Originally Posted by
george wilson
Do you guys know that you can see as little as .0001" (One TEN THOUSANDTH) of an inch of light, emerging from under a square held against a slightly out of square surface?
Many squares themselves are not made to those specifications. And,unless the surface you are checking isn't SMOOTHLY ground fine enough, light will STILL be seen under the square. LV and LN planes are certainly ground fine enough!!
It is good to have your own "Bureau of Standards" in your own shop. Mine consists of an ultimate granite straight edge 2' long,2 black granite squares(which sit upon a granite surface plate, and look like book ends),All steel squares made by Starrett and OLD Brown and Sharpe,from 2" to 24", and 3 hand flaked cast iron flat straight edges, Called "Camel Back" straight edges in the machinist trade, from 18" to 4'. And, four hand flaked cast iron surface plates, and a black granite surface plate, and a 4' precision ground steel straight edge. I left out a 6" cylindrical steel square. A number of Chris Vesper squares, too. The Vesper squares checked out o.k. against my 6" mint Starrett steel square. Chris has a cylindrical square that he uses. It sets upon a black granite surface plate,and is a very accurate type of square. Generally used in a laboratory setting.
For your information, a PINK granite surface plate is considered the ultimate, as it has a lot of quartz in it, making it harder to wear out. One, except for a small one, has not come my way. I'll never even come close to wearing out my black granite one !!!
If you want to be SURE of your findings,you need to acquire at least a few of these tools if possible. I'd say the most important things for you would be at least a few precision squares. Chris Vesper makes precision squares. I'd choose the ALL METAL versions rather than the wood inletted handle ones, as wood can swell a bit,or shrink(the swelling is what MIGHT slightly throw off the beam's squareness to the handle (Chris might argue that point! And, it is a nit picking one, as the wood isn't that thick, to be moving metal).
I have been lucky to get these precision tools for a fraction of what they retail for.The 24" squares retail about $2500.00, or so. Unfortunately the man who ran the used machinery business, where I acquired 99% o my things is going out of business tomorrow. Too bad. But I have too much stuff as it is!
Anyway, acquire a GOOD STEEL square or two for yourself. I don't mean aluminum ones either. Steel should be your choice, and NOT a cheap $15.00 import.The only thing I use one of those for is checking the squareness of the tool rest against the belt of my Square Wheel belt grinder. I could open a business of checking people's squares and straight edges for accuracy. But,I'm just too tired and HATE mailing things.
As a historical sideline,it is interesting that the ancient Egyptians used black granite to make their master measuring rule. Every year, all the carpenters had to come to the palace and have their measuring STICKS checked against the granite master. Even then, they had a bit of "Bureau of Standards", and for more things than the measuring sticks. I don't know what happened to the unlucky guy whose measuring stick wasn't accurate enough. I'm pretty sure the stick would be destroyed. I hope the owner wasn't beaten!
In the middle ages in Germany, a traveling Brew Master would arrive yearly to test the beer being made locally . He would pour a little beer on the chair he sat upon for a certain length of time. When he got up, if his pants didn't stick to the chair, the local brew master would be flogged!! I suppose they thought the beer needed to have sufficient "substance" to it to be nutritious. I know that seems like a strange way of doing things, but people had their own way of doing things back then. Medieval people were very AFRAID of clocks, for example. They thought that clocks MADE time, not just record it. They were afraid they'd get old if they hung around the clock much!! This is a true story. Don't dismiss it.