radial arm saw. I then built a 12:1 table that mounts on top of the original table and has clamps to hold the sheets down as I feed the sheets across the table. A 1 x 3 clamped to the sheets, becomes the edge guide. The router is then moved out about 3/4" and I make another pass, etc.

Here are some images of the setup:



The device was easy to build and set up but I found the results to only so so. The radial arm saw is not as stiff as you might think it is for a close tolerance operation and the plywood should be kept in one place rather than feeding it for a better scarf. The sheets are too limp to hold their location, even with good downpressure, while feeding. Here is a photo of the reulting scarf on two stacked sheets. (Note the sheets are narrow as I only needed to scarf the sides and bottoms, rather than the full sheet widths.)


If I had it to do over, I would probably just hand plane it.

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The concept seems simple enough. Just take off a thin, wedge shaped piece off both edges, flip them over and glue them up. But how to make it uniform?

As an example of the lengths guys will go to do this:


Funny how for small projects, a good sharp hand plane seems to always work. It's slow...but that may be the secret. You ease your way into it, never taking too much. Unless you are striving for a perfect pencil line joint, this is good enough. The gap filling qualities of epoxy is the great equalizer.

The only thing I've not seen tried is to build a large, sheet sized jig to run the edges through a jointer. If the angle is set right and you have a fence for it to ride against, the bevel should come down right....as long as you get even pressure on top so that it feeds through evenly. Or you might be able to build a jig where the only thing protruding is the edge you want beveled off and run a long bed power plane back and forth over it, with the fore and aft edges riding on the jig. Or....well....see above.

I used the glass taped butt joint on my Spindrift. It works and the only modification I've seen for that is if joining larger sized panels (1/2" plus ply) is to route or sand a shallow groove (maybe 1/16") for the tape to lay in, so the end result is perfectly flush. But if you are coating with epoxy, the adjacent level can be brought up flush enough that even that is not neccessary.

It's all just too much fun!

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  • 2 weeks later...
Funny how for small projects' date=' a good sharp hand plane seems to always work. [/quote']

It just amazes me at the lengths people go to avoid using hand tools. It's almost like they were scared of them. Of course, it is a lost skill for the most part. But the more I do with hand tools the more I like them. I am not about to give up my table saw! But neither am I giving up my hand tools either. They both have their place in my shop.

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The best reason I can think of for wanting to use power tools (and it is a good one) is to get a level of precision that mere mortals find hard to achieve by hand. I suspect that was what motivated Bruce to build his jig....the desire to find a way to get that perfect joint.

And well worth it if you were building 100 of something...or maybe even 5 or 10 if it was complex to do and it mattered. But for most of us, only building one of something...I'm finding that you do the best you can and move on. Most of the time it will turn out ok.

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Howard - what I meant was that if you are doing a smaller job- such as a "one off" it's often more time consuming to set up a power tool to do the job, than it is to just grab a hand tool and DO IT. I should have been more thorough in my post.

Certainly if you are doing multiple iterations, you should set up the jigging, and use the power- you do get a level of consistency that is not really possible with hand tools in the same period of time. That's for certain.

I use a power hand plane to rough down scarf joints in my work. But then I have that tool. And it's no more trouble to use it than a hand plane since the plywood must be set up alike for either. But it's really not hard to get a GOOD joint on a plywood scarf with a hand plane. The layers in the ply give you an excellent guide to the straightnesss and flatness of the cut.

On the other hand, for scarfs in solid wood such as stringers, rub rails, etc. I go straight to the table saw and a jig I made long ago for that purpose- MUCH quicker and simpler.

I think the real key is using the APPROPRIATE tool for the occasion.

Oh and while we are on the subject- the Gougeons sell an attachment called "The Scarffer" that attaches to the sole of a circular saw for cutting scarfs in plywood. Used to have one and when you have dozens of scarfs to make, it's a REAL jewel. Big time saver also. The one we had went all over the boatyard cutting scarfs. EVERY one borrowed it :D But we were building big boats and scarfing many sheets. 6 thirty plus footers under construction at that time.

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That is what I suspected you meant, but thanks for clearing it up.

I have found that I often will go looking for a tool or process for some job, when I already have the means to do it. Example...a couple years ago I built a rudder for my boat. I built a screwy jig to use my router and a flute bit on it so I could get a "perfectly" true fairing job on it. I didn't and could have done it in a fraction of the time if I had gone after it with the hand tools I already had. The outcome would have been almost the same.

But any excuse for a new too.....

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That should have been "new tool".

The one we had went all over the boatyard cutting scarfs. EVERY one borrowed it Very Happy But we were building big boats and scarfing many sheets. 6 thirty plus footers under construction at that time.

Charlie: When was this and what yard? BTW....I seen your posts in no fewer than 4 different forums...and you still manage to find the time to build boats. Either you don't sleep or you are much better at time management than me! :lol:

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lol- Yeah- Laura fusses at me sometimes about spending too much time on the box. I spend time on at least 6 forums regularly.

This was back when I was building my own 35 foot trimaran, at Pelican Creek Boatyard. A do-it-yourself yard on an island in the ICW over by Jacksonville Florida. We had 9 boats being built or rebuilt there at one time. Time was late 1970s and earlly 1980s.

First pic, the tri (Different Drummer) sailing in the open Atlantic. Second pic, under power in the Elizabeth River off Norfolk.

The boat is a Cross 35. Now sails Galveston Bay.



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  • 1 year later...

Quick and dirty scarfing method -

The method I used to make the scarfs for my Spindrift was quick - and dirty!!! But it worked great. I simply used a belt sander and ground away everything that didn't look like a scarf!

First I made a line across the end of each sheet to be scarfed marking the top edge of the scarf. Then I stacked all the plywood with the ends placed at the line on the sheet below. This formed a stair step at the correct slope for the scarf. The whole stack was well supported to keep the plywood from flexing under pressure and clamped into place with a sacrificial strip protruding from the bottom edge to keep from feathering the edge of the lower panel too much.

Then I started sanding away with a coarse belt on the beltsander keeping it moving. It was easy to gauge my progress by the steps and by the glue lines revealed as I removed material. The wavering glue lines told me when I was getting uneven and I could bring them back to straight by spending a little more time in the high spots. Once I was close to the bottom of the steps (which meant I was close to the scarf surface), I switched to 80 grit to give a more precise surface and better control. Or you could switch to a hand plane.

This created a bunch of dust so do it outside. The scarfs were plenty accurate and the surface was great for getting a good bond. The epoxy with a little silica filler took care of any voids which would be very slight anyway.

I wouldn't hesitate to use this method again!

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  • 1 year later...


I modified the edge of a work bench to scarf long lengths of plywood. It works on short scarfs too. This one is for 1/4" ply, set to the maximum depth of a 7.25" circular saw (about 2.5" which gives a 10:1 scarf).

There is a constant stream of plywood scarfing jigs showing up on forums.  That is the first one that I've seen that makes any sense.  I think that 10:1 might be overkill tough.  Never had a single failure on 8:1 bevels. 

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I've never had a failure with 8:1 either Tom. I set the saw at it's maximum depth, which just happened to be 10:1 on 1/4" stock. In this case, many of these scarfs would receive considerable bend, so the extra slope made me use as much as the blade depth permitted.

For what it's worth, I've found the simpler the jig, the more likely it'll get used or be successful. No set up, just clamp the stock and cut.

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  • 9 years later...

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