I know about the square triangle 3-4-5, and I know how to use it get a 90° angle.

But I can't think or understand how people use it (accurately) to get 90° on a big plywood sheet.

The most "proper" way that I can think of it mark my reference edge two points 4 units apart, and using a compass or very static (non strechable) line trace two arcs (3 and 5 units) to get the intersection. But I don't have any line that I would trust for that, not any compass large enough (and precise enought to reference from an edge instead of a point in a plane) to do that.

I could have measuring sticks 3 and 5 units in lenght and just join everything to get the needed point, but how to accurately mark this point if you have the sticks on the way?

So, how do you do it properly?


This is not far off from your original description, but consider that a set of yard sticks of reliable precision will do what you require. Drill a small hole in each one at the 1/2" point (or 1" if you like) and again at the appropriate multiple plus the 1/2" or 1" offset. Place a brad at your zero point, which could be a salvage board under the edge of the primary surface and place another brad at the appropriate distance. You now have two arc-devices which will provide you with the third point when the brad-holes align at the ends.

If precision is required, use your salvage board with the two brads and tap the edge of the brad into the work surface until half the diameter is "buried" to compensate for the thickness of the brad.

You can use any material, of course, and drill holes to your specifications in order to fit the working surface.

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  • This works like the compass/line method that I described. I was asking if there is another method do it in a more practical/smart way. I've seem so many people mentioning just the 3-4-5 method without a description of how they use it that I thought that there must be some clever way to use it that I still haven't figure it out. :) Let's see if someone can bring a out-of-the-box way of doing that is clever and easy. – Luiz Borges Jan 20 '17 at 16:39
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    I suppose the reason that the 3-4-5 method is so common is that it works and may be the easiest method you might find. It's another modification of this method, but if you use heavier stock for the triangle arms, it can be the saw guide for a circular saw or similar tool. You ask "how do you do it properly" which is a question with many different answers. Mazura's answer of big tools is a good one, but might be expensive for a one-off or infrequent project. – fred_dot_u Jan 20 '17 at 17:07
  • Apart from saws designed for that (which would be really expensive for a diy shop), I never met a square that I could trust over great lengths. It might be "good enough" for most projects, this is why I'm trying to find a good method to get a good perpendicular line to use with my track saw. – Luiz Borges Jan 20 '17 at 17:35

How do you use the '3-4-5' rule to accurately make a square cut on a sheet of plywood?

You don't. Well at least, I've never used it to cut plywood.

I either use a drywall T-square, or a measuring tape and a chalk line (working off one of the factory edges). The 3-4-5 is for squaring up prebuilt onsite framing or concrete forms (or for anything longer than your longest straight edge). It's for taking the rack out of something; plywood can simply be measured and marked in two locations, have a line scribed and then cut.

Also, the error tolerance deceases as you increase the multiplier and vice versa; straight edges and carpenter's squares are going to be more accurate than your ability to 3-4-5-measure-stuff that's less than 5'.

Meaning, I wouldn't use that rule for small stuff, which would be any sheet of plywood that doesn't have at least one factory edge left, from which it comes more square than any cut I've ever made.

If it doesn't have a factory edge left, (you make one) make your first cut, which then becomes your reference point for further measurements.

If you really need them to be square and true, a table saw or someone who owns one will be you new best friend.

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  • Not trying to dismiss your answer, but you didn't actually answer my question. There are many reasons to get plywood square, like: some sheets aren''t perfectly square, you might be using a offcut without a square corner, your squares might be off (a my biggest square is off by about 1 mm, not a huge deal with drywall, but a terrible thing for cabinets), you table saw might no be able to handle a large sheet safely and accurately (mine doesn't), and so. Also I've seen many forum posts over the years mentioning the 3-4-5 specifically for squaring plywood. – Luiz Borges Jan 20 '17 at 9:39

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