Replies

1. 
   ring | Sep 15, 2009 07:26am | #1

   The tolerance (between diagonals) you are looking for depends on different factors. Obviously, the bigger the object the more tolerance you can allow. Checking a large cabinet for square, I'm not bothered by a few millimeters difference, but that same difference would indicate a real problem if it was on the drawer of a jewelry box. Checking diagonals is just a convenient measuring tool, like any other. How accurately you want to use the tool depends on what you need in any given case.

   David Ring

   http://www.touchwood.co.il/?lang=e&id=1

2. 
   mrbird90 | Sep 15, 2009 03:33pm | #2

   I just made a bunch of drawers that were 33" wide and 22" deep, my criterior for cross corner measurement was 1/32 or less and in almost all cases the drawers were right on. They were dovetailed both front and back using a Porter Cable jig.

3. 
   oldusty | Sep 15, 2009 05:14pm | #3

      Mac,

           The two diagonals should be the same when square . When one side is longer by an 1/8" you are out of square by that much .I squeeze the drawer box or door for that matter diagonally on the long line to push it back to equal for a close to square item.The diagonal measurements are a good quick indication of square .

          Caution , just because the diagonals are equal will not ensure you don't have a parallelogram , these also have equal diagonals.

          regards  dusty

1. 
   DonaldCBrown | Sep 15, 2009 06:46pm | #5

   "Caution , just because the diagonals are equal will not ensure you don't have a parallelogram , these also have equal diagonals."Nope. See attached. The basis of the equal-diagonals rule is to insure that you have a rectangle and not a parallelogram. A rectangle with equal sides is a rhombus.

   
1. 
   BenM | Sep 15, 2009 07:20pm | #6

   I'm sure you know he meant "trapezoid".  That can have equal diagonals and be way off square.  Not that too many of us build anything trapezoidal but it would be a mistake to measure diagonals to see if table legs were square to the apron.

1. 
   oldusty | Sep 15, 2009 08:11pm | #8

   Thanks Ben ,

2. 
   DonaldCBrown | Sep 15, 2009 11:59pm | #9

   "I'm sure you know he meant "trapezoid"."No, I didn't know that. But I don't believe that, in general, a trapezoid (four-sided figure with two parallel sides) has equal diagonals. Isn't that true only for an isosceles (equal base angles) trapezoid?

   
1. 
   BenM | Sep 16, 2009 07:33pm | #13

   I stand corrected.  Thank you.

2. 
   oldusty | Sep 16, 2009 02:03am | #10

    Hi Donald ,

               The message in my post was , it may not be square even if the diagonals are equal . I knew as soon as I wrote it , that it may have been the wrong word    (parallelogram), but the message was correct.

      If some spent as much time helping the OPs as splitting hairs all would be good.

       Please let me know if you find fault in my theory, I am open to any new math.

1. 
   DonaldCBrown | Sep 16, 2009 02:38am | #12

   oldusty-I'm sorry I came across as nitpicking. I was trying to clarify the issue by pointing out the assumption behind the "equal diagonals" test: That is, IF we have cut the pieces to form a square (or any rectangle), then unequal diagonals will indicate that the figure is not square (and is a parallelogram).You are absolutely correct that equality of diagonals, by itself, is not an adequate test of squareness.I promise a better effort to comb out any split hairs from future posts.Respectfully,Don

2. 
   RalphBarker | Sep 15, 2009 07:50pm | #7

   "Caution , just because the diagonals are equal will not ensure you don't have a parallelogram , these also have equal diagonals."All squares are parallelograms, but not all parallelograms are squares. ;-)If I trap a zoid, will I have to feed it forever?

   Edited 9/15/2009 12:53 pm by RalphBarker

4. 
   Tony Z | Sep 15, 2009 05:55pm | #4

   To attain square, you got to begin with spot-on stock preparation.  For example, for drawers:  make sure all pieces are the exact width and length.  When cutting to length, make sure each end is shot square on both axis.  When assembling, make sure your bench top is flat.

   Get back to your basics and take your time.  Measuring diagonals is a test of methodology.

5. 
   hammer1 | Sep 16, 2009 02:26am | #11

   The important part of measuring diagonals is to make sure the opposing sides are of equal length before checking the diagonals. On buildings we use Pythagorean's theory. With this you know what the sides are and the diagonals. Pretty handy when staking out for a foundation. I prefer to use a square for smaller projects. If you aren't careful where your tape is put in the corners and where you measure to, you can be off. A little variation on a diagonal can mean more variation in the overall square of what you are measuring. The diagonal only tells you half the story, multiply the variation by 2.

   Beat it to fit / Paint it to match

6. 
   mike4244 | Sep 16, 2009 11:14pm | #14

   You're question has started a feud over what an out of square drawer is called,paralellogram,rhombus,trapazoid,and maybe a few more.

   To answer your question on what is the acceptable allowance, .00001 is my personal tolerance. Any more than this goes in the woodstove, then I start over.

   This is my excuse for not having any drawers.

   drawerless mike

1. 
   bones | Sep 16, 2009 11:25pm | #15

   I was scratching my head at your response till i got to the end.  Very good!I was married by a judge - I should have asked for a jury.George Burns

7. 
   dmdnk | Sep 18, 2009 08:49am | #16

   Squarig a box by measuring diagonals takes both understanding of the process and practice.  It's extremely accurate if you know how it works and if you know how to make the proper adjustments to bring the box into square.

   Once the individual parts are made properly, assembled, and the box is lying on its back (or face) the most important factor is that the top edges of opposing sides of the box must be parallel to each other.  If the box has any twist to it, squareness can't be determined by measuring diagonals. 

   If it is a small box, lying it on a flat surface will make the top edges parallel.  For a large box- like a cabinet- you will have to align the top edges.  This can be done by sighting across one edge to the opposing edge to see if they line up.  If they don't, place a shim under the low corner of the box to bring the edges into alignment. 

   After you have the top edges aligned in both directions (BTW the box doesn't have to be level for this to work- just properly aligned!) you can measure diagonals.

   If the measurements don't match, go to the corner with the longer dimension and shift it to shorten it's length.  If one diagonal is 1/8" longer than the other, you will need to shorten the long corner by 1/16".  This is because as you are shortening one diagonal you are lengthening the other an equal amount.

   After shifting the box, check that the edges are still parallel- you may have shifted your shims.  When the diagonals are equal the box is square!

   This works just as well with frames and doors.  As long as there is no twist (assuming the individual parts were properly made) measuring diagonals is very accurate and quick.

   By the way- Not a stupid question.

   David

    

   
   

   ---

   Edited 9/18/2009 2:10 am ET by dmdnk

1. 
   CStanford | Sep 18, 2009 06:35pm | #17

   Yup, all bets are off if the piece has twist.

1. 
   oldusty | Sep 18, 2009 07:50pm | #18

   Well we beat this subject up pretty good , imo this was a very useful thread for folks learning for the first time as well as the experienced .

          thanks to all             dusty