Joining boards end to end
What's the best way to do it when creating a 20' oak top? April 2, 2002
Question
I'm a furniture builder who doesn't typically build anything taller or wider than 8 feet, but am working on a built-in bookshelf that will have a 20' by 1' by 4/4 oak top. Since I can't find oak in 20' lengths, what is the best way to join boards end to end? I'm thinking of making two 10 footers that can be joined together and finished on the job site. Any suggestions?
Forum Responses
Time to order a fingerjoint router bit and make some clamping jigs for end joining glue-up for making 20' long boards.
A spline might work well. Biscuits would tie it together. A half lap joint is another possibility. My preference would be biscuits with a small v groove or bead to mask the joint.
KV 516 planetary joint (ball fastener) will work great here. They do not require clamps.
I agree with the above post. The KV ball type draw bolts are perfect for no-glue dry joints such as office and library counters. You will need access to the undersurface of the top if it is to be assembled in place. Also a couple of inches of side play is needed in order to engage the bolts.
From the original questioner:
I ordered some KV draw bolts and a finger joint router bit. Can the draw bolts be used in conjunction with finger joints? Or would this be overkill?
From contributor R:
Why not scarf and make the joint less noticeable? I scarf shorts together all the time. I believe it's the premium way to join for long lengths.
From contributor D:
Contributor R, what kind of equipment do you use to scarf wide tops? I have seen plywood scarfed, but haven't figured out how to do a similar cut with typical shop tools.
From contributor R:
As I understand the question, his top is 1' wide and 20' long, easily spliced together in the field or shop using a bench saw or, in my case, a slider. You could also skill saw the angle in the field. Oak is easy to find in that width, but you could splice together multiple narrow boards before gluing to width. Maybe the term scarf is misleading, as I've always known a simple long grain splice to be called a scarf, just no key, bolt or strapping. There is also a joint known as a hooked splice that is more complex but doesn't leave a knifed edge glue joint, which some frown upon. I like the way a tapered long grain joint looks as opposed to a cross grain butt.
Pocket screws (Kreig Jig or similar) would give you any length boards you want, are fast and easy and cheap, and are quite strong. We sometimes use it for things like this, or for adding wood edges to tops, where you drill pocket holes at 6-8" o.c. in the underside of the top and just glue and screw the edge to the top with the top upside down. Most important is prepping the wood edge, with smooth and flat wood to work with. With the screws, you don't need clamps and can do any size top easily.
From contributor D:
Contributor R, the plywood that I have seen scarfed is angled over a 6" length at a very shallow angle. You would need a very large blade to cut a 15 degree angle, for example. You must be describing a 45 degree angle.
The idea behind the long, shallow angle, I would assume, is to allow downward pressure to clamp the joint. Also, I imagine it would be stronger with 6" of glue surface compared to about 1" that you would get with a 45 degree cut.
I agree that the scarf joint, assuming you get it tight, would blend away better then a butt joint.
If I was doing the top I would stagger the joints so that they weren't in a line across the top, assuming you can get the thing in the room in one piece.
I've made scarf joints with a router on a jig set at a 15 degree angle. The wood sits flat in the jig, the router is held at the desired angle by the jig and you shave away at the wood with a straight bit.
It would be very hard to scarf a joint on a saw because you must not burn the wood at all. The rubbing from the blade on a long scarf would generate a great deal of heat and therefore a very poor strength joint. Instead, go with the router.
We have done the scarf joint with a home made jig and a router, which works okay, but is a little tricky to get glued up. We use multiple strips on a 12" wide top, stagger the joints, and use Hoffman dovetail keys to hold the butted ends together for gluing.
If you keep in mind the match up of the grain, you will find scarfing most rewarding. I keep my scarfs no less than 2' in length and by doing so, with the grain matching as well as possible, an untrained eye will not notice them. Clamp one on top of the other (grains running same direction), flip so they are lying side by side and scribe your line. Then unclamp the pieces and cut them on the table saw, raising the blade a little at a time. A shop-made jig/sled works great for this! With both pieces cut, fit them back together, mark a line across them, mark c of w/p from the line on both pieces, drill 3/8 hole for dowel. This helps keep pieces flush during the gluing process, and is strong and cheap as fasteners go. Glue up using wood Jorgenson clamps, clean the joint with a damp towel (be sure to cover entire face of glue joint), let cure and sand lightly. You will be glad you took the time.
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).
Comment from contributor A:
I have done very high strength scarf splices using a band-saw to create a stepped scarf splice. That is, each end of the splice goes in at 90 degrees for about 1/4 the total thickness, with the remaining center 1/2 of the thickness spread across a long angle. Getting the joint to fit perfectly *inside* is the key to the strength. I have put blue chalk on one piece, then dry-fit the joint, then shaved down high spots with a block plane or chisel (do not sand, as sanding will seal the grain and prevent glue from penetrating). One particular scarf I did in a sailboat mast that was being re-designed (lengthened) later had the mast snapped in an accident, but it broke over a foot past the joint.
