Router Tables Session 7: Lock Miters

Lock miters are a great but challenging way to put corner together. What's great about a lock miter joint is that when it's done, we literally have a miter. We can't see any end grain on the outside of the joint. And there's a tongue and groove nature to the joint that gives you a lot of mechanical interlock. A lot of glue surface inside the joint.

The challenge is getting the bits set just right height and fence working in two planes set up can be kind of fussy but I'm gonna give you some tricks. First thing you got to know is what part of the bid I'm gonna be talking about when I start locating the bid in the router table, what I want you to pay attention to is this inside corner right here, that little spot. And what I'm gonna be talking about is getting that inside corner on the center of our material. So trust me on that for a second. We're ready to get the bid in the table So far so good.

Now test pieces have to be exactly the same thickness as your project pieces. Additionally, we wanna make sure that the cutter you're gonna use is the right size for the material you'll be working with commonly with lock miters. There's a range of sizes that they'll cover. I think this one is half inch to five eight. We've got that particular cutter source for you.

So before you buy your bit, make sure it's the right size for what you're gonna be with on my material I'm gonna find center using the George method the George method being finger and pencil, make a line, finger and pencil, make a line. What that shows me is at the center of that board is probably about there or so now when that comes on to my router table, remember inside corner centered on the thickness of our stock, And I'm gonna call it right about there. Looking at the relationship between that pencil line and the inside corner I showed you earlier in the bit. We wanna get it done close, but as frequently happens test cuts along the way are gonna tell us if it's actually right or not. So don't pull your hair out, going crazy over that setting.

Here's the next thing we need to look at. Where do we put the fence we gonna bring the fence forward and make sure that my fence halves are in the right position for this size cutter. Now we're gonna talk three planes. We're gonna talk about the top of our material the face of the fence and that 45 degree carbide surface on the router. But as a result the router bit has to be facing out this way carbide forward material goes against the fence.

Test pieces being exactly the same thickness as your project pieces are three planes then are top of material. Face of fence, 45 degree cutter all have to line up. We make that happen by putting a ruler into that inside corner bringing the fence out until the bottom of the ruler kisses across the top of that carbide. Now right there it's hitting it. I'm a little too far back, gonna lock one end get finer control here.

adjust Let's try that with me so far on this cutter on your setup, what we're gonna do is feed two pieces horizontally on the router table. We're not gonna use the fence this way just yet. We're not gonna do that until we know that the height of the bid is correct. Next thing you'll see me do is feed and feed. And then we'll look at the result and see what we need to do next.

The bonus points question for you is I wanna know if that bit is in fact cutting in the center of my material. How am I gonna check that? Think about it for a second. I'm gonna take my two pieces. Flip one, rotate the other slide them together if we're flush oh my gosh.

When we are flushed, that never happens. If we're flush across the face the bid is perfectly centered. This is where things get fussy. We have to get the height of that bit dialed in perfectly so that when I do this, I'm flush across the face. Doesn't always happen.

In fact, pretty rarely happens on the first try. Let me show you some sample pieces here. And what you wanna do is keep sample pieces like this and your shot. Here's scenario number one the sharp edge created by the lock miter cut is higher than the adjacent face telling me then the bid is too low in the table. So because I'm a long time between lock miter setups.

I keep these pieces so that when I get to this stage, the question is, well if it's not right, which adjustment do I make. Raise the bid or lower it. So I've held onto these pieces for a long time. Alternatively, look at this one, the knife edge, the sharp edge is below the adjacent flat face. So on this one that tells me the bid is too high.

And then I kept a third set just to show this is where we're going. This is where we wanna end up. We wanna end up with those two surfaces perfectly even with each other. Let's go back and look at my Poplar I'm working with today. Yeah, I'm real happy with that.

So I'm gonna say somehow magically we got there on the first try, doesn't often happen but you have to be very very patient at this stage of the game and make sure that that set up the bit height has just right. Now if you change the elevation of the cutter raise it or lower it on. What else do we have to do? We have to readjust the fence position. I'm gonna unplug my router so I can get a pencil in here.

Keep in mind that we've got this relative position here. We've got this imaginary line created by this inside corner. If the bit is lowered, this 45 degree surface drops and I have to adjust the fence accordingly, move it back. If the bid is raised, then the carbide comes up gonna have to scoot fence forward just a little bit. So every time you make a height adjustment change you also have to make a fence adjustment change.

Step one get the height of the bit perfect. So that I'm perfectly centered on my material. Next thing we need to do then is examine the resulting edge and in this edge, and I specifically mean the knife edge. Along the top of our material one of three things will happen. We're gonna get a nice perfectly sharp edge like I have here.

Or when we look at this edge, it might be blunt. So instead of coming to a perfect corner it may have a flat spot on it. Or instead of we do have a knife edge but at the trailing end of the cut we might get a snipe and over cut. So in those two scenarios, if it's blunt the fence is too far forward. We're not taking enough wood off and you've got to move the fence back.

If we have a knife edge, if we have a sharp edge but it snipes at the end of the cut and over cuts the fence is too far away from it. So your first micro adjust get the height of the bit right. Then examine that knife edge. See if you're getting an over cut. Now, when I look at this one, I'm gonna say I can feel it, but I bet you can't see it.

There's just the tiniest bit of snipe on this edge right here. So I'm gonna go ahead and bring the fence forward. And I'm gonna intentionally come too far forward 'cause I wanna show you what that flat spot looks like. Snipe tells me I had to come forward, which I did let's see what this cut's gonna do. Good news is I made a correction.

Bad news is I made too much correction. This is the flat spot I was talking about. The fence is too close to you or not taking enough wood off. We're not producing that knife edge. We're not gonna get a good lock miter.

So I've got to go back just a little bit. Let's unplug and do this set up that we did earlier with the straight edge again. Once we're messing with the fence, when we know the height of the bid right. And we're just messing with the fence, you really only have to make one cut 'cause we don't have to make pieces together anymore at this point we just have to look at that knife edge and make sure we're getting what we want out of that. I'm gonna mark this one so we know that we've already examined that go with that one.

Looking at this edge, no flat spot, given it the field test this is our trailing edge, no snipe. So what that fence adjustment, we took care of that problem. Now make your life easier probably want it to be easier like I want my life to be easier pay attention to how I'm doing these test cuts. Notice that I'm cutting long grain, not any grain either one would work, but the dynamic of woodworking is it's easier to cut the same direction as the grain than it is to cut across the grain. So my advice is do your test cuts that way just to keep things simple.

Now we are ready to go. That means on our real cut. We're gonna feed one piece on the horizontal plane. The other piece on the vertical plane in order to get our resulting lock miter. This is the cool part.

When we get that joint to work and put it together. That's what we got miter on the outside. so we can't see any grain the Canton mechanical interlock inside the joint that does a couple of things for us. One is that look at what that mechanical interlock all the glue surface that we're getting. Additionally, it's easier to clamp a lock miter than a conventional lighter because these parts can't slide past each other.

If this was only a miter anytime I put pressure on this piece it wants to slide downhill on the other piece, in this case because they're locked together. It's not gonna do that. Comes to gluing in fact you gotta be careful because the tolerances are so tight. It's a great fit between this tongue and this groove that if you get a puddle of glue in the bottom of that groove that can prevent the pieces from coming together. One of the things that's cool about this is using this procedure on plywood.

If I wanna make a column of some kind instead of making it out of solid wood I can make it out of a veneered material. Get the strength of plywood the mechanical advantages of a lock miter. And when it's done, it'll look like a solid wood plywood 'cause of course it's veneer on all four sides on the outside. A lot of benefits to using a lock miter little bit fussy to set up. If I change thickness of material everything goes away here.

In other words, when I change my material thickness that'll affect the height of the bit and the fence location. So you're always working in two planes. What I would do, I'd save these scrap pieces. So when I come back to this thickness, I have a gauge I can use to set that bit height and fence location. That'll get you close.

But like I said, every time we change material thickness you're gonna have to reset both bid height and fence location. But what the benefit we'll get out of this as the advantages of that lock miter joint, which we've talked about mechanics and beauty, putting this together with two contrasts and materials, really look cools imagine one of those is cherry. One of those is maple. I've done that quite frequently 'cause wanted to find out if I'm taking the time to make a lock miter I want people to see the lock miter, take your time get the hang of making that joint. And you'll be using lock miters on your boxes in no time.