Re: large sundial

[John Davis]

Tony Moss added:   >With the photoetching process I use for sundials I've settled on a line >width of 0.35mm.  The technology will allow much finer lines to be >achieved but at the expense of depth, very long etching times or missed >bits which then have to be engraved by hand.  A line depth of 1/2mm >doesn't sound much but is likely to survive weathering for centuries I >believe so that is my standard.   Although we have strayed rather far from the original posting about the resolution on large dials, it is interesting to look at the same effects on small engraved ones.  I believe Chris Lusby-Taylor and Tony are right to say that 8" is about the smallest practical size for 1-minute graduations.  I have seen a hand-engraved 10" dial from the 1860s with 1-minute markings; they were very fine and beginning to suffer from erosion (it's been exposed to the weather on the outskirts of London for its whole life).  It is worth noting that the depth of the lines on these hand-engraved dials is far less than Tony's half a mm - I think 0.1mm is nearer the mark.   The minimum linewidth that I can print is around 0.08mm, though I usually don't go finer than 0.1mm (about 4 thou for those still imperial!).  However, after etching the resulting line is around 0.2mm wide, depending on etch depth.  Luckily, the larger features automatically etch deeper than the fine ones.  The undercutting of the resist sets the closest spacing achievable: for a 12" circular dial with the toe of the gnomon offset by about half the radius from the centre, the printed gaps between 1-minute markers are around 0.6mm, so for a 6" dial it would be very borderline to keep the photoresist from failing.   Of course, if anyone wants a really fine scale, the anodisation technique is the answer as the finished lines get thinner than drawn, rather than wider.  Thus lines at 0.2mm spacing are feasible.  Most customers want brass, though!   Regards,   John ------------------------------------------------------------------- Dr J R Davis Flowton, UK 52.08N, 1.043E email: [EMAIL PROTECTED]