In an attempt to save money (and not clutter up my work space with more tools which I don't have anywhere to store) I created a mitre box for my circular saw.

You may have seen a few other instructables about turning a circular saw into a table saw, and I have a similar project in the works once I finish my work bench but first I needed a mitre box so I borrowed the idea and turned it up side down.

Materials:
MDF or ply wood (left over from other projects)
Structural pine (left over from my work bench [instructable hopefully coming soon!])
Other pine (again, left overs)
Screws (who doesn't have a bunch of these laying around?)

Step 1: First Stop Block

Put your circular saw on the MDF, lining up the front end of the foot plate with a straight edge (this is the edge we will be working off for the whole project to keep things square). Draw a line from the front mounting hole in the foot plate parallel to your working edge. This is going to be the pivot point for your saw to make angles cuts on your mitre box.

Make a mark a few centimetres away from your working edge past the pivot line. This is where the front edge of your stop block is going to be.

The stop block is how you will quickly and accurately line up the saw each time you want to make a perpendicular (90 degree) cut.

Take a piece of scrap pine with a straight edge. Use your quick square (or any square if you don't have a quick square {I'm not sure how you could live without a quick square!]) to line up your scrap pine block perpendicular to the working edge. Line the end of the block up with your mark and screw it in place.

Step 2: Mark and Drill the Foot Plate

Put your circular saw in place hard against the stop block and check the front end is still lined up and parallel with the working edge of your MDF.

The first line you made should be visible through the front mounting hole on the foot plate. Mark each mounting point and remove the saw.

Pre-drill each marked hole, replace the saw and put screws in place. Make sure you are using screws with a flat bottomed head to give the best holding contact with your circular saw.
You don't want it to slip around when you are using it. Don't tighten the screws all the way at first. While they are still loose you can move the saw around a little (depending on how tight a fit it is between the thread of the screw and the hole in the foot plate) while it is still loose pull the saw hard up against the stop block, then tighten the screws all the way.

Step 3: Second Stop Block

For my mitre box I only wanted two angles of cut. 90 degrees and 45 degrees. A proper compound mitre saw can cut to any angle within about 90 degrees (some a bit more), but for my use these two angles are enough.

To set the second angle loosen the pivot point screw and remove the other screws.

Using your quick square swivel your saw around till it is at a 45 degree angle to the working edge of your MDF. Put another scrap pine block against the foot plate of the saw on the other side to the first stop block. Clamp it in place, and check it is still at 45 degrees to your working edge and screw in place.

Step 4: Mark and Drill Again.

As before, mark and pre-drill the screw holes to hold your saw in this position.

Step 5: Cutting the Slots

Attach the circular saw to the perpendicular mounting on the MDF sheet and clamp the sheet so that the blade would pass through over the edge of your workbench.

Plunge the blade as far through the MDF as you can to make the cut hole.

Repeat for the 45 degree angle.

Step 6: Fixing the Sub-frame

The sub-frame for the mitre box is just two structural pine beams. I intentionally left them much longer than the mitre box top so I could put quick referance measurements on for doing rough cuts quickly, and to also allow me to clamp on a stop block for making repeated cuts of the same length each time. But more on those later.

The first pine support I simply set square with the working edge of the MDF, double checked by inserting a piece of metal in the 90 degree cut slot (and checked with the quick square) and screwed in place. Countersink the screw heads in any place where they might come in contact with the saw foot plate to avoid damaging it. I didn't take a photo of this (although I should have).

Before attaching the second pine beam I set the box up on my work surface (resting it on the second pine beam), set the saw to its perpendicular cut position and had a look at how it was sitting. As you can see in the first photo I was again fortunate in that the pine supports are high enough that my saw blade doesn't hit the surface underneath. This means that I don't need to worry about plunging too deep when I make cuts.

As you can also see in the first photo if I attached the second pine support to the out side edge of the MDF sheet it would be useless as a fence to cut against. The material against the pine would not be cut. To fix this I chose to mount the second pine support further in, meaning it needed to also be cut by the saw blade. I chose what looked to be a good enough position to cut through most things thinner than the support its self (or maximum cut depth of my saw) which just happened to be when the cut line from plunging the blade through the MDF just showed on the outside of the pine beam.

Step 7: Re-cut the Blade Slots

Once the sub-frame is attached you will need to re-attach the saw to its mounts on the MDF sheet in both the perpendicular and 45 degree positions to cut the sub-frame.

If you are fortunate like I was that your blade doesn't go all the way to the surface under the mitre box you can simply make sure there is nothing under the box and plunge down. If you are not so lucky you will need to either boost the box up a bit to protect the surface under it, or clamp it over the edge of your work surface again.

As a safety concern you might need to attatch a sacrificial block on the outside of the sub-frame. With my cuts the blade passes outside of the sub-frame (see the third photo). If there was something in this position it would also be cut. If this is a piece of timber that could be bad, if it where a finger it would be disastrous!

By screwing or glueing a block of scrap wood over it, and re-cutting with the saw) you can cover this and make sure the saw blade is not exposed outside of the mitre box anywhere.

Step 8: Quick Mesurements

Starting on the underside of the MDF sheet, you can make some quick rough cut measurements. Start marking from the closest edge of the blade slot and mark back. I chose on the main direction to mark in 5cm increments from the blade. While under the MDF I only marked the flat face, once past the edge of the MDF I marked on the pine beam, for the full length (about 80cm from the blade). On the shorter secondary side of the blade I marked 0.5cm increments, again measuring from the closest edge of the blade slot.

It wasn't done in time for the photos, but I have also marked distances from the 45 degree blade slot, to allow me to make properly dimensioned cuts based on the outside length of the desired result. The marks for the angled cuts are made in red ink and on a 45 degree angle so I don't confuse them.

Step 9: Use

Using the mitre box is a little different to using a compound mitre saw.

First mark up your wood to the point you want to cut. Flip the mitre box over and line the cut mark up with the edge of the blade slot.

Clamp the work piece to the underside of the MDF, and flip it over.

Attach the circular saw, and make your cut.

If the cut does not go all the way through your work piece you can either add a spacer between the work piece and the sub-frame to get it more in the center of the blade, or rotate and re-clamp the work piece to cut the remaining material.

The work piece needs to be clamped to the underside of the MDF to make sure the blade passes through it. If you leave it to sit on the surface under the mitre box the blade won't cut all the way through, and you will end up with a concave slot in the work piece rather than a clean cut (although this could be useful for some purposes)

If you need to make several cuts to the same length you can clamp a scrap block on to the pine beam which extends from the sub-frame out the back of the MDF to use as a stop. then simply butt the work piece against the block, and clamp it to the underside of the MDF sheet, make your cut and repeat. It's a little slower than a proper compound mitre saw, but once you get used to the process not by much.

Step 10: Notes

A few quick notes about this circular saw mitre box.

Accuracy:
The mitre box will only be as accurate as you are when you make it, and as accurate as your tools will allow. Each time you set it up try to use the same procedure, holding the saw in the same way, taking out the slack against the locking screws in the same direction each time. If this changes it could change the angle of your cuts a bit, or the length of your finished piece might not be quite what you wanted. The more reliable you can be in attaching your saw the better the result will be.

Speed:
While you could make many cuts in a very short time, be sure to slow down and measure, set up, clamp and re-check. The faster you go the worse the end result could be.

Durability:
Constantly screwing and unscrewing the circular saw in place will weaken the MDF face. Each time you put the screw back in the same hole it widens the hole a little, and if the thread doesn't align with the previous entry it will get a weaker and weaker grip each time.
To fix this you could try bolding through the MDF sheet rather than screwing, counter sinking the bolt head on the underside to keep it out of the way.
Otherwise you might need to replace the mitre box every once in a while. Fortunatly it's mostly made from scrap and doesn't take long to

Dust:
Using your circular saw in this way also makes it easier to attach a vacuum cleaner to collect most of the dust from making cuts. As in the photo on this step I have used some left over (from other projects) PVC pipe and gaffa tape to secure the end of the vacuum hole to the ejection port on my saw. It does a great job and really helps keep the amount of saw dust in my work area to a minimum.

While it is a good way to get more out of your tool, it is not quite as good as a proper Compound Mitre Saw, but it's a good stand in for those of us who can't afford one.