Thread: checking a square

Hey guys,

I was in a group environment (shared table saw at a workshop on box making, which my wife actually suggested we could do together, oh boy imagine the toys I can buy if SWMBO gets heavily into woodworking).

I am sure most would be aware in this situation it's best to check everything is set right before you start cutting (in case someone before you has changes something, intentionally or accidentally) .I was helping someone (buddy system, who was fairly new to woodworking), and I was explained how what can happen is a new person to the table saw can mistake the height adjustment hand wheel with the angle adjustment hand wheel, they soon realise their mistake, but this results in the angle of the blade no longer being at 90 degrees.

I grabbed my trusty engineers square out, raised the blade (unplugged the machine of cause), and did a quick check, sure enough it was out (probably not enough to effect the project, but I thought it was good to help instil good habits). As I started adjusting the angle of the blade, one guy came over and said "It's at 90 degrees, I just adjusted it".

At this point it was engineer squares at 20 paces

Years ago, I was shown a quick way to check my square with the tools available in the workshop (obviously not to tight tolerances, but enough to feel confident with your woodworking project). I was surprised by how few in the workshop, even very competent woodworkers actually knew how to check that their square was actually square.

I have received a lot of help over the years from forum members, so I thought I would make a little post, in the hope that I may help someone who perhaps is struggling with getting tight joints (when it's the square that is letting them down).

So below is what I have been taught, the aim with this approach is to use what is already in your workshop to get a fair idea on if your square can be trusted for woodworking purposes. This is just how I was taught, and so far it has served me well. If you have a better/easier/more accurate way to check, please do add your approach to this thread.

I have a engineer square that I picked up at the TTTG sale that I know is a bit out, so I figured this would make a good example piece.

The first thing you need is something that you know is straight. For the precision I am after this metal rule should work, but I will do a quick check (by eyeballing it I could probably skip this step, but errors will be doubled).

On a piece of paper draw a line using your rule (you need a sharp pencil or pacer), and make a mark about half way along the rule.



Turn the rule 180 degrees, so the same face is facing the line but now on the top side of the line. Line the mark up to the same point on the rule (so say 150mm mark).



also line up the rule at both end.



If the line you just drew meets up with the rule all the way along, you have a straight (well straightish edge, I am not talking to within 1000's of an inch here). If not you don't have a straight edge.

Now you know your rule is straight, this effectively means you have 180 degrees to work off (a straight line). Next get your engineers square and put one side against the rule



Draw a line on the other face of the square, making sure the square and rule do not move. Now holding the rule in place flip the square over so the same face is against the rule, and the other face is on the other side of the line you just drew.



draw a second line



If these lines are not either one line or parallel to each other your square is not at 90 degrees, as 90 plus 90 equals 180. If you have got the mark right at the bottom and your square is out, you will have one line at the bottom, breaking into two lines at the top



The actual 90 will be from the bottom point to a point that is half way between the 2 lines at the top.

You can also use this same principle when using a carpenters square, try square or combination square. You can use a piece of timber that is fresh off the jointer, or a machines side of ply or mdf etc. to rest your square against.

Well hopefully their will be at least one person who reads this and didn't know and finds it useful, and perhaps someone might read it with some suggestions of a better way and I might learn something new.


Cheers,

Camo

Sorry about the quality of the pictures, but hopefully it helped explain things.