Square

[Carpenter84]

I’ve been on and off looking for some kind of square, be it a cylindrical square, or granite, or whatever. I have no precision square of any kind to actually take advantage of my squareness comparator I made.

I came across this, eBay square

What’s it For? Any use to me? Says 90*, seems reasonably inexpensive, even if I make a lower offer.

[4GSR]

Here's all the squares you'll ever need.

https://www.ebay.ca/itm/NICE-STARRETT-Machinist-Square-Set-No-20-6-3-453E-Die-Makers-Square/263947551164?_trkparms=aid%3D777003%26algo%3DDISCL.MBE%26ao%3D1%26asc%3D20140620091411%26meid%3D824987a35c5f4c138a1546767672ce0a%26pid%3D100009%26rk%3D1%26rkt%3D10%26sd%3D223135748216%26itm%3D263947551164&_trksid=p2047675.c100009.m1982

Seriously, that one you are looking at in my opinion is too small to use.  Unless you are doing something small. Get you a couple of precision squares like the Starrett No. 20 in a 3" and at least a 6" to start with.  A cylindrical square is okay or a granite square, which is what I have.  I do have a B & S Machinists square with a 20" blade on it, and had it calibrated when I got it being within a half thousandth in 20". I find my 3" square being more handier than any of the others.  I also have one with a 1-1/2 beam I use for aligning small taps for tapping holes with.  Your choice to do what you want. You're going to find out that just one square is not going to be enough to work with.

[Carpenter84]

Holy expensive. Haha.

Are you using those squares to set a squareness comparator? Or just for scribing?
I have a full set of these but I'm sure they're far from precision. I just use them for quick scribing. I'm not sure is trust them set a squareness comparator.

[woodchucker]

Look at garage sales.
The square you are looking at is too small.

The purpose of the bevel is to be able to see against a knife edge (the bevel) whether any light will shine past.
I have a large Starret about 10" possibly smaller, possibly larger.. I'm at work and can't measure it.
it serves me well. As well as my starret combo square (for rough squaring) although it is dead on.

Even a cheap set may be in your accuracy range.
http://www.shars.com/products/measuring/squares

[Carpenter84]

But just to clarify, you're suggesting I use these squares as a standard for my squareness comparator?
(Yes, I will pass on the small bevel square)

[chips and more]

A cylindrical square is nice to have , but pricey.

[f350ca]

Made one a while back from a piece of HT 4140. Its out by a couple of 10ths, but you find the plumb sides and use them. The pin was used to drive it on the face plate, replaced it with a flush set screw.