Ernie you do beautiful work and the agony in which you speak, I feel for you. My point was that if you use one instrument to measure as your own standard for your project then the accuracy that you seek is in how you use it and how you layout your work. It doesn't matter if your measuring instrument is accurate to some arbitrary standard or is the same as someone else's measuring instrument. The earlier post here by Donn was he used two tape measures interchangeably, that is a recipe for failure any day of the week. In my day to day work I use a thirty foot tape measure. That is the only measuring instrument I allow on the project, accuracy is all in how it used not just the instrument.
Generally speaking, I agree with you totally - a single ruler should be used for an entire project. This will, or should, make every assembly accurate
relative to every other assembly in the project. In most cases, that's fine. In some cases, it's not fine enough. There are times when it's necessary to be accurate to a standard. For example, when two or more people, working in different locations, are making assemblies that will ultimately be united in a single piece of work. If their respective rulers aren't truly identical, misalignment can, and will, occur.
ln my book on harpsichord building, I give measurements in both metric and Imperial, although I use neither. I've found that the smaller an assembly is, the greater the need for accuracy. So, whenever possible, I use a micrometer and/or a digital caliper that reads in the thousandths. That said, the measuring device is only one part of the accuracy problem. Regardless of what measuring device you use, if you're marking your wood with a crayon you cannot expect any consistent level of accuracy. That's why folks who make dovetail joints using a knife instead of a pencil to mark their wood.
Then there is the problem of your machinery. Rip a 3" wide slice off of a piece of wood using your tablesaw. If you measure, using a digital caliper, in various places on your 3" slice, most of us will find some variation in the width of the board. In most cases, this won't matter - in some, it might. This can be caused by inherent inaccuracy in the machine, and/or user error. However, for most of us, this is the best we can do. It's why we use a tablesaw instead of a chain saw to dimension our wood. It's the best we can do.
These are some of the things that keep me up at night. It becomes an unhealthy obsession that, after all these years, I struggle with. Help me, please!:BangHead:
