A mitre box is a handy woodworking tool used to guide a backsaw, so you can make clean and precise cuts with it. In this instructable, we're not just making any plain-old mitre box: This mitre box has an adjustable width, so you can use it to cut wood of all sorts of shapes and sizes. In fact, you can adjust the mitre box's width from 0 all the way up to, well.. the length of whatever saw you want to use really. It doesn't even have to be a backsaw; it works just as well for e.g. a hand saw, as you can see in the first picture. Moreover, making this mitre box is actually easier than making a regular fixed-width mitre box. The trick is that we're just going to make two sides of our "box", clamp them into a Workmate/workbench's vice, and let that workbench do the hard work. That is, the workbench will form the bottom part of the mitre box, as well as provide the adjustment mechanism. Additionally, you can also secure your workpiece by clamping it down to the workbench.

The reason I ended up making this funky-looking mitre box was because I needed to resaw some small wood strips into thinner strips. (see the third picture) Because I don't have a table saw or a bandsaw, I resorted to cutting these strips by hand. It would be a pain to cut these strips without some sort of mitre box and (un?)fortunately, the only one I had wasn't wide enough. Sure, I could've just made a bigger fixed-width mitre box, but where's the fun in that if you can build a variable-width version in less time?

Step 1: Tools and Materials

Here's what you'll need to get going:

Pencil
Carpenter's square
Double-sided tape
Clamps
Sandpaper
Hand saw / backsaw
Small sheet of wood (As you can tell, the wood doesn't have to look pretty, but it should be quite strong/dense though.)
Workmate/workbench (Anything with a Workmate-like vice will do.)

Step 2: Making the Mitre Box

Building our mitre box only consists of making the two pieces shown in the first picture, so you'll be done before you know it:

First clamp your sheet of wood to the workbench and cut two 24 cm x 12.2 cm-sized pieces with e.g. a hand saw. (You may want to choose a different size for your mitre box though; see the notes below.)
Use double-sided tape to attach one piece to the other. (see the second picture) This ensures that both pieces will have the exact same shape in the end. Plus, you'll only need to make half as many cuts from now on.
Use a pencil and a carpenter's square to copy the drawing in the third picture onto your taped-together pieces. (A .pdf of this drawing is also attached.)
Make the remainder of your cuts (except for the mitre box's slot) using a hand saw.
Cut the mitre box's slot with the saw it's meant for. You can also cut multiple slots if you intend to use the mitre box with multiple different saws. (As you can tell in the first picture, I ended up cutting two slots: one for my backsaw and a wider one for my hand saw.) It's especially important to be precise when cutting these slots, as they have to be square/perpendicular to the workbench's surface. Be sure to check the notes below for tips!
All done! You can now take your two pieces apart again and remove the double-sided tape. You can also do a little sanding to clean up the edges.

Notes:

The size of your mitre box mainly depends on the following two things: First, the length of the mitre box's slot depends on which saw it's meant for, so you'll want to make the mitre box more/less tall to accomodate for this. Second, if you want to cut multiple slots, you can make the mitre box a little wider to ensure there's enough space between the slots.
When making straight cuts, the main problem to prevent is that you simply start cutting and notice at some point that you're going off in the wrong direction. At that point it's already too late; the saw's course has already been set from the very first stroke. (I should know.. I eyeballed my first attempt at cutting a slot for my backsaw, so of course it ended up slanted. I then "fixed" my mistake by covering it up with a proper hand saw slot.. no one will ever know :) )
To make sure that you're making the right cut, give these tips a try:
- When cutting with a hand saw, you can simply clamp down a block of wood right next to your cut, using a carpenter's square to line it up properly. This block already helps a lot in keeping your saw guided.
- When cutting with a backsaw, you can't use a helper block for the entire cut since the saw's stiffening rib would get in the way at some point. However, it's sufficient if you cut an initial groove with a helper block, and then remove the block. (see the fifth picture) You may also want to cut an initial groove along the side, such that your saw is guided in that direction as well. (see the sixth picture)
- There's no need to put much force on your saw when cutting. You just do the back-and-forth part.. gravity will take care of the rest. (.. assuming you're not doing woodworking in space?) You don't need to worry about keeping your saw in the right position either. Once the first couple strokes are made, the cut basically guides itself. In fact, if you do put too much force on your saw, you might just slightly force it in the wrong direction and the saw will start to fight back (because friction), so a lot of your energy would then just go to waste anyway.

Mitre box.pdf
Download

Step 3: Using the Mitre Box

All right, time put our newborn contraption to work! While this mitre box does require some more set-up than a regular mitre box, you do get an adjustable width, and you can also clamp down your workpiece much more securely (using bar clamps instead of cam pins). Here's how it works:

Put the workpiece that you want to cut somewhere on your workbench. If needed, put some sort of riser block underneath to raise the workpiece to the desired height.
Place the two sides of the mitre box into the workbench, and close the workbench's vice to keep the two mitre box sides locked in position, as shown in the first picture.
Note that you can adjust the distance between the two mitre box sides to the bare minimum, such that it already provides a bit of clamping pressure on your workpiece.
If needed, you can add a clamp to the workpiece itself such that it can't move at all.
If the workpiece is too small to add clamps, you can still fix its position by clamping down extra blocks next to the workpiece instead, as you can see in the second picture.
Once you've got your workpiece in position, go ahead and make that cut! If all is well, you barely need to put any force on your saw and the resulting cut should be nice and smooth; hardly any sanding required.

Notes:

There should be some leftover pieces back when you made the mitre box. You can use those as riser blocks to put underneath your workpiece. (I happen to have chosen my measurements such that these leftover pieces raise the workpiece just above the bottom of the mitre box's slot.)
You should have enough room to add clamps to your workpiece using the 4 cm gap between the vice's jaws. If your workbench's working surface has holes in it, you can add bar clamps through those as well.
You can only do this if you can separate the two parts of your bar clamps though. In case of my el-cheapo bar clamps, there was a metal bumper at the end of the bar that prevents you from separating the two parts. You can simply use a hacksaw to cut it right off; problem solved :) While it has happened more than once that the two parts now accidentally fall apart, there have been many more occasions where it comes in handy to be able to put bar clamps through small spaces.

Step 4: Extra: Making Angled Cuts

While our mitre box is mainly suited for making cuts at a 90 degree angle, you can also use it to make cuts at any other angle. You just need to think outside the box ... no seriously, just literally put your workpiece outside the mitre box. The mitre box will still do what it's supposed to, which is to guide your saw, and you can angle your workpiece however you want. Have a look at this step's picture to see what I mean: You can simply position the two sides of the mitre box at some distance, then clamp down your workpiece outside the box at the desired angle; done. It doesn't quite work like making angled cuts in a regular mitre box, where the slot is angled instead of the workpiece, but who cares, right?

How do you go about positioning your workpiece at a certain angle though? Our mitre box by itself certainly won't be of much help. However, this can be easily remedied using e.g. a set square (one of those plastic triangle thingamabobs used for drawing 30/45/60/90 degree angles): Line up the set square against the mitre box, then line up the workpiece against the set square to position it at the desired angle. Now clamp the workpiece to the workbench; remove the set square and you're good to go. (In case you don't have a set square laying around, or you need an angle other than 30/45/60/90 degrees, it should be easy enough to make your own helper triangle out of wood.)

There's one last problem though: making angled cuts with our mitre box as-is only works well if the cut is somewhere near the edge of the workpiece. The mitre box would get in your way if, for example, you wanted to make a 45 degree cut in the middle of some long strip of wood (e.g. a skirting board). This problem too can be fixed: You could make a version of the mitre box where the sides are less wide (than the current 24 cm), which should give you a lot more space to put your workpiece in the desired position and angle.