How do I measure "squareness"

[KingArthur900]

Hello all,
I'm a hobbyist woodworker, with a question for the machine world. I have a specialty cast-iron fixture that is designed to hold a dial indicator for setting-up woodworking machines. It's made by Oneway Manufacturing, and is called the Mult-gauge. (http://oneway.ca/workshop/multi-gauge.htm)
It's generally rectangular in shape, and the manufacturer says three of the sides are precision machined to 90 degrees.
I don't need machine-shop-level precision when working with wood, but I would like to use this fixture as my reference standard when checking my woodworking try-squares.
Just out of curiosity, I wonder how one would measure the "squareness" of one side to the next. I guess my questions is, not owning a precision square, how would I measure the outside corners of this fixture?
I've searched Google, and this site, and I don't seem to be able to concoct a search that yields any results, so now I'm posting my questions.
I appreciate any suggestions you might have.
Thanks, in advance,
Eric

 

[DMS]

Check out this thread

[thread]16473[/thread]

I will also add that if you don't have a surface plate, the table of a table saw is pretty darn flat.

 

[Fabrickator]

As DMS suggested, you can check your tri-square using the tablesaw top against any 3 of the sides of this fixture. If you want to get more technical than eyeballing a possible gap for light passing by...get a .0005 feeler gauge. Unless you are checking square over a 6' length, either one should be accurate enough. The 3,4,5 trick works good too.

 

[KingArthur900]

Check out this thread

[thread]16473[/thread]

I will also add that if you don't have a surface plate, the table of a table saw is pretty darn flat.

Beautiful! Just the sort of stuff I was looking for. I actually have a surface plate (Starrett, in the box, from the thrift store!), so I'm good to go. Thanks for the fast, helpful info.

 

[Andre]

I'm surprised nobody mentioned a squareness comparator. But if it's out of your price, skill, or you just don't want do do that there are other ways.

Since your a woodworker, you probably have a planer, jointer, or other machine with a fairly flat plate you can use as a makeshift surface plate. Then I would check it with a machinist square, or good quality combination square.

All the best.

 

[cjtoombs]

DMS, flatness of the table saw table likely depends on the table saw, and I would not make a comparison unless I was able to measure it. Mine is out by about 1/8" over its length, but then, its a HF special, too. It's out so far, you can forget dadoing on it, as it winds up with different depths on each end of the dado.