5.18.
MOUNTING THE KNIFE
171
devices which incorporate mechanisms for tilting the knife a specified
number of degrees. These devices presuppose that the front face of the
knife fits snugly against a metal plate which then truly parallels the knife
face. Obviously, if the face of the knife is curved, this will not obtain, nor
will it if a chip or spur of glass or a piece of debris prevents the proper
positioning of the knife. Thus, one must use considered care in setting
the knife in such a holder. These holders also ordinarily are equipped
with a device which indicates the proper height of the knife edge.
Other microtomes, such as the original Porter-Blum instrument, do
not have as elaborate knife holders, and judgment or homemade jigs
have to be used to establish the angle and height. To do this visually
without a jig, it is absolutely essential that the operator bestir himself
from his comfortable chair in front of the microtome and move around
to the side of the instrument, some distance away. With this perspective
he can judge the verticality of the front face without too much difficulty.
This writer knows of laboratories where an appropriately tilted line has
been drawn on a nearby wall, and the front face of the knife aligned
parallel with this by sighting on it.
In our own laboratory we have devised a simply constructed jig for
Porter-Blum microtomes which will set the same gross angle of the knife
face every time, and also indicate its proper height. This is illustrated in
Fig. 20(a)-(c). Its success depends on the fact that the Porter-Blum knife
holder incorporates a key which fits into a keyway in its mount and in the
jig. Thus, this holder is always returned to its mount in the same position.
We have used this jig in our own laboratory for several years now and
have found it an invaluable aid. No doubt comparable jigs could be
devised to orient knives for other microtomes.
The front face of the knife of course is fractured glass, and can never
be expected to be perfectly plane. The reader has already been warned
not to use a glass face which is decidedly wavy in making a knife for
then one cannot know exactly how the face may be oriented in the
immediate vicinity of the knife edge. The relatively flat glass faces produced by free breaks [Chapter 5.13, Figs. 14(c) and (d)] therefore offer
decided advantages. Thus, standardizing the gross angle can give only an
approximation of the important clearance angle in the vicinity of the
cutting edge. Once one understands the situation it is preferable to err,
if at all, by making the knife too nearly vertical. Then, as one starts to
cut, the symptoms of backward tilt may appear and remedial measures
are easy. Tilting the knife 1-2° may be enough to make all the difference
MOUNTING THE KNIFE
171
devices which incorporate mechanisms for tilting the knife a specified
number of degrees. These devices presuppose that the front face of the
knife fits snugly against a metal plate which then truly parallels the knife
face. Obviously, if the face of the knife is curved, this will not obtain, nor
will it if a chip or spur of glass or a piece of debris prevents the proper
positioning of the knife. Thus, one must use considered care in setting
the knife in such a holder. These holders also ordinarily are equipped
with a device which indicates the proper height of the knife edge.
Other microtomes, such as the original Porter-Blum instrument, do
not have as elaborate knife holders, and judgment or homemade jigs
have to be used to establish the angle and height. To do this visually
without a jig, it is absolutely essential that the operator bestir himself
from his comfortable chair in front of the microtome and move around
to the side of the instrument, some distance away. With this perspective
he can judge the verticality of the front face without too much difficulty.
This writer knows of laboratories where an appropriately tilted line has
been drawn on a nearby wall, and the front face of the knife aligned
parallel with this by sighting on it.
In our own laboratory we have devised a simply constructed jig for
Porter-Blum microtomes which will set the same gross angle of the knife
face every time, and also indicate its proper height. This is illustrated in
Fig. 20(a)-(c). Its success depends on the fact that the Porter-Blum knife
holder incorporates a key which fits into a keyway in its mount and in the
jig. Thus, this holder is always returned to its mount in the same position.
We have used this jig in our own laboratory for several years now and
have found it an invaluable aid. No doubt comparable jigs could be
devised to orient knives for other microtomes.
The front face of the knife of course is fractured glass, and can never
be expected to be perfectly plane. The reader has already been warned
not to use a glass face which is decidedly wavy in making a knife for
then one cannot know exactly how the face may be oriented in the
immediate vicinity of the knife edge. The relatively flat glass faces produced by free breaks [Chapter 5.13, Figs. 14(c) and (d)] therefore offer
decided advantages. Thus, standardizing the gross angle can give only an
approximation of the important clearance angle in the vicinity of the
cutting edge. Once one understands the situation it is preferable to err,
if at all, by making the knife too nearly vertical. Then, as one starts to
cut, the symptoms of backward tilt may appear and remedial measures
are easy. Tilting the knife 1-2° may be enough to make all the difference