As the title states what is the best way to cut a square hole inside a sheet of plywood? One consideration, I would like to reuse the cutout for a door to the same hole....
What comes to mind would be something like the Fein Multimaster oscillating tool. There are plenty of knockoffs out there if you are not into the Fein expense.
Bas put me onto the Terratek TPMT15VB from Amazon and it seems to perform fairly well for the money.....
Gary, I do this sort of cut with my Fein tool. I chop out DT sockets in table legs this way too.
Make a scoring cut all around your outline, then start to work on the thru cuts.
You can also make a plunge cut with your circular saw or a suitable jigsaw blade (it is possible to plunge a suitable jigsaw blade without a pilot hole, it just takes more skill).
However, with any sort of cut you will always lose at least the width of the blade's kerf on all 4 sides so it is important that your design can accommodate this loss if you wish to reuse the "waste" piece to fill in that same space afterwards.
Of the choices offered so far, the semi-circular blade on a Fein Multimaster (or equivalent) is probably about as narrow a kerf as you are going to get and it is potentially as good a choice as any.
I don't have experience with the Fein Multi, however, I would tend to think the straightest cuts would be with a circ saw and a plywood blade. +1 on the straight edge. But you don't necessarily need to do a frame if you don't want. I have made these cuts before with no issues. Now for your door. What particular is this projects use? What style door? (i.e. - recessed, flush, face frame) I would think you could add a border or frame to make you door fit properly.
Why not make a template and use a plunge router? Only make a part way through cut, then finish with a jig saw. Using a pattern trim bit clean up both door and hole. Add stops either to door, or frame and you are done.
I plunge cut with the circular saw (you can clamp a straight edge to the ply as an edge guide for a really straight cut), and then use a hand saw to finish up the corners, Any difference in kerf/edge on either piece is easily cleaned up with a chisel, plane or rasp. Hardest part is getting into the center of a full sheet. You can lay the ply on the floor supported by 2 x 4s or a sheet of styrofoam insulation so you can kneel on it to get a more comfortable cut.
+1 on the multi tool. Another option is to make four cuts (two rip, two crosscut), then glue everything back together (sans center piece) with biscuits or a spline. Not as strong, more work, and in general a hassle, but hey, I'm not a nice person.
What size door are you putting in and what type of plywood? Another option might be to cut the door from a piece of scrap. I have some pieces of the textured pre-primed siding left over from my coop build.
It seems that 2 parallel straightedges which guide/sandwich a jigsaw could give a pretty straight cut with minimal tear out if the cut lines are thoroughly scored first.
A couple of DeWalt jigsaw wood blades measured 3/64" t so you'll lose 3/32" total around the entire perimeter of the door. That undersized cut out could be edge banded and trimmed to make up the blade kerf loss. :dontknow:
1) +1 for Bas.
2) If an 1/8" kerf loss is acceptable (1/8" all around your door), use a circular saw and finish the corners by hand.
3) Using a multi-tool (IMO) is too tedious (if the door opening is bigger than a switch plate!), but HF sells one for $20-$25.00. I have two (they're a little heavier than the competition and get warm to the touch, but otherwise, cut quality rests more on blade quality than machine quality). If you go this route, be sure to use a block on top of your work to register the blade against and maintain perpendicular plunges and square & straight edges around the door.
What size door are you putting in and what type of plywood? Another option might be to cut the door from a piece of scrap. I have some pieces of the textured pre-primed siding left over from my coop build.
Plunge router will have the least amount of tear-out, however with a frame, you can hide all your tear-out, bad edges and tighten up the door a good bit.