I'm new to woodworking. What's the best way to cut accurate round corners for a sheet of plywood to make a countertop?
From contributor M:
Use a coffee can or anything in your shop that's round as the template. The exact radius rarely matters, but consistency does. We use a coffee can or other to draw the radius. Then we jigsaw it off. We are usually sawing through 1.5" of plywood and the blade always kicks out at the bottom, meaning that the radius cut isn't square like the flat surfaces are - it flares out in the bottom a bit. A belt sander and a 60-grit belt solve that problem rather quickly. Just takes a little finesse.
1) Choose your radius, for instance the coffee can mentioned above.
2) Trace the radius on one corner of a 12" X 12" square piece of 1/2" MDF.
3) Rough cut the radius with a jigsaw to about 1/16" outside the line.
4) Finish to the line. Easiest is a stationary edge sander. My second choice would not be a belt sander; I would clamp the piece vertically in a vise and fair out the curve with a sanding belt – cut - holding each end in one hand and sanding directly down on the cut with a sawing motion until the cutline is fair and transitions smoothly to the straight edges of the square piece. Making this template is the most important part of the process. Spend some time and make it nice! (Actually smooth is more important here than exactly round...)
4) Hold the template on the ply corner with the two adjacent straight edges flush with the plywood edges and draw the radius on all the corners that you want to shape.
5) Again, rough-cut the radii close to the line.
6) Place the template as mentioned above with the straight edges again exactly flush with the plywood edges.
7) Clamp on the two opposite corners of the template away from the radius. Make sure the template is clamped firmly and hasn't moved.
8) Rout to the template with a straight, top mount bearing bit. For 3/4" ply, I would use a 1/2" shank spiral upcut bit. Make sure the bearing lines up properly with the template and the cutters will engage the entire piece of ply. Start the router on one of the straight edges away from the radius and cut firmly around the corner until you hear (and feel) the router stop cutting the plywood on the adjacent straight edge. If you cut close enough to the line in step 5, one pass should be sufficient.
9) If necessary (but it shouldn't be), dress with a sanding block before you remove the template.
Here's another tip. I generally stick a piece of 150 grit sandpaper to the bottoms of my routing jigs with spray adhesive - a little more insurance that they won't move.
Figure out your most commonly-used radii, and reproduce four of them on a 12"x12" template. 1/4" or 1/2" MDF work well as templates. Apply laminate to one side and the edge all around, 120 grit sandpaper to the other side, mark the radii on the template, and you'll have a useful template for years of use (until somebody else decides they need it more than you do). If your band saw has a relatively fine tooth blade, laminate the side first, then cut the radii. No band saw? (What?!) You can get the same result by rough cutting the radii with a jig saw, and a fixed disc sander or a disc sander plate installed on your table saw. Use the same method as above using a hole and nail to a mounting plate clamped to the sander or saw table at the proper radius.
In use, place the desired radius on the material to be cut, draw the radius, rough cut outside the line with a jig saw, clamp the template in place, and finish the edge with a top bearing 1/2" pattern bit.