square by measuring diagonals - how accurate is accurate?

[Kyle Iwamoto]

And of course the fun begins when you install known square cabinets and the walls you install them on are not square!

Oh now lets throw some gas on this already heated debate....

This whole thread is pretty entertaining.....

[Thomas Bennett]

Quote:
Originally Posted by Bill May


Assume ad=bc=3 and ad=bc=4

The only way for ac and bd to be equal would be if they were 5.
If that is the case then we have 3,4,5 triangles which means that adc and abc are 90 degrees


I'm confused! First ad=bc=3 and then change it to 4?
Do you mean ad=bc=3 and ab=dc=4??

[Ken Fitzgerald]

Oh now lets throw some gas on this already heated debate....

This whole thread is pretty entertaining.....

That's why they invented caulk........or use of a compass.

[Jon Bonham]

And of course the fun begins when you install known square cabinets and the walls you install them on are not square!

There's the key. I challenge anyone to find a square wall in my house.

[Tony Bilello]

....cabinets measuring 32" high by 30 inches wide. ......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. This is correct. You must use the same measuring technique on both diagonals. Tony B
2. How close do the two measurements need to be in order to call the cabinet square? Within a 1/32nd? Or??? There is no reason to not get it right on the money. Tony B
Jesse Tutterrow

There are various techniques for squaring up. I just measure diagonally using a tape measure (on the outside). Then put a pipe clamp across the longer of the two measurements and close in on the measurements until they are the same. If for instance, one measurement is 40 1/4" and the other is 40", I know I have to split the difference and crank the longer diagonal to 40 1/8". Then re-measure again until both are exactly the same. This should take well under 60 seconds to get them both exact. For lower units, I recess the backs about 3/8" with a dado prior to assembly. Then take a 3/8" piece of plywood and square it to the exact dimensions of the inside of the recess and insert it in there - glue and nail or staple. If you cut a hair short, just keep it tight in one corner so everything is square. After glue dries, remove clamps. This is a very simple process.
For those of you that dont believe that exact diagonal corners makes an exact square, just do it by eye and dont worry. Nobody will ever notice the difference - maybe.

[Don Dorn]

I didn't take it as that heated and I'm learning allot about this.

[robert micley]

i have made a lot of cabinets. very few are perfectly square. but i have put a square in many top end cabinets in the face frames or inside with the square on the bottom or top . once again very few are dead on. it is almost impossible because to cut exact lenghts of plywood or rails and stile they would have to have zero warp- in another words they need to be dead flat. but you can get very,very close.

[John Mark Lane]

I just read thru this and my head is going to explode. I've been using corner to corner for more years than most of you have been alive. But one thing I learned from being a boat carpenter is, if you don't have something square to begin with, it doesn't much matter whether you can build something square to fit within it. The real skill is in making something that is not square, but fits.

Carry on.

[Dave Lehnert]

If I get some free time this week, I will get in the shop and make some cuts to show what I am trying to say.

Threads like this is good for the brain.

[Keith Outten]

Measuring diagonals is a perfectly acceptable means of inspecting your cabinet if it has 90 degree corners, which means it is a rectangle or square. The accuracy of your inspection is the result of the accuracy of the inspection tool and the technique you use.

How square your cabinet is involves the application of a tolerance. Ask any professional machinist.
.

[Sam Babbage]

Measuring diagonals is the best method of determining the squareness of an assembly. Clamping pressure can bow components making them read 'out of square' if tested with a square but the diagonals will give a true reading. Also, diagonals can be used to check 'non-square' assemblies, like a chair that tapers from the front to the back. In this case it is important that the angles are even, for fitting the seat, equal diagonals will ensure proper alignment.

To the people that use a clamp over the longer diagonal, of course that method works, but it is easier to just adjust the clamps that are already on the job. You simply make the clamps 'more parallel' with the longest diagonal.

As for the accuracy, it really is a matter of horses for courses. If I'm assembling a dining table, then 1mm or 1.5mm difference in diagonals would be perfectly fine. If I'm assembling a small carcass that will have inset drawers or doors then 0.5mm difference might be too much.

[Norman Hitt]

If I get some free time this week, I will get in the shop and make some cuts to show what I am trying to say.

Threads like this is good for the brain.

I understand what you are trying to say, HOWEVER, if the ends of the pieces you are trying to glue up are not SQUARE to the sides of those pieces, AND the lengths of the opposing sides are not exactly the same, they will never glue up square unless you leave gaps in the joint, in which case ANY method of measuring would be moot. Making all the pieces square just takes practice and attention to detail, no different than all the practice it takes to make a properly fitting dovetail. One poster said he couldn't get square ends even using his sled, but that only means that the sled needs tweaking of either the fence so it is 90* to the path of the cut OR the slop needs to be removed from the guide bar that fits in the slot in the saw table.

Please understand that I am not trying to be nasty, but just trying to point out WHERE the problem is, because diagonal measurement, (done properly) is the most accurate and easy method to determine squareness of a rectangle or a square and is the reason it has been used for centuries, think Pyramids.

[Marty Paulus]

Measuring diagonals is the best method of determining the squareness of an assembly. Clamping pressure can bow components making them read 'out of square' if tested with a square but the diagonals will give a true reading. Also, diagonals can be used to check 'non-square' assemblies, like a chair that tapers from the front to the back. In this case it is important that the angles are even, for fitting the seat, equal diagonals will ensure proper alignment.

To the people that use a clamp over the longer diagonal, of course that method works, but it is easier to just adjust the clamps that are already on the job. You simply make the clamps 'more parallel' with the longest diagonal.

As for the accuracy, it really is a matter of horses for courses. If I'm assembling a dining table, then 1mm or 1.5mm difference in diagonals would be perfectly fine. If I'm assembling a small carcass that will have inset drawers or doors then 0.5mm difference might be too much.

Oh boy Sam. Now you have thrown in the metric system! This is going to get even better now! Have to save this one for beers later.

[Lee Schierer]

A 1/32" is .03125". Assuming your opposites sides are exactly equal in length, if your 30 x 32 sides are off 1/2 degree, then your diagonal difference would be .382 or more than 3/8 of an inch. If you diagonals are different by 1/8 your sides are out of square .2 degrees.

I think getting within 1/16" should be close enough for most work.

[Steve Milito]

Quote:
Originally Posted by Bill May


Assume ad=bc=3 and ad=bc=4

The only way for ac and bd to be equal would be if they were 5.
If that is the case then we have 3,4,5 triangles which means that adc and abc are 90 degrees


I'm confused! First ad=bc=3 and then change it to 4?
Do you mean ad=bc=3 and ab=dc=4??

Yes, sorry for the typo,