Spring Marked Planes

by Herbert P. Kean

From Plane Talk, Vol XII, no 2 (1988)

Published by Astragal Press ©1988
and reprinted here by the kind permission of the author and publisher.

Spring marks on molding planes is a subject that continues to generate spirited discussions among plane collectors. Articles have been written about spring, but nagging questions still remain. This article, I hope, will answer a few of them.

Springing is tiling a molding plane from its vertical position as shown in Fig. 1.

Fig.1[Full Size]

The primary fuction of spring is to minimize any vertical, or near vertical, sections of the cutting profile. These areas are at right angles to the blade and consequently are scraped rather than cut. They are shown parallel, or near parallel, to the sides of the blade in Fig. 1a, but are no longer so in the sprung plane in Fig. 1b., and consequently will be cut rather than scraped.

Springing brings out two coincidental advantages:

1) the mouth opening becomes more uniform, as the profile doesn't extend as far upward into the ever-widening throat opening; and

2) the diagonally downward push on a sprung plane holds it in the cut, while the unpsprung plane needs downward and sideward presssure to keep it from drifting.

Rober D. Graham Jr.'s article, in Kenneth Robert's Wooden Planes In 19th Century America, 2nd ed. illustrates the above phenomena. However, there is another pertinent point to be considered. Although the laws of trigonometry dictate that a sprung plane requires a wider body than an unsprung one, it becomes obvious from fig. 1 that the height of the plane is reduced by springing. The overall effect is that springing generally results in less material being used, particularly in the case of the expensive laid-on steel on the plane iron. Fig. 2 shows a sprung and unsprung plane, either of which could cut this particular molding easily. But notice the added amount of laid-on steel that the unsprung blade needs, and how much more grinding is required to bring it to final shape.

Fig.2[Full Size]

In the article in the Jun 1979 Chronicle of the EAIA, Frederic A. Shippey presented data in an effort to correlate the measured spring angles of 218 molding planes by manufacturer, the pitch of the iron, the type of molding, the existence of integrated fences or boxing, and the width of the plane. He found little statistical correlation. However, recognizing that a new reason for spring might exist, Fred and I went back over the 218 diagrams and found some significant relationships:

1. The spring angle was neither selected nor calculated. It merely fell out ot whatever angle best minimized the vertical sections. Usually this occurred by "splitting the quirks." i.e. paralleling the centerline of the quirk (or pointed section) with the vertical centerline of the blade. Aside from reducing scraping, this produced a symetrical bevel and easier sharpening. Perhaps this is what Holtzapffel hinted at in his Mechanical Manipulates and Turnings, Vol. II, 1856, when he wrote "The spring is also partly determined by the position which is most favourable to the maintenance of the form of the cutter in sharpening it."

2. If the cutting profile showed no vertical sections, the spring angle was produced by making the sides of the blade perpendicular (or nearly so) to the base line of the profile. As previously shown, this minimized the material and labor required.

Fig. 3 shows some varieties of spring that cover the above considerations. The profile base lines of (a), (b), (d) and (e) are not totally perpendicular to the vertical sides of the planes. Although planes (d) and (e) could have been tilted more, (a) and (b) are best as-is, as their quirks are properly split. After looking at a couple of hundred sprung planes one thought became clear to me. There is nothing magic or critical about the actual angle. In most cases the planemaker got it "good enough" and didn't worry about theoretical perfection. It's easy to see where improvements could have been made, but I'm sure they used the industrial practice still in effect in production shops today: Good Enough is Best!

Although not obvious, (e) a scotia
has a near vertical section if left unsprung.
Fig.3[Full Size]

Here are the most common questions concerning spring: Why are nosing planes, hollows, rounds, astragals, and most beads unsprung? When the profile takes in a large portion of the circle (approaching semicircular), spring can create drag, or a miscut on one side. Also, when the baseline is already perpendicular to the sides, you can't achieve any reduction in laid-on steel. The planes below all fall into one or both of these conditions and are best left unsprung.

The nosing plane, because of its deep profile, could have
quite a different sized mouth opening from top to bottom.
Most planemakers prevented this by "contouring" the mouth,
i.e., cutting back the front side of the throat to equalize
the opening. (See Philip Walker's article on spring in
PLANE TALK Vol II, No. 2, P. 16, Summer 1977). To reduce
scraping sometimes two oppositley skewed irons were used,
eliminating the "deadly" right angle between the vertical
section and the blade.
[Full Size]

Why are large cornice planes (crown molders) unsprung? You might thing that here is where all the subtleties of cut and economies of material and labor would require springing, but no. When the base line of the molding is perpendicular to the sides of the plane, and the vertical elements are equally distributed, that's as good as you're going to get it.

Fig.5[Full Size]

Why are the spring marks on the the front of the plane? If the user is to align the plane by them, wouldn't they be better in the rear? If you try it, you'll see that the starting position of the stroke puts the nose of the plane in a good line of sight. Once the stroke starts, it really makes no difference where the marks are.

Who put the spring marks on the plane, and why? If the owners put them on, the lines would be bolder, and less consistent in their application. And the owners would have no reason to put the marks on both the front and the rear of the plane. Out of 42 spring-marked plaens, 8 had marks front and rear, 6 had marks so light they could hardly be seen. Some had marks completely extraneous to the concept of spring. Out of 8 unsprung planes, 6 had marks similar to the unsprung planes, and of these some both front and rear.

All the evidence points to these marks as layout lines, which leads to the conclusion that the maker put them on during the early stages of production. It's possible that where only one set of lines were used, e.g. with "mother" planes, the maker chose the front, knowing that the marks would also server for visual alignment. Where two sets were used, the makers, (or even the owners), could have deepened the front marks.

I hope this article helps you make up your mind about spring marks. Check your planes; see how many fall within the boundaries of the analysis presented here. I'm sure some of us will come up with additional information. We may never unlock the total story of spring marks, but it's fun trying.

Spring Marked Planes: Addendum

Special thanks to Gerry Kmack for his help in scanning these pages.

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