Determining Square for Machine Setup and Q.C.

Here's advice on shopping for reliable squares, checking for square on machines or cut parts, and setting up machinery to be true and square. April 6, 2011

I have a problem. My new Starrett 12" combo square is different than my CNC'd aluminum framing contractor square (cheapo) and my architectural plastic triangle (good quality). Every one of these is off more than a 32nd at one foot and/or to each other. What can I trust? Or is this standard, and should I just suck it up? I do my best to keep cabinets and face frames perfect, but with the results I'm seeing from this, it's got me crazy. I generally only use my Starrett.

Forum Responses
(Cabinetmaking Forum)
From contributor P:
I like the digital squares.

From contributor T:
I have a Starrett 12" combo and it was out of whack from the day I got it. I was pretty mad about it and never used the thing. Then I decided I paid too much to just let the thing sit, so I contacted Starrett, told them the story, and they had me send it in and they calibrated it for free. Now it's good. Mostly what I use, though, is a 4" machinist's square. Stay away from the cheap Chinese junk, though. I had a set of those and took them back. They weren't even close.

From contributor M:
I am not sure how your shop is equipped, but you should not be relying on squares to make square assemblies. Use the squares to set up your machines (Starrett would be my choice) then use the accurate machines to create square joinery. I cut face frame parts on my panel saw. As long as the clamps are placed accurately and not over-tightened, the frames are as perfect as can be expected.

When we need a square for layout or odd assemblies, we use the panel saw to cut a square of the appropriate size. My Griggio panel saw is accurate to .2 mm over 8 feet. That is based on the 5 cut test method. Your chop saw can achieve good precision as well.

There is a way to check the trueness of a square. Use a straight reference edge on your table saw or jointer. Scribe a line (use the scribe awl), then flip the square and do it again. The two lines should be parallel.

Aluminum framing squares can be trued using a punch. Search it on google.

Drafting squares are for drafting, not assembly. They are thin, usually opaque, and hard to read in a 3D environment. Machinist squares are designed for this type of work. An accurately cut HDPE or phenolic assembly square is perfect for the workbench. I have used HDPE type cutting boards sold at Wal-Mart and cut them to a precise 90 degrees. This is a very cheap and durable option. They survive being dropped and glue will not stick to them.

From contributor B:
Equal corner to corner measurements are as basic and as accurate as it gets. Square a cabinet with this method, then check your squares against it. Trash the ones that are off.

From contributor D:
Just check your squares for 90 degree angles. This is done by placing a square against a straight board and using a sharp pencil to draw a line, then flipping the square over and drawing a line on top of the first one. These two lines should be one on top of the other. Any divergence means the square is not accurate and should not be used.

From contributor I:
I've done well with an inexpensive set of machinist's squares (made in India, I think) from MSC. Combination squares are worthless as squares.

From contributor V:
I haven't squared an assembly with a square for 35 years. I use various small squares to set machines, fences, etc. but never for assembly work. I learned to measure the diagonals and compare. Far more accurate.

I was given a few corner sticks by the old guy that taught me, and these work fine. Two pieces of 3/4 x 3/4 stock, with a sliding dovetail along their length - one male, one female - and a nail sticking out 90 degrees at each end. The fit on the dovetail is snug enough to move, but only if you intend to move it - not loose. The sticks can extend in length and the nails are used to determine one diagonal, then checked against the other diagonal. Especially handy for repetitive work, where a tape will work for one time assemblies. Of course, these are called radii sticks in our shop since one end is also fitted with a sprung hole to hold a pencil for laying out curved work. My collection of framing squares is up on the wall, preventing some dust from hitting the floor.

From the original questioner:
Thanks for all you responses. I would truly rely on my tools if I could define which square to use to set up my machines in the first place. Great tips here - thanks.

From contributor M:
When setting up machines, don't rely on squares too much. In the end it is the machined part that matters. The 5 cut test for squareness is better than any precision square on a panel saw. And the "two miters equal 180 degree test" is the most accurate test for a miter saw or setting arbor tilt. Setting a jointer with tools will get you close, but the real test is jointing perfect, no snipe, no bow boards. Knowing how to read the ripped edge of a board will tell you more about your table saw fence than any dial gauge and straight edge.

There are things that do require precision tooling, but it usually has to do with the mechanisms inside the machine. Edgebanders need to be set with precise measuring tools, for example.

From contributor S:
I highly recommend the measuring and layout tools from Woodpeckers Inc. You need to have a baseline that you can trust - these folks offer lots of tools that you can depend on. I'm not a shill for the company, just a satisfied customer.

From contributor G:
Just wanted to emphasize the diagonal (corner to corner) method of obtaining perfect squareness on any assemble step of the construction project. As far as I've ever known this is how it's done.

You're just going to drive yourself crazy first trying to find a square that is square and then when you do, you will have trouble keeping your square square square (stepped on, dropped, left on the jobsite, not there when you go back, etc.).

One last trick for, say, squaring up your peninsula cabinets with the sink run is the 3/4/5 method, i.e. one leg is 3 (or 30" or 33" or 60"), the second leg is 4 (or 40" or 44" or 80") then the diagonal measurement is 5 (or 50" or 55" or 100"). Once again, perfect square.

Don't forget to factor in that the world is round. Just kidding.