Have you noticed the numbers on modern daceting diagrams like L/W = 1.25 P/W = 0.564 C/W =0.175 and not known what they mean? Don't worry lots of facetors are not sure what they are.
They are ratios of the length of the dimensions on the diagrams on the faceting diagram above the numbers. L/W is the length width ratio. If your rough is say 11 x 14mm then this ratio says you can probably get a 10 x 12.5mm stone from it but if your rough is 11x13mm you can't.
C/W is the crown height. Add the girdle width (say 3% of W or 0.03) and a bit for cutting and you can work out how much you have to have to cut the crown. So if your stone is 11mm and rough for the crown is 1.5mm, in this example you need at least 2.3mm to cut the crown if the top of the stone is already flat, so you have to reduce the size of the stone when cutting the girdle to leave enough rough for the crown.
This is because P/W = 0.175 and allow say 0.03 for the girdle as that is about 5% of the height of the stone in this case, and the finished crown plus girdle height is 0.205 times the stone width. Then you need to add a bit too to cut the rough away from the table.
C/W + P/W + girdle width (say 0.02) is the finished height of the stone if the girdle is 2% of W. Allow a bit more for cutting, say 2mm, more if there is a big change in pavillion angles or your design has a preform.
The stone I am cutting has pavillion angles starting at 55 degrees and finishing at 40 degrees. This means I needed about 0.2W more material than the finished height to accommodate the material cut by the 2nd to 6th rows of facets. When I thought I could get a 12mm stone from the rough, I'm getting a 10.0 mm stone.
The diagrams show where P C L W etc are measured. The ratios let you calculate the remaining measurements by measuring W. Just a note of caution, the height of the girdle is not included in the disgram. Nor is the rough cut from the temporary meet point or any preform as the pavillion is cut.
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