When dealing with large project parts or plywood it can be tough to get good results cutting them down to size. This jig allows you to get accurate & straight cuts every time using a circular saw. I have always referred to it as a Door Board and it is super cheap and easy to make. Additional info as well as tons of other builds shop stuff can be found here
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Step 1: Material Selection & Cuts
I used 1/4" MDF (medium density fiberboard) for the project. I tend to make a lot of my shop jigs & fixtures from MDF. It is a very stable material and it cuts nicely. The only drawback is the dust isn't very good for you so I would recommend some sort of dust mask or to make your cuts outdoors. Plywood or harboard would also work well for this. 1/4" thick material is plenty adequate as the material to be cut supports the jig from bowing.
Cutting your material can be done numerous ways. I used a bandsaw as that was quick & readily available to me. A jigsaw or circular saw would also work. The great thing about this jig is that the cuts DO NOT need to be super accurate at this point. DO however use a factory edge for the top piece. This will ensure when you finish the door board it makes straight as an arrow cuts.
Step 2: Sizing & Tips
The exact size of your pieces will depend on what circular saw you will be using for your jig. In my case my boards were approx. 6" wide & 10" wide. DO NOTE: the wider bottom piece should extend just past the blade and also past the motor housing a few inches. The narrower top piece should have that factory edge to have the saw ride against. Keeping all this in mind will allow for you to clamp the door board to your work piece without the saw getting in the way. Length will be however long you want to make the jig. I have made one that is 8' long for plywood rip cuts & one 4' long for plywood crosscuts. 2' long boards come in pretty handy as well.
Step 3: Glue Up
To make it easy to know where to apply the glue I like to put the top piece in place & mark it with a pencil. Then just add some wood glue & clamp the 2 pieces together. Any glue that squeezes out of that factory edge should be cleaned up at this point so as to not interfere with the saw later.
Step 4: Final Cutting to Size
First things first, run your saw in the channel created in the glue-up. This will make the bottom part to the size of your saw. NOTE: This jig will only be good for that model saw now. You may want to mark the jig to indicate that saw if you have multiple saws. Purely for aesthetic reasons I also trimmed the back edge & ends flush on my table saw.
Step 5: Get Cutting!
At this point your new door board is ready for use. Mark a line on your workpiece where you need it cut. Then position the jig edge up against your cut line, clamp the jig in place & make your cut. I also added a handle to make it a bit easier to carry. I show how to make those in this instructable. I highly suggest making one of these as it really improves the accuracy of cuts and makes breaking down plywood & sheet goods a breeze. Thanks for checking it out. Let me know what you guys think in the comments section.