More about knife marks per inch

Chipped or torn grain frequently occurs when machining wood against the grain. As previously discussed in FDM January 2006 and January 2009 chipped or torn grain occurs when wood splits ahead of the knife below the surface and then fails as a cantilever beam. The most severe chipped grain occurs when the grain is sloped at about 15 degrees.

Other types of wood failure generally occur at 20 degrees or greater slopes of grain. Chipped grain can be reduced by reducing the chip stiffness, i.e. the thickness of the chip, by decreasing the feed rate or increasing the cutterhead revolutions per minute (rpm).

The earlier FDM articles also show that the shallowest defects occur at 20 or 30 knife marks per inch with moderate rake angles of 15, 20, and 25 degrees. Rake angles of 10 degrees or less generally produce high cutting forces which may accentuate chipped or torn grain. Lower rake angles may reduce the frequency when machining against lower slopes of grain, but generally produce deeper chipped grain when machining against steeper slopes of grain of 10 or 15 degrees.

The previous description indicates that chipped or torn grain can be at least minimized at moderate machining conditions. These conditions first include a combination of number of knives on the cutterhead, feed rate, and revolutions per minute to provide 20 or more knife marks per inch. These conditions also require a moderate depth of cut and rake angle. A peripheral milling demonstration against the grain was developed to show that chipped or torn grain could be substantially reduced when machining against the grain with the aforementioned conditions.

An eight-inch diameter cutterhead was fabricated to be mounted on a metal milling machine without a chip breaker or pressure bar (Figure 1). Three layer, laminated red oak specimens were fabricated for the milling (Figure 3). The red oak specimens were laminated so that the two outside laminates would be milled parallel to the grain and support the middle lamella which could be milled against a 15-degree slope of grain. The red oak specimens were maintained at eight-percent equilibrium moisture content conditions before machining them against the grain of the middle lamella. The specimens were milled at one-sixteenth inch depth of cut, a common finish milling depth of cut. The specimens were milled at 10 knife marks per inch and 20 knife marks per inch (Figure 3).

Reducing knife marks 

The chipped or torn grain is substantially reduced by increasing the knife marks per inch from 10 to 20 (Figure 3). The end clip or split of the middle lamella is also substantially reduced as knife marks per inch increase from 10 to 20 (Figure 3). Overall, reducing chip dimensions and cutting forces as well as changing the direction of cutting forces reduces the severity of chipped or torn grain.

For instance, an eight-inch diameter cutterhead has a greater tangential cutting force parallel to the feed direction than a one inch or smaller router bit at the same width of cut. Further, the smaller diameter cutter has more of a lifting action upon the workpiece when conventional or upmilling then a larger diameter cutterhead.

Once again Figure 3 show the surface quality benefits of increasing knife marks per inch. Although the figures show the benefits up to 20 knife marks per inch, increasingly knife marks to more than 20 would probably further enhance surface quality. Obtaining the desired number of knife marks per inch requires the proper sharpening and/or jointing of the cutting edges or knives into the same cutting circle.

Calculating the knife marks per inch was discussed in FDM January 2009. Figures 3 shows that reject parts may be eliminated and subsequent processing such as finishing cuts or sanding can be substantially reduced by optimizing knife marks per inch.

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About the author

Harold A. Stewart wrote a tooling column for FDM magazine, looking at ways to improve the cutting process. He was previously an educator and consultant to wood products companies.