square by measuring diagonals - how accurate is accurate?

[Jesse Tutterrow]

A friend was helping me build a set of shop cabinets measuring 32" high (without the toe kick) by 30 inches wide. We started having a discussion about measuring the diagonals to check square. So, I thought I would ask your opinion.

Here is what we were discussing:

1. There will be a slight variation in measurement based upon what part of the end of the measuring tape is on the corner.

2. How close do the two measurements need to be in order to call the cabinet square? Within a 1/32nd? Or???

Thanks in Advance,

Jesse Tutterrow

[jared herbert]

There is always some spring back and movement after you take the clamps off, etc so it is hard to get them exact. I usually get them as close as I can in a minute or two, usually 1/16th and they seem to work just fine. You can straighten them up a little when you put the back on too, if it is cut square. Jared

[David Helm]

The measurements will be a accurate as your measuring tool is. Measuring diagonals is an old time honored method for squaring. If you are doing cabinets I would get them as close to zero variance as you can. If I were doing a foundation I would be happy with 1/16.

[Terry Beadle]

The square tolerance is directly related to the relationship of the cabinet and its surrounds. If it is to slide into a very square under counter situation, then you need to allow a 1/16th clearance on each side or use a hand plane to fit. The squareness of a free standing box or cabinet is not critical except to the eye. Humans can tell square very accurately when their attention is brought to the issue but usually if it's with in a 1/8th inch of square, that's not going to draw their attention. A picture frame is another matter.

If you are getting a 30 inch or larger cabinet within 1/32nd, you are gold IMO.

Dead square is a goal in a non-square world.

[pat warner]

Maybe for ball park impressions but for anything close to accuracy: A measurement with little value.
There are other and more accurate ways of determining squareness inside and out, not a vague opposite corner to corner guess with a rule that can't get into the corner itself! Don't count on it.

[Kyle Iwamoto]

1/16" for shop cabinets? You guys are good! I would not even measure....

[Jon Bonham]

Maybe for ball park impressions but for anything close to accuracy: A measurement with little value.
There are other and more accurate ways of determining squareness inside and out, not a vague opposite corner to corner guess with a rule that can't get into the corner itself! Don't count on it.

You'd be surprised how accurate it is, which is the reason it's been used for hundreds for years. The bigger the item you're measuring, the easier it is to use. Even minute variances will make for huge measurements differences on big items. For something small, it's probably easier to use a square.

[Mike Henderson]

Maybe for ball park impressions but for anything close to accuracy: A measurement with little value.
There are other and more accurate ways of determining squareness inside and out, not a vague opposite corner to corner guess with a rule that can't get into the corner itself! Don't count on it.

Uh, what would you do that's more accurate than a corner-to-corner measurement? I'm assuming an accurate and well fitting measurement device to measure the distances so my question relates to the technique itself and not some improved measuring device.

Mike

[Peter Quinn]

I'm with Kyle on this one. How accurate for shop cabs? Who cares, its not the opening for the door on the space shuttle. I'm making a set of shop cabs right now and come to think of it I didn't even check for square so much as one. I may have put a framing square in one corner to check for gross errors, thats it.

A nice way to check for square is with a shop made pinch stick. It lets you compare inside measurements in a real accurate fashion. Its making real accurate adjustments with clamps and plywood that has been glued and screwed that gets tricky. So the measurement may be dead accurate, but will the cabinets? I'm getting real good at scribing inset drawer fronts in less than perfect openings. That's really where the sum of all your errors comes back to bite you in the butt. Drawers of all kinds and inset doors like to operate in square openings. For over lay doors and applied drawer fronts. again, who cares!

[Will Overton]

I agree with Pat to a degree.The principal is dead on accurate, but the ability to measure exactly in the corner alludes some of our measuring devices.

In reality, if you do the math, you only need to measure one diagonal. Pythagoras proved that in a right triangle, A squared + B squared = C squared, where A and B are the sides and C is the hypotenuse (diagonal).

If you know your sides, top and bottom of your cabinet were cut correctly, the math has to work. If a cabinet is 3 feet wide and 4 feet high, the diagonals will both be 5 ft.

3² + 4² = 25

The square root of 25 = 5, so if your cabinet is square, both diagonals must equal 5 feet. If one of them is correct, the other one has to be correct. If one is wrong, so is the other.

[Lee Koepke]

I agree with Pat to a degree.The principal is dead on accurate, but the ability to measure exactly in the corner alludes some of our measuring devices.

In reality, if you do the math, you only need to measure one diagonal. Pythagoras proved that in a right triangle, A squared + B squared = C squared, where A and B are the sides and C is the hypotenuse (diagonal).

If you know your sides, top and bottom of your cabinet were cut correctly, the math has to work. If a cabinet is 3 feet wide and 4 feet high, the diagonals will both be 5 ft.

3² + 4² = 25

The square root of 25 = 5, so if your cabinet is square, both diagonals must equal 5 feet. If one of them is correct, the other one has to be correct. If one is wrong, so is the other.

The 3 -4 - 5 rule is how an old carpenter taught me as a teenager to verify that the batter boards / layout for our building foundations were square.

You can use any multiple of 3-4-5 to verify a square corner.

[Bill May]

measuring corner to corner is not accuratte at all if your peices are cutting correctly its just means your pieces are paralelle

[Gary Hodgin]

I don't know how close is close enough until it's too late. I try to get the stuff as square as I can. I'm always uneasy about using a tape measure especially on deeper drawers or cabinets. I now use a veritas bar gauge for most measurements. Recently, I ordered a couple of these "square checks" from Rockler, but haven't used them yet.

http://www.leevalley.com/US/wood/pag...85&cat=1,43513

http://www.rockler.com/product.cfm?page=18032

[Dave Lehnert]

measuring corner to corner is not accuratte at all if your peices are cutting correctly its just means your pieces are paralelle

What he said.
When I started woodworking I would struggle with this all the time. Your case would look out of square (because it was) but your diagonal measurements were the same. I had a hell of a time trying to get anyone to understand what I was trying to say.

[Cody Colston]

measuring corner to corner is not accuratte at all if your peices are cutting correctly its just means your pieces are paralelle

One has to assume a certain level of competency here...like the ability to cut pieces to the exact same length. It a person can't do that, the argument of how to check for square becomes moot.

Measuring diagonals is the accepted method...and has been for a long, long time. The acceptable tolerance will depend on the size of the cabinet.

My .02 - Just like everything in woodworking, strive for perfection and you may one day achieve excellence.