3-4-5 Square Clarification when squaring boards, items, plywood, etc...

[David Ruhland]

Is there an easy way to square a plywood panel or "trapezoid" piece of lumber using the 3-4-5 method? I thought i could simply draw two lines, measure using the 3-4-5 method (or construction master calculator using rise/run/ diagonal) I squared up several cutting boards today and a couple were quite painful... meaning i would be off by 1/16 on one end... I finally got there, but it took me awhile!

Where/how do i start when trying to do this when i don't have a factory edge?

Take a look at the example below. not a square corner on it...

TOP WIDTH is 15 5/8 "
Bottom Width is 16 1/2 "

Left hand Height is 10 1/8
Right hand Height is 10 1/2

How would you go about making this square?






399374

[Matt Day]

Joint one side, Table saw to make the long sides parallel. Radial arm saw, miter saw, or sled/miter gauge on TS to make the other sides perpendicular.

Could also do a number of Other things after the TS rip. Clamp a straight edge, use a good square to align, and flush cut with a router for instance.

[Ken Combs]

You have to have on 90deg corner for 3-4-5 to work. That's the first step, no matter what tool or process you use. In my case, I'd put it on the TS sled and cut one corner. RAS, miter saw, or framing square and handsaw will also work. Then do the 3-4-5 layout from that corner.

[Mike Kees]

Take it to my slider. Piece of cake.

[Richard Coers]

You have to have on 90deg corner for 3-4-5 to work. That's the first step, no matter what tool or process you use. In my case, I'd put it on the TS sled and cut one corner. RAS, miter saw, or framing square and handsaw will also work. Then do the 3-4-5 layout from that corner.

You don't need a 90 degree corner, you need a starting point and one straight edge. Start at one corner, measure up the straight edge at a unit of 3 and mark a point, then take a trammel and set it at a unit of 4 and make an arc with the pivot point at the first corner. Now set the trammel to 5 and strike an arc with the pivot point at the end of the 3 measurement. Where the two arcs meet is the point where the other two lines are drawn to.

[Wayne Lomman]

Richard has it exactly right. This method gets the job geometrically perfect with the most basic of tools. Cheers

[Pat Barry]

I would just put it on my tablesaw with the long edge against the rip fence and trim it to 10 inches. Tjat makes the two long edges parallel. Then use the table saw with my crosscut sled and retrim one edge. Then, keeping the same edge as a reference, flip it left to right and retrim the opposite edge. Clean up the corners by a simple roundover or small chamfer.

[Mike Cutler]

Can you square it using the 3-4-5 method?, or mark it?

Richard has it right for marking and laying out lines on panels.

If you just want to square that piece of material, that is relatively simply. Joint one edge, rip the opposite edge for parallel, place on some type of known 90 degree to blade sled, slider, fence, etc. Cross cut the first side, flip it over and crosscut the second side to length. You should now have a board square at all four corners, the dimension of your choosing. This assumes that you have some type of mechanism to cut 90 degrees to the blade.

[David Ruhland]

You don't need a 90 degree corner, you need a starting point and one straight edge. Start at one corner, measure up the straight edge at a unit of 3 and mark a point, then take a trammel and set it at a unit of 4 and make an arc with the pivot point at the first corner. Now set the trammel to 5 and strike an arc with the pivot point at the end of the 3 measurement. Where the two arcs meet is the point where the other two lines are drawn to.

I think i am starting to understand how to apply this to a larger piece of material say perhaps a 49"x97" piece of melamine that comes from Menards that needs to be square.

[Art Mann]

I strongly recommend you take the time to construct an accurate crosscut sled for your R4512 table saw. The effort will pay off over and over and over again.

[David Ruhland]

I strongly recommend you take the time to construct an accurate crosscut sled for your R4512 table saw. The effort will pay off over and over and over again.

Built one of those a couple weeks ago...works great as long as i put the "square end" against the fence of the crosscut sled! LOL....

What I am really trying to get at here, and i guess i didn't communicate it clearly is "how to square larger panels" that dont fit on a table saw or crosscut sled. (think 4 x 8 sheets of plywood that are being repurposed and dont have one square edge to start from here)

[Gary Ragatz]

What I am really trying to get at here, and i guess i didn't communicate it clearly is "how to square larger panels" that dont fit on a table saw or crosscut sled. (think 4 x 8 sheets of plywood that are being repurposed and dont have one square edge to start from here)

If that's where you're headed, I think you want to follow Richard's approach for marking up the sheet, then proceed with a track saw, if you have one, or a circular saw with a carefully positioned guide rail.

[Tim Bueler]

You don't need a 90 degree corner, you need a starting point and one straight edge. Start at one corner, measure up the straight edge at a unit of 3 and mark a point, then take a trammel and set it at a unit of 4 and make an arc with the pivot point at the first corner. Now set the trammel to 5 and strike an arc with the pivot point at the end of the 3 measurement. Where the two arcs meet is the point where the other two lines are drawn to.

True this!

Another method: Starting from one straight edge, preferably the longest edge, find the center point. From each corner of that edge, using a tape measure, trammel or whatever, strike an arc somewhere near the opposing edge and approximately in line with the center point (using the same, arbitrary, radius from both corners, 65" radius will get you close enough on a 48" wide sheet). From where those 2 arcs meet strike a line to the center point on the original straight edge. That line will be perpendicular to your original straight edge. You can measure both directions from there.

[Mike Kees]

David ,I should have expanded on my slider comment above,sorry about that. Richard has an excellent method/desciption of 3,4,5, this will get you lines laid out square,now you just have to cut them. If you cut much melamine you need a method to make one edge straight. This is where a sliding saw pays for itself,cut one eight foot edge straight and go from there. I remember struggling to get things square,I never owned a tracksaw. I think this would be the answer,cut the straight edge with the tracksaw and then take it to your tablesaw and crosscut box.

[Mike Cutler]

What I am really trying to get at here, and i guess i didn't communicate it clearly is "how to square larger panels" that dont fit on a table saw or crosscut sled. (think 4 x 8 sheets of plywood that are being repurposed and dont have one square edge to start from here)

David
It is for this reason that folks are willing to spend some rather big$$$$ for either tools/machines, or layout tools.
Simple Plane Geometry is the basis for the layout lines, exactly as Richard detailed. You may not be able to draw a right angle accurately over a length, but you can easily strike an arc and work the form backward from there. Once again though, this relies on an accurate method to establish the relationship.
Large panels are tough for the person working alone, in a small garage type shop.

[Jim Becker]

David, assuming your sled's fence is perfectly perpendicular to the blade, just be sure to use the SAME reference edge for both 90º cuts relative to that edge. It does require you to flip the material to do this but then you will have three sides with the correct relationship and can then complete the last cut using either of the two previous cuts to get a "perfect" rectangle. I do this a lot on my slider to square up various random off-cuts so that I can use them. Obviously, this gets more challenging as the size of the workpiece increases, however...

[Lee Schierer]

The first step toward getting square cuts is to align your table saw. Contrary to popular belief, saws do not always come properly adjusted from the factory. My son purchased a Rigid 4512 last year and the first thing we did after getting it assembled was to check the alignments. We used a dial indicator attached to the miter gauge to true up the blade to the miter slots. We started at about .007" out of alignment and ended up at .001", not perfect but better. Then we adjusted his fence alignment using the same set up. On his saw the fence was surprisingly close, but we got it even closer. The miter gauge on that saw is not a precision piece of metal. We used a high quality drafting triangle to get it square, but any time he changes it it has to be reset. I prefer the Kreg miter gauge, which I use on my saw.

Since you have built a cross cut sled, have you taken the steps needed to make it square to the blade using the five cut method? If not, you should.

One more point. Many squares you buy are not "square" Lower cost combination squares are notorious for being out of whack as are most carpenters squares. Thee are methods for checking and adjusting both.

[Tom Bender]

A panel that is too big to fit on your power saw is a good place for the 3,4,5 method.

Make one long side straight. Lay out a 3,4,5 triangle on it and you will have one 90 degree angle. Layout another 3,4,5 triangle on the opposite end of the straight side. You now have two parallel sides and a pair of 90 degree angles between them. Measure the desired distance along the two parallel sides and connect these endpoints. Trim to the lines and you have a rectangle.

[Jacob Reverb]

Or just measure diagonally, corner-to-corner, one way and then the other. Diagonal lengths should match.

[johnny means]

Or just measure diagonally, corner-to-corner, one way and then the other. Diagonal lengths should match.

This doesn't necessarily prove square, it only proves out of square.

[Mike Henderson]

This doesn't necessarily prove square, it only proves out of square.

I think that if you make sure the lengths of the parallel sides are exactly equal, the diagonal measurement will prove that something is square. If the parallel sides are not of equal length, you can have equal diagonal measurements and the panel will not be square.

So in your picture, if the top line and the bottom line are exactly equal length and the two lines on the sides are exactly equal length, equal diagonal measurements will prove square.

Mike