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In my article "Spring Marked Planes" (PLANE TALK 1988, No.2), I asked the reader to check over their planes in an effort to come up with an additional information about spring marks. With the responses I received, I was able to piece together more of the puzzle of this intriguing aspect of planemaking.
Carl Bopp said that when planes and irons were made with standard heights, the advantage of "less material being used" in sprung planes was lost. However, he countered this with the fact that savings in labor on the sprung plane, both in shaping the sole and grinding the iron, was much more significant. In addition, a blade shaped for a sprung plane would last much longer than one used in an unsprung plane: the reason being the much larger regrind area of the former (see Fig.1).
The quote in my original article form Holtzappfel's Mechanical Manipulations and Turnings, Vol.II, 1856, can be expanded to include his mention of another advantage of spring, namely "The spring is also partly determined by the position which is most favourable to the maintanence of the form of the cutter in sharpening it; as the obliquity [*] of the sole of the plane causes the cutter, when advanced through it, also to shift sideways and cause a disagreement between thir figures." I have to admit that originally I had trouble fullying understanding theis quote, but Carl gave me a good enough explanation so I could produce the sketch below.
I still maintain that springing originated in order to elimitate the scraping difficulties with certain blade countours. But, considering how many planes are sprung that could function just as well unsprung, I have to conclude that other considerations motivated the planemakers to continue its use. The list of the advantage of spring is impressive:
Fig.1[Full Size]
Special thanks to Gerry Kmack for his help in scanning these pages.