Build a Really Good Tri-Square

I wanted to make a tri-square (or try-square--some claim both are acceptable), and I wanted to use my wire-feed welder.  I also wanted a larger than normal tri-square. 

This is my new square finished and ready to use.

The square is all steel from pieces I already had in my workshop.  I used a piece of 1/8" x 1 1/2" steel about 17" long and some 1/2" x 1/2" square tubing.  I cut four pieces of the square tubing 3/4" long each.  I also cut four pieces 10" long each.  These will be used to make the handle leg of the square.  I also cut a piece 3/4" long from the end of the 1/8" x 1 1/2" bar.  It will become a spacer or shim in the bottom end of the handle.

The chop saw is from an earlier Instructable.

The pieces for each half of the handle are clamped to a flat piece of aluminum angle in my vise.  Any burrs from cutting to length have been ground away for a good fit.  The two 10" pieces in each half of the handle must be parallel to each other.

Weld the edges of the pieces at their ends with little more than a tack weld.  These welds are visible in the photo from step 4.  Welding a bead where the pieces lay next to each other is not necessary and will only complicate matters later. 

UPDATE--The square worked out quite well, but had a slight inaccuracy when comparing lines scribed after flipping the handle over to the other direction. Upon close examination I discovered the square steel tubes used to make the handle were not uniform, but actually had a little bit of a diamond shape to them in places. I broke the welds holding the blade to the handle. Then I used a sanding drum on my radial arm saw to grind the front and back faces of the handle so they were flat and true over their entire length. See this Instructable for how I can do the same process with a piece of wood to be glued. Then I welded the handle back onto the blade. But, instead of working with the handle in two halves as shown here, I worked with both halves joined together in one piece. Now the square is quite precise. (Aug. 20, 2011). 

In the photo you can see one of the handle halves connected to the blade of the square with two small welds.  This proved to be what you should not do.  Hot weld material always cools, and when it does, it contracts.  That means pieces move, and the parts of the square are no longer square.  While both welds were made at the same time, one had apparently cooled a little and when the second one cooled, forces pulled the square away from a 90 degree corner.  I had to cut these welds apart with a cutting disc on a Dremel tool and start over.   

You can align the blade to the handle so they make a perfect right angle by any method you choose.  Use a square you know to be true.  Use the 3-4-5 method.  Once aligned, I clamped the two together to keep them in alignment as you see in the photo with the previous step.  Then I made light welds on both ends of the 3/4" inch piece that is between the two longer parts of the handle half.  This means forces present from hot metal contracting are acting across the width of the square's blade, not in a place where they can cause the blade to twist out of a 90 degree corner.  Still, there may yet be the need for some adjustment later.  (In this photo you can see the light welds on the edges of the ends of the square tubing, too.)

The two sides of the handle must be aligned and parallel to one another.  I rested the square's handle on a flat surface so the blade was vertical.  I brought the second half of the handle against the blade of the square and clamped the ends.  Notice a shim mentioned earlier in step 1 has been placed between the two parts of the handle at the end opposite the blade.  This method of aligning the halves of the handle worked quite well.  Weld the second half of the handle to the blade as in step 4.  Weld the halves of the handle and the shim together by welding on their end edges.

At this point your project looks like a square, but you cannot assume the handle and blade are precisely 90 degrees from one another.  I am checking by scribing a line on a piece of paper taped down so it cannot move.  The handle of the square is resting against the edge of a piece of machine cut plywood I use as a reliable straightedge. 

A geometrical theorem states that two lines perpendicular to the same line are parallel.  Flip the handle of the square to the right and scribe a second line with a very small amount of space between it and the first line. 

The lines I scribed are much longer than what is shown here.  But, even here you can see the lines are parallel and do not converge on either end.  I used this method of checking a square in another Instructable.

I wish I could say my square was perfect when I checked it.  But, it was not.  I had to cut into the welds shown in step 4 at the top of the handle to loosen them on both sides of the square.  The welds at the other end of the 3/4" lengths of 1/2" x 1/2" tubing were still intact and I was able to tap the square's blade enough to make the square accurate without breaking these welds.  Then I made the welds I had cut apart again and checked the square for accuracy once more.  I do not know if heat had caused the blade to twist out of position with respect to the handle, or if my alignment was not as good as I had thought.  But, eventually I got it right and the square is very accurate now.   

I could scribe the blade with markings to denote graduations of 1/8", and I could drill a hole in the blade for hanging the square, but those things do not seem very important right now.