I’ve written on a bit about the Omni Faceting Machine head design in my GemologyOnline post.
One thing that I noted was the Omni Facet Head has the index pin on the bottom of the head. At the time I wrote the article I noted that I didn’t like it because you have to life the head up to see what index you’re on. Since then I’ve realized that there are other drawbacks to having this located on the bottom of the faceting head.
Have a look at this photo. You can see that I have a stone that I wanted to preform on the dop, and the head is set to 59.5 degrees. The lap on the machine is an old Crystalite 600 plated lap, about 1/2″ thick. I’ve lowered the Omni Facet Head so the stone is almost touching the lap.
Now look at the close-up photo. Notice that the metal parts that house the Cheater and Index Pin are bulky and now contact the edge of the Splash Pan. This severely limits the movement of the stone across the lap.
Here are two photos showing a top view. Notice that you only have about 1″ of travel across the lap before the stone either hits the lap screw or the Facet Head hits the edge of the Splash Pan.
What this means is if you are cutting a stone at an angle higher than about 50 degrees, you will have to cut the stone through the girdle cut-out in the Splash Pan. This means you will need to change the orientation of the Swing Arm (and consequently the stone’s orientation to the Platen and lap).
This is bad – in order to maintain the same alignment of the stone to the lap, you should be cutting swinging the stone from the 6 o’clock position to the center of the lap as much as possible. By forcing you to cut in the girdle-grinding position, you have much less travel (due to the limited size of the Splash Pan cut-out) and you are also not cutting on the optimum areas of the lap. It’s also one of the reasons I’ve been frustrated trying to cut rectangular and step-cut stones – moving the Swing Arm position changes the orientation of the stone to your lap.
A final gripe is that if you’re cutting a facet with the Facet Head low and then swing the Quill up to look at the stone, you run into the problem of knocking the Fine Adjustment Arm against the Splash Pan. Irritating and it will change the angle that the Facet Head is set to.
BTW – the stone in the pic is a nice big piece of Afghan Aquamarine 17x9x10 mm, Internally Flawless. I can’t remember how many carats. It’s a beautifully-formed crystal with sharp edges and termination at one end. Pale, as all Afghan Aquamarine tends to be, but very very bright once cut. I’ll post a pic once I’m done.