Checking Plywood Sheet for Square

[Don Dean]

I am ready to move from the making firewood stage of my woodworking learning to making some shop projects stage of my learning.

I will be making 3 projects for my shop over the next several months. First will be a table saw out feed table (Wood Whisperer plan), next a plywood workbench (Fine Woodworking plan), and finally a assembly table with a torsion box top and a bottom with drawers and doors (Wood Whisperer plan). I selected these projects to improve my skills and I need them for my shop.

My question. How do you ensure a sheet of plywood is square and if not square what is your process for squaring a sheet of plywood?

Also I have wonders why plywood is not the thickness as sold (3/4" = 23/32"). Then I realized that 23/32" is 18mm. Has plywood makers gone to the metric system or is it all imported? How do you deal with this less than 3/4" thickness when planning a project as 23/32" measurement seems cumbersome?

[glenn bradley]

Oh my, so many replies possible. I'll do a few and others will surely follow.

Good plan on making shop fixtures to practice your skills. I frequently make shop fixtures with odd joinery or doors just to tryout new things.

NEVER assume a factory edge is straight, the compounded errors of using a factory edge as a baseline will cause you grief. Unless you have a large slider or a TS surrounded with feed surfaces you will probably use a guided circ-saw to initially breakdown your sheet goods. I use stiff foam insulation (http://www.lowes.com/lowes/lkn?action=productDetail&productId=15357-10477-15357&lpage=none) on the floor and a shop made guide. I buy or cut the foam into 2ft x 4ft pieces for versatility. When not in use they store with the sheets.

Plywood is undersize and not consistently. The undersize router bits are good for when you get lucky, save your money. The measuring does seem clumsy. You'll notice most plans assume 3/4" sheet material. It is generally up to you to adjust. This is why I use plans as (read in a captain Barbosa voice) "guidelines", not absolutes.

I'll stop here as there is plenty to say and a great group of folks here to say it. I don't want to flood you with only my opinions.

P.s. Save yourself the grief and future rant post by going to a lumber yard and buying 'shop' or 'cabinet' grade ply. Don't go to th BORG. I prefer BB ply for shop fixtures but the recent "shortage" drove the price up and like milk, coffee and gas . .. it ain't comin' down.
http://www.sawmillcreek.org/attachment.php?attachmentid=54205&d=1167872143

[Jason Roehl]

Other than what Glenn said, if you wanted to check the squareness of a sheet of plywood, first check all lengths. If you have 2 pairs of equal-length sides, with the matching sides opposing each other, you have at the very least a parallelogram. It could be a square or rectangle, too. To check that it is also a rectangle, measure the diagonals. If they match, you have a rectangle. If not, you don't.

[Tom Veatch]

What Jason said will tell you if the sheet is a rectangle (all corners square). If not, you can check each corner with the 3-4-5 method - measure 3' along one edge, 4' along the adjacent edge and if the diagonal of those measurements is 5', the edge is square (assuming the edges are straight). If not square, lay out a cut line square to one edge, then what Glenn said.

[Dave MacArthur]

http://www.sawmillcreek.org/showthread.php?t=78303

same topic today, more answers there.

[Wayne Cannon]

Plywood thickness isn't even uniform across any single sheet.