I have a similar problem, out of square machinist and wood handled tri-square. I have the following squares I use:
1 – Standard metal framing square, not particularly square, but works fine for framing and laying out stairs. I don’t expect great accuracy, it doesn’t deliver great accuracy, we’re both happy.
2 – plastic speed squares, you know, the bright orange or yellow triangles with a hook along one edge. Not particularly square, but not too bad. I use these things all the time for construction marking. Anything structural, framing, decking, things where a little slop is acceptable. These things are fast, convenient, cheap, and plenty close enough.
3 – Starrett hardened combination square. This thing is dead on. I checked it on the surface plate at work against multiple references. It was pretty pricey but I’d recommend it hands down. It operates smooth, very accurate, and you can do a lot with it. If I could only have one square in my entire shop it would be this one.
4 – el-cheapo combination square from HD. It turns out this thing is pretty accurate. I guess it doesn’t cost much more to mill it square than it does to mill it out of square. The faces are not hardened, the quality is not there, it sticks when it moves. Not a particularly quality tool but it is pretty darn square. For a beat around square I’m pretty happy with it.
5 – A set of Brown and Sharpe machinist squares. These are my standards. They are dead nuts on. I got a set of 4 and they are all less than a mil out in any dimension. Kinda pricey. I don’t typically use them except to check or verify other tools. In general I wouldn’t recommend having a set. I don’t really get much utility from them other than the comfort of using them as references for other tools.
And now my problem children – I wanted a beat around machinist square for tool setup, and at the same time I thought I’d look like a cool woodworker if I had a rosewood handled brass pinned tri-square. So I picked up one of each from a local woodworking specialty store. I admit both were cheap and I ended up getting what I paid for.
6 – 6 inch machinist square. It’s out of square about a 64th, around 15 mils, at the end. It is more out of square than my jointer. I can mill a board square (checked with a reference square) and this tool will say the board is out of square. So I’m hating this thing. I think I’m going to try to square it myself by lapping edge of the blade. If it doesn’t work, oh well, I’m not out that much money and I haven’t ruined a tool I care about. If it does work I’ll have a tool I can be happy with.
7 – 8 inch wood handled tri-square. This thing is so far out of square it’s a joke. It is close to a 32nd or more out at 6 inches. It kinda looks cool and I drag it out and set it on the bench when I’m in the shop but the reality is this thing is junk as a square. I don’t know if it’s possible to adjust these things and have them remain adjusted. The blade is pinned to the handle with brass rivets. It’s not clear to me that it’s possible to adjust it without ruining the handle to blade joint. I’ve considered lapping the blade to true it but it’s not worth my time.
So my conclusions and recommends:
1 – You get what you pay for.
2 – if you can only have one square, save you pennies and buy a Starrett or Brown and Sharpe combination square, you’ll never be sorry.
3 – if you cant afford a high end square try a cheapo from your local big box hardware store. You might be surprised, I was.
4 – if you bought a square that is guaranteed to some accuracy and it doesn’t meet that accuracy, take it back. I figured it would be a crap shoot with any low end tool so I’m inclined to try to clean it up myself (probably a waste of my time)
5 – If you like the looks of the brass and rosewood handled tri-squares, then by all means buy one. But if you want it to be actually square take a reference with you and confirm it before you buy it.
I know the old saying, it’s wood working, not machining. But the reality is if you want your joints to come together clean with the least muss and fuss then having your tools setup properly and starting with boards and joints that are square and true will only help. A 32nd out in 6 inches is an 8th out in two feet, and a quarter out in 4 feet. Try to get those joints to line up.
