Miter Clamps

One doesn’t have to do a lot of woodworking before coming up against the necessity of creating tight miter joints. It seems like many simple projects require miter joints at some point, creating challenges for new and experienced woodworkers alike. While cutting a miter joint doesn’t require the degree of skill that dovetails do, getting a tight miter joint, without any gaps, has led many woodworkers to despair.

The key to those tight miter joints is clamping. Having made the mistake of trying to make various types of miters, without clamping, I can testify from experience that it’s not worth trying. Clamping the pieces together, regardless of the assembly method used, is a requirement for ensuring alignment and tight miters.

Fortunately for us, there are a number of different clamp that have either been developed specifically for use as miter clamps or which can be adapted for use as miter clamps. Which one is used in a particular build depends on what the woodworker has available to them, and what they prefer working with, more than the abilities of the individual clamps, as most do more or less the same thing. However, some may be better than others in certain applications.

Attachment Methods

Before picking a clamping system to use, it’s necessary to decide how the mitered corner is going to be attached together, as that can affect which clamping systems will work. If the clamp gets in the way of installing the fasteners, then it’s clearly a poor mix and other options must be considered for that project.

Regardless of the fasteners used, mitered corner should always be glued. However, that can be problematic, because it is essentially gluing to the end grain. When glue is applied to the mating edge, the glue is quickly absorbed into the wood fiber, often without leaving enough to bond the joint together. To overcome this, apply a heavy coat of glue to both surfaces, then give it a few minutes to soak in, before attaching the pieces together. If all the glue has soaked into the wood fiber, apply more before attaching.

Nails

Nails are the most common means of attaching miter joints together. As the joint requires that the fastener goes into the end grain of the wood, nails are an excellent choice, providing a solid joint. A minimum of two nails should always be used from each direction, when nailing miter joints.

The problem with nailing a mitered joint is that the force of the hammer’s blow won’t just drive the nail into the wood, but can also have the effect of pushing the wood to one side, out of alignment with the joint. This problem is best handled by using a pneumatic brad nailer or finish nailer, rather than trying to drive the nails manually with a hammer.

Screw

Screws are rarely used with miter joints, but can be. The basic problem with using screws, is that the screws are going into the end grain. As the screw threads enter into the end grain, they break it up, weakening the joint. While that joint will last for some time, it is questionable how long it will last, especially if the screws are inserted and removed repeatedly or placed under stress.

Better screw results are generally received by using the pocket hole method. This allows the screws to be inserted at an angle to the grain, rather than parallel to it. That difference reduces the amount of grain breakage from the fastener, allowing for a stronger joint. A minimum of two screws should be used from each direction, whenever screwing miter joints together.

Biscuits

One of the uses for biscuit joinery that was pushed heavily when the biscuit joiner was first introduced to the market was miter joints. This has given many people the idea that biscuits are the ideal way of making miter joints. However, biscuits actually sit loosely in the slot cut for them, so they don’t automatically align the joint. Depending on them to do so can result in sloppy joints.

That’s not to say that biscuits can’t be used for miter joints, just that they should be used in conjunction with a good clamping system to align the joint and that the joint should be left clamped long enough for the glue to dry. With those precautions, the biscuit does add strength to the joint.

Dowels

Many people avoid dowels, but it is my favorite means of mitering joints. Doweling has been used for the construction of furniture since long before nails were invented. It still provides a very strong joint, with the choice of leaving the dowel exposed or hidden. However, on mitered joints, it is extremely difficult to make a hidden doweled joint; we’re basically limited to exposed joints.

That’s not an issue, if we make the dowel joint into part of the project’s design. I like to use contrasting hardwoods for my dowels, making the dowel distinctive. While not a feature that would necessarily be wanted on fine furniture, for more casual furniture it provides a nice detail that speaks of the quality of the project. People tend to be impressed by doweled furniture, because it is so rare.

In order to dowel a mitered corner, the pieces must be firmly clamped in perfect alignment, with the outside surfaces of the corner visible. Then the hole locations are marked and drilled. The hole nearer the outside of the corner is usually drilled one dowel deep (referring to standard fluted dowels cut for joinery), while the hole towards the inside is drilled one and one-half deep to ensure that the dowel goes into the second piece.

With the hole drilled from one side, put glue in the holes or around the end of the dowel and drive it in as far as it will go. For the dowel hole closer to the inside of the corner, once the dowel is driven in, drive a second one in after it, pushing the first one all the way to the bottom of the hole. Then, once all the dowels are driven in, cut them off flush with the surface of the wood.

A Note About Cutting Miters

One of the most important parts of making any miter joint is cutting the wood that is being joined together. Yet many woodworkers, especially new woodworkers, consistently end up with joints which don’t come together properly. The accuracy of these cuts is extremely tight and unless the saw is set up properly, it is unlikely to come out well.

The issue boils down to not properly setting up tools, something that experienced furniture makers understand well and the rest of us struggle with. Saws and other tools are not generally properly set up, right from the factory, but are close enough that we usually think that they are.

Take a table saw, for example. There’s a stop for the blade, when it comes straight vertical. But that stop is rarely set to make the saw blade come out exactly perpendicular to the saw’s table. Check it out sometime, putting a square up against the table and the side of the blade. It’s likely it will be ½ to 1 degree out. The same will happen with the fence, if it is checked.

Before using any power tool for cutting miters, verify that it is set up properly in all three axes, even a power miter saw. Then avoid using miter gauges, unless you’ve spent the money to buy a really good aftermarket miter gauge. Rather, if cutting miters on a table saw, make a miter sled for it, to ensure that the angle being cut is exactly right.

One of the best tests for any saw’s setup is to make a cut, then flip the board over and see how the blade tracks with the edge of the cut. A saw that’s off will take off more material at one side of the cut, than it will at the other, if you’re just trying to shave off the edge of that board.

If the saw being used leaves blade marks on the surface of the wood, it can be enough to keep the two surfaces of the miter joint from mating fully and leave a gap. To overcome this, true pros cut their miter joints a little long and then use a shooting board and plane to make the final surface. That eliminates the blade marks, while bringing the size of the workpiece down to the exact right point, ensuring the best possible fit.

Clamp Styles

There are a number of different clamp styles which can be used for clamping and attaching miters. One requirement of any of them is that they leave space for inserting the fasteners, even while holding the wood pieces together. Not all clamps do that well. It’s also important that the outer edge of the corner remains visible at all times, so that the miter joint can be visually inspected for alignment.

Corner Clamp

Corner clamps consist of two slots to hold the two pieces of wood, set at 90 degrees from each other. The wood is held by screw handles, similar to what’s found on F-clamps. This provides very tight position control for the boards, allowing the woodworker to fine-tune their joint. I have four of these, allowing me to clamp a frame all the way around, if needed, to assemble it, even though it is possible to use this sort of corner clamp to clamp one corner at a time.

One problem with using corner clamps is that they only hold the board at the corner, without any support for the remainder of the board. This isn’t a problem for short boards, but it can be for longer ones, In those cases, it can be useful to cut a piece of scrap to the thickness of the corner clamp’s base and use it to prop up the far end of the board. That will help prevent misalignment from the board being twisted or sagging.

Band Clamp

Band clamps are perhaps the worst possible option for clamping miters, especially mitered frames, as they cover up the very part of the board where the fasteners need to be installed. However, band clamps can be useful in the center of a miter joint, where no fasteners are being installed, when a long miter joint is being made. They are one of the few clamping options that can be used in the center of a long joint.

Spring Clamp

There are a number of different types of spring clamps on the market which are designed for use with miter joints. Some can be used for other types of joints as well. The big plus to these clamps is their convenience; but they can be unreliable if the spring tension is not high enough.

The big problem with most spring clamps is that the clamp is dependent on the friction interface between the pads on the jaws of the clamp and the wood that it is in contact with. If there is not enough friction, either due to the choice of material used for the pads or the amount of spring pressure that the clamp provides. Manufacturers must balance between providing enough spring pressure to make the clamp have a good grip on the wood, while still possible to operate with one hand.

Collins Spring Clamp

Collins has overcome the problem common to other spring clamps by the unique design of their clamp and how it is installed. The clamp itself looks like some sort of a large, strange wire paperclip. It’s installed with a tool which looks much like a pair of snap ring pliers. But this combination overcomes the friction issue that plagues most spring clamps, while also providing more spring force.

One of the major differences between the Collins and other spring clamp is that there is no “pad” for the clamp to wood interface. Rather, the ends of the wire are sharpened, allowing them to dig into the wood, ensuring a solid clamp to wood interface, which won’t slip. The sharpened end of the wire does create a hole in the wood; but it’s a hole along the size of a brad, making it extremely easy to fill.

Collins clamp are designed for using several together, when needed, such as for long mitered edges. It is the only clamp, other than a band clamp, which can be used in the center of a long miter. This makes it a useful clamp to have in one’s toolbox, even if they use other types of clamps for mitering as well.

F-clamp

F-clamps really aren’t designed for clamping mitered joints; but they can be used for such, if a couple of cauls for clamping the miter. For this a couple of pieces of scrap will be required, preferably the same size and thickness, although they can be thicker or thinner and still work.

Cut notches at 45 degrees into the cauls, as shown in the drawing below. The actual size and location of these notches is not important, as long as the flat area provided is large enough for the pad on the F-clamps that are going to be used and that the notches are in the same position on both cauls. It is also important that the cauls be attached to the workpieces in such a way as to leave the corner exposed.

cauls, f-clamps, notches
Cauls for f-clamps

To use the cauls and F-clamp, first attach one caul to each of the parts to be mitered together. Small F-clamps can be used for this. Then use a third F-clamp, across the notches cut into the cauls, to draw the mitered corner together and hold it while the glue is drying.

Fixing an Open Miter Corner

Despite our best efforts, we all end up with mitered corners which don’t turn out right. The most common problem is to have a gap between the two pieces, either just at the very corner or along the entire length. There are a variety of reasons this can happen; but it all boils down to something either not fitting together properly or not being clamped together properly.

If there isn’t much of a gap, it may be possible to solve this problem without the use of any wood fillers, especially on a long miter joint, such as might be found on a face frame to side panel on an end cabinet. To fix it, take a screwdriver with a round, chrome-plated shaft, and use the side of the shaft as a burnisher, working both sides of the joint to displace wood fibers, pushing the joint closed. For small cracks, this can be done so well that there isn’t enough displacement of the wood fiber to be visible to the casual glance.

If the gap is too large for this or the gap is in the surface of the miter joint, rather than in the corner, then it will be necessary to use some sort of wood filler. However, rather than buying a wood filler that isn’t going to match the wood in the project, it is better to put a small amount of glue into the crack, then use some of the sawdust from cutting that same wood, working it into the glue and the crack with a finger. The result will be much less visible than any sort of wood putty on the market.

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