How to Make a Mitre Box

A mitre box is an important woodworking tool which can be used to help you put accurate angled cuts into wood.  Mitre boxes are quite simple to make, and you can put one together out of scrap wood quite easily.

Mitred Architrave With a 45 degree join. Photo: Michele Schaffer

The time you invest in making a mitre box will save you time, waste and hassle in the future by ensuring that each piece of wood you cut in the future, whether for shelving or architrave, will be a straight, clean cut.

The name “box” is a bit of a misnomer, since the box is open on two ends and at the top, so that longer, larger pieces of wood (such as architrave and skirting) can be slid into the box and the ends can be cut smoothly.

If convenience is important to you, then you can purchase mitre boxes made of a strong plastic or rubber.  However, even store-bought mitre boxes need replacing periodically because the guide slots are prone to damage. Once you get the hang of making mitre boxes, you will probably find that it’s just as easy (and far cheaper) to make your own and swap them out when they start to show signs of wear.

Fitting Your Mitre Box

If you are going to be cutting a lot of a particular size of wood – for example, a large cornice, or some unusually sized architrave, make a box that will fit this.  Match the size of the bottom board to the size of the wood you are cutting, to ensure that the wood sits true while it is being cut.

Making Your Mitre Box

To make your own mitre box you will need:

• Three pieces of wood
• A workbench with clamps
• A tenon saw
• A combination square
• A screwdriver and some matching screws

Assembling the Box

• Clamp the three pieces of wood together so that they form a ‘U’ shape. Position the base of the box so that it is slightly elevated – the outer sides should stick out by approximately ¼ of an inch, forming “legs” for the box.
• Mark the centre of the elevated base with a pencil so that you can accurately place the screw holes.
• Drill three holes on each side – one at each end and one in the middle, through the outer sides and into the inner piece of wood which makes up the base of the box.

• Screw the wood together – choose a long screw so that you get a firm, solid bite.
• Using a combination square, mark 90 and 45 degree angles (or whatever angles you are going to be cutting frequently) on the upper surface of the box.

• Cut the lines you have marked, using your tenon saw.  Cut slowly and carefully to a depth of around 2 inches (50mm).

Using a Mitre Box

To use your mitre box, place a piece of flat, scrap wood into the base of the box and rest the wood (or other material) that you want to cut on top of that wood.  Use the guide slots on the mitre box to start the cut.

Insert the saw blade into the slot and gently draw the saw back and forth to open up the cut, while holding the wood firmly in place.  The slot will guide the cut from the top to the bottom of the board, ensuring that the cut is perpendicular to the edge of the board (or at a consistent angle all the way down.  This creates a consistent, precise finish.

When you are making a 45-degree cut, the saw can easily fall out of the slots, so cut carefully, using short strokes. If the wood you are cutting does not fit snugly in the box, hold it in place firmly with a clamp.

Architrave. Photo: Simon Wheatley

Cutting And Fitting Architrave

One of the main uses of a mitre box for interior design is cutting architrave – the timber moulding that fits around doors and windows.  Architrave is designed to hide the joint between the wall or ceiling and the timber casings, hiding movement or shrinkage, and keeping the wall looking clean and professionally finished.

It is important that architrave is neatly cut, and that the corners are mitred.  Usually, the corners meet at right-angles, so the mitre cut on each piece should be 45 degrees (half of the right angle).  However, this is not always the case.  In very old properties, the door casing may no longer be square, so you should measure the real angle before you start cutting.

Mark out the casing, leaving a suitable margin for the hinges. Make sure that the margin is big enough to allow you a safe nailing area, but not so large that it restricts the knuckle of the hinges.  Mark margins both horizontally and vertically. Mark the angles that you need to cut on the architrave, and use a mitre box to ensure that the cut follows the marking accurately.

To place the architrave, position the head piece first of all.  Line it up against the horizontal margin markings, and make sure that the corners are in line with the vertical margin markings.  Tack the architrave in place with three nails, one in the centre and one on either side.  Do not put the nails all the way in at this point, simply use them to hold the architrave steady.

Next, position one of the side the side pieces so that the mitred edges line up neatly with the sides.  Again, tack the first side in place but leave the nails protruding slightly.  Finally position the other side in the same fashion.

Once you have all of the joints lined up, drive a panel pin into each mitre joint at an angle across the two pieces, to ensure that the joints remain flush.  After securing each joint, drive all of the nails home and punch them so that they are slightly lower than the surface of the architrave.  Fill in the hole with liquid wood, and allow this to dry.  Sand the surface flat, and paint it.

Information is accurate at time of publishing, however, may be subject to change.