If a cutter requires resharpening during the cutting process (due to dulling or breakage), removing the cutter itself and resharpening it may result in a cutter the shape of which is identical to its original shape, but an attempt to re-position that cutter to the same depth that it was previously will almost certainly fail because the reference point is the needle-sharp tip of a nearly microscopic cutter. If, however the entire spindle is removed, the cutter may be advanced in it (or replaced) and resharpened so as to be in the same position relative to the spindle (because it was ground initially in this spindle, and presumably you haven't changed the settings of the cutter sharpener). The spindle, in turn, can be replaced to exactly the same position in the machine because it has large banking surfaces to locate it.
This capability is even more important because a matrix is typically not engraved using a single cutter but rather a succession of cutters of increasingly fine proportions. A removable spindle facility (with appropriate cutter grinder) allows each of these to be ground and positioned exactly to depth.
Benton went one step further and designed a cutter grinder the grinding wheel of which was dressed in such a way that it always retained the same relationship to the spindle holding the engraving cutter.
Removable spindles were a feature of Benton pantographs and their derivatives, and were also employed in certain commercial machines intended for matrix work (e.g., certain models of Gorton machines).
Despite the logic of Benton's method of removable spindles, they are not strictly required for matrix engraving. It is possible that in practice this capability is a necessity for production or large-scale commercial matrix engraving, because its lack would mean restarting each matrix from scratch if a cutter dulls or breaks. But the requirements for small-scale or artisanal matrix making are less stringent.
So the aspiring matrix engraver need not despair if they lack this capability. We know from the documentary evidence in the film Making Faces that Rimmer's pantographs did not have removable spindles. He still managed to accomplish marvelous work with them. (Of course, Rimmer was an extraordinary craftsman. Look at the details of the way he handles his pantographs in the film - especially the way he instinctively takes up the backlash in the vertical travel of his Ogata pantograph at time 32:01.)
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