Question: How can chipped or torn grain from solid wood machining be eliminated or at least minimized?
Chipped or torn grain occurs when short particles of wood are broken out below the surface, usually by wood splitting ahead of the knife and then failing as a cantilever beam (left and below). Chipped grain is associated with sloped grain. The split follows the grain ahead of the knife and below the surface. Chipped grain is caused by the knife cutting against the grain with high rake angles, large depths of cut, machining very dry or wet wood, or too few knife marks per inch.
The mechanical properties and dimensions of wood vary as moisture content varies. Moisture content problems generally result in machining defects of the wood surface or tool wear. Problems discussed here are related to planing but could apply to other processes where a wedge-shaped tool removes a chip from a workpiece by mechanical failure. Some mechanical properties related to moisture content are shown in the table on the next page.
Studies have shown the most severe chipped grain occurs at green moisture content or with over-dried wood below 6 percent MC. Chipped grain is worse with the combination of low tensile strength perpendicular to the grain, resistance to splitting and high modulus of rupture in static bending, particularly at extremely high or low moisture content.
Study results from planing hard maple at 8 percent MC and 1/16-inch depth of cut have shown the most severe chipped grain occurs when machining against slopes of grain up to 15 degrees. Also, moderate rake angles of 20 to 25 degrees produce the shallowest chipped grain at 30 or more knife marks per inch.
At rake angles equal to or less than 25 degrees, the depth of defect increased as the number of knife marks decreased. At 10 knife marks per inch, the depth of defect was approximately the same for all rake angles. After planing with a rake angle of 25 degrees, the depth of defect increased more rapidly as the number of knife marks decreased. Generally, moderate feed rates that provide 20 or more knife marks per inch provide the shallowest chipped grain.
Generally, wood should be machined with moderate rake angles 15 to 30 degrees at 8 percent moisture content, 20 or more knife marks per inch, and moderate depths of cut of 1/16 inch.
However, many wood machining operations exceed one or more of these conditions.
Although moderate rake angles, depths of cut, feed rates and moisture content reduce chipped grain, proper machine maintenance and knife sharpening are paramount to minimizing waste from machining against the grain.
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