Thread: checking a square

How accurate (for square) is a sheet of paper?
I often use a sheet of A4 to test for square on printed paper plans and on-screen diagrams.
Maybe not extremely accurate, or accurate enough to check a purchased (manufactured) square for accuracy.
But the best thing I've found about a sheet of paper to test square (or linear measurement) is it's so thin compared to a square or rule, that it sits flat on what you're measuring, so no lateral (visual) errors.

So, you've discovered it's out of square. How do you get it back to square, besides buying another and discovering it also is out? I'm talking about carpenters type square here.
Could anyone please advise me on this, I know how to get a 600 X 900 square correct. If anyone needs to know, you use a ball pein hammer. If it's over square, tap the outside corner to stretch the metal, tap and check each time, if it's under square, tap the inside corner stretch the metal. Check each time so that you don't go over, taps may need to be a bit harder if it doesn't move.
Kryn

I the scribe a line in my bench method, flip it round and check it the other way (as above) To do this I used my Great Great Grandfathers Rosewood, Brass and Blue steel square, 170ish years old and still deadly accurate (about 600 x 300) so it gives a good reading over a bit of distance. All my others are checked against this line. If it's an adjustable CC it won't be out anyway, I have 3 and if you treat them with respect they stay square. If its a cheapy you could try and fix it but other wise throw it away and buy a good one. If your square is out your whole job will end up a bl__dy mess and waste your time and money.
If you go to buy a new one, check it in the shop/market etc because many of the brand new off the shelf ones are way out already.
Photo attached

IMG_0317.jpg

KB,

What do you mean by a carpenter's type square? The large ones that are just a fixed piece of right-angled metal?

If it was something super sentimental that you were determined to keep and reuse, you could carefully file the shorter side until it was right. That seems like a lot of work though...

If you're talking about a combination square then you can very, very carefully file the internal parts against which the square is pressed when it is captive until it is square.

Hope that helps (I'm not sure it did...)

Cheers,
Luke

There are two issues here.
Is the square really square?
Is the TS blade set to 90°?

So we go to the home center to buy a square. How do we KNOW that it is accurate?

Pick the square that you want. Pick another brand of square and verify the accuracy of your square. Do this with as many different brands as the home center carries. Use a key or knife to scrape off any finish that might interfere with the accuracy.

Is the table saw accurate?

Take a 10 cm by 7.5 cm piece of Baltic Birch plywood and rip it approximately in half. Be sure to use a lot of pressure to keep the timber against the fence. After the cut and the Baltic Birch plywood has been pushed all the way through the cutting area, remove power from the Table Saw. When the blade has stopped spinning take the off fall piece and flip it end for end. On the TS table slide the two pieces of Baltic Birch plywood together along the cut. It there a gap at either the top of the two pieces or at the bottom? The saw needs adjustment.

I say Baltic Birch because it almost always has parallel sides. That is the key to accuracy.

BTW - If you're off by 0.5 mm the error is 1.5° when using 19mm plywood.

Originally Posted by KBs PensNmore

So, you've discovered it's out of square. How do you get it back to square, besides buying another and discovering it also is out? I'm talking about carpenters type square here.
Could anyone please advise me on this, I know how to get a 600 X 900 square correct. If anyone needs to know, you use a ball pein hammer. If it's over square, tap the outside corner to stretch the metal, tap and check each time, if it's under square, tap the inside corner stretch the metal. Check each time so that you don't go over, taps may need to be a bit harder if it doesn't move.
Kryn

On smaller squares use the same principle but use a centre punch rather than a ball pien.

I use the reference up against a newish piece of ply of MDF - its actually been around for millennia and it's more than good enough for most small scale woodwork.

The following is for those if you that want a little more.

Assuming you have a super sharp pencil point, the limit to all this is the resolution of the human eye to discriminate between two clear adjacent lines. In theory this is ~0.1mm, but for those of us with average pencil points and/or vision issues its probably closer to 0.2mm or even more and it could be as much as 0.5mm
But lets stick with 0.2mm for the moment.
For a 150 mm square that's an uncertainty of ~1 part in 750 or 0.076º
For a 300 mm square its 1 part in 1500 or 0.038º
The bigger the square the more accurate this method is but for small squares it may struggle to see any differences.

This method can tell you how square the inside of the stock is against either side of the blade but it doesn't tell you how square the outside of the stock is relative to the blade. The back of the stock is assumed to be parallel to the inside but is it? That has to be check and the way to do that is with a vernier callipers.

A calliper in good nick should be able to measure this to 0.02 mm.
For a 50 mm wide stock this is 0.02 parts in 50, or 1 part in 2500 so the eye are still the limitation.

If the square has sharp corner edges the outside edges of the square can be checked with a vernier calliper and Pythagoras.
One problem with this method is that most wood workers are unlikely to have a large enough set of callipers to measure a square bigger than about a 75/100 mm square.
The other is that old squares tend to rounded edges

The limit is the shortest distance being measured, i.e. 0.02 in 75.00 mm
This is 1 part in 3750 so this method may be about 5 times more accurate than the eye method but only for the outside angle of the square.
The calliper can then be used to measure how parallel the blade and stock are for the other angles

I just checked out my 75/100 mm engineers square this way - the outside angle is 0.057 +/- 0.015º over 90º
(A tolerance of 2 parts in 3750 is ~0.015 degrees)

Either way its an alternative check and if you have the gear then its worth doing especially after you drop the square.

Originally Posted by justonething

Sure, why not.
By the way, the Pythagoras triangle becomes really useful if you have to build a right angle structure considerably bigger than an engineering square. but that's is not checking the trueness of a square.

You don't need Pythagoras to check a large structure. Just measure both diagonals. If they're equal, it's square.

Cheers,
Joe

Originally Posted by BobL

For a 150 mm square that's an uncertainty of ~1 part in 750 or 0.076º
For a 300 mm square its 1 part in 1500 or 0.038º

If you need your square to be that accurate, chances are you're not working with anything resembling wood...

I have to say that I am now curious to check my squares in this way, but I only have 300mm verniers so I'll just have to take Irwin and Starrett at their word for tolerance

Originally Posted by elanjacobs

If you need your square to be that accurate, chances are you're not working with anything resembling wood...

Exactly