Knife Cutting Problems

Grain direction in wood influences surface quality of a machining operation. When the direction of cutterhead rotation is against the grain, and the workpiece is fed in a direction opposite to that of cutterhead rotation, the chips produced become splinters and are broken away rather than cut, producing an irregular, rough surface. If direction of cutterhead rotation is with the grain, a smoother surface results. Torn grain is very likely to occur around knots and wood having wavy or interlocked grain. It can be eliminated or at least reduced in severity by increasing cutterhead velocity and/or the number of knives or, machining at a lower rate of speed. Using high-speed still photography, the mechanics of chip formation and the production of a good surface were observed: 1) at cutting angles between 10 and 20 degrees, the chip forms and moves along the face of the knife and the blade does not scrape along the cut surface; 2) for removal of excess material, the chip is lifted, splitting off the surface ahead of the knife provided that the grain direction is such that it runs out of the surface with respect to cutterhead rotation. Concerning the direction of cutterhead rotation with respect to feed, i.e., “up-cutting” versus “down-cutting”, it is pointed out that the major advantages of down-cutting are: 1) chips are not split ahead of the cutting edge, thus discouraging torn grain; 2) the cutting edge is in contact with the work for a shorter distance, thus increasing tool life; 3) less power required to operate feed mechanism because down-cutting tends to draw workpiece through the machine; 4) for the same knife cuts per inch, down-cutting permits a higher production rate; 5) principle cutting forces tend to hold the workpiece against the reference surface of the machine thereby reducing vibration and flutter of the work. Disadvantages of down-cutting: 1) machines cannot be hand-fed because the tendency of the workpiece to be drawn into the cutterhead could result in accidents and injury; 2) feed mechanism must be back-lash free and the stock firmly clamped; 3) more power is required to make a cut because the knife cuts across the grain.

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