Which faces of a machinists square are square?

Ok, this is a stupid question, but are the inside faces of a machinists square supposed to be square, or only the outside faces? Or, put another
way, are the blade and base faces both parallel?
I've got the old "never take two clocks to sea" problem -- I've got two squares and I piggyback them and I can see light between the blades. So now I don't know which one is off...
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Ben Jackson
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Ben Jackson wrote:

If you measure the width of the base and beam at several places it'll tell you if they are _not_ parallel -- measurements all over the place will indicate that the edges aren't straight, measurements that trend from one point to another would indicate a taper. About the only thing that this won't pick up would be parallel but wavy faces.
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Tim Wescott
Wescott Design Services
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Sounds like you need a third square...

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No, sounds like he needs to scribe a line based on the square, flip it over, and see if it stays on the line. Repeat with the other square. One of 'em will describe a "v" instead of one line, when flipped.
Dave Hinz
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You are just no fun!
wrote:

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Instead of "piggyback", put them blade-to-blade on a known gauge plate, then look for light. If they're both good, and your gauge plate is flat, you'll get a blackout join. If not, borrow a known good one, and figure out which (or both) are worn or bent. (Maybe instead of borrow, you could haul yours down to a shop that has the facilities, and ask as a favor to check them out)
LLoyd

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Hey! Don't sweat it! File down the one that is bent until it mates with the straight one.
Bob Swinney

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Ben Jackson wrote:

If you have access to a surface grinder with precision angle plate then clamp the short leg of the square to the vertical face of the angle plate so the long leg is horizontal, and take a cleanup grind (very light) on the long edge.
If your square is out, that is.
GWE
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snip---

That doesn't address the inside of the square, which is just as important as the outside. Fixing a machinist's square that isn't isn't as simple as it may appear.
Harold
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Why no do it the same way you adjust any other square. Use a GOOD straight edge. Lay the long edge of the square on the SE and draw an internal and external line. Flip the square and see where the square lines up. To open the square lay it on a solid flat surface and use a hammer to strike the inside corner. (this stretches the metal and opens the legs some) If you need to close the legs you hit the outer corner.
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Steve Williams

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AAAAACKKKKKK! BANG on a precision square? (tearing hair and screaming while running with eyes closed)
Steve, we're talking straight, parallel, and square to within a few ten-thousanths! This ain't framing carpentry!
LLoyd
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another
And do not be surprised if the grinder bends the blade down even with a very light cut and your square is worse than before. Been there, done that. Jim
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Ben Jackson wrote:

machinists
another
two
If you've got a name brand square, the edges are supposed to be parallel AND square with the world, that's why you pay the big bucks these days for Starrett. There's a number of ways you can check them out, drawing a line and flipping the square, then drawing another is a crude way to do it, but effective. Most of the others involve surface plates, sensitive indicators, surface gauges and/or check squares. Lautard has a lot on squares in his books, one section details how to lap a square for correction purposes, another section is about how to make your own precision square. He also tells how to make a cylindrical square.
Stan
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Yep. Guy Lautard am my HERO. (non-pro, totally-PRO home shop machinist)
LLoyd
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Lloyd E. Sponenburgh wrote:

http://www.lautard.com /
Enjoy (really!)
Ken.
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Both sides are supposed to be square.

There are different grades of squares. Some are within .000080" (eighty millionths) and some are only .0008" (eight ten thousandths) and there are others in between. Now if you have two that are of the .0008" variety, it would be very possible to see light through them even when brand new. IIRC, they are classified grade 1-4 or something like that. Then there are master squares. Of course one of them could be bent or worn, as well.
Dan
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There's a good article in the most recent HSM on indicators that briefly describes how to check a square with a simple fixture and a good indicator.
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Hi guys..new to the group. What I would do if I had to check my square. If you have a known angle plate, place it on a surface plate, set up a dial indicator and run the indicator down the blade and see how much "run out" there is on the blade. This can be done inside and out, or you can send it to the inspection department and let them deal with it. ;-) Tracy
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