Build a Really Good Tri-Square





Introduction: Build a Really Good Tri-Square

About: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying posting things I have learned and done since I got my first ...

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.

Step 1: Cut Materials

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.

Step 2: Fit Up the Sides of the Handle

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). 

Step 3: Welding the Blade to the Handle Half -- What Not to Do

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.   

Step 4: Weld the Blade to the Handle

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.)

Step 5: Fit the Other Side of the Handle

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.

Step 6: The Moment of Truth

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. 

Step 7: Flip the Square and Make a Second Line

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. 

Step 8: Check for Convergence

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. 



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    Another good thing about the design you used is that you could easily make custom angle jigs or "not squares".

    1 reply

    Yes, you could do that. One of the great things about Instructables is that your idea may trigger an idea in me for a slight adaptation that does something I need, but you never considered, etc. When I made this square I was eager to do a welding project. Since I have thought about ways to make a nice square (or angle guide) without welding for the sake of those who do not have access to welding equipment. Thank you for your comment.

    Nice job, getting a true square can be a real pain but it looks like you've done a brilliant job!

    3 replies

    It worked. "Brilliant" probably exceeds the bounds of reality. Thank you, anyway.

    The majority of the squares at my school don't line up, and now I can prove it with your method of testing.

    Your comment reminds me of one day when I was about age 13 and in a shop class. I had learned a little about electricity from working with my father, who had an electrical wiring business. We were using a wood lathe in class, but there was only one lathe. High up on a big shelf we could see an older lathe just sitting there. We got it down. Very quickly I saw that a neutral wire had been disconnected near the switch. I reconnected it and we began using this second lathe. The next time we met for class the older lathe was back up on the shelf. The teacher said we could not use it "because it had a short." For some reason, he just did not want us using that older lathe. Teachers sometimes do not want to be embarrassed by students who know too much. I hope you do not find any resistance to what you are able to show, but you may. Still, you can use the technique later in life to check squares you might build or buy. Thank you.

    Phil, as always you have done a good work!. By chance a week ago I also needed to build a squad for a little job of welding, but I did it heavy, with just two pieces of mild construction steel, 0.2 inch thick x 1 inch width. One arm is about 10 inches length and the other 12. It took several attempts to get an approximate 90 degrees angle. As you say, it is not easy keeping parts welded in a precise position, because the weld "pulls" when get cold.

    5 replies


    Thank you. After I try to weld something I am always amazed that factories can produce welded precision items, like earth movers. You may notice I tried to keep my welding to an absolute minimum on this project in order to avoid injecting a great deal of heat that would later distort the square. I also carefully planned where I would locate the welds so I could keep the stresses from altering the placement of the parts as much as possible. The welds needed for this square do not need a lot of strength. Even if they are weak and of a poor quality, they will suffice for what is required from them. I must say, though, I contemplated whether I was being foolish to attempt welded construction of a precision device. Thankfully, the end result was good.


    Surely you know about pounding on a square with a hammer to open or close the angle of the legs.  Even though your steel is fairly heavy (5 - 6 mm), you may be able to adjust your square for perfect accuracy.  Pound in the area of the red circle to close the angle, in the area of the green circle to open the angle.

    tweak square.JPG

    Thanks, Phil, I read your instructions here, months ago, they are very clever. But my square is not plain, its arms are welded one over the other, to provide an edge for back (or support, or lean, I don't know the English word, I choose it among a lot from Google Translator).  As a result, the thickness of the elbow is more than 10 mm. But its accuracy is enough for my works. I did for my son an angle iron support , 3D axes X, Y, Z, and happened that his wall was slightly out of square, mean that my precautions were excessive.


    If I understand you correctly, this image is like your square. I would call the joint a lap joint. The two pieces overlap one another. If you welded only where you see the two red areas, it would be similar to what I did and I think you would have less distortion from the heat of the welds, even if the strength of the welds is less because the beads are smaller. It is just my suggestion.

    tweak square.JPG

    Phil, I did different. first put a tack weld on the inside of the elbow, then make the correction of square, and then other on the outside. Anyway, that's it, and I have no plan to do another try-square at the moment. Should I will do it, I will consider your suggestion. Thanks, very much.