From time to time lumber and furniture manufacturers inquire about an increased incidence of chipped grain, torn grain, or tearout. The defect results from a split initiated at the knife edge and continuing along the grain until the cutting forces cause the chip, loaded as a cantilever beam, to fail in bending away from the workpiece. The defect, tearout, has been amply described. The defect related to knife marks per inch was further described here.
Tearout is more severe, deeper, and more frequent when lumber has been over-dried. Over-dried lumber has been described as being more brittle, which fractures more readily along the grain. Hardwood and softwood lumber become over dried when their moisture contents are under about 8 percent. Further, over-dried wood may not recover its strength properties fully or at all by increasing the moisture content.
Wood may or may not become more brittle as lumber dries below 8 percent moisture content. However, the strength properties continue to change, in particular the tensile strength perpendicular to the grain decreases. The modulus of rupture in static bending increased and the tensile strength perpendicular to the grain decreased below 8 percent moisture content in a published study by Norman C. Franz in 1958, Analysis of Wood-Cutting Process. Table 1 shows some strength data for three diverse species of wood at three moisture contents (1.5 percent, 8.0 percent and saturated). The modulus of rupture continues to increase from 8 percent to 1.5 percent moisture content; however, the tensile strength significantly decreases from 8 percent to 1.5 percent moisture content. This combination of changes of strength properties would accentuate the severity of tearout in over-dried wood. The split along the grain ahead of the knife edge would be elongated.
The three species (sugar pine, yellow birch, and white ash) in Table l have diverse physical and mechanical properties. These species are a conifer, a diffuse porous hardwood, and a ring porous hardwood, respectively. Many species probably react similarly when over-dried. Any number of improper kiln operating procedures or mechanical problems could cause over-dried lumber.
Reaching low EMCs
After examining Table 1, some may question if equilibrium moisture content conditions (EMC) under 2 percent could be reached in a dry kiln. This author personally recorded EMC's below 2 percent numerous times with readings from a sling psychrometer at room temperature on cold winter days in the Wood Technology Laboratory in Michigan. Consequently, EMC's below 2 percent could easily be attained in conventional dry kilns which frequently have temperatures up to 180F.
Almost every year from the late fall to early spring inquiries about more frequent and severe chipped grain or tearout increase. The increased frquency and severity at these times of year can usually be attributed to over-dried lumber, particularly at and near the surface of the lumber. Extremely low EMC's result from heating cooler and dryer air. In addition, the wet bulb temperatures may be too high, The wet bulb temperatures are frequently too high because the air velocity over the wick or sock is too low or the wick has not been changed and has become encrusted. Either situation can cause higher wet bulb temperatures. Consequently, the kiln controls sense a higher EMC inside the dry kiln and vent what the controls sense as humid air. The air is replaced by colder dryer air which is heated up to the kiln operating temperature creating very low EMC's.
Although lumber may be about 6 percent moisture content, the zone at or near the surface may be near one percent. The wood at and near the surface may well have reduced tensile strength perpendicular to the grain as depicted in Table 1.
When machining kiln dried lumber, the zone at and near surface, where the tensile strength perpendicular has been reduced, will be machined first. Hence, the more frequent and severe tearout will probably be initiated near the surface which has been over-dried. The tearout can generally be minimized with moderate rake angles, depths of cut, and 20 or more knife marks per inch as discussed in FDM January 2006 and 2009. However, tearout or chipped grain may not be eliminated once lumber has been over-dried.