Confession time: I avoid miters if at all possible. Tight mitered corners on a box look great, but there’s a lot that can trip you up. First off, you need to cut clean miters in every mating part. Not only that, the angle of those miters needs to be dead-on to avoid a gap in one or more of the corners. And I mean dead-on.
At assembly time, miters conspire against you. They slip and slide to avoid alignment. Plus, the porous end grain soaks up glue that leads to a weak joint.
MY ALLY IS THE SPLINE.
So it’s with admiration that I observe other woodworkers step into the ring with a miter. From their examples, I learned a method to tame some of the unruliness of this vital joinery technique. The secret is the spline. Splines are found all over, incidentally. They appear in construction as well as medicine. To sum up, a spline is a separate component that reinforces an assembly. I consider splines part of the loose tenon family. The drawing illustrates this concept with our current topic.
I see two quick benefits. First, the spline registers the two parts in relation to each other. This eliminates the sliding around that occurs once glue hits the surfaces. The splines allow for clamps to hold the joints tight. The other benefit is how a spline increases the glue surfaces. This results in a stronger assembly. This attribute holds greater importance as the project size increases. (Many times in small boxes, you can get by without splines.)
MITERS FIRST.
Of course, cutting miters on the workpieces is where you start. As important as it is, it’s not the focus here. Once this is done, there are two remaining steps to add a spline to the joint: the fi rst is to cut matching slots in the mitered faces. The second is to make a spline.
THE SLOTS
Think of the spline slots as mortises, and you have a good idea of some of the considerations you need to make in the slot size and location. The right drawing on the previous page illustrates this with a table saw setup.
When you place the slot closer to the “heel” of the miter (about 1/8" from the edge in 3/4"-thick stock), this allows you to cut a deep slot and use a long spline. Avoid extending the slot through more than half the thickness of the workpiece.
SETTING UP THE SAW.
In the drawing, the blade is tilted to 45°. The principle behind this angle is that the slot is perpendicular to the face of the miter.
The rip fence locates the slot consistently on each mitered face. The distance between the blade and the rip fence determines the slot’s position.
CUTTING SLOTS.
When the project is made up of narrow parts, I guide the workpiece with the miter gauge. An auxiliary fence backs up the cut. For a wide plywood or solidwood panel, the rip fence is the only guide you need. You simply slide the long, mitered tip of the workpiece along the fence as you would for cutting a groove. A featherboard keeps the workpiece flat on the saw table and the slots consistent.
THE SPLINES
The splines that fit the slots must fit snugly but still slide easily into place. Take care here. I find that PVA wood glue tightens up the fit. Liquid hide glue (often my first choice) lubricates the parts for assembly.
If we stay with the mortise and tenon comparison, it should be clear that the splines need to be strong for a solid assembly. There are several ways to reach this goal.
SPLINE OPTIONS.
For solid wood assemblies, like the one in the main photo, I use solidwood “crossgrain” splines, as the margin drawings explain.
A strip of 1/8" plywood or hardboard gives you a strong spline with a fit that’s ready-made for a 1/8" saw kerf when assembling wide plywood panels. In these situations, the edges are generally covered and the ends of the splines are hidden by strips of edging. For a long miter in solid wood when the spline will show, I use a different approach. Here, a plywood spline can be cut to fit short of the slot ends. Then hardwood caps are added to create the look of a hardwood spline.
Miters shouldn’t be scary. Adding splines is a simple technique that yields big benefits.
SPLINE GROOVES ON THE TRAY TABLE
In some ways, it’s easier to wrap your head around a 45° miter. It’s just more common. In the tray table on page 50, Shop Manager Marc Hopkins took a different tack that I think is worth highlighting, since we’re on the topic.
There are two items to note about his approach. First, rather than use a miter gauge to guide the workpiece, he went with a tenoning jig. This holds the workpiece vertically. The piece is clamped in place, so it’s less likely to shift during the cut. The jig shown here is a shopmade version that rides along the rip fence for easy setup and use. You can find the plans for it at www.WoodsmithPlans.com. A commercial tenoning jig would work as well.
The second detail is the blade Marc used. To avoid a fragile tip between the kerfs, Marc installed a very narrow 1⁄16"-kerf blade. These aren’t common, an alternate choice is to use a 40-tooth blade from a portable circular saw.
One last item: note that the rip fence is located to the left of the blade in order to make the cut.
Go Vertical. A tenoning jig is another way to cut the slots for a spline to reinforce a miter joint. Here, three parts join together with 60° miters. So the table saw blade is only tilted 30°. A digital angle gauge works better than the saw’s scale.
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