by Duncan Miller

This month I will show you how to scale a GemCad diagram to a different L/W ratio. This is very easy if the diagram is a fully meetpoint diagram, without a preform. You note the initial L/W ratio from the Print Preview and then click on Scale in the Edit menu. Here you check the X box because you want to change the proportions in the X direction, then enter the appropriate numbers to divide by the initial L/W ratio and to multiply by the one you want, and press OK. The next menu offers to round the indexes to the nearest notches. As we are working with a 96 index wheel it will use that. Press YES – horrors, see all those factional indexes! – then press OK, and they will vanish.

OK, now let’s do it with an example. I am going to use the same mixed oval 1:1.30 design as last month. Here is a screen shot of the design.

Here is the Edit Scale menu box with the appropriate entries.

And here is the index rounding option.

Pressing YES produces a terrible list of fractional indexes, but OK in the next menu will round them. The end result looks like this.

If there are grey cut-offs because the diagram now is too long, you can Scale All to shrink it a bit for the diagram to fit. (I did that here.) The preform doesn’t rescale, so it is meaningless. You can remove it with Preform Delete. If you are going to save the rescaled diagram you should change the heading using Edit Heading/Footnote. The final diagram is below.

There are several things to notice about this rescaling. Crown star facets 9 will need tweaking to meet. The girdle indexes have changed from the 02, 08, 13, 21 set to 02, 06, 11, 19. This is the same as given for a L/W ratio of 1.50 in Table D-5 of Long & Steele, in the table below. This means we can use the Long & Steele recommended preform angles for L/W 1.50 to cut a preform for this design. And remember that in GemCad you can change the pavilion culet angles easily to deepen the stone if necessary, using the New angle option in the Facet dialog box that appears if you click on a facet.

Extract of Table D-5 SIXTEEN FACET GIRDLE OUTLINES FOR SELECTED OVALS (from Long & Steele, Facet Design Volume 1 Ovals, second edition)

L/W ratio

Indicies (96 wheel)

Angle (degrees)

Typical size (mm)


02, 08, 13, 21, etc.

45.0; 43.5; 42.1; 40.1

5×6, 10×12, 15×18


02, 08, 13, 21, etc.

45.0; 43.4; 41.8; 39.6

9×11, 18×22, 27×33


02, 08, 13, 21, etc.

45.0; 43.1; 41.4; 39.0

4×5, 8×10, 12×15, 16×20


02, 07, 12, 20, etc.

45.0; 43.3; 41.3; 38.6

7×9, 14×18, 21×27


02, 07, 12, 20, etc.

45.0; 43.0; 40.6; 37.6

3×4, 6×8, 9×12, 12×16


02, 07, 12, 20, etc.

45.0; 42.6; 39.7; 36.2

5×7, 10×14, 15×21


02, 06, 11, 19, etc.

45.0; 42.7; 39.2; 34.9

2×3, 4×6, 6×9, 8×12

 Scaling other designs that have preforms means you will have to construct a new preform from the rescaled design, using the technique described in the MinChat of October 2018.