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Box Beam Construction1/23
I have an upcoming job to make a dozen or so paint grade box beams out of 4/4 poplar. Since the lengths are 18-24', I will need to join for length. I'm wondering if dominos would be acceptable for the butt joints, rather than finger jointing on the shaper. Seems like it may go a little quicker as well.
I use toggle bolts. Some refer to these as "Dog Bones." I have a CNC so it is fairly easy to route for a couple of these.
I do the finger joints on a shaper to get length, then clean up the edges and edge glue up to get my widths. I've tried the dominos and had them fail on 18' long stock when someone picked it up flat and not rolling it to vertical before lifting it. So we do the finger joint and then stagger those joints in the bigger glue up. Once that's dry you can't break those without getting really stupid, like putting them flat on top of a Prius. Truth! Just last week! It didn't come back so it must have survived.
Does anyone scarf join for length? the old 7 to 1 ratio is fine, but I have done them more like 5 to 1 and they and perform fine.
I once made an industrial scarf-o-lator to use to reclaim a surplus of 10' to 12' base and crown moldings. Put a big old skillsaw on a platform that rode on linear rails at an angle. Turned all those 'short' parts into 20' long parts.
I did 2 jobs last year that needed long styles & rails, I used a 12-1 ratio because they were going through the moulder. Made them the same way as David. I don't think I've used that joinery for 20 years
12:1 scarf joints theoretically maintains the full structural strength. We use the same ratio in composite(carbon/fibreglass) repairs.
The reality is unless the part you are making is a beam the original strength is orders of magnitudes higher than required.
6:1 or 5:1 scarf joints are incredibly strong.
Our current scarf-o-maker is about a 6 to 1, and we can't break joints in 4/4. In diffuse woods like maple and Alder, it is almost impossible to see the joint.
The beams in the photo have 18'+ long mains that are scarfed together. These were 6/4 and the corners were tongue and grooved. In Alder, the joints cannot be seen up close, much less down on the floor.
The reason for the 12-1 ratio is that is the way I was trained. Could have been a boat builders standard.
Not really the same, but we have used dominos for this type of thing on long/ wide runs of baseboard, it works well given enough dry time. We do a domino scarf hybrid!! The only reason I know this is pretty strong, is because one time I glued a few lengths together oriented the wrong way in a long hall, the painter helped me fish it out the balcony end for end nothing broke 10' each side of joint, also with dominos and the angle the joint just slides together when you tap the end, if you go too far tap the other end to get it back, we dont even clamp them!
The scarf is amazing for hiding the joint. I once did some maple trim around the top of an oval photo booth and scarfed the seams the customer went crazy trying to figure out how I did it.
Ok, so most everyone is doing scarf joints. How is everybody doing them? Jig on a tablesaw? Some special machine? Do tell please.
We use a jig on the tablesaw with a fence at the angle and 2-3 DeStaCo clamps. Fast and easy.
We also use the Festoool and track, and lay one piece directly on top and clamp the fence in place and run the saw. Somewhat faster than the tablesaw, but it has a few more things that can go wrong.
Yes, the old 12 to 1 ratio came from boatbuilding as I recall. More surface needed with less effective old time glues. But it is a killer in the material use department. Say you have a 12' piece and you need 13'. Put a scarf 12' long on the 12' piece and put an 11' scarf on a second piece 12' long. Put them together and you have a 13' piece of stock. And two large, long triangles you don't know what to do with.
By far the best Scarfulator was the Skillsaw with a big fat blade on a carriage that travels back and forth on linear bearings. The bearings are above the moldings. The moldings are the end of one on the left, and the other is the end of the one on the right, and they come together under the jig, laying tight against the fence and directly on top of each other. Air clamps were then engaged, then the saw fed thru the two molds or molder blanks.
We moved quickly to scarfing the molder blanks so they could go thru the molder after the scarf joints. I think we had one come apart in what must have been thousands of pieces.
We stocked 20' lengths of crown and base molding in an attempt to garner carpenter loyalty at a time when the builders were flush, and did not care. Of course there was a $5.00 surcharge on the 20 footers along with the per-foot charge. We could make a hundred 20' pcs in an hour, so it paid well.
The way it went for us was we could not sell the 10' and 12' base and crown. Everyone ordered 16' - even if there was not a 16' wall in the house. So, I went them one better and promised them all the 20' moldings they wanted.
Well, that worked well until the Owner came by and stopped it all. "We are not here to help carpenters" he said in his usual drunken drawl. 'We are here to make money from their employers". Bright boy. Later did 18 months for price fixing. Plead ignorance - did not know price fixing was against the law. His famous quote was " Profits were eroding, we had to do something!"
I just used a slider, with lamellos
first are these load bearing beams or just for show? Second lets not forget they are paint grade so any strong joint can be used, they can even be made up of two 8/4 boards glued together with lap joints and then they can made as long as anyone wants. If the are just for show they can be made out 3 pieces of 4/4 and run through a planer to make the bottom flush and then you have some simple way to attach them to what ever structure you are applying them to. I now we run into arch. that are constantly wanting us to mill brackets and such out of large peices of solid lumber and you be surprise how many we make are just hollow boxes. We of course prime everything and make sure no joints show and make one and submit it for approval and never had one turned down yet.
Thanks for all the responses, great info. I think I will skip the finger jointing and scarf them. I was thinking about a jig on the radial arm saw to cut the miter, then using dominoes for alignment, staggering the joints. These are paint grade, non structural so I think they will be plenty strong. Thanks again
Clamping the two long halves of the joint can be hard to do. Especially with slippery yellow glue. A way to iff it is to drop 2-4 grains of sand along the open joint, then clamp it up. The sand - only 2-4 grains! - is enough to lock the two together and prevent sliding.
Salt also works.
Sorry if I am not understanding the scarfing part, but are you scarfing in thickness, or across the width of the board. I am trying to visualize a jig that could scarf in thickness at about 12” long on 1” material on anything over about 4-5” wide?
I scarf on the width, square off the face of the board. I also clamp against a straight edge and glue up for width with staggered joints. no salt or sand needed!