> my favorite layout guy once explained the notion of datum lines (I > think that's what they are called). Anyway, if you use an arbitrary > line (datum) in the center of your board, and reference all > measurements to it, the tolerances are only from that imaginary 1/2 > way point to the edges. It's better than if the reference is on an > edge, then the tolerance adds up along the entire length.
When fabbing your own boards, there's a few places where accuracy or precision are lost. First, printers aren't perfect. Each printer has a characteristic imprecision. Mine makes plots slightly thinner (like, 0.1%) than true. As long as all your plots are oriented the same way, these should cancel. Second, when you etch a double sided board, misaligning the two masks causes inaccuracy in the traces on those two planes. Third, when you laminate the core with the outer layers, misaligning those causes more inaccuracy. Lastly, if you don't drill exactly in the right spot, each hole can add inaccuracy. Especially when the drills are used to align the various layers. In my case, when laminating the masks, the datum edge is always the first one through the laminator. I don't have much choice about that. _______________________________________________ geda-user mailing list geda-user@moria.seul.org http://www.seul.org/cgi-bin/mailman/listinfo/geda-user