Typically, an ohm meter is connected between a hard stop, insulated
from the rest of the machine, and the portion of the machine that
contacts the hard stop. In some cases, the ohm meter is replaced by a
single ‘led’ or an electronic circuit that controls several 'leds’
that light in sequence.
The meter reading or light sequence is dependent on the time the hard
stop & the machine part that contacts it are in contact. The stone
being cut is in contact with a turning lap (usually 6 or 8 inches in
diameter) on the faceting machine. When enough of the stone has been
ground away, contact is made between the hard stop and the rest of the
machine. Due to slight irregularities in the surface of the lap, some
parts of the lap allow contact to be made before other areas. This
results in the contact being made and broken as the irregularities
pass under the stone.
The result is a varying meter reading. In every meter using a de
arsnoval movement (the type with a needle), there is a certain time
required to move the needle from zero to full scale. If the time the
contacts connected to the meter are closed is shorter than the time
require for the meter to reach full scale, some reading between 0 &
full scale will be indicated. The longer the contacts are closed, the
closer to full scale the reading will be.
Cutting of a stone proceeds at a uniform rate. Because of this uniform
rate, the contacts stay closed longer & longer as more of the stone is
ground away. Thus the meter reading will progress toward a full scale
reading. If the cutting of each facet at a given setting is stopped
at the same meter reading, the facets will be the same.
All of this assumes a perfectly constructed & aligned machine. These
conditions are almost impossible to attain at an economical cost. Some
variations in facet size or placement may be present. However, the
irregularities are all but invisible unless the stone is view under
magnification by an experienced person. When cutting for competition
where irregularities aren’t tolerated, most faceters use the meters
or other indicators to get ‘close’, the final position is determine by
careful examination under magnification (many times as high as 30X).
The contacts used in meter approach may be replaced by a device
called a ‘strain gauge’. This approach is more costly and requires the
machine to be designed for the strain gauge. The hard stop, contact
approach can be retro fitted to existing faceting machines.
It should be pointed out, that the automatic faceting machines used
by large cutting houses are not of this type. Many industrial cutting
machines are computer controlled & cost over $50,000. New faceting
machines used by hobbyist faceters cost (on average) $1,000 - $5,000.