Using a Mitre Box and Saw
The trick to cutting a perfect mitre angle in a mitre box is correctly measuring and marking for your cuts.
You commonly find mitre joints in the construction of picture frames, mouldings, skirtings and where two pieces of wood are angle-cut or bevel-cut at the same angle then joined to form a corner. Most often, two pieces that have been cut at 45 degrees are joined to make a 90-degree corner. For a perfect fit mitre, cuts must be precise. If they are off even one degree, the corner will be noticeably out of true.
The most inexpensive way to make angle or bevel cuts in narrow stock is to use a mitre box, which is available at most hardware stores. If you have a lot of joinery to cut, consider buying a power mitre saw.
Before placing the piece in the mitre box, support it on a scrap of timber or other suitable material. This allows you to saw completely through the work without damaging the bottom of the mitre box. Place the timber to be cut against the far side of the mitre box, positioned as it will be when in use, and make the cut with the saw. Hold the work firmly against the back of the box with your free hand whilst cutting through.
If you find it very difficult to make a perfect mitre join, wood filler comes to the rescue! Use wood filler to fill in any gaps between the joins and sand by hand with fine-grit sandpaper to match the joins.
You need to use the correct saw for perfect mitre cuts. This is a Tenon Saw. These saws have fine teeth and a support along to top of the blade to prevent the blade from bending whilst cutting.
It's very easy to use a mitre box but you do need to think about the cuts that you need to make. Make a mark on the moulding or trim for the angle that needs to be cut at each end - and double check just in case.
You can clearly see that I have marked a line on my piece of trim for the direction of the angle. Insert the backsaw into the correct groove angle to cut.
For cutting the opposite corner you need to cut an opposite angle!
When using a mitre saw to cut a picture frame you will cut four pieces in exactly the same way as that on the left.