Zero Clearance Throat Plate for Cicular Saw
Author: Suwat PhruksawanDate: 25 July 2005
When a project requires the use of sheet goods such as plywood, I normally rough cut each project parts from the 8'x4' sheets a bit oversized and then run the final cuts to dimensions with my table saw to get the smooth edges. This works fine if the parts are small enough for me to maneuveur them around the table saw.

Sometimes, however, I need to cut the pieces that are too large to handle safely on the table saw (for example, a large plywood table top). Since I can't use the table saw, I have to cut it to final dimensions using circular saw alone. And this is why I came up with the Zero Clearance Throat Plate idea to get the cuts from my circular saw as smooth as possible without having to use the router or some other time-consuming means to smooth the edges.

This jig is nothing more than a simple base plate with a routed slot for inserting a piece of 1/8" or 1/4" thick Lexan or acrylic where the saw blade will cut through. The base plate can be made from a scrap piece of 1/4" or 1/2" plywood or MDF or whatever material that is smooth, flat and stable. I made mine from a 1/4" thick piece of left over laminated floor plank. It is strong, flat and has slick surface which is an ideal for smooth gliding over the work pieces.
I chose to use 1/4" thick base plate with 1/8" thick clear acrylic so that I don't have to sacrifice much of the cutting depth from my saw. The 1/8" acrylic is rigid enough at this dimensions (about 2" x 3.5"). But if you want to add extra structual strength, you can use 1/2" thick base plate with 1/4" thick Lexan, too.
Creating The Base Plate
Start by setting the depth-of-cut to maximum and measure the distance from the front of the base to the tip of the saw blade. Take a note of this distance ("A"). Then do the same measurement but with the depth-of-cut set to minimum. Write this distance down as well ("B").
Trace the layout of the circular saw's base plate on to a blank sheet. Set the distance from the front of the plate to the front of the blade slot to be a little less than the distance "A" measured earlier. This way, the blade slot will only be as long as necessary. It will also provide more mounting surface for the acrylic throat plate later.

Cut the base plate out of the traced line. Use the jigsaw to cut the blade slot and route it smooth.

Flip the base plate over and draw the throat plate mounting slot. This slot should be about 1/2"-1 1/2" wider than the width of the blade slot on each side. The top of the mounting slot should be about 1" from the front of the base plate. The distance from the front of the plate to the bottom of the mounting slot should be a little less than distance "B" measured earlier.

Use the straight bit, set the depth to be the same as the thikness of the acrylic or Lexan sheet and route the mounting slot. Then, pre-drill the throat plate screw holes and drill & counter bore the base.

To make it easier to register the jig's base to the saw's base plate, I attached the thin strip of hardwood (about 1/8" x 3/4" x the length of the jig's base). This side wall is also make it easier to run the saw against a straight edge during the actuall cutting.

Once done, drill the mounting screw holes and install the jig on to the saw's base plate using machine nuts and bolts.

Making The Throat Plate
The next step is to cut the acrylic blanks to fit the mounting slot. Because it's pretty easy to do, I made several blanks at a time. These blanks can be used for various cutting angles or as the replacements. I used clear acrylic to make it easier to see during the actual cuts on the work pieces.
Creating The Zero Clearance Saw Kerf
To create the Zero Clearance saw kerf, install the throat plate on the base. Set the saw on top of two pieces of 2x4 scraps. Make sure the saw blade can go up and down between the two scrap pieces.
Loosen the depth-of-cut locking mechanism and set the saw blade to the highest position (i.e., min cutting depth.) The saw blade should be well clear from the edge of the throat plate at this point. Without tightening up the locking mechanism, turn on the saw and slowly lower the blade down to the lowest position (i.e. max cutting depth). The zero clearance saw kerf should be created as shown in the pictures above and to the right.
Test Cuts
For comparison, I first test the cross cut on a piece of ply wood without using the zero clearance throat plate. The picture below clearly shows a visibly rough edge with a lot of tear outs.
Then I reinstalled the throat plate and proceed with the next cut. (A little side note here, as you can see in the picture on the left, the added benefit of using this throat plate with clear acrylic is that it is easier to line up the saw blade with the cut line.)

As shown on the picture below, with the zero-clearance throat plate, the result of the cut is markedly improved. The tear outs is virtually eliminated.

Below is a side-by-side comparison between the edges created by the cuts with and without the throat plate installed.
Even for the low-end saw and a cheap blade I have, with the use of this jig, the quality of the cuts are quite impressive. With a decent saw and a good blade, combining with the use of a good straight edge, this could make circular saw a precision tool that can be used for the final dimension cuts by itself. And with the very-simple-to-make throat plate blanks, we can easily afford to have a fresh one for every project and every cutting angle.