The importance of tooling clearance angles.

The rake or hook angle is generally considered the most important element of tool geometry for wood machining. The clearance angle is also important, particularly in combination with low rake angles, because the workpiece springs back after separation from the chip. This springback is common in resilient materials such as MDF and other wood products. The clearance is provided by the clearance angle as measured from the new surface of the workpiece to the clearance tool face.

When cutting MDF, tool wear as measured by edge recession and cutting forces was less for rake angles between (but not including) 10 and 30 degrees. Small rake angles ( 10 degrees or less) characteristically have high cutting forces that increase the indentation and compression in the cutting zone at and near the knife edge. Higher frictional and cutting forces result when the newly formed surface and adjacent workpiece subsurface spring back just behind the knife edge. Both the cutting and frictional forces contribute to extreme temperatures, which increase tool wear and damage the workpiece. The damage results in subsurface crushing, fuzzy or raised grain, glazing and even burning. Tool force data were taken and analyzed to emphasize that cutting forces can be reduced by providing larger clearance angles in combination with small rake angles in order to reduce wood machining defects and tool wear.

Methods

A sample of 46.5-pound per cubic foot density MDF was obtained from a single source and maintained at 8 percent equilibrium moisture content conditions. Test C2 tungsten carbide (WC-6% Cobalt) blanks for the cutting edges were all taken from the same batch. All the MDF and C2 specimens were randomly distributed for the tool wear tests.

The reported results here are a portion of a study including seven rake angles. Three clearance angles (5, 10 and 15 degrees) were tested in combination with a 10-degree rake angle in turning tests previously described. The cutting combinations were replicated three times at a 0.005-inch depth of cut and 550 rpm. The measured dependent variables were the normal and parallel tool force components.

The turning test was intermittently interrupted 12 times so tool forces could be correlated to length of cut. The initial and final forces were also recorded. The length of cut between any two radii of the disk was easily calculated by the length-of-arc equation for a spiral, which is generated by the tool path on a lathe.

The tool force components parallel (F01p) and normal (F01n) to the direction of tool travel relative to the workpiece (Fig. 1) were recorded by an oscillographic recorder attached to a lathe dynamometer as previously described. The length of cut (spiral) varied slightly for each combination but was approximately 33,000 inches long.

Results and discussion

Tool forces increased more rapidly for low clearance angles in combination with a 10-degree rake angle (Fig. 2). The normal (F01n) and parallel (F01p) forces were higher for a 5-degree clearance angle than for the other clearance angles. The forces for a 10-degree clearance angle were higher than for a 15-degree clearance angle. According to these results, a 10-degree rake angle should have at least a 10-degree clearance angle.

The normal force exceeds the parallel force for the 5-degree clearance angle at approximately 22,000 inches length of cut. When the normal force exceeds the parallel force, the knife is essentially plowing instead of cutting. The increasing positive normal force indicates how much the tool was being pushed away from the workpiece and indicates increased workpiece compression. Charting the results shows vividly the increased tool forces (tool wear) for a small clearance angle.

Tool forces have been related linearly to rake face and clearance face recession. Consequently, these results reflect how the tool edges receded or were worn away for the three clearance angles tested. The edges would recede most for the 5-degree clearance angle.

Wood machining situations frequently exist where rake angles and clearance angles are too small. These situations include sawing, shaping, moulding and routing. Often, at some section of a pattern tool's knife edge, the true rake angle is 10 degrees or less and a side or back clearance angle has not been ground along sections of the pattern. As a result, tool wear accelerates at these knife sections with small rake and/or clearance angles and tool costs for maintenance and replacement are increased. In addition, workpiece surface quality decreases rapidly. The workpiece subsurface can be crushed and other common wood machining defects (raised grain, fuzzy grain, glazing and burning) can become more severe. Because of these defects, the workpiece requires more subsequent processing, such as sanding, or may even be rejected. Generally, insufficient clearance angles, particularly in combination with small rake angles, increase tool wear, machining defects, and most machining costs.

Sufficient clearance angles cannot be provided in some machining situations, such as milled-to-pattern knives. These knives are manufactured to be sharpened on the rake face. The pattern is milled into the extended clearance face of the knife. Consequently, no clearance angle exists. Milled-to-pattern knives such as for molders, generally dull more rapidly and cause many of the defects already described. Also, the milled-to-pattern knives generally require frequent sharpening. When possible, cutter-heads and machining situations should be designed to provide satisfactory clearance angles.

CUTTING TOOLS

These tool force data emphasize that cutting forces can be reduced by providing larger clearance angles to accompany small rake angles, thereby reducing tool wear and wood machining defects.

Smaller rake angles have higher cutting forces. The normal force usually increases faster than the parallel force. The increase of the normal force indicates the material is being compressed and forced under the clearance face of the knife edge as it dulls. The increased compression of the material leads to more springback or recovery, which in turn requires even larger clearance angles. Rake angles less than 10 degrees may need clearance angles as large as 15 degrees or more.

Although these results pertain to tungsten carbide when machining MDF, an optimum wedge or knife angle exists for each combination of rake and clearance angles, tool material and workpiece material. Additionally, a high rake angle of 25 degrees may have a lower optimum clearance angle of 5 or 10 degrees. Optimum tool geometry may vary from high-speed steel to polycrystalline diamond. However, sufficient clearance is necessary for good machining.

Conclusion

These results indicate rake angles of 10 degrees should have a clearance angle of at least 10 degrees and that smaller rake angles may require even larger clearance angles. MDF is generally machined along the panel edge so that the tool edge essentially cuts across the fibers and causes springback; hence, these clearance recommendations should be satisfactory for machining MDF and other reconstituted wood products. Although exceptions may occur, these recommendations provide a basis for tool and machine design. The guidelines can be modified as additional results from rake and clearance angle studies become available. When wood or wood products are machined, a sufficient clearance angle is paramount for good tool life and surface quality, particularly when rake angles are 10 degrees or less.

The author is a senior research assistant of the Mississippi Forest Products Laboratory. This article appeared in the October 1991 issue of Forest Products Journal.