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CUTTING INSTRUMENTS. 37

occasional errors should be expected. When these are discov-
ered, the particular instrument should be discarded at once and
its place supplied by a new one.
The spoons and discoids are ground easiest by so placing
them on the stone tliat the motion in grinding will be parallel
with the length of the circular edge, and during each stroke from
end to end of the stone the instrument shaft should be rotated in
its length on its point in the line of the length of the stroke so
as to bring every point in the circular edge on the stone. In this
movement, the point of the instrument is first toward the near
end of the stone, and with the progress of the stroke the angle
is progressively changed so that in the center of the stone the
shaft is perpendicular to the stone, and at the end of the stroke
the point will be toward the distant end of the stone. In the
progress of each stroke, the circular edge is ground at the proper
bevel in every part of its length. When this stroke is once
learned, it is easily and rapidly done and these circular edges
may be very accurately groimd. The spoons may also be ground
Avith fine emery disks in the engine. The spoon should be held in
the left hand and the rapidly rotating disk so placed that its
motion is from the edge. By carrying the disk regularly around
the circumference, the edge may be given the proper curve. After
a few efforts a particular way of holding the instrument for
obtaining accuracy of bevel in all parts of the circumference of
the edge will be readily found. In sharpening a pair of spoons,
the disk should revolve to the right for one, to the left for the
other. The emery disk will not make as fine an edge, however,
as a good Arkansas stone.
The special points in grinding the gingival margin trim-
mers are: to carefully maintain the bevel of the edge and the
correct angle of the edge with the shaft of the instrument. They
should be very sharp, but the bevel should be rather short.
Burs and finishing files are ground with knife-edged slips
of Arkansas stone. These may be obtained from the jeweler's
supply houses. A very hard stone is essential. A strip of fine
emery cloth on a flat board completes the necessary outfit. The
stone is used as in delicate filing of the leaves of blades of the
bur or finishing file. The edges of the stone should be flooded
with oil. When the edge of the stone has become dulled or
rounded by use, it is ground to correct form on the emery cloth.
Every blade of the bur or file should be made sharp and the
work should be so regularly done that the edges of the different
blades will be of equal height. This may be done by the dentist