Checking a Square for Square

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Forum topic by fivecodys posted 08-20-2015 04:07 PM 1423 views 0 times favorited 21 replies Add to Favorites Watch
View fivecodys's profile


999 posts in 1632 days

08-20-2015 04:07 PM

Good morning Lumberjocks.
I bought a new machinist square. It’s a Robert Larson 885-1006 6-Inch Engineer Square.
My question is this: How do I check it to make sure it’s accurate?
I have a couple of plastic squares that I used for drawing back in school and I have an Orange Speed Square that I got at Lowe’s a year or so back. I checked these against my new square and it looks dead on to me.
Is there a better way to check it?

Amazon sent this to me in an envelope wrapped in bubble wrap. The envelope was pretty ragged by the time I got it but I could not see any damage to the square.

Thanks in advance for your comments.

-- There' are two theories to arguin' with a woman. Neither one works.

21 replies so far

View bondogaposis's profile


4723 posts in 2347 days

#1 posted 08-20-2015 04:24 PM

Simple, lay it on a board that has a straight side. Then using the square, mark a line that is at right angles to the straight edge of the board. Then flip the square, if the edge of the square lines up perfectly w/ the marked line then it is square.

-- Bondo Gaposis

View jmartel's profile


7884 posts in 2146 days

#2 posted 08-20-2015 04:24 PM

Find a known straight edge, like the edge on a sheet of plywood/mdf. Draw one line off the square perpendicular to the edge. Flip the square over and draw another line. If they are parallel, it’s square.

-- The quality of one's woodworking is directly related to the amount of flannel worn.

View JayT's profile


5625 posts in 2207 days

#3 posted 08-20-2015 04:26 PM

On a piece of straight scrap, use the square to draw a line perpendicular to the edge the full length of the blade. Flip the square over and draw another line starting at the same exact point on the edge. If the two lines are exactly on top of each other, it’s square.

Edit: or what they ^^ said. :-) I must type slow.

-- In matters of style, swim with the current; in matters of principle, stand like a rock. Thomas Jefferson

View HokieKen's profile


4985 posts in 1134 days

#4 posted 08-20-2015 04:58 PM

All of the above will work. However, if you checked that square against 2 others and they all look dead square, it’s square. It’s highly improbable that all 3 squares are out the same amount in the same direction.

One word about checking using the method mentioned above, I like to use a striking knife or a razor blade to scribe the line and to darken the wood with a marker first. That way I get a crisp, fine line. A pencil leaves to wide and jagged a line to suit me.

-- Kenny, SW VA, Go Hokies!!!

View Woodknack's profile


11610 posts in 2376 days

#5 posted 08-20-2015 05:06 PM

HokieKen beat me to it—if you hold two squares back to back, the chances that one is the reciprocal of the other is infinitesimal, that three are reciprocals is all but impossible. So for example I have a Larson engineer square and a Lufkin combination square, I check them for square against each other by holding them back to back with a window in the background. When I do that, there is no light between them, so I would call them deadnuts.

-- Rick M,

View fivecodys's profile


999 posts in 1632 days

#6 posted 08-20-2015 05:48 PM

Thanks fellas.
I appreciate the information.

-- There' are two theories to arguin' with a woman. Neither one works.

View Ocelot's profile


1975 posts in 2634 days

#7 posted 08-20-2015 05:54 PM

I also have a pair of 1-2-3 blocks. It’s impossible for all 12 lines of the block to fit tight to a square unless both the block and the square are square. 2-4-6 blocks would be better but they are quite a bit more expensive. The 1-2-3 blocks are less than 15 bucks.

View Woodster73's profile


19 posts in 1071 days

#8 posted 08-20-2015 05:56 PM


what are 1-2-3 and 2-4-6 blocks?

View oldnovice's profile


6845 posts in 3363 days

#9 posted 08-20-2015 07:30 PM

The way we used to check or triangles back in drafting (does anyone remember high school drafting class?) is the same way to check a square which is what was described above by Bondo!

The finer the line the better the check.

Use your table saw table. Put the square in question on the front/back edge on your TS table and check it again st the miter head slots. If you trust your TS then you should trust this method too!

-- "I never met a board I didn't like!"

View woodworkerguyca's profile


30 posts in 1040 days

#10 posted 08-20-2015 08:01 PM

All great methods, very good question. Off the top of my head, I can’t think of any method to know how close to square it is (like a jig for a micrometer), but holding it against a mirror and looking down the perpendicular edge should show pretty well.

View joey502's profile


535 posts in 1514 days

#11 posted 08-20-2015 10:35 PM

123 blocks are a machinist set up tool. They are ground flat, and sqaure to within .0003” or so depending on how much you spend. The are 1”x2”x3”. 246 are the same thing but larger in size.

View joey502's profile


535 posts in 1514 days

#12 posted 08-20-2015 10:42 PM

I use the 123 blocks to check the inside of my squares and a 123 block on a granite surface plate to check the outside.

View oldnovice's profile


6845 posts in 3363 days

#13 posted 08-20-2015 11:44 PM

If you are really concerned, take it to a machine shop that has a CMM, coordinate measuring machine, and they can remove all doubt!

-- "I never met a board I didn't like!"

View BobAnderton's profile


255 posts in 2786 days

#14 posted 08-21-2015 01:40 AM

Watch this William Ng video called “5 cuts to a perfect crosscut sled” and think of drawing lines with your square instead of making cuts with the tablesaw. You will be able to determine exactly how out of square your square is to the thousandth.

-- Bob Anderton - Austin, TX - Nova 3000 lathe, Alaskan Mark III mill, Husqavarna Saw

View Pendragon1998's profile


74 posts in 1572 days

#15 posted 08-21-2015 02:38 AM

Ok, so say you buy a vintage square and discover it is not quite square. Can combination squares be adjusted back to square?

showing 1 through 15 of 21 replies

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