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I want to cut a mostly rectangular hole out of a piece of 3/4" plywood about 21" wide by 32" tall. I would like the corners to be somewhat rounded (about a 1" - 2" radius), and I want to rehang the cut out section back in the main sheet as a door. I know that I could simplify my life by cutting the door and the opening from separate pieces of plywood, but I'd like the grain to match which is why I want to reuse the cutout piece as the door.

What is a good way to go about making this cut keeping the straight parts parallel and the rounded parts clean and uniform to reduce the amount of reworking and cleanup needed to finish the job?

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    More info would be good, since as it is the problem is hard (or unsolveable). Assuming this is going to be a kind of dog flap? Is is absolutely necessary to re-hang the cutout or could you get a second piece of plywood (that would make your life immensely easier, could do with jig saw or plunge router)? Dimensions of the little "door"? (May either need a slanted side or a larger clearance, depending on how small it is, or it will lock in the hole). Acceptable to cut through the whole piece and re-glue afterwards, losing 0.4-0.5 mm breadth (thinking bandsaw)? – Damon Sep 18 '15 at 9:01
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    Thing is, bandsaw would be ideal since blades are slim and it nicely cuts round corners as well as straight lines, but it needs an "entry" and the maximum size of the work piece is limited. Jig saw needs a hole as entry, which is ugly if you want to reuse the cutout piece. Router with a guide made of 4 pieces of wood (or a fence) would be ideal too, but you won't find a bit smaller than 8mm that goes through 2'', so there will be a large gap. Circular plunge saw won't do corners... – Damon Sep 18 '15 at 10:38
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    ... and blades are kind of thick, too (like, about 8-10x as much as bandsaw), so again, large gap, and you'll need to do round corners separately. If you are not afraid of muscle work, you could use an azebiki saw along a guide for the entry, and then follow with a jig saw. That might actually work and produce an acceptable width gap... but you'll probably need half an hour to make the entry through that plywood. – Damon Sep 18 '15 at 10:43
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    Have you considered veneering the finished piece? That would give you a consistent grain. You are already using plywood, so it's not that veneer would cover precious hardwood. – PeterK Sep 18 '15 at 18:33
  • So does that mean you don't intend to finish it or paint it or anything? – Matt Sep 18 '15 at 18:50
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There are generally speaking two ways people go about this kind of work these days, using a router or jigsaw.

Router

With the router you'd use a suitable straight-cutting bit with a bearing guide in conjunction with a template made from thinner material, running the bearing carefully around the inside perimeter of the template while being careful to avoid the router wandering into what will be the door.

A good material for the template would be 1/4" (6mm) tempered hardboard, but thinner plywood or even solid wood could be used. The rounded corners of the opening would normally be drilled using a suitable bit, either a Forstner or a flat (spade) bit. A hole-saw could also be used if one of the right size is available.

Jigsaw

Internal jigsaw cuts are normally started in a drilled hole but some guides show a technique where you carefully plunge a suitable saw blade (with the saw running) into the material. With the latter technique in particular I would do some test cutting on scraps of your plywood to refine technique before committing to the final cut. Practice doing the curved cuts would also be advisable.

The radiused corners would generally be cut freehand with a jigsaw (working to a marked line obviously) and while you can cut the straight sides freehand as well you're more likely to ensure good results if you create a basic fence setup.

At the simplest this would just be four lengths of wood clamped at a suitable distance from the marked lines, this distance being the measurement from the side of the blade to the edge of the jigsaw's baseplate or 'shoe'.

In use the baseplate would then be carefully run along the edges of each of the boards to make the straight cuts, again being careful not to drift away from the edges into the door.

Note: if both faces of the plywood need to have the best surface possible then it's worth investing in a good blade made specially for clean cutting of plywood. Some blades give a good top surface, others a good bottom, but the best will give good results on both sides of the board (generally these will be reverse-tooth blades, where the teeth face downwards).

  • I disagree with using a fence for cutting the straight sides with a jigsaw. In my experience, variation in the wood tends to make the blade twist, and you need to rotate the body of the jigsaw to keep the blade on the desired line. – Martin Bonner supports Monica Apr 24 at 9:38
  • A multi-tool (like a Fein) might be a good choice to make the initial cut for a router - it is likely to produce a thin enough cut. – Martin Bonner supports Monica Apr 24 at 9:41
  • @MartinBonner, I don't think this is a simple binary decision, go fenced or unfenced for straightness of cut. Obviously consistency of the material is a factor, but from what I've seen if you let the blade do the cutting like you're supposed to problems with steering become practically a non-issue. – Graphus supports Monica Apr 24 at 10:16
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If the tools in the wonderful answer from Graphus are not available to you two saws would do this for you just as well.

  • regular hand saw
  • keyhole or compass saw

Keyhole/Compass
(source: globalsources.com)

First the hand saw would be for the long straight cuts and you could switch to the keyhole saw for the curves. The first real problem you have is making the first cuts to fit the saws in. I suppose you could drill in with a small bit (that you are not attached to) and pull it along carefully.

I understand that you are looking for nice parallel cuts so this question covers some techniques that should help: When using a hand saw, how do I cut a straight line?

I see your door vision a protential issue since you would usually need a nice gap between the door and the frame. Especially over time one the weight of the door and wood settle. I think the trade off of a larger space might be advantageous in this case.

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Use a hand held circular saw with a guide to cut the straight portions then a jig saw to cut the radius corners, being careful not to run the circular saw ti to the curves. It could all be done with a jig saw and a guide rail for the straight edges, but it would take longer and be more difficult to achieve straight lines.

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