Scarf joints are fundamental to boatbuilding. There is pretty much no other basic joint as strong for making something "long" out of somethings short. I first encountered scarfing when I built my Bolger Gypsy; through this project, I learned how often scarfs are used in boat building: masts, rails, etc.
There are many methods for cutting scarfs. Luthiers use them, too:http://www.youtube.com/watch?v=g29dInBtvcAhttp://www.youtube.com/watch?v=ECmsQWsFtiY
That second link, Steve's Guitar Making, has some excellent woodworking throughout the series...handplaning to thickness, for example, and lots of chisel work. Very inspiring.
I've tried several ways to cut scarfs, and the bottom line, for ME, the best way I've found (again, for ME) is with a router and a jig.
I built one of these jigs years ago, for my old router (since retired to the scrap pile, alas) and this one is, I think, an "improvement" on my first try. The basic idea is well known, and I got the example from the Fred Bingham book Boat Joinery and Cabinet Making Simplified
For this round, here's what I did:
(1) 3/4" 'whitewood' base and for the angled rails.
The rails are the critical part for this jig if you want good scarfs. Here's how I did mine (the first one, years ago, was done less precisely):
(a) cut the angles with a jig saw; the right angle corner needs to be precisely square so you have a good reference point.
(b) the angled pieces were clamped together side-by-side and the angles hand planed to "uniformness." That is, I cared less about the actual angles than about them both being the same.
(c) both "ends" (square to the base) were hand planed to uniformness also so that I could 'index' the length.
(d) It is IMPORTANT to glue and screw the angled pieces uniformly to the base board...so that the resulting platform for the router base is straight, square and even side-to-side. This is the reason for the hand-planing above. Since I created an "index" of the back end square to the base (the part glued to the base board), I could use that to make sure both pieces are mounted in the same 'plane.'
I hope that's clear. If the angled pieces are off a little, the scarf won't be square and the glue-up won't yield a true piece.
(e) Width of the jig
I did this jig 6" wide, just because. The pieces I'm working are about 2-1/2", but several of them are curved. Obviously, this is not a scarfing set-up for scarfing sheet goods together.
(f) The jig shown in the pictures is 5:1, which is not really a good scarf...it needs to be AT LEAST 8:1 or more properly at least 12:1 slope for anything structural. The jig shown is for making cosmetic scarfs!
Note that if you go to classicplastic
and mention 5:1 scarfs, you'll get yelled at (properly so, I guess). Those guys do some incredible (read...beyond MY ability or patience) boat restorations. But, 5:1 worked for me within my material constraints for COSMETIC scarfs that are not structural in any way.
A 12:1 scarf joint is stronger than the wood itself...I know, I've tested it. It's also stronger than my wife...I know, she's tested it. (She likes destroying things I've built that I claim are strong...the 12:1 scarfs have so far resisted her efforts).
(2) Index lines
In the picture, you can see 1/4" index lines on the jig; these are for moving the 'stock' forward and are not REALLY necessary. But...I've found that trying to shave off bits by lowering the router bit, even with a plunge router, does not work as well. Well, maybe for someone else it would, but for me it did not.
So, I set my router to a depth of about 1/4" and shave by moving the stock. I tried using the plunge depth turret on my router to get a couple of cuts out of one wood position and for some reason, it just did not work very well. Well, I think the 'reason' has to do with bit depth and chunks of wood being taken off... YMMV.
(3) Length of board for jig
I'm going to be scarfing LONG pieces, approximately 10 feet long, so I wanted a good base at the jig end. My board is 2 feet. When I do the full stock (10 feet long), I'll prop up the pieces along the length, but like the idea of having a good couple of feet at the jig end for support and "alignment."
(4) Router Sub Base
I made this one out of 1/2" hardwood (birch) plywood, and it is screwed to the plunge base of the router. My first one, made years ago, used regular B/C plywood and did not work as well. In my opinion, it is worth the few bucks to get hardwood plywood for stuff like this.
I glued "stops" on my router base to keep it from being possible to route out the angled pieces of the jig itself. These were just 3/16" wide bits of luan I had as scrap...nothing fancy...they just stop the base shy from the bit contacting the angled upright rails.
Incidentally, the bit here is a 3/4" straight bit. I like a 3/4" bit for hogging wood with a hand router (non-table mounted), but that's probably governed by personal preference.
(5) Using the Jig
Couple of notes:
(a) Patience. The scarfs shown below took about 1 hour to cut. It's cut a line, shut down the router, move the piece, cut another line, etc. It takes a while.
Interestingly, if you read the comments to the luthier link above about Steve cutting guitar neck scarfs on a band saw, he says "go slow"....so, perhaps patience is a theme in scarfing.
(b) As mentioned, I found plunging the router to deeper depths did not work as well as moving the work piece with a shallow router setting. Again, patience is a virtue?
(c) The second to the last pass is likely to get into the jig base a bit...
My first scarf jig for the router, made years ago, had a sacrificial piece mounted at the base. This time, I figured that was just too much trouble, so I left it off. I'm only nibbling away, after all, and only near the end of the cut. I figure I could replace the "base" of the jig, reusing the angled uprights if desired, as needed. I'll bet I get many years of use out of this jig before such replacement becomes necessary.
(d) The final cut is likely to be pretty shallow, and I found that slightly scoot the piece over (rather than a full "index line" amount) resulted in a better end...the end of the work is hard to support properly, so it chips out rather easily which results in a poor edge (and joint, as viewed from 'above'). By barely scooting the work over, again using patience, the edge can be feathered quite finely and nice and straight.
(e) In the picture, the "spacer" piece between the angled upright and the work is a sacrificial piece of uniform thickness. The other piece is simply clamped around the work to help hold it still since clamps cannot extend into the jig...the ends of the work can "chatter," so anything to help hold it stationary helps.
This is a first cut with some oak scrap.
First, the two pieces cut. They were cut separately, though I was tempted to cut them together as per the luthier techniques.
Next we see the two sides of the glue line for the joint:
The first one is not as good as I hoped, but not bad.
The other side was quite acceptable (it runs in the picture from lower left to upper right).
The faces are not shown here. They are "not bad," but for these test cuts on scrap, I did not sand to the degree required, so the 'reveal' is less desirable than I'd like.
The resulting piece is true straight (as measured with several metal straight edges) along both planes. The jig produced a joint both straight and true.
Now, my next challenge...cut this quality scarf on some curved pieces. Wish me luck.