Checking squareness of a T-square

Question

There are well known ways to self-prove a straight edge, combination square, or machinist square, to the accuracy needed in woodworking. Those methods only need the tool itself, a pencil or knife, a flat surface (for straight edges) or a flat surface with a true edge (for the squares.) They also amplify the error by twice, which helps the human eye spot it.

Is there a comparable way to check the squareness of a "T square"-like square? I'm thinking of things with a stock/fence that is below the plane of the blade/beam, such as an architect's drafting T-square, a drywall square, a shop made circular saw cross cut guide or router dado guide, etc.

You cannot flip them over like a combination square because then the fence is not registering against anything. Referencing against the opposite edge of a piece of wood just reproduces the same angle, not a flipped angle; even if it didn't, you are then trusting the two reference edges are parallel.

I am aware of the 3-4-5 method, but I don't feel like I can read a ruler or measuring tape to the mere thousandths that the straight edge/combo square truing methods afford. You can compare to another square, but often the square I want to test is significantly larger than my most trusted 12" square.

Is there a clever method I am not aware of?

Answer 1

I am aware of the 3-4-5 method, but I don't feel like I can read a ruler or measuring tape to the mere thousandths that the straight edge/combo square truing methods afford.

This method can in fact be perfectly accurate and was what I was going to suggest primarily, as it's one standard way of laying out for a self-built T-square, or anything where you need a large 90° drawn from scratch with accuracy¹.

I wouldn't use a tape for this, too much wiggle room and the tab will be a hindrance, but of course you would need a 1m or yard rule instead which not everyone has or wants to get².

Is there a clever method I am not aware of?

Add a lip to the straight edge you reference off so you can register the fence against the edge once the square is flipped over.

Technically add two pieces, with enough space between them to easily pass the blade or beam. Although it should be doable for any woodworker to plane a scrap of wood down to a uniform thickness there's no reason not to use existing material if it's on hand and fits the bill. I'd go with tempered hardboard or MDF as, barring edge damage, these should be absolutely consistent (but confirm). The pieces can simply be glued in place with superglue, but just double-sided tape might actually be sufficient and not introduce any unevenness.

There is another method that may work, but no guarantees. That is to use the factory corner of a sheet of ply, MDF or particleboard. These should be dead square just as the factory edges should be reliably straight, hence their commonly suggested use as the straight edge against which we test try squares etc. Again, you can't assume it's a perfect 90 so you need to check it, but how.....?³ :-)

---

¹ Because I didn't have a reference square at the time I made my first wooden try squares this is the method I used. They're dead-nuts square as far as a fine pencil line can show.

² Although I personally find mine indispensable. Although not a frequently used tool the few time I've needed it nothing else would do.

³ Back to the 3-4-5 rule again.

Answer 2

If you happen to have a piece of plexiglass make a small edge cut with a thin razor knife. Place the t square on the edge and slide to the razor blade then make a line or scribe a line the 48” length. Flip it to the other side registering the same cut line razor blade, scribe or mark a line and simply compare.