Wood machining defects generally result from a combination of factors that cause one or more of the mechanical or physical properties of the wooden workpiece to be exceeded. These factors may include workpiece design, tool geometry, and/or machining parameters. The photo on page 58 shows a workpiece that was affected by all of these factors. The semicircular pattern was machined with a full bullnose cutterhead in the conventional up-milling feed direction. The cleavage failure occurred because the machining stresses exceeded the tensile strength perpendicular to the grain and/or the shear strength parallel to the grain of the oak workpiece.
The workpiece was designed so that the cutterhead is up-milling against the most adverse slope of grain (about 10 to 15 degrees). A double spindle shaper with counter revolving cutterheads would allow the workpiece to be down-milled with the grain for half of each segment of the workpiece. Some additional sanding may be required where the apposite revolving cutterheads overlapped major cleavage failure and subsequent part rejection would probably be avoided.
In wood machining, the milling cutters actually have two major rake angles: the radial and axial rake angles. The radial rake angle is the deviation in degrees of the tooth face from a radial line to the cutting edge. The axial rake angle is the deviation in degrees of the cutter. Shaper cutters, router bits, and moulder knives may also have combinations of chamfers or curved cutting edge segments that connect the axial and radial cutting edges. These oblique rake angles are measured in the plane normal to the cutting edge on the chamfer or to the tangent of the curved cutting edge.
The oblique rake angle can be calculated from the equation:
A = oblique rake angle (degrees)
B = radial rake angles (degrees)
C = axial rake angle (degrees)
D = chamfer or angle of tangent (degrees)
tanA = tanB cosC cosD + tanC cosB sinD
Inspection of the previous equation indicates that a low axial rake angle would exist at the top and bottom of a full semi-circular bullnose cutter. By segmenting the cutting edge into upper and lower percent round arcs, the radial and axial rake angles (the hook and face shear angles) may be increased to improve the surface quality by reducing the cutting forces. Again, some additional sanding may be required where the upper and lower cutting edge arcs intersected, but a reject would be avoided. Increasing the radial and axial rake to moderate angles between 10 and 30 degrees may substantially reduce cutting forces to reduce wood machining defects.
Another way of reducing cutting forces to minimize wood machining defects is to machine the first half of the semicircle, turn the part over and machine the second half of the semicircle. Once again, some additional sanding may be required where the cuts overlap. The cutting forces may be further reduced by successively cutting to the desired dimension with more than one pass on the top and bottom of the pattern. A rough cut pass or two followed by a finishing cut reduces the stresses in the workpiece which cause wood machining defects.
Still another method to reduce cutting forces and excessive stresses in the workpiece is by changing the orientation of the tool axis. Shapers and routers generally have the cutting tool's rotational axis perpendicular to the workpiece surface. Reorienting the cutting tool's axis at an angle or even parallel to the surface may change the direction of the cutting forces to sufficiently reduce the critical stresses to reduce wood machining defects. A moulder has one or more heads that shape all four surfaces. Coordinating parts of the moulding pattern or partial roughing cuts among the cutterheads may also sufficiently reduce the stresses that cause wood machining defects. Most wood machining defects can be eliminated by reducing cutting forces that cause critical stresses that exceed the strength properties of the wooden workpiece.
Still another method is to reduce the stiffness of the individual chips being removed by each flute or cutting edge. Reducing the depth of cut or rough cuts followed by a finish cut will help reduce the chip stiffness. Another way to reduce chip stiffness and perhaps splitting ahead of the knife edge is to increase the revolutions per minute (rpm) of the cutterhead and/or reduce the feed rate of the machine. Both increasing the rpm and reducing the feed rate reduce the average chip thickness or stiffness so that the chip may be separated from the workpiece without splitting. Consequently, a satisfactory surface and workpiece may result.
Once again moderate rake angles and machining parameters produce satisfactory surfacing results. Number of parts produced may drop, but productivity may increase by following guide lines or consider changing machining conditions. Paramount for satisfactory wood surfaces is selecting the proper tool geometry, woodworking machinery, machining parameters and maintaining all of the above. These all include proper moisture conditioning, gluing and finishing to maintain a satisfactory surface.
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