 In order to minimize rounding errors it was good practice to place fiducials in the same grid (or some multiple of it) that was used to place the parts, however, this isn't always possible on high-density boards nor is it a requirement any more with modern high-precision machines.Most placement machines are fed boards for assembly by a rail conveyor, with the board being clamped down in the assembly area of the machine.Each board will clamp slightly differently than the others, and the variance—which will generally be only tenths of a millimeter—is sufficient to ruin a board without proper calibration.