Woodworking for the handy, and not so handy
Occasionally the need arises for ‘box beams’, that is a three-sided wooden structure fabricated to represent a large, solid piece of wood. They sometimes are fitted over an engineered beam for cosmetic reasons, or the weight of solid wood beam would not work in the application for some reason. Most common cause is economics as large beams of clear, straight grain, dry lumber can be vey expensive. So we create a hollow representation of a beam that looks very much like the real thing cosmetically but will not function structurally.
My most recent project was a dozen box beams made to appear as 6 x 6 clear Douglas Fir, ranging in length from 6 to 16 feet in length.
Typically, the three-sided ‘boxes’ are joined with butt joints, which is relatively simple. But this guy demanded miter joints, which is a completely different (as in difficult) animal.
Ripping perfectly straight 45° angles over the length of a long board can be a challenge. As you can see in the pic, this is required once on each side and twice on the bottom board. Interesting.
Before we get to the cutting, let’s visit the material. In this case it was clear, dry, vertical grain Douglas Fir. The planks came square-edged and I required that they be as close to straight and flat as possible. Because the 1 x 6 ( ¾" x 5½") planks were to form a box beam no smaller than 5-3/8" x 5-3/8", I had almost no leeway as far as machining them to optimal straightness. This stock was excellent but for a few pitch pockets, so curves and twists were not much of a factor.
My shop includes a 7.5 hp, European table saw to which I have added a Steff power feed. After going over each board and identifying which would be a side or bottom, them grouping them by similarity of grain and color, and choosing a ‘face’ or exposed side, I begin the milling.
The customer told me the beams were to be fit to an uneven surface and he wanted a rabbet to make scribing easier, as less material would have to be removed to fit tightly against the ceiling.
The rabbet is done on a router table (or shaper) with a simple straight cutter. Pretty straightforward with the notable exception that fir is somewhat brittle when dry, so slow goes it.
The power feed combined with the heavy, solid table and fence makes for excellent results. I set the blade to the proper angle using an electronic device. Set it to zero on the table keeping the device square to the blade and on the blade on the same line.
The side pieces are relatively simple. I set the long point of the angle as close as possible to the edge of the plank using as much of the available dimension as possible. If doing the feeding by hand, firm, even pressure against the fence must be maintained, as the slightest movement away from the fence will create flaws in the joint. When milling the bottom board it is almost impossible to make the second cut because the sharp point of the first angle will tend to want to slip under the fence and bind the motion and cause it to waiver.
Solving this requires gluing up the first miter joint and allowing it to set/dry overnight. Then the new 90° corner will provide a solid smooth surface against the fence (right photo). Accomplishing the miter joint from this point on is actually fairly simple.
I am a big fan if splining and there is a relatively simple way to place the spline notched in the faces of the miter joint as pictured here:
But it was not necessary in this case. On to the glue up.
I thought I might try the joint without the spline as I have come to be very impressed by Titebond III (a multi-purpose, waterproof PVA type adhesive) for use in all instances except joints that will live underwater. Once the glue is applied evenly on both faces, I had to decide how best to hold them in place until the adhesive set, in most cases, overnight.
It has been my experience to over-think these problems, going off on tangents that include complex fabrications. Because many of my projects are unique, I do not have the benefit of legacy jigs and table setups waiting for re-use. My projects all come from our customers, in most cases experienced, talented builders who occasionally come across a situation that their normal tools and know-how cannot accomplish. This fact is what I like so much about my work here at Economy Lumber — problem solving.
To combat the tendency to complicate matters, I try to apply the K.I.S.S. (keep it simple stupid) philosophy. Start modestly and increase complexity as needed. This shot shows the first joint completed, but the setup and process is the same. Clamp or screw a straight scrap of lumber to a table to provide backpressure as you press the two faces together. Leave a small bit of table exposed for the second plank’s edge to rest on while the pieces are joined.
The secret ingredient, the thing I use to hold the pieces together, is a GREX 23ga. Pin nailer. Set the pressure to ensure the pin is entirely embedded and they are invisible. Then, working from the middle out toward the ends, pin the joint together every 12 - 16".
If there is some curvature in one of the boards or some other reason the joint is balky, I use increased hand pressure or a temporary small clamp to straighten things, then place an extra pin or two or three in the area to hold it until dry.
The glue is applied from the pint-size bottle (which I fill from a gallon jug as needed) in a straight full line down the middle of the joint face, then spread using a 1" foam brush. A great feature of Titebond III is its clean-ability. Keep a damp, cotton cloth handy at all times and clean as you go. I also wipe down the table between joints to keep any excess glue from getting on the faces of the ensuing boards. These box beams will be stained or treated with a clear finish and glue residue will possibly inhibit the stain or finish from being applied/absorbed evenly. I spend a good deal of time wiping the wood down during this process.
When the joint is pinned, I move it to another table to dry. The joint is sufficiently firm to place the first joint as shown at left to set and during the second joint you may need a small piece if 1x scrap as a ‘spreader’ to assure the point of the joint is completely closed. A final wipe down and they are set to dry overnight.
These joints are exceptionally strong, even without the spine or biscuits or Domino floating tenons. In fact, the contractor that applied these to an exposed plank ceiling told me when he tried to break down the cutoff pieces for disposal, some of the planks split instead of the joint. High praise!
The final picture of the box beams installed. The smaller beams are fake and installed 1/2" below the ceiling. Conditioned air is distributed through the gaps to heat and cool the room. Kinda cool, no?
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