Updated 17/9/09

Serif - WEBPLUS X2

I was using my combination square, similar to the one shown below to set my fence on the joiner - I thought it was square, but the resulting joints were not at 90°, but 89.xx° or 90.xx°, I did not measure them.

So I got another square out of the cupboard and used that, with perfect results. A little later in the day, I wanted to find out why the combination square was no longer square, after a quick examination, I could see no reason why it was not accurate. This prompted me to look more closely.
I noticed that the edges of the bladed had microscopic nicks, some very fine burs and an accumulation of dirt and other non-descript substances on the edges. I carefully, with some 1000g wet and dry, bonded to some plate glass (perfectly flat surface), polished the edges of the blade to remove the debris and burs. this did help a lot but still did not give me that perfect 90° I always had before.

I then looked at the cast part of the square and noted that where the blade registers with the cast part (both before and after the knurled knob), that there was also an accumulation of debris - now I am not talking about 1/128", but more like 0.001" if not less. I carefully scraped this off with a wooden cocktail stick and some acetone, to reveal the shiny metal once more.

Combination Square Tune-Up