APPENDIX A:
Skew

by John Whelan

From The Wooden Plane: Its History, Form, and Function
By John Whelan,
Published by Astragal Press ©1993
and reprinted here by the kind permission of the author and publisher.


SKEW

There is a way to fool the wood into thinking it is being cut at a lower pitch than that of the plane being used. This is to move the plane along the board with the plane body at an angle to the direction of motion. (This is a good way to start a bench plane cut in any case, to ease the bump as the blade hits the edge). To see why this is so, visualize a single fiber of wood being lifted by a plane blade (Fig.A-l:3). If the blade is set at 45 pitch and is not skewed, the wood fiber is lifted at an angle of 45. At the same pitch but with the blade skewed, the fiber does not climb straight up the face of the blade but travels along a diagonal. As in climbing a hill, straight up is steepest, traversing diagonally up is less steep --you must walk further to rise the same distance. (To quantify this, the sine of the effective pitch is the product of the sine of the actual pitch and the cosine of the skew angle.) As a result, the effective pitch of a plane at common pitch is 42 if used at a 20 skew angle (a common value) and would be 30 if used at a skew angle of 45. In the extreme case of a spill plane at common pitch with blade skewed at 55 the effective pitch is 24, almost as good as could be obtained using an unskewed razor. This decrease in effective pitch is gained with no loss in the clearance angle.

Fig.A-1:3[Full Size]

Note that the apparent greater keenness of the blade is not due to a "slicing cut". The blade edge does not move across the fiber as a saw does. In a slicing cut, the minute irregularities of the blade edge (visible under a microscope no matter how well the blade is sharpened) act as saw teeth. It is possible to use a plane to get such a cut, but the action is different: the plane body is aligned with the board and is moved in a diagonal path across it. This is not a usual practice, although a circular motion of a finely set plane is recommended by Krenov for smoothing wood of difficult grain.

Skewing the plane body is acceptable as required for greater apparent keenness, but would be poor practice when using jointers. In planes where skewing the body is not possible (rabbets, panel raisers, shootboard planes, and other such) it is common practice to mount the blade at a skew within the plane body.

Decrease in effective pitch is not the only advantage to be gained by skewing the iron. The skew is normally in the direction which tends to push the fence against the workpiece, and decreases the user's effort to this end. The shavings are directed laterally and choking is less of a problem in rabbets and similar planes. And in cross-grain work, as with a dado, the wood fibers need not be lifted vertically but are peeled back from one end to the other.

The mention of slicing cut, above, prompts some comments on sharpness. There are several kinds. The kind which is best on a carving knife is quitedifferent from the sort needed in a plane blade. The carving knife is used with a sawing motion, and depends on tiny irregularities, like saw teeth, for its cutting action. Meat fibers are more flexible than those of wood, and simply pushing a blade into the meat makes it yield, rather than cut, until the penetration is large. A saw-tooth edge catches individual fibers and tears them apart by pulling sideways as the slice proceeds. This sort of edge is produced by a butcher's steel. The steel bends the metal at the extreme edge of the blade back and forth until it breaks off in minute slivers, leaving a ragged saw-like edge which doesn't have the acute angle needed in a razor or plane blade.

In sharpening to get the razor type of edge, the objective is to remove metal from the bevel with as little distortion as possible. The angle of the blade bevel on the sharpening stone should be constant. If it is allowed to rock, not only is the sharpness angle increased by the rounding of the edge, but the pressure on the extreme tip bends the metal in the same fashion as does the butcher's steel. A certain amount of this is hard to avoid, and a "wire edge" is a frequent result of stoning, which is then removed by stropping. The larger the wire edge, the more is the edge like a butcher's and the less satisfactory it is on a plane iron. Too large a wire edge is a sign that the hardness of the blade is too low, that the temper has been drawn too far.

Fig.A-1:4 [Full Size]

The plane iron should be sharpened by removing metal from the bevel only. The front of the iron should be laid flat on the stone only to remove any metal bent in that direction as the wire edge. There is a great temptation to tip it "just a little bit" to facilitate this. Doing so changes the effective pitch of the blade. Even if this is your objective, creating two perfect planes to meet in a straight cutting edge is more difficult than relying on the flat face of the blade (and it should be perfectly flat) for one of them.

Fig.A-1:5 [Full Size]

In addition to skew, there is another factor which enters into the apparent pitch; unfortunately in the opposite direction. In molding planes, the curvature of the blade can increase the apparent pitch - to vertical in the extreme case of the curve becoming perpendicular to the sole. In an astragal, the cut at the lowest part of the semi-circle is parallel not to the sole, but to the side of the plane. To make this clear, let us use an analogy due to Holtzapffel (1875) and picture the cutting of a large astragal with rabbet planes. We would cut the lower part with the plane on its side, as on the left of Fig.A-l:4. In cutting a small astragal with a conventional blade, this cannot be done. In effect the lower part of the astragal (at the right in Fig.A-1:4) is being cut as if by a side rabbet. But the blade is not pitched with respect to the side, as it is in a proper side rabbet, but is perpendicular to the surface it is cutting. The action is that of a scraper blade held upright and although skewed (at the pitch angle), this does not help in this case and the effective pitch is still vertical. It is not cutting, but scraping. The sketch of Fig.A-l:5 shows that the blade section at A is cutting like a bench plane, but the part at B is scraping.

Fig.A-1:6 [Full Size]

How is it that it can work at all? The saving feature is in the relative thickness of the shaving to be removed. Fig.A-l:6 shows an exaggerated cross-section of the shaving. As one moves along the blade toward the bottom of the astragal, the effective pitch is increased but the amount of wood it must remove decreases. At the bottom, the pitch is vertical but the shaving thickness is zero, and a scraper works just fine. The bulk of the wood removal is done by the horizontal sections of the blade edge. The strictly vertical segments of any profile cause no problem. It is the variation of cutting angle as the slope changes that is objectionable.

Fig.A-1:7 [Full Size]

Holtzapffel (1875), troubled by this, designed several types of cutter to correct the problem. While he reported that they worked satisfactorily, his ideas did not gain general acceptance. A rare type of plane uses one of these. It has a cutter in the shape of a gouge directed almost horizontally, hollow side down. The rear was straight and bent upward to form a normal shank. The example shown (Fig.Appendixl:7, showing a spare blade in front of the stock) is a short molder intended to convert a square 1/4 in. (6 mm.) tongue to a semicircle. The same principle has ben seen in planes to cut V-grooves and in a hollow groover.

Molding planes, except for hollows and rounds, are rarely skewed. As an extreme example, an astragal, if skewed, would have effective pitch decreased on one side of the curve, but would suffer an increase on the other side to beyond vertical. As will be seen below, profiles are usually arranged to have equal degrees of curvature on either side of horizontal. Skewing hurts on one side as much as it helps the other, and little is gained. In planes without spring, however, skew can help. It is used in double iron nosing planes, for example.


Other section of this Appendix:
THE CUTTING ACTION OF PLANES
SPRING

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


19Apr01
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