A circular saw cutting guide is very handy for making accurate and repetitive cuts in large wood panels. Ex. MDF or plywood sheets 4′ x 8′ (approximately 1.20 m x 2.40 m). It also avoids having to calculate the cutting line of your circular saw every time which saves significant time.
The one I propose you to make has the particularity of being made of two parts of 49″ (1.24m) nestable, so you can use it on sheets ranging from 24″ (60 cm) to almost 96″ (2.40 m). This is an inexpensive and easy to do project that you will not regret your investment.
Let’s start with a plan in good standing…
One 3/4″ x 24″ x 49″ MDF sheet (1.9 cm x 60 cm x 1,244 cm), another 1/4″ x 12″ x 49″ (0.6 cm x 60 cm x 1,244 cm) and a 3/8″ x 3/4″ x 24″ (0.95 cm x 1.9 cm x 60 cm) stick made of UHPM* polyethylene will suffice.
You could make a wooden stick, plywood or other material but UHPM (*Ultra High Molecular Weight, PE-UHPM notation) is a tough, rigid plastic and has the advantage of having a very low coefficient of friction (remember your physics classes…). Its cost is low and it is an easy material to find. It is perfect for this kind of project.
Realization of the rails
Divide the 3/4″ thick sheet into two equal parts along the entire length. Cut two 3″ x 49″ tabs into the 1/4″ sheet. Finally, make a groove along the entire length of 3/8″ x 3/4″ at 3″ from the edge.
Glue the 3″ tabs on each of the pieces along the groove.
Using a grooving blade and a wood chisel, make the 1/4″ thick posts on the table saws.
Make the mortises with the mortiser or the drill press and then finish with a chisel.
Check the interlocking of the two guides. The UHPM polyethylene stick must slide from one end to the other without catching or being obstructed during its passage.
Be careful when assembling the two rails so as not to break the tenons.
Realization of the guide plate
This is the guide plate that will accommodate the circular saw and slide along the rail.
You will need to adapt the dimensions of the plate to your circular saw. In my case, it is a Hitachi model C7DB2.
Warning ! Sensitive souls refrain…
Disassemble the base of your circular saw and drill 3 holes 1/4″ diameter, which will be used to hold the screws that will secure the plate and the saw.
Cut the plate into the 1/4″ thick MDF sheet and duplicate the 3 holes (1/4″ diameter) of your base. The holes should be flared so that the countersunk screws do not protrude.
Before drilling, glue the base of your circular saw and the MDF plate with double-sided tape. You will get a perfect alignment, clean holes and no splinters.
Assemble the base of your saw and the guide plate. I used butterfly nuts because I will not always need the guide to make my cuts. It will be easier and more convenient for assembly and disassembly.
Re-assemble your saw at its base. Then turn on your saw and go down slowly to make light of the guide plate. This will have the advantage of producing clean cuts without splinters. This is the same principle as the zero clearance insert plate. See the article.
Be very careful during this operation.
Cut the PE-UHPM stick to the same length as the plate and stick it on it with Epoxy type glue. You can use finishing nails to hold it in place.
It’s time to check the operation. Take the opportunity to check the alignment and squareness of the whole. The plate must slide perfectly from one end to the other of the two guides.
The final step is to size and align the rails with the saw blade. Assemble everything and sit on a surface that allows you to do this safely. Make a pass with your circular saw along the entire length of the two rails.
It’s over ! You are ready for hours of pleasure…
To improve the slippage of the plate on the rails, apply two coats of water-based polyurethane varnish on the assembly. You can also wax the underside of the plate and the top of the rails, it will glide like a charm.
To increase the adhesion of the rails to the materials to be cut, you can glue some sheets of sandpaper on the underside. A grain number 120 should suffice.
After several years of use, I invested in a Festool TS 55 REQ diving circular saw whose guide offers the same benefits.