The third thing which is the elephant in the room — machine deflection and how squarely it is assembled. Some suggested mods at: http://www.shapeoko.com/wiki/index.php/Shapeoko_3#Modifications — that said, it really is astonishing the possibilities which these machines afford — Clockit on the ShapeOko forums was making clockwork mechanisms which one could make move just by blowing on them.
Thanks @mbellon, @WillAdams for the assistance in this small brain fart. Once I put the correct dimension of the end mill and then did a test cut, I came out less than .005 (5 thousands of an inch) off of the correct dimension.
I will do the testing and calibrating of my machine so I know exactly what I can cut and then off to make sawdust!
Given that the measurement was constant, after some thougth, it occurred to me that the tool might not be what was thought.
Knowing the actual size of a tool is critical for precision machining. In machine shops, each job is often costed with new tool costs included - no reusing tools. The old tools are use for machining things flat and other tasks where the diameter isn’t critical to the operation.
That’s a very believable number for the SO3. @WillAdams can provide details of stiffening methods. It’s my understanding that with some work, that number can be lowered a fair amount. Good routers, good collets, stiffening, metal bed…