Reply To: Checking My Combo Sqaure
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Here’s a slightly cumbersome test for an Engineer’s Square – it’s easier with a big wooden square – but if you are careful it does work.
By Pythagoras’ theorem of 3-4-5 for a 90deg triangle, if we have 4″ of ruler sticking out of the Square, and if we measured 3″ along the casting and scribed a tiny line, the distance from that line to the tip of the rule must be 5″ EXACTLY.
So using a pair of sharp dividers, set them to exactly 3″ using the engraving of the rule to ‘catch’ the tips precisely. Then put the tip into the corner of the rule/casting and scribe a tiny line on the casting at 3″ along.
Then using the graticules on the rule again, set the dividers to 5″ EXACTLY, and see if distance from the tip of the rule you set to 4″ to the scratch is indeed 5″. If it’s not then it’s not square.
(Make it a little easier by having more then 4″ of rule sticking out- say 5″ – then again you can line up with the graticule at 1″ with the dividers, as opposed to trying to catch the end)
If the square is 1/32″ out over 10″ as in the OP’s then you will be able to spot the error.
Fiddly as I say, but it is surprising how carefully this can be done as the graticules on the rule make it easier to set dividers very accurately.
This does work a treat on nice big items like old big carpenter’s squares where you can scale up to amplify the error and also where even a good Engineer’s Square is too small to test squareness over a long distance.
- This reply was modified 7 years, 7 months ago by Richard Kelly.
- This reply was modified 7 years, 7 months ago by Richard Kelly.
- This reply was modified 7 years, 7 months ago by Richard Kelly.