How can I trim plywood flush to an edge that is not 90 degrees?

Question

I am building a small door. I have a frame that is glued and nailed together that is 1 inch thick shown here:

1/4" birch plywood will be glued to both sides of this. The plan was to glue oversized pieces and then trim them to the frame. The difficulty is that the longer edges of the frame are cut at a 3 degree angle so that the door will not snag on the jamb. I can trim the plywood with a router on the short ends, but what is the best way to trim it to the angled edges?

In retrospect, I should have cut all the pieces square, done dowel joints or something similar so there are no nails in the way, put on the plywood skin, then trimmed the edges at the proper angle with a circular saw.

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What I ended up doing

I up-voded all the answers because they all seem like sound advice to me. Thank you, everyone. The accepted answer most closely matches what I actually ended up doing.

I initially trimmed the plywood with my circular saw within a 16th of the desired edge with the intention of sanding it down flush. I have a clamping cutting guide so I used that. But then sanding was taking a very long time and I was afraid I would round off the door edge in the process so I trimmed it closer and noticed that the edge of the door was bowed in. At that point I decided that fortune favors the bold and trimmed the full edge of the door, plywood faces, framing, and all, with my circular saw at 3 degrees. Did it by creeping up on the right guide position with multiple cuts. I have a nice fine-toothed blade on the saw so I saw no tear-out. Nails didn't get in the way because they were already driven deep. Worked pretty well but running over the door face 100 times with the saw left stains. Not ideal but I plan on painting the doors anyway.

And yes, I do have a preference for methods that involve 10 minutes of prep work and 10 seconds of actual cutting. Thanks again!

Answer 1

Using a circular saw is a good option and it's not as scary or difficult as you might think.

First, if not already done, punch any nail-heads below the surface. You can fill those later. Then, with your saw unplugged (or battery removed) and the guard raised, position the saw as if it were making the cut and set the blade angle by flushing it up with the edge of your frame. This will give you an exact match, far better than trying to measure it and/or relying (guessing) that the scale on your saw is 100% accurate.

All you have to do then is clamp a straightedge down on the work and use that as a guide to run your saw along. A 2m (6") spirit level works well for this. Get the distance you need to set the straight-edge back from your cut-line by measuring from edge of your saw base to the edge of a tooth on your saw blade using a combination square. (Just make sure your saw is unplugged when you do this.)

To avoid splintering the edge of your plywood, cut one side at a time and either make a very shallow first pass cut with the blade depth set to just score the surface or use a clamped straight-edge and a Stanley knife to score the cut-line prior to making your cut proper.

Take your time and you'll be fine. It's a tried and tested method so just measure accurately and clamp your straight-edge down firmly, and you'll get a really professional look with crisp, flush, beveled edges - and no tearout.

Answer 2

If you have a hand plane plywood edges are easy to plane. Size of plane does not matter, this can be done with block plane if this is only type available.

Without hand planes I would trim close with router, then remove the rest with hand sanding. Start with P60 grit.

Answer 3

If you cut the bevel on a table saw, just tip the blade 3° the other way, flip the frame over, raise the blade just high enough to cut through the plywood and be done with it.

If you used your circular saw, tip it to 3° again, clamp a straight edge down to the top of the door skin, and again, set the blade just deep enough to cut through the plywood.

If you have both the TS and circular saw, but the blade on one won't tip the opposite direction, use the other tool to cut from the other (long) edge of the door so the tip angle matches up.

In both cases, I'd suggest cutting just proud of the frame by 1/16" or less (assuming the frame fit is exactly what you want now), and do a finish sand or plane to the final dimension. Not only will this give you a nice clean edge, but it will finalize the edge if there was any tearout on the plywood and will prevent you from cutting into the door if you don't get the blade alignment exactly in line with what it was when you cut the initial bevel.

Answer 4

Looks like the top will be less that 1/64” too proud if you just flush trim at 90 degrees instead of worrying about matching an angle etc... 0.25 inches rise with a 3 degree pitch gives about 0.0125 (1/80) of an inch for the run. So less than a third of a millimeter. I don’t think it’ll be visible or even physically interfering with the function based on those numbers.

If it is somehow tight, sanding for maybe 2 minutes matching the structures frame angle of 3 degrees will get you there.

Answer 5

You could screw an angled baseplate under you router and run this against an edge instead of using a flush trim bit.

Take a piece of plywood (slighly larger than your router base). Cut a hole in the middle for the bit. Cut one straight edge on the plywood base to run against a fence clamped to your door to prevent altering the angle. Screw the plywood base under the router. Use some wedges or distance rings on one side to get the angle. Might be a bit fiddly to get the proper angle.

Answer 6

I can trim the plywood with a router on the short ends but what is the best way to trim it to the angled edges?

Do you want the plywood edge to be square to the face, or cut at the same 3° bevel that the sides have?

If you want the edge trimmed square to the face, an easy way to do that is to temporarily attach another piece to the bottom of the rail, so that the edge is even with the line you want to trim to. You can use thin double-sided tape, a few dabs of hot glue, or a few brads. Or just clamp the frame to your workbench and use the edge of your bench if it's nice and straight. In any case, the idea is to just supply a straight edge to guide the bearing.

If you want the edge of the plywood to match the edge of the frame, you'll need to tilt your router. First, extend the line from the edge along the edge of the plywood face, so that you can see where the corner should be. Use your table saw to trim the panel to that width, so that the face of the plywood is the final size, and only the thickness of the plywood needs to be trimmed. Next, cut a strip 1/8" thick and at least as long as your frame from a piece of scrap. Set your router on the edge of the frame and put the strip under the base, about 2 3/8" from the edge, adjusting the position until the bit matches the angle of the edge, and secure the strip in place. Now the router will sit on the edge of the plywood and the edge of the strip, tilted the correct angle, and the bearing will ride on the edge of the frame and guide the cut.