# checking a square with a mirror



## t4d (Sep 11, 2014)

I've collected a couple combination squares, and a try square and have several framing squares and also a speed square. I'm not sure that I truly trust any one of them. Can't afford a Starrett!

I've done the usual test where you use a factory edge on a sheet of plywood and then mark a line, flip the square, and then mark a second line and look for divergence. Seems like one time a particular square looks correct, then the next time I test it might not look right.

Question: is there a way to test a square using a mirror? Maybe something similar to the trick where you check the reflection in your handsaw to be sure that you're sawing straight? Not sure how to do this with a mirror and a square.

Need some help.........all ideas welcome


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## hwebb99 (Nov 27, 2012)

Check all of the squares against each other. If they match chances are that they are square. Using the method you already tried is the best. If you have a jointer it should produce a better edge than the factory edge on plywood.


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## Tree Hugger (Sep 1, 2011)

Have you got any buddy's that work in a machine shop ? 
Lots of ways to check it there from granite inspection tables with precision angle plates to CMM's ( coordinate measuring machines).
I was just tweaking the shoulders in some tenons today , checking with my knife edge Brown and Sharpe die square (yeah overkill, but don't need where I work now .), looks like the danged sled needs tweaking.
Crazy money for precision tools nowadays , but then again it seemed high 38 years ago.
Maybe look for a precision combo square at a pawn shop. With the contractor grade stuff you get what you pay for.


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## GeorgeC (Jul 30, 2008)

What you are doing is a very accurate test.

Either the edge of the plywood is not straight or there is a problem with your technique.

George


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## woodnthings (Jan 24, 2009)

*Your method ...*

You said:
"I've done the usual test where you use a factory edge on a sheet of plywood and then mark a line, flip the square, and then mark a second line and look for divergence. Seems like one time a particular square looks correct, then the next time I test it might not look right."

There's one issue with that method. If you register the top of the bar on the square against the test surface and mark then flip it and mark again, you will now have 2 lines to compare. They may or may not be parallel which is the desire condition. If they are NOT parallel, the amount of error is actually only half of the difference. So now you have to correct for that difference by hammering or bending the joint to reduce it.

Here's my point. The top of the bar is your "test" reference, but you have not yet tested the bottom of the bar.  The bar may not have parallel edges for some reason. It's not going to be a huge difference, but the potential is there regardless. The bottom is the part of the square you will use the most often in a cabinet corner to check for square. The top of the bar is used more often in the construction phase. 

I would use a marking knife when doing this test because it will be more accurate. Another method is to test all your squares with the bar facing to the left at first. Check to see if all those lines are parallel, testing them against each other. Then you can flip them so the bar is facing to the right and repeat the test. If one of the lines is "off", there is an issue with that square.


Another method is to place the square on a flat surface holding them vertically and butt them together so the blades can be compared one against the other. Check for a sliver of light that can be seen between the edges of the blades. This method checks the bottom of the bar, but only to another square.

WE are splitting hairs here and usually not necessary for woodworking accuracy. :no:


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## Steve Neul (Sep 2, 2011)

I don't think I would trust a sheet of plywood to be square. I would check the squares against the cast iron top of a table saw.


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## bzguy (Jul 11, 2011)

Steve Neul said:


> I don't think I would trust a sheet of plywood to be square. I would check the squares against the cast iron top of a table saw.


Steve, the plywood factory edge they are using for a straight-edge, not a squareness test, that is done with the parallel lines.
The other post saying it's 1/2 the error when flipped I think is also incorrect, pretty sure it's twice the error?


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## Dan3103 (Mar 24, 2015)

1/2 the error. If you had a square that was one degree off to the right and flipped it, it would be one degree to the left, total two degrees. The two would be twice the error.


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## Bill White 2 (Jun 23, 2012)

Buy a relatively inexpensive drafting square. I use on all the time to cross check my WWing squares. It is a Staedler Mars 965 10-45, and is dead square. Best reference tool I have. Might not be accurate enough for aircraft use, but sure does work for my needs.
Bill


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## GeorgeC (Jul 30, 2008)

bzguy said:


> Steve, the plywood factory edge they are using for a straight-edge, not a squareness test, that is done with the parallel lines.
> The other post saying it's 1/2 the error when flipped I think is also incorrect, pretty sure it's twice the error?


Correct on both counts.

George


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## woodnthings (Jan 24, 2009)

*the correction is 1/2 the amount*



GeorgeC said:


> Correct on both counts.
> 
> George


The difference is 2X or double the the amount, so to correct it you only need 1/2 the amount. :yes:


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## bzguy (Jul 11, 2011)

Steve Neul said:


> I don't think I would trust a sheet of plywood to be square. I would check the squares against the cast iron top of a table saw.


This is what I do also, top of Delta unisaw!


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## FrankC (Aug 24, 2012)

bzguy said:


> This is what I do also, top of Delta unisaw!


When you use the plywood factory edge or a straight board you can scribe a line with a knife, gives a very fine line to check against.

One again it needs to be said, "This is woodworking, close enough is good enough".


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