For molders such as the quarter-round there is a way of minimizing the variation of cutting action along the curve. The profile is placed on the sole at an angle, called the spring angle. In use, as shown in Fig.A-l:8, the plane is held at this angle with the vertical. The device was used on some of the earliest known molders, including ogees by Robert Wooding (England, 1710-39). Now the equivalent positions of the miniature planes (assuming spring of 45º are as shown in Fig.A-l:4B. At worst, they are acting like planes with soles at 45º. (The effective pitch, if the blade is at common pitch, is then 55º.)
There are two other advantages gained by using spring. One is the direction of force toward keeping the fence on the work. The second, as was pointed out by Phillip Walker (1977), concerns the mouth clearance. In making the plane, the throat is sawn into the stock with the minimum clearance at the lowest point of the profile. As the sole is cut away to form the profile, the mouth openings of the higher points of the profile are larger due to the wedge shape of the throat. Note that in Fig.A-l:9, a quarter round without spring, the distance from blade edge to the front of the mouth is much greater at the top of the curve than at the bottom. If it is correct at the bottom, it is too wide at the top. This effect is minimized by using spring, to keep the different parts of the profile as nearly as possible at the same level. A quarter round without spring has the top of the curve a distance of one radius above the bottom, while with a 45º spring, the distance is 0.29 of a radius. Thus the mouth widening is reduced to 29% of that of the unsprung plane.
While a spring angle of 45º has been used in some of the examples above for simplicity of argument, this is larger than is common in practice. The reasoning is complicated by the fact that the plane is cutting downward, and not in line with the stock, being guided by the fence. Another reason is that a large spring is a bit awkward to use in practice. Spring angles of 30º or less are more usual.
None of the above implies that spring is essential to good performance. It is almost never seen in Continental planes, and in many common profiles (astragal, bead, etc.) it cannot be used and the planes work well. But the advantages are sufficient that, where possible, it is almost always used in the English-speaking world.
It remains to be explained why astragals and beads do not use spring. At first glance, it is natural to assume that the cutting action progresses in line with the body of the plane, as it would if unsprung; and that, of course, the far side of the curve couldn't be cut. Not so. The progress of the cut is vertical, as the fence does not permit movement along the line of the body. In fact, a sprung astragal would cut the profile, but not very well. The real reason is that the spring, as outlined above, improves the effective pitch on the near side of the curve. It makes it worse on the far side, where the effective pitch is beyond vertical (pitch greater than 90º).
Perhaps this will be clearer if we look at an example in which the spring has been carried to the extreme of 90º. The plane body is then positioned on its side, but its motion is still vertical. Take away the plane body and consider the position of the blade (Fig.A-1:10). It is held at a horizontal angle equal to the bed angle of the plane, say 45º. The left side of the astragal is then cut well, with an effective pitch of 45º. The top of the curve is scraped, effective pitch of 90º and skew angle of 45º. The right side of the astragal is not cut or even scraped well: the blade is leaning forward (effective pitch 135º).
For further discussion of spring see Kean ( 1988, 1989 ) and other references cited therein.
Other section of this Appendix:
THE CUTTING ACTION OF PLANES
Special thanks to Gerry Kmack for his help in scanning these pages.