How to square a medium sized plywood panel?

[James Baker SD]

I am trying to make a rectangular panel approximately 31" x 29" from 3/4" baltic birch. The original corners on the ply were horribly out of square (if I am to believe my Starrett square). I tried to cut one edge (with a track saw and guide clamped down) perpendicular to another that I arbitrarily chose as my reference edge (seemed close to straight with my straightedge). Starrett square says it is pretty close, but not perfect. I then measured 31.5" from my new edge in several places to clamp down my track saw guide again, and made the second cut. My square says this one is terrible as well.

I checked the square a few months ago and haven't used it much since, but I guess its time to check it again, but in the meantime, what a proven method of doing this? I could probably rip a 2nd edge parallel to my chosen reference edge on my table saw although the panel is big and heavy enough that I might have a hard time holding it tightly against the rip fence. But getting the 90 degree edges is still a problem. Even 29" is a much larger cross cut than my table saw can handle. I've failed 3 times now with the track saw and guide and I am running out of waste (I started with the panel oversized just in case this happened) so I have to get it right pretty soon.

Thanks, James

[ray hampton]

I can not help to true your square both there is a way to check the plywood for squareness by using your tape measure , measure from a corner to the opposite corner, then measure across the other two corners, if the wood are square the measures will be the same

[Sam Murdoch]

Factory edges are never very good and with Baltic in particular you need to create a new edge - this you now know :). The square can give you no accurate info if you are always going off the factory edge. Start with getting 2 edges parallel. Cut one edge. Flip your sheet to rip the other edge. For cutting one board (multiple equal size cuts warrant more explanation and set up options):

1) To establish parallel for your track saw guide measure from your new edge at top and bottom of your 2nd cut - some guys set razor blade points into their pencil marks at top and bottom of the cut line then push (lightly) the guide up to the razor blades. Yes, clamp the guide, pull the blades. AND ONE MORE THING before you let 'er rip. Measure from your good ply edge to the back edge of the guide (the non cutting edge) at both top and bottom. Those dimensions should be right on and so confirm that you are about to cut a parallel edge.
Go ahead and cut the 2nd edge.

2) Now set up to do the cross cut. Use your square to give you an idea of square from both edges. Check and double check until you are satisfied. Check square from both edges again. Using a good framing square if you have one - OR make a true square from a piece of 1/2" or 1/4" ply on your table saw as big as you can comfortably cut. This smaller piece ONLY NEEDs 2 edges to be square for your set up. Set your razor blade at the bottom edge - set your guide to that blade and move the top edge to your square until satisfied. Clamp - pull the razor blade and cut.

3) With 2 parallel edges and a matching square edge you can establish your final cut as Ray suggested by measuring from each corner. Those pencil marks or razor blade stops can be confirmed for accuracy with your framing square or your plywood square. Cut and be square!

Yikes :eek: Good luck.

Sam

[HANK METZ]

Lay it out, measure the diagonals from corner to corner, if they agree, it's square enough- I never trust a carpenter square for large panels, it's not accurate enough. O.K you say, how do I start? With the 3-4-5 rule and it's multiples. Pick any corner, Draw a base line at 18" long, draw another at a right angle 24" long, measure tip to tip and it should be 30".
http://www.maths.surrey.ac.uk/hosted-sites/R.Knott/Pythag/pythag.html#sidesCalc

- Beachside Hank

[Mike Cutler]

James

Having just finished cutting 25 panels that are 31"x31" and 31" x 35", I can feel your pain.
I have an EZ rail system, but any track saw will do. I made a 4'x8' table to support the plywood and screwed a known, good, jointed edge of 4/4 material, about 6" wide, to one side to act as a reference. Perfectly flat with no distortion, or deviation along its edge.
Using Sam's techniques, I then found the perfect 90 degrees from that referenced edge. It took some trial and error, but I got it. I then locked that EZ Rail in place so it couldn't be moved out of the 90 degree reference.
The first cut went slightly into the referenced edge and created a known cut point.
From then on it was just a matter of sliding the material under the rail and cutting, after some check, and cutting to one side of the line, or the other.
Basically I made my own "manual" panel saw.
The deviation in accuracy was less than the width of a pencil line over 4 feet.
I actually used the panels to square my JessEm Master Slide. It's the most accurate it's ever been.:eek:,;)

Take a morning and make a cutting table. You won't regret the time spent.

[Sam Murdoch]

What Mike says here - Perfectly flat with no distortion, or deviation along its edge - in regards to your cut table is essential for the best in achieving true straight and square cuts with a track saw! Good lesson, that one :rolleyes:.

[david paul miller]

I can not help to true your square both there is a way to check the plywood for squareness by using your tape measure , measure from a corner to the opposite corner, then measure across the other two corners, if the wood are square the measures will be the same
I had the same problem getting a panel square, using a square I knew was 90 degress my panel showed the square was off , but measuring diagonally it was dead on, I was confused until my college son showed up and explained an isosceles trapezoid. ( measures the same diagonally but is not square)

[ian maybury]

Thanks Sam, the razor blades sound like a good move.....

ian

[Guy Belleman]

+1 for making a cutting table, but I also found that making a large sled for the table saw also led to producing nice square panels. Of course the sled has be set correctly, but once done it is one of the most used tools.

[Jerome Hanby]

That's a great point. That diagonal trick was only ever intended to get an assembly square, not for verifying that a particular part was four square.


I had the same problem getting a panel square, using a square I knew was 90 degress my panel showed the square was off , but measuring diagonally it was dead on, I was confused until my college son showed up and explained an isosceles trapezoid. ( measures the same diagonally but is not square)

[Brian Kincaid]

measuring diagonally it was dead on...isosceles trapezoid. ( measures the same diagonally but is not square)

That's a bummer and suprisingly easy to do if you flip the work and use the same crosscut fence. It's also easy to diagnose because your parallel edges will be different lengths.

If your parallel edges are the same length (both sets) and the opposite corner measurements match each other you have a perfect rectangle.

-Brian

[Myk Rian]

Check your square.
The usual way is lay it against a flat edge, like your fence. Draw a perpendicular line.
Flip it over. If the line and edge meet, it's OK.
If there is space, take a nail set and punch some dimples in the inside corner to stretch the square open.
If the lines overlap, punch the dimples in the outside corner to close it up.

[James Baker SD]

my square is a big starrett builder's square, the angle is adjustable by loosening a thumb screw. I checked it a few months ago and it has mainly just hung on the wall since then, but I will check it again, both against a starrett engineers square and with the "draw a line, flip it over" test. If it is off, all I have to do is loosen the screw, get it square, tighten the screw and adjust the scale if necessary.

I usually found that purchased plywood was pretty good about having square corners. This particular sheet was by far the most out of square I have encountered. Now I will doubt all future purchases, but I would check the corners and start from the best one. Just none of these was good enough to start with.

I use the diagonal measurement when doing glue-up to make sure a carcass is square, but never used it to draw cutting lines. I would still have the problem of cutting to my line even if I got a perfect 90 degree lines, or knife marks, on the wood.

Biggest problem, and I know the solution, is that I have too much junk in the way to have an organized work area. Trying to live in half a garage that the wife insists still gets a car (my car at that) inside. Too much useless junk that I need to get rid of (if she will let me) to make some room for what is important. I cannot handle big sheetgoods on my table saw so I try to break them down with a track saw, but the table saw still has to serve as the table that supports the sheet during the cut--not good as it is too high for comfortably controlling the track saw and I am tripping over dust collection flex hose etc on the floor around it, but that is all my fault and I know the solution.

Thing that really blew my mind was that the starrett square said one corner was pretty good, so I measured from an edge to do a parallel cut with the track saw. After the cut, I expected the new edge to be square with the 90 degree edge of the original. Seems if two lines are parallel, then if a third line is perpendicular to one of them, it is perpendicular to both. Measuring the width of the panel after the cut, the width was off a little from top to bottom (31" between the width measuring points). By a little I mean one was a pencil line long and the other a pencil line short so maybe 1/32" error over 31". But when I put the starrett on that new "90 degree" corner, the ruler gapped from the newly cut edge almost 1/8" at the end of its 24" length. This still seems mathematically impossible, so I called it a night, started this thread, and figured when I was rested I would go out an measure and check everything again.

[James Baker SD]

OK, I did the line, flip, compare and my square had gotten out of adjustment. Took about 30 minutes of tweaking the 90 degree stop screw inside the head, but I finally got it adjusted so that it passes the flip test. Now, checking the corner I was basing everything on, I have about 1/16" gap at the end of the ruler and that pretty much accounts for the problems I was seeing. Don't know how it got out of square hanging on the wall, but I feel kind of stupid I did not check it earlier. Thanks for all the ideas and hints here. Of course I did not get the hint I really wanted--"tell your wife you need a sliding table saw" :)

[Harvey Melvin Richards]

I made a squaring sled. It's 1/2" MDF with an aluminum miter bar. It's probably 32" deep, and 42" wide. There is a Hardwood lip on the back, and it's left end is even with the blade. It will always cut my panels square, and larger ones can over hang the edges. If I have to square something larger, I use my router and a known straight edge.

[Brian Kincaid]

Of course I did not get the hint I really wanted--"tell your wife you need a sliding table saw" :)

Tell your wife you need a sliding table saw. (or a tracksaw with a bridge and fence.)
-Brian

[Thad McCulloch]

Or make something she wants using your new Exaktor EX60 sliding table that you added to your saw (my approach)!