Machining Wood

ll is convenient to analyze machining as the action of a culting tool on a piece of wood or workpiece, with the cutting action that takes place referred to as chip formation, wherein a portion of wood called the chip is separated from the workpiece. Chip formation involves the geometry of the tool, the condition of the wood and the motion of the tool relative to the orientation of the structure of the wood.
The objective of machining always should be the paramount consideration. The approaches used may differ drastically, depending on the objectives sought. These objectives may he classified as:
Severing: To make two or more pieces from one, for example, splitting firewood or bandsaw-ing rough parts from a plank.
Shaping: To impart a specific shape to the workpiece, in some cases a flat-planed surface,
in some cases a flat-planed surface, in other some specific contour. Joining a flat surface on a cupped board is one example, millng an ogee molding is another . Surfacing: To create a surface of prescribed quality, for example, sanding a surface prior to finishing, or jointing edges that are suitable for gluing.
In most cases, two or even all three of the above are involved concurrently. In ripping boards into strips, for example, one might want the resulting surface to be true enough and of appropriate condition to be glued. In most machining the objective is the workpiece, and the nature of the chip removed is irrelevant. An interesting exception is knife-cut veneer, where the chip itself, the veneer, is of primary interest, and the surface left on the workpiece becomes one face of the veneer that will be removed by the next cut. Although the average woodworker is not involved in making veneer, the user of veneer should understand the machining process involved
Let's survey the interrelationship of the workpiece, the tool and the chip formation in machining.

The workpiece
The aspects of wood that affect machining have already nature of wood in terms of its three-dimensional properties is particularly important. Density variation among and within species is also of obvious importance, as is unevenness of grain, especially in ring-porous hardwoods and uneven-grained softwoods. Heartwood extractives in some species are particularly abrasive and contribute to tool dulling. Defects such as knots create both irregularities of grain direction and variations in density. Structural irregularities such as wavy or interlocked grain cause special machining problems. Moisture content influences machining as. it affects the strength of wood, and so do stresses or checks developed in drying.
Strength of wood is, of course, the bottom line. The relationship between the strength ol wood parallel to the grain and perpendicular to the grain is perhaps the most important part, although every other factor affecting strength in turn affects machining.

The tool
At the business end of a cutting tool, where chips are being formed, the tool geometry can be described in terms of a cutting edge formed by its intersecting face and back surfaces, or planes (1). The critical geometry of the cutting edge is usually defined in terms of its direction of motion:
î = the rake angle [also called the cutting angle, the hook angle, the chip angle and the angle of attack) is the angle between the tool face and a line perpendicular to the direction of travel of the edge. /3 = the sharpness angle, the angle between the face
and back of the knife.
ó = the clearance angle, the angle between the back of the knife and the direction of travel of the edge. As required by circumstance, cutting-tool geometry can be varied considerably. The sharpness angle will always be a positive value. The cutting angle and clearance angle can be negative values. In the case of the
between tool and workpiece.
 

1�The business end of a culling tool consists of an edge formed by ils intersecting face and back surfaces. Its geometry can be described by the rake angle, î (alpha), measured from a line perpendicular to the direction of travel to the tool face; the sharpness angle, p (beta), measured between the face and back of the lool; and the clearance angle, ó (gamma), measured between the hack of the knife and its direclion of travel.

2�Cutting action idealized. Energy Is consumed in severing the wood to form the chip, in deforming or rotating the chip, and in friction of the tool face against the chip, plus friction of the tool back against the new surface of the workpiece.

3�Cutting action in reality. The wood does not fail until ultimate stress is reached, and stress is always accompanied by strain. As the cut proceeds, the workpiece deforms ahead of the tool, severs, and then both workpiece and chip spring back to some extent.

Types of cutting action
There are two basic types of cutting action. The first is called orthogonal cutting, in which the tool edge is more or less perpendicular to its direction of motion and where the cut is in a plane parallel to the original surface of the workpiece, with removal of a continuous chip. An ordinary plane peeling a shaving from the edge of a board is one example. The second type is referred to as peripheral milling, in which a rotary cutterhead carrying one or more cutting edges intermittently contacts the work surface. Each cutter proceeds on a curved path, and removes a single chip. Virtually every cutting situation can be compared to either orthogonal cutting or peripheral milling. Note that as the cutter-head radius increases in peripheral milling, it approaches orthogonal culling.
Visualize a cube of wood with its sides oriented in the radial, tangential and longitudinal planes. According to notation reported by W.M. McKenzie, orthogonal cutting is described by two numbers. The first is the angle between the cutting edge and the cellular grain direction and the second the angle between direection of cutting and the grain direction. Thus there are three basic cutting directions: 90�-0� cutting, 90�-90� culling and 0�-90� cutting (!). By considering each type of orthogonal
cutting, some common types of machining can be
understood more clearly.
 

1�The three types of orthogonal culling. The first number is the angle between cutting edge and grain direction; the second is the angle between direction of cutting and grain direction.

90�-0� cutting (planing along the grain)
Parallel-to-grain culling is best typified by the standard hand plane. The chip forms as the plane is pushed along the board. The typical cutting action involves a cyclic sequence of events (2), The iron separales fibers lengthwise to begin a chip (A). As the knife advances, the separated chip slides up the iron. The chip is now a cantilever beam that resists bending. It lengthens itself by failure of the wood in tension perpendicular to the grain well ahead of the knife edge (B). Finally, the chip is so long that bending stresses equal the strength of the wood and the chip breaks (C). The iron advances to the fracture point and begins to lift ihe next segment of chip (D) and so on. The chip, produced in a long jointed curl (see Figure /, page 144), is referred to as a Type I chip in 90�-0� cutting.
The typical plane cutter or iron is set al an angle of 45�. Sharpening to an angle of 30� leaves a 15� clearance angle. If the rake angle becomes too great, the friction of the chip upon the iron face would increase and Ihe efficient bending and breaking action would be lost-
ward compression and ii smaller component of upward lifting are transmitted to ihe chip. Failure may occur as a diagonal plane of shear, bending the fiber structure, so curl of deformed cell structure. This is classified as a Type II chip (3). The cutting edge produces the surface as it dislodges cell structure. Greater force is required because of the compression resistance. Where the tool is well controlled and a reasonably thin chip is taken, chip formation takes place quite uniformly and an excellent surface is produced. Some special hand planes with a cutting angle of only about 30� are designed to take
As small (or even negative) cutting angles are used, the grain. The knife edge produces the surface as it shears free the cell structure. As the wood fails in compression, the damaged cell structure packs up against the cutting face and may form a wedge that transmits force and causes failure out ahead of the knife edge, often below the projected cutting plane. The failure is erratic and leaves an irregular surface, and is accompanied by an irregular chip of mangled cell structure. This is Type III chip formation (4). With very low cutting angles, a smooth surface and uniformcutting action occur only when a thin enough chip is taken to form Type II chips. This is the cutting action of scrapers.

2�The cutting action in planing wood. The cut begins at A. The chip bends as it slides up the knife, and the wood fails ahead of the edge due to tension perpendicular to the grain, B. Finally the chip breaks, � whereupon the next segment of the cut starts, D. In 90�-0� cutting this is known as a Type I chip, produced by a relatively large cutting angle.

3�At small cutting angles, the face of the knife produces more forward compression than upward lifting. Failure occurs as a diagonal plane of shear right At the cutting edge. With, enough force and a thin chip, the workpiece surface can be left in excellent condition. This is a Type II chip in 90-0" cutting.

4�At very small (or even negative) cutting angles, force is transmitted mainly as compression parallel to the grain. The damaged cells pack up against the culling face, often causing erratic failure ahead of and below the edge. This is a Type III chip in 90�-0� cutling. The snowplow effect can only be avoided by taking a very thin chip, whereupon it becomes Type II cutting. Cabinet scrapers work this way.

The quality of cutting depends mainly on two factors: the grain direction and the mechanics of chip breakage. Figure 2 on page 147 assumes perfectly straight grain, which in reality is more the exception than the rule. Usually some degree of cross grain exists wherein the fiber direction either rises ahead of the projected line of cut or leads down below it. The former case is termed cutting with the grain, the latter cutting against the grain (I), Cutting with the grain is preferable, since the splitting of the wood associated with chip formation projects harmlessly into the next chip segment which subsequently will be removed. The cut produces a new surface generated by the continuous severance at the tool edge. Culting with the grain is very efficient because most of the chip segments fail readily due to cross grain. The woodcarver will find that cutting with the grain at an acute angle to the grain is a fairly efficient way to remove large amounts of slock. At the same time, 90�-0� Type I chip formation using a hand chisel can be painfully undesirable in carving. The case in point involves carving with the grain, where a splinter-type chip slides all the way up the face of the tool and jabs the carver in the hand (2). I have numerous scars on the outside heel of my left hand thanks to this situation. I collect another scar every time I fail to wear a glove
By contrast, cutting against ihe grain can result in chip formation where the splitting projects below the intended plane of cutting. The resulting surface is called chipped or torn grain. In some cases, as in planing the edge of a flatsawn board with spiral grain or in passing a flatsawn board with diagonal grain through a surface planer, the board can be alternately turned end-for-end so that culling will occur with the grain. In other cases, however, as with the bulge of grain direction associated with a knot, some cutting must take place against the grain. To minimize the depth of torn grain, the breaking length of the chip segments must be controlled. One approach is to take an extremely thin cut, in which the chip segments break frequently. Otherwise, a "chipbreaker"  must be introduced, such as the cap iron on a hand plane (3). The cap must be located suitably close to the cutting edge and must fit tightly enough to the face of the iron so the chip will not lodge but will slide up easily and bend beyond its breaking point quickly. The Face and mating edge of the cap should be shaped as precisely as the cutting edge of the iron itself, for the �a� iron is an integral pan of the cutting mechanism.
The importance of the clearance angle in the cutting process should be appreciated. As with the "springback" of the wet sponge, it is also true that some deflected cell structure will recover alter the chip lorms. In order for the back of the cutting edge to clear this material, fric-tional drag and pressure against the back of the knife must be eliminated. If the cutter were infinitely sharp, of course, little clearance would be needed, and recommended clearance angles of up to 15� may seem excessive. However, such large clearance angles are probably safeguards against less-than-perfeel sharpening. If the back (beveled side) of the cutter is not perfectly flat, the clearance angle is reduced. As will be pointed out in discussing sharpening, it is crucial that no portion of the cutter be deeper than the cutting edge itself.
In 90�-0� cutting with a hand chisel, the back of the tool itself guides the cutting direction. The clearance angle is effectively zero, and springback is automatically compensated by the angle at which the tool is held. Frictional resistance is overcome by whatever force is applied to advance the chisel.
Most hand planes used along the grain involve 90�-0� cutting, whether they operate on the whole surface of the wurkpiece (as in planing a flat surface) or whether they plow a groove or form a rabbet. Spokeshaving down a canoe paddle is another example of 90�-0� cut-ling where tool geometry and depth of cut are fixed by tool design and adjustment. Rough-shaving an ax handle with a drawknife and taking long shavings along the grain with a pocketknife are also 90�-0� cutting; in these cases the cutting angle, clearance angle and depth of cut are controlled by the way the tool is held.

	1�In ihe real world, the wood grain is rarely parallel to the cutting direction Usually ihe fibers are rising ahead of (he line of cut, and cutting with the grain leaves a very smooth surface (A). When the Fibers lead down below the line of cut, cutting against the grain leaves a chipped surface (B). We usually reverse either the work or the tool, to go with the grain.

	2�In woodcarving, ihe splinter-type chip resulting from 90-0� cutting with ihe grain can slide up the tool face and jab an unwary carver In the hand.

3�In 90�-0� cutting with a hand plane, the cap Iron minimizes frorn grain by breaking the chip near the cutting edge.

Peripheral milling (machine planing)
Orthogunal cutting in the 90�-0D mode has a counterpart in peripheral milling in the cases of the typical jointer, single surfacer, spindle shaper and router� wherever a revolving cutterhead operates along an edge or face of a board.
The cutting action is modified by the path of each cutting edge, which by combined revolution of the cutter-head along the surface of the workpiece follows a trochoidal path (1/. Each cutting edge takes a curved chip from the workpiece. Customarily, rotation of the cutterhead moves each knife in a direction opposite ihe relative direction of the workpiece, representing the up-milling condition (1, inset). In most rotary cutterhead designs the cutting angle is decreased to between 10� and 30�. This requires more power, but the chip type produced approaches a scraping Type II or Type III chip rather than a splitting action, as in Type 1 chips, and there is less uncontrolled splitting ahead of the knife edge. The surface generated by the overlapping cutting arcs of successive edges is wave like. These waves are often visible and are known as knife marks. Figured shows an extreme case of knife marks in crudely planed structural lumber�only four to the inch. The marring of the surface is plainly visible. However, in finish lumber the best surfaces are produced by 12 to 25 knife marks per inch. In this case, the height of the waves is typically quite small, and may not be seen easily with the naked eye. Their visibility is the result of crushed or buckled cells, rather than the actual surface irregularity of the waves (3). When the number of knife marks per inch exceeds 30, unless the culling edges are extremely sharp, the surface may actually get worse. The chip gels so small that each cutting edge does not bite, but rather rides over the surface, as with the table knife and ihe wet sponge. Frictional heat also may be produced and the resulting surface, although apparently smooth,

	1 In peripheral milling, each cutter actually follows a trochoidal path relative to the work piece, the result of culter-head rotation plus feed. Each cutter takes a curved chip from the workplece, usually by up-milling (inset).

2�Crushed cells delineate knife marks�about fc.ur lo the inch�on this eastern hemlock board.	3�Light reflection reveals closely spaced knife marks on this butternut board