One of the posts asks why the predominant convention is used (zero at the lower left corner of the part). I believe that this is because this is a drafting standard. The convention is to create a datum from which all other dimensions are measured. Most commonly this is the lower left corner of the part for a 2D drawing. So when putting the part on the mill, the convention was carried over. It doesn't have to be that way.
For most of us with milling vises with the back jaw fixed and the moveable jaw in front, this can be awkward, because you have to touch off the zero point every time you reinsert a new part. One way to avoid that is to index off the back, fixed jaw. But if you index off the back left edge, the dimensions are not to the drawing's datum. So you can make a case for indexing off the back right edge and reversing the sign of all X and y movement. Or maybe we should all go out and buy a new $600 milling vise with the fixed jaw in front...
Re the drafting convention, interesting bit of history: when I was in college, I signed up for a work-study program at the local bomb factory. Now I was an aspiring electrical engineer, but had to wait almost 3 months for the security clearance, sitting in an office outside the secured area (where all the interesting work was being done). For make work, they gave me a programmed instruction course on True Position Dimensioning. In TP dimensioning, all features are given a location and tolerance w.r.t. the datum. The text explained that this was to eliminate build-up of tolerances when features were dimension in a chain, like a series of holes. Cadcam has rendered much of this less important. Re getting the security clearance: the FBI finally interviewed me (a bearded kid from Berkeley ca. 1965) and asked me a lot of mindless questions, like "What magazines did your reference (my godfather) read when he was in college...(15 years before I was born!) This was back in the day of J Edgar Hoover and all of the FBI types had crew cuts and wore black suits with narrow black ties. It dragged on and on, no clearance, until by happenstance I was invited to a faculty party and introduced to the Dean of Engineering, who asked what I was studying. When he heard about my situation, he said he would contact the Atomic Energy Commission rep on campus...lo and behold, 2 days later my clearance was granted! I eventually did get to do EE work at the bomb factory, which was interesting and educational.
Craig
