Preforming: What’s The Best Way?

by Fred Van Sant

With the new bells and whistles added onto faceting machines in recent years, preforming options have expanded. It used to be that when you cut an octagon shape you always started with the dop arm at 90° and cut 8 facets using a positive stop (Method 1). You cut each one until no sound could be heard between the stone and the lap. It took a long time for each facet.

Now with digital readouts, contact switches that turn on a light when the facet has reached the proper depth of cut, and better positive stops, we should take a second look at our options and ask ourselves, “Could I be doing it a different way that might be better ?”

But what is better ? Certainly a more accurate shape is better, and certainly making a preform of the same accuracy in a shorter time is better. One could also throw in the consideration of safety–one method may be less prone to making errors than another. But accuracy and speed is probably the best answer. So it depends largely on what the cut stone is for. If it’s a commercial stone speed may take precedence, but if it’s a competition stone accuracy will be most important.

The advantage of making eight girdle facets at 90° is that it is a one-step process. But it is slow, and to be sure your preform is accurate you have to measure the relative lengths of the girdle facets made. Also you have to cut close to the edge of the lap, where any wobble in the lap is magnified. Another limitation is that you must cut the pavilion starting at the girdle. There is a distinct possibility that your culet point will not end up in the exact center of the dop axis. This method is limited to only those shapes which can be preformed at a single mast-height setting. From a historical point of interest, R. Long and N. Steele once proposed the Mast Height Adjustment, or MHA, method of making a preform. This method, while possible, did not prove practical, and was superseded by next method described below, the Centerpoint Angle Method.

Method 2, the C.A.M. (Centerpoint Angle Method), is a two-step process–you cut a set of facets at the same indices as your girdle facets, at predetermined angles, to make a temporary culet point, then at 90° make a second set of facets (girdle facets) all around the stone to make a level line. It can be used to preform any shape. The accuracy of this method depends on how close you can cut to the given angles. If the CAM facets do not come to a perfect point, then what ? Do you juggle ?, lower the mast a hair and recut ? When you have the point made, how do you know it is in the exact center of the dop arm axis ? This last question is answered in the article “CAM Preform–The Cone” in the previous issue. The centering of the temporary culet point makes it possible on rounds to cut a set of facets without adjusting the mast height. You can cut a round brilliant type stone with the culet point off-center, but you will have to adjust the cutting angle for every facet in every set.

The CAM preform is being used more frequently in recent years. Originally popularized by Long and Steele for making odd-shaped preforms, some cutters prefer it even for round designs because it is faster, and because it is possible to be sure the culet point is centered. Once the shape is made the pavilion can be cut from the girdle to culet, or if the CAM angles are computed using the Zero Vertical Loss Method, as given on my own designs, the pavilion can also be cut starting at the culet. The Zero Vertical Loss Method is a method of computing a set of CAM angles such that the true pavilion angles can be cut using the culet made by the CAM preform as the permanent culet. The lowest-angled true pavilion facet’s lines will coincide with the face of the CAM facet which it cuts into.

The other popular preforming method is Meetpoint. Here again you must start with a culet point which usually must be centered, so starting with a cone culet is a good option. The disadvantage of any method which starts from the culet is that the stone shape is the result of a series of meets, and it is more possible to get an accumulation of small errors which affect the girdle outline. The advantage is that many non- symmetrical shapes can be cut with a fairly high degree of repeatability.

As you can see by now, the preform method you use is determined sometimes by the design, sometimes by the use to which the finished stone will be put, and by the capabilities of your machine.

Let’s discus competition stones. Where you do have a preform choice, you should choose the one which gives greatest accuracy. This is determined by your machine’s capabilities. If you can’t read your protractor to at least an accurate tenth of a degree, or if your positive angle stop allows too much flexibility, then if you’re cutting an ECED design you should stick to the old one-step direct girdle at 90°; But if you have a tight, well-calibrated machine, you might try a comparison check between the old method and one or two of the others. Use the same material–glass will do–and cut two or three preforms the same pattern and same size. Time yourself. See how long it takes to finish a preform for each method. Then appraise the finished preforms. The results might surprise you.

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