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?
Best Answer
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.