Reinforcing Mitered Corners with a Spline

Miters are a great way to put boxes like this together. What I really like about it is that because we've got a mitered corner there's no end grain showing on any side. One of the downsides to miters is because it's an end grain to end grain glue up there's not a lot of strength there. Also, it's pretty neat to dress up these miters, make 'em look a little bit cooler. I'll show you what I mean.

Look at this box, we've still got our miters, same material, working with maple here, but what I've done differently on this one is I've cut splines into it and the splines are made of a contrasting material. They're made out of walnut. So we got a couple of cool benefits here. One is the spline is going crossways across that miter, so it reinforces the joint. The other thing that is neat is that it looks cool.

On this side, I've cut with a dovetail bit, so it gives the appearance as though we have dovetailed that corner when we put this all together. Splines are a great way to reinforce corners and they look nice. So let's talk about splining here on the router table. Big deal here, you've got to have a carriage to hold the material to cut for the splines. There's no way we could do this freehand.

So what we want out of this is a device that's gonna go on your router table, securely hold the material in place 'cause if it walks as it goes through that's gonna adversely affect your splines. It's gonna make it hard to get parts to fit together well. So it's really gotta be nice and solid. I'm gonna do my cuts with a dovetail cutter. I've also got on the table just a 1/8" spiral bit.

Lots of options for spline cutting. The 1/8" spiral bit is what I used for the second splines on the first box I showed you. We can go with 1/8", 1/4" we can use whatever bit we want for that. I do like the spiral cutters. I think they provide a better surface finish inside the cut so when we slip the spline inside there we get a better fit between the spline and the slot that we've cut.

As I've said in this case, I'm gonna use a 1/2" dovetail bit, more specifically it's a 1/2" 14 degree. I'm gonna disassemble just a little bit here to help you see what I'm talking about. One of the things that we wanna check for is the height of the cutter. So as that bit comes up, we wanna make sure that it's gonna cut into the miter but it's not gonna cut so deeply that it comes through the inside corner of our box. So, really easy way to make that happen is simply, put the box into the sled and then set the height of that bit, watching the top of the cutter relative to the inside corner of the box and that gets me in a good position right there.

Now these side supports are very cool because we need a couple things to happen here. One is we want repeatability. So once we create or generate a position for one of the splines, we wanna make sure that we cut that uniformly. What I mean is that I can use this to set the distance from the cutter, router's unplugged so we can get in close like this, from the cutter to what will be the edge of the box. 'Cause the box is gonna go up against this face.

Lock that in place. Now the way that's gonna work is, box comes in, make the cut, box comes in, make the cut. So it creates uniform spacing from the cutter to the edge and that's gonna give us our best repeatability, our best look. On the other side of the world here, we put this one in, that's gonna provide support for the box over here. Remember I talked about the jig being solid so that things don't walk as we're goin' over the cutter.

We also wanna make sure that the box can't move while it's goin' over the cutter because that'll adversely affect our cut. All right, that locks me in. So what we've done is set the height of the bit, positioned these guides so that it controls the position of the box. And with a little electricity here we'll be ready to make a cut. If we're doin' a 1/8" bit a 1/4" bit the approach is gonna be exactly the same.

Make sure the top of the bit isn't gonna penetrate inside the box. This results in the spline profile inside the box. Next thing we wanna look at is, specially in the case of these dovetail splines, how do we make the component that's gonna fit in here, so it's a good fit, looks on the box? I'll show you that next. The dovetail splines are actually easier to make then you might think.

Let's look at the dovetail bit set up first, router's unplugged. Here's what's goin' on here. You gotta use the same cutter that we cut the female with so that cutter's still in the table, now we're gonna use the fence on the router table. And notice that down here the bottom edge of this carbide cutter is just slightly below the surface of the table. That's our starting point.

Face of the fence is forward so that we're gonna make just a minimal cut. Think about what we've got goin' here. The inside dimension of that dovetail slot, that we cut for the spline, is a 1/2" 'cause we used a 1/2" dovetail cutter. So the material from which you cut your spline should be just a little bit over 1/2" thick, 9/16 or so would be fine. Then what we're gonna do is stand our material on edge, pass it over the cutter, rotate it, pass it over the cutter and that results in this dovetail shape being produced in our material.

Now, the more the fence is forward, the more the fence is toward me, the bigger that tail will be, the tighter the fit. The more I move the fence back, the more material I remove, the looser the fit. So, the way we work this is, do a test cut, grab your material, your box, and then slip that in and see what you've got. When we have a good fit then we can go to the next step which is actually to get the spline material ready. Remember that as you adjust the fence you're always making two passes.

So tiny movements in the fence have a pretty big result in your spline material 'cause it's twice the effect. So real small changes in that fence. Once we've got that to fit, then we can take this to the table saw and rip this so we can get just the spline material which is then gonna look like that. That can then get glued in. Once the glue is dry use a hand saw and cut it flush, and sand it and you'll end up with splines that looks like this.

So key to this, good solid splining jig on your router table, make sure that everything is nice and rigid so that the box isn't walkin' while you make the cut. Fine-tune your fit between the spline material and the slot that we've cut. When you glue it together you can then come back sand 'em nice and flush. Not only are you gonna get a great lookin' box when you're done but you're also gonna do a lot toward reinforcing that miter to really increasing its strength.