Fuzzy grain occurs when a wood fiber or groups of 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 one or more contributing factors, such as tool geometry, machining parameters, moisture content and species. Many of the factors discussed for dealing with raised grain (FDM, May 2006) apply to minimizing fuzzy grain.
Low density, more fuzz
Low-density wood or wood species generally have more fuzzy grain. For instance, the early wood of Southern yellow pine may frequently exhibit fuzzy grain. Reaction wood, compression wood from conifers or tension from broadleaf species (Figure 1), may develop fuzzy grain from leaning trees or wood in or near knots.
Reaction wood generally 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, as a result, frequently have fuzzy grain. Fuzzy grain from machining low-density or reaction wood can be accentuated by poor tool geometry.
Small rake or clearance angles or dull tools may cause extreme tearing of the fibers below the machined surface of reaction wood in some species (Figures 2 and 3). The tool edges formed by low rake and clearance angles or dull tools may indent and bend the fibers, particularly weaker reaction wood fibers, until they tear out below the surface (Fiqure 2) on end grain.
Clearance angles should generally be 12 to 20 degrees and rake angles should generally be 15 to 25 degrees to minimize fuzzy grain.
Since machining parameters other than tool geometry (FDM, May 2006) may cause or accentuate fuzzy grain, machining parameters may be adjusted to minimize fuzzy grain. These parameters include the feed rate (fpm), revolutions per minute (rpm) and number of knives of the cutterhead.
The combinations of feed rate, rpm and number of knives of the cutterhead 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.
Again, as with raised grain, 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, as well as raised grain.
Minimizing a change of moisture content below fiber saturation point can also minimize fuzzy grain of a wood workpiece. 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 generally be dried, conditioned and equalized to 6 to 8 percent moisture content to minimize wood machining defects, such as fuzzy or raised grain. 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.
Summing up, proper tool geometry, woodworking 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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