I like David's solution and I am in awe of his stone-cutting capability. However, to return to your problem where the ellipse has been returned without any markings, why not draw an ellipse of the same specified dimensions on paper with the axes already marked on it? This could then be matched to the "close-to-perfect" slate ellipse to locate the axes and the centre. It probably would not be necessary to go to the trouble of drawing a complete ellipse. A polygonal approximation might work.
On 31 October 2016 at 08:22, David <da...@davidbrownsundials.com> wrote: > I cut my own slate ellipses, often up to 3cm thick. In the process of > marking out the ellipse I will have drawn both the major and minor axes. I > make a small indentation with a metal scribe at both ends of both axes as > well as the centre point. These indentations are deep enough to be > noticeable but shallow enough to be erased easily on the cleaning-up > operation after the eventual painting/gilding of the finished inscription. > The indentations can be made more prominent by painting them with a spot of > light-coloured acrylic paint. After the ellipse has been cut out (which I > do with a series of short straight cuts from a bench-mounted, water-fed > circular saw, followed by a hand-held water-fed cylindrical abrasive drum) > the axes are easily drawn through the still-visible marks. > If the job of drawing and cutting out has to be done by another person, > such as at the stonemason's yard, they could be asked to leave similar > marks to help you with the alignment of the inscription. > David Brown > Somerton, Somerset, UK > > On 30/10/2016 21:29, Karl Billeter wrote: > >> On Mon, Oct 31, 2016 at 08:24:04AM +1100, Karl Billeter wrote: >> >>> On Sun, Oct 30, 2016 at 02:37:04PM +0000, Frank King wrote: >>> ... >>> >>>> Almost the first task is to find >>>> the centre and the axes. Clearly >>>> you cannot fold a slate in half >>>> and the traditional way to proceed >>>> is to put a large sheet of paper >>>> over the slate and crease it down >>>> all round the rim. >>>> >>>> You then cut round the crease and >>>> attempt to follow your procedure! >>>> >>> Wrap the slate with a reflective strip (smooth, shiny plastic? thin >>> polished >>> metal?). Playing around with a laser should find the focii. >>> >> You could also try balancing the slate on rod to find the centre but it's >> probably too hard to get the required accuracy and it might be a little >> risky! >> >> K >> --------------------------------------------------- >> https://lists.uni-koeln.de/mailman/listinfo/sundial >> >> > > --- > This email has been checked for viruses by Avast antivirus software. > https://www.avast.com/antivirus > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > >
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