Raised grain is simply an uneven surface that frequently occurs as a result of a moisture content change such as swelling or shrinking, or springback after machining a wooden workpiece.
Raised grain generally results from inherent or mechanically induced density variations of the wooden workpiece. An example of this is the denser latewood and less dense earlywood of Southern yellow pine. When the moisture content of Southern yellow pine changes below fiber saturation point, the denser summer or latewood shrinks more as the moisture content decreases and swells more as the moisture content increases (Figure 1).
The less dense earlywood can also be compressed by the denser latewood when machining (Figure 2). The compressed earlywood may expand when exposed to moisture or springback after machining which raises the wood above the machined surface (Figure 3).
Sometimes the repeated cycling of compression between the earlywood and the latewood can cause separation between the early and latewood (Figure 4), often referred to as shelling or shake. Generally, all peripheral milling methods such as planing, routing, shaping, etc. can cause subsurface compression (Figure 5), which in turn may cause raised grain.
Reducing raised grain
Machining parameters can be set or adjusted to minimize and perhaps eliminate raised grain. Tool geometry that may accentuate raised grain are small rake and clearance angles. Small rake and clearance angles increase the compressive forces on the workpiece surface during peripheral milling such as planing.
Consequently, dull tooling causes greater compressive forces. The gradual rounding of the knife edge reduces the rake and clearance angles. Dulling may even cause negative rake and clearance angles at the knife edge which greatly increases compressive forces and enhances raised grain. Therefore, moderate rake angles from 15 to 25 degrees and clearance angles above 12 degrees may reduce raised grain.
Paramount for reducing raised grain is proper tool maintenance to maintain the rake and clearance angles. When knife jointing is required, such as for planer knives, the joint should be as narrow as possible. A jointed knife essentially has a zero-degree clearance angle at the knife edge. Maintaining optimal tool geometry can reduce raised grain.
Optimizing machining parameter combinations of feed rate, revolutions per minute (rpm), and depth of cut can also minimize raised grain. Generally, combinations of feed rate, rpm and number of knives in the cutting circle should be set to provide 16 to 60 knife marks per inch, although some exceptions to this recommendation exist. Since too much recycling of compressive stresses may cause growth ring separation (Figure 4), too many knife marks per inch may cause raised grain or growth ring separation.
Since the number of knives, cutting edges and rpm of most woodworking machines are fixed, the number of knife marks per inch are obtained by adjusting the feed rate. The next easiest is to change the number of knives of a cutterhead or flutes of a router bit. An optimal combination of machining parameters to reduce wood machining defects usually exists.
Too shallow of a depth cut can also increase raised grain, particularly with small rake and clearance angles or dull knives. A shallow depth of cut can be caused by too low of a machine depth setting or by too small of a bite per tooth caused by machine settings previously discussed. If the depth of cut and/or feed per tooth are too small, the knife edges will do more rubbing and crushing and less cutting of the wooden workpiece, particularly with dull knives. The additional rubbing and crushing of the wooden surface may also increase surface glazing and burning as well as tool wear.
Minimizing a change of moisture content below fiber saturation point can also minimize raised grain of the wooden workpiece. As with other woodworking defects such as from drying or gluing, wood seasoning to the proper moisture content will minimize wood machining defects such as torn or raised grain. Wood products should generally be dried, conditioned and equalized to 6-8 percent moisture content to minimize wood machining defects such as raised grain.
Proper wood seasoning minimizes the shrinking and swelling of the wooden workpiece in service. Proper wood drying or seasoning also minimizes the amount of subsurface crushing or compressive failures which causes raised grain. Proper wood seasoning is essential for wood machining as well as for other wood processing.
Recommendations for minimizing raised grain of wood also generally apply to wood composites with varying or uniform densities. Proper tool geometry, woodworking machine settings, tool and machine maintenance and wood seasoning are all paramount to producing a satisfactory surface.
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