Fuzzy grain occurs when wood fibers, usually above the surface, aren't cleanly severed from the workpiece surface. In extreme conditions, wood fibers may be torn out from below the surface.
Fuzzy grain results from contributing factors such as tool geometry, machining parameters, moisture content and species. Many of the factors discussed for dealing with raised grain (CabinetMaker, May 2006) also apply to minimizing fuzzy grain.
Low density, more fuzz
Low-density wood often has more fuzzy grain. For instance, the early wood of Southern yellow pine frequently exhibits fuzzy grain. Reaction wood, compression wood from conifers or tension from broadleaf species (Figure 1) may develop fuzzy grain.
Reaction wood often has lower-density, gelatinous fibers, which are not as stiff as normal wood. Consequently, they're not severed as cleanly. Species such as cottonwood are known to have large zones of reaction wood and fuzzy grain. Poor tool geometry accentuates fuzzy grain from machining low-density or reaction wood.
Small rake or clearance angles or dull tools may cause tearing of fibers below the machined surface of reaction wood (Figures 2 and 3). Tool edges formed by low rake and clearance angles or dull tools may indent and bend fibers, particularly weaker reaction wood fibers, until they tear out below the surface (Fiqure 2) on end grain.
Clearance angles should be 12 to 20 degrees and rake angles should be 15 to 25 degrees to minimize fuzzy grain.
Since machining parameters other than tool geometry (CabinetMaker, May 2006) may cause or accentuate fuzzy grain, parameters may be adjusted to minimize fuzzy grain. This includes feed rate, revolutions per minute and number of cutterhead knives.
The combinations of feed rate, rpm and number of knives should provide 16 to 60 knife marks per inch. The combinations may have to be adjusted upward or downward for the optimal number of knife marks.
Too shallow of a depth of cut can increase fuzzy grain, particularly with small rake and clearance angles or dull knives. Too shallow of a depth of cut or too short of a bite per knife edge reduces the initial incising of the wood fibers and their subsequent clean severance from the surface.
The additional rubbing and crushing from a shallow depth cut can enhance fuzzy grain.
Minimizing a change of moisture content below fiber saturation point can also minimize fuzzy grain. Machining wood or wood products at high moisture contents can cause severe fuzzy grain, especially sanding or abrasive planing processes (Figure 4).
Wood products should be dried, conditioned and equalized to 6 to 8 percent moisture content to minimize wood machining defects. Wood generally springs back more at higher moisture content, which causes additional rubbing.
The recommendations for machining solid wood generally apply for machining wood composites, such as particleboard and medium density fiberboard. The lower-density cores of many wood composites frequently have a higher occurrence of fuzzy grain. Moderate tool geometry and machine settings generally produce a satisfactory surface when machining wood composites.
Proper tool geometry, machine settings, tool and machine maintenance, and controlling workpiece moisture content are all paramount in producing a satisfactory surface when machining wood and wood composites.
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