Whilst trying to sand a wooden block square recently, continually chasing around each corner with an Engineer's square and sanding it back, I kept finding that the last of the 4 angles was not square :confused:
Puzzled, I started to go through all the options.
1. I was going mad
2. An uninvited black hole had entered our Universe and distorted Space Time such that Euclidean rules no longer applied
3. The square, was not.
As you can see in the attached photo it was option 3 - checked against several known good squares. Made a nice sound as it hit the bin. Lesson from this is be careful buying cheap tools from market stalls . . .
ok...so here's a similar dilemma... 3 squares, 2 x 4" one 6", all new, all marked to DIN whatever... none agree with the other 2 over the full length of the blade... all seem to agree with a 321 block - how to determine which is the 'rightest' of the three?
I guess there are several areas of a square which need to be accurate. The internal angle, the external angle, the straightness of the inner edge on the blade and the inner edge on the stock and the straightness of the outer edge on the blade and the outer edge of the stock.
When you compare a square with the 321 block you are using the internal angle, which sounds like it might be 90deg on all the squares in the 'corner'. When you compare one square to another you must be using the outer angle on one vs the inner on the other. If they are different then the outer edge is not parallel to the inner edge.
All this suggests that you can't rely on any of them for the outer angle, only perhaps the inner. Were they expensive?
BS939 covers the specification for squares, but at £141 :surprised: I don't own a copy ! But I do know that the square marked 'conforms to BS939' has given good service over the years and seems to be OK.
BS939 may well cover the specification for a square. But this is where British Standards fall down, surely a square is square or its not, it cannot be within a given limit. To put it in context, you cant be Nearly A VIRGIN !
Surely a square must be within a certain limit, albeit a fairly tight one. Otherwise you are suggesting that every atom is perfectly in line, and that they are all perfectly at 90.00000 recurring degrees relative to the other part. Not only would that be difficult to build, it would be extremely expensive. As soon as you picked it up you would have to reject it due to the heat from your hand distorting the frame if that was your pass or fail criteria.
We need to use limits all the time in Engineering - think of all those tolerances we put on dwgs. They aren't to make it perfectly square, they are to make it square enough for the intended application.
A quick way to check whether a square is "square" is take a piece of material with a straight side, put the square against it, and draw a line, then turn the square over and draw on top of the original line - both lines should line up with each other, if you see what I mean.
No square (or indeed any other measuring instrument) can ever be "perfect": temperature, the material its manufactured from, how the item is treated (in manufacturing, transport/shipment, and also treatment by end user) all affect accuracy (a square can only be accurate to a given tolerance over its length).
Cost is obviously a factor (items mass manufactured in the far east to keep cost down are obviously never going to be as accurate as more expensive items where more care is taken in manufacturing / handling).
Personally, when I buy new measuring equipment, I try it on a known standard, and if possible I'll try to compare it to other measuring equipment which I have.
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