Checking and Squaring a 4'x8' plywood sheet

[Don Dean]

How do you check and see if a sheet of plywood is square? Do you use the 3-4-5 method or measure both opposite corners to see if there the same?

Once you determine that the sheet is not square how do you square the sheet?

[tim rowledge]

Personally I cheat by just trimming the ends on my sliding table. I know it's set square so it makes life very simple.

If you check the two diagonals, remember to check at the edges as well since you can have equal diagonals on a trapezoid!

[Alan DuBoff]

If you check the two diagonals, remember to check at the edges as well since you can have equal diagonals on a trapezoid!Can't you only have the same measurement on the diagonals if the sheet is a rectangle (i.e., 90 degrees at the corners)? Otherwise they won't be the same, AFAIK.

[Deirdre Saoirse Moen]

A trapezoid would have diagonals the same length.

[Norman Hitt]

Can't you only have the same measurement on the diagonals if the sheet is a rectangle (i.e., 90 degrees at the corners)? Otherwise they won't be the same, AFAIK.

NOPE!!!

As long as any two sides are of equal length, you can have equal diagonal lengths as well, BUT......only if the two angles in each "pair" are the same. Of the two "pairs" of angles, each pair will have a different angle from the other pair.

Note: I'd draw you an example, But I flunked Sketchup 101.:rolleyes: :D

Example: A sheet of Ply, measured along the edges is as follows;

Side A = 90", Side B = 40", Side C = 90", Side D = 48"

If the angle between Side A & Side D is the Same as the Angle between Side C & Side D, then the angles Between Sides A & B, and Sides B & C will be the same, and the Diagonals WILL be EQUAL, but it will not be a square cornered rectangle.

Hope this makes sense.

[Alan DuBoff]

Maybe I'm missing something (not unheard of) or I have a thick head (also not unheard of :D). A trapezoid can have equal measurements, but only if it's an iscoseles trapezoid, meaning the base angles are the same, and hence the two non-parallel sides would be the same length.

A trapezoid only guarantees you'll have 2 parallel sides, so you're not guaranteed that you will have sides of the same length. If you measure them, and they are, you're in biz...

Now, you're not even guaranteed you can get 2 parallel sides on a sheet of plywood, AFAIK. :D

Even so, if both sets of opposite sides were equal, this would also be true, but it is an iscoseles at that point, just as a rectangle or even a square would be.

Tim's solution of using his sliding table saw will only work if the two sides are parallel, which they probably are, but I'm just pointing out, if you really wanted to get technical about it you would measure the angles at the corners. Buldings aren't square, rooms aren't square, and most furniture and woodwork is not square either.

My guess is that the sheets start out being prefectly perfect, as a machine can produce, but that as wood changes, so does the shape. I'm sure most ply is pretty stable, but I'm sure things must change as the moisture contents changes.

[Bob Russell]

:confused: NEW GUY 1ST post Would using a framing sq. on the 4 corners tell if its sq? btw i am a long time lurker and have greatly benifited from the info here . As a newbie im not much help but ill try when i can. thanks Bob

[Gary Keedwell]

A trapezoid will not have equal diagonals. The sides may be the same lengh but not the same measurements from corner to corner. Thats why on a glue up you squeese the corners till they are equal.

Gary K.

[Cliff Rohrabacher]

Personally I cheat by just trimming the ends on my sliding table. I know it's set square so it makes life very simple.

If you check the two diagonals, remember to check at the edges as well since you can have equal diagonals on a trapezoid!

What he said. When your slider is set up right you get square so close you can't measure any run out. Closer than 1-MOA even.

[Mark Pruitt]

.....if I have a sheet of plywood that isn't square, my preferred method of squaring it is to use a straightedge and my circular saw.

Measuring opposite corners will tell me whether the sheet is perfectly rectangular or skewed. If the distance is the same then all is well.

[Byron Trantham]

I'm dimensionally impaired so I quickly fired up my CAD package and drew a trapezoid. Yep both diagonals are the same! Hum. To use the corner to corner method and BE SURE, you would have to also measure the two parallel sides. So, if the diagonals are the same AND both parallel sides are the same then its square. Boy you guys can come with some bizarre problems!:o

Just curious, has anyone ever come up with out of square sheets?:confused:

[tod evans]

Just curious, has anyone ever come up with out of square sheets?:confused:


30+ years butchering boards never seen a sheet of cabinet grade anything out to where i could measure it with a tape measure. .02 tod

[Byron Trantham]

30+ years butchering boards never seen a sheet of cabinet grade anything out to where i could measure it with a tape measure. .02 tod

Now I feel better. Some things in life should just be so.:D

[Mark Pruitt]

...A trapezoid is a four sided shape with at least two opposite sides parallel. That is the only criterion. Opposite angles can be congruent or not; likewise both "bottom" angles and both "top" angles can be congruent (what Alan referred to as an iscoseles trapezoid) or not. A rectangle is a type of trapezoid.

BTW, I have had sheets of plywood off the rack out of square. Hasn't happened often but very irritating when it does happen as it indicates sloppiness IMHO.

[tod evans]

[quote=Mark Pruitt

BTW, I have had sheets of plywood off the rack out of square. Hasn't happened often but very irritating when it does happen as it indicates sloppiness IMHO.[/quote]

where are you buying your plywood?

[Kev Coleman]

I think the definition of a trapezoid is a quadrilateral (http://mathworld.wolfram.com/Quadrilateral.html) with two sides parallel (http://mathworld.wolfram.com/Parallel.html). In such a case, I do not think that the diagonals would be equal.


However a a parallelogram is a quadrilateral (http://mathworld.wolfram.com/Quadrilateral.html) with opposite sides parallel (and therefore opposite angles equal). In this case the diagonals would be equal. Squares and rectangles are a form of a parallelogram.

I cant beleiveI still remember my high school geometry. Hope this clears up the confusion.

[Don Dean]

OK Creekers, thanks for the information on parallelogram, quadrilateral, etc. Now lets say I have checked and the sheet of plywood is not square, how do you square the sheet?

[tod evans]

OK Creekers, thanks for the information on parallelogram, quadrilateral, etc. Now lets say I have checked and the sheet of plywood is not square, how do you square the sheet?

sliding table saw...02 tod

[Harvey Chute]

A more common problem for me is the 4x8 sheet that is cupped or bowed.

A recent bookcase carcase I assembled measured equally on the diagonals, but wasn't square because of slight bows in the plywood. (Exaggerated in the diagram).

So now I'm hoping the backing will be strong enough to force the standards straight and produce a square carcase. Argghhh.
40279

[John Hemenway]

I think the definition of a trapezoid is a quadrilateral (http://mathworld.wolfram.com/Quadrilateral.html) with two sides parallel (http://mathworld.wolfram.com/Parallel.html). In such a case, I do not think that the diagonals would be equal.


However a a parallelogram is a quadrilateral (http://mathworld.wolfram.com/Quadrilateral.html) with opposite sides parallel (and therefore opposite angles equal). In this case the diagonals would be equal. Squares and rectangles are a form of a parallelogram.

I cant beleiveI still remember my high school geometry. Hope this clears up the confusion.
Well I hate to jump on you but I don't think you remembered all the details of HS geometry. :(

An equilateral trapezoid ( by definition, trapeziod has only two parallel sides) has diagonals that are equal but the parallel sides are not the same length! This is the only flavor of trapezoid with equal diagonals.
See:
http://id.mind.net/~zona/mmts/geometrySection/commonShapes/trapezoid/trapezoid.html (http://id.mind.net/%7Ezona/mmts/geometrySection/commonShapes/trapezoid/trapezoid.html)

A parallelogram has two pairs of parallel sides but corners are not square. It's a rectangle or square that has been skewed. The diagonals ARE NOT the same. If diagonals are equal it's a square (four equal sides) or a rectangle.

After digressing into HS geometry ...
Measure diagonals. If equal, measure length of parallel sides. If equal it's a square or rectangle.

If the sheets are not square, use them anyway. You'll be thought of as 'ahead of your time' or ... :D

[Bruce Benjamin]

A more common problem for me is the 4x8 sheet that is cupped or bowed.

A recent bookcase carcase I assembled measured equally on the diagonals, but wasn't square because of slight bows in the plywood. (Exaggerated in the diagram).

So now I'm hoping the backing will be strong enough to force the standards straight and produce a square carcase. Argghhh.
40279

I get those from time to time but even more annoying than that is when only one of those sides is either bowed in or out. In other words, in your drawing the two sides on the left will be at 90 degrees and the two on the right will be at greater than 90 degrees. How do they do that? It's not often but I've had that happen on various types of sheet goods from a good grade of plywood to a sheet of Masonite. I can't imagine how a the factory would be able to cut something like that but I've checked and rechecked and that's what I get. Lay my EZ Smart saw guide on the edge and straighten it out and it's good.

Bruce

[Rob Russell]

:confused: NEW GUY 1ST post Would using a framing sq. on the 4 corners tell if its sq? btw i am a long time lurker and have greatly benifited from the info here . As a newbie im not much help but ill try when i can. thanks Bob

Hey Bob - first post - glad you decided to come out of the shadows. Now ya just better hope that you don't get confused with me - that'd be a fate worse than death!

Rob

[Charlie Plesums]

:confused: NEW GUY 1ST post Would using a framing sq. on the 4 corners tell if its sq? btw i am a long time lurker and have greatly benifited from the info here . As a newbie im not much help but ill try when i can. thanks Bob

Welcome to the creek. And thanks for your input. And please tell us wheere you can buy a framing square for a reasonable price that is "really" square. :confused:

Actually I played with quite a few, quite a few years ago, and have one that is reasonably good, but it isn't good enough to align a saw, etc.

So what is really square? I use a 5 sided cut to align my saw, which has stayed within a few thousandths of an inch over 4 right angles and perhaps 10 feet of cutting. Far better than my framing square.

[Norman Hitt]

A more common problem for me is the 4x8 sheet that is cupped or bowed.

A recent bookcase carcase I assembled measured equally on the diagonals, but wasn't square because of slight bows in the plywood. (Exaggerated in the diagram).

So now I'm hoping the backing will be strong enough to force the standards straight and produce a square carcase. Argghhh.
40279

Harvey, I recently had the exact same thing on an aquarium stand's case. I tried something that worked pretty well for me. I placed a short 2 x 4 close to the top and bottom on the concave side of the dividers that were bowed, and then placed another short 2 x 4 vertically across the other two short 2 x 4's and put a clamp from that vertical 2 x 4 to the cupped divider at the middle of the bow/cup, and then tightened the clamp to about twice the cup amount past a straight line. I then took a spray bottle and sprayed a fine mist of water on both sides of the divider and let it set 4 days in the clamps. When I removed the clamps it was straight enough that I could install the faceframe with only slight pushing on the dividers to get them to align with the face frame stiles when glueing/clamping it up. I haven't installed the back yet, but the dividers appear to be remaining straight bacvk there, but I will check them again before installing it and if they are bowed, I will use the same treeatment before installing the back if necessary.

Hope this helps.

[lou sansone]

Wow ... I can see where engineers get a bad rap ... too much information here.
I agree with tod

how square do you need and how to get it square if it is not.
1. measure all the sides to make sure you have something with parallel sides.
2. diagonal measurements to check for square
3. If its out, use your $20,000 sliding table saw to get it back in
4. use a Sheetrock square if you don't have a sider and some type of guide set up

Boy I am so glad I live in a house where nothing is square. I never have this problem :)

Lou

[Dan Forman]

OK Creekers, thanks for the information on parallelogram, quadrilateral, etc. Now lets say I have checked and the sheet of plywood is not square, how do you square the sheet?

You can use a framing or sheetrock square to lay out the cut, then use a circular saw with a guide, either commercial or home made. First however, you must make sure that the square is really square. I have a framing square that is, and a sheetrock square that isn't. Find a surface whose edge is known to be straight, and is as deep as the long leg of the square. Align the short leg of the square with this edge. Next, draw a line along the long leg of the square, perpendicular to the edge with which it is aligned. Now flip the short leg over to the other side, and see if the long leg still lines up with the line you just drew. If it does, the square is true. If not, it's out of whack.

I haven't actually done this next part, as I found that my framing square is indeed square, but I've read that you can true the square with a hammer by striking the corner section, on the outside to move the leg inward, or on the inside to move the leg out. Apparently the hammer rearranges the metal enough to effect these changes. I'm not sure if this would work on a sheetrock square, due to the way it's built, with overlapping pieces. If that doesn't sound like much fun, there is another way that makes use of basic geometry and doesn't invove trapezoids.

If you are cutting a sheet into a number of smaller pieces, and just want to make sure you are starting with a sqare cut, you can start anywhere beyond a foot or so of either end in the long dimension. First make yourself a large compass like thingy out of a narrow strip of wood about three feet long, with a pencil secured in one end, and a nail for a point in the other end. The longer the compass, the more accurate the cut will be. Figure out where you want to star cutting, and make a mark on the edge of the sheet.

Then make two more marks, one on either side of the first mark, 12 inches from the initial mark. Now draw a one inch line perpendicular to the edge of the sheet for each of the two outside marks (use something to guide the line so that the points will remain equidistant from the center). This will give an accurate point away from the edge on which to anchor the point of your makeshift compass. From each of the outside marks, make short arcs which will intersect each other directly over your initial mark. You can now draw a line (with an accurate straightedge) connecting your first mark with the point of intersection of the two arcs, which will be square to the edge of the sheet that you are working from. You can cut this with a guided circular saw, and be cetain that those corners are square. It would be a good idea to mark them so that you can keep track of them. Continue cutting up the sheet in this way until the pices are small enough to get onto your table saw where you can trim them to final size. Best to allow a little extra room on the rough cuts.

Hope this helps.

Dan

Edit: I guess Lou came up with the short answer while I was typing this.

[Brent Harral]

I use a lots of sheet stock and I rarely come across an end that is square. I made a couple simple but accurate panel squares to square one end before I cross cut using a circ saw guided system. Easy and no math involved!

<img src="http://pic14.picturetrail.com/VOL563/2750473/9448194/153831356.jpg">

[tod evans]

just now i cut 3 pc of 3/4 baltic and 2 pc of 5/8 mdf core oak veneer on the slider. and measured the off cuts from the "squaring cut" with calipers, less than .01 on the baltic over 60" the mdf was out .02-.03 in 96" both measurements are to small to see with a regular tape measure. for most everything i build i feel safe calling the sheetgoods i buy square out of the box. the deviation in the mdf could very well be from the pressure i applied to the calipers as the material is softer than the baltic...02 tod

[Harvey Chute]

[quote=Norman Hitt]Harvey, I recently had the exact same thing on an aquarium stand's case. I tried something that worked pretty well for me. I placed a short 2 x 4 close to the top and bottom on the concave side of the dividers that were bowed, and then placed another short 2 x 4 vertically across the other two short 2 x 4's and put a clamp from that vertical 2 x 4 to the cupped divider at the middle of the bow/cup, and then tightened the clamp to about twice the cup amount past a straight line. I then took a spray bottle and sprayed a fine mist of water on both sides of the divider and let it set 4 days in the clamps. When I removed the clamps it was straight enough that I could install the faceframe with only slight pushing on the dividers to get them to align with the face frame stiles when glueing/clamping it up. I haven't installed the back yet, but the dividers appear to be remaining straight bacvk there, but I will check them again before installing it and if they are bowed, I will use the same treeatment before installing the back if necessary.
quote]

That is a good idea - and even better to hear that the technique actually works. It sounds like you did this even after the carcase was assembled - in my case I can do the clamping to the sides prior to assembly so it should work even better. Thanks Norman!

[Norman Hitt]

That is a good idea - and even better to hear that the technique actually works. It sounds like you did this even after the carcase was assembled - in my case I can do the clamping to the sides prior to assembly so it should work even better. Thanks Norman![/QUOTE]

Yeah, Harvey, I was out of town (and the shop off and on for about three weeks after I glued up the carcass) and that's when the two center dividers (Birch ply) bowed. The end panels were Red Oak Ply, and they didn't bow, so I only had to correct the two center dividers. I used a fine mist spray bottle, but I did get both sides of each divider fairly damp.

[Steve Clardy]

This threads amazing:) ;)

I have yet to have a 4x8 or even a 4x12 sheet measurable out of square with a tape measure.

On the off chance I GET one out of square after cutting, I use a standard framing square.

Years ago while shopping for a new one, I had an old hardware guy show me a trick while sifting through a stack of them, wondering which one was square. He simply picked three of them that all fit together. [After we went through a dozen or so]

[Jason Roehl]

Here's a site to help clarify some things:

http://www.mathleague.com/help/geometry/polygons.htm#trapezoid

Unfortunately, they do not have a picture of an isosceles trapezoid, which IS the only case where a trapezoid would have equal diagonals. However, a parallelogram only has equal diagonals if it is a rectangle. For example, scroll up from the trapezoids section in the link above, and take a look at the parallelogram and rhombus (parallelogram with 4 equal-length sides) sections. The bright green parallelogram definitely does NOT have equal diagonals, nor does the light brown rhombus. (The others don't either unless they're rectangles, but the two examples I gave are obvious on-screen).

["Gary Brewer"]

Squares, rectangles and "isosceles" trapezoids have equal diagonals.
An isosceles trapezoid is a trapezoid with the non-parallel sides ( also called legs ) equal.
A trapezoid and isosceles trapezoids have two and only two sides parallel. ( i.e. exactly two sides, no more or no less ).
In an isosceles trapezoid the lower base angles are equal and the upper two angles are equal. ( upper angles don't equal lower angles though )
I've taught this too long ( 34 years )