Large volumes of hardwoods and softwoods are machined, generally, along the grain for furniture, windows and moulding. Because lumber or wood parts cannot always be machined with the grain, chipped or torn grain frequently occurs.

As discussed in FDM January 2006,  "How to reduce chipped grain" chipped or torn grain occurs when wood splits ahead of the knife below the surface and then fails as a cantilever beam. Chipped grain is associated with sloped grain. The split follows the grain ahead of the knife and below the surface.

Chipped grain is accentuated by knife cutting against the grain with high rake angles, large depth of cut, machining overly dried or wet wood, and too few knife marks per inch.

Generally, wood should be finished machined at least 20 or more knife marks per inch. Increasing the knife marks per inch reduces the stiffness of the chip, which in turn reduces the length of split ahead of the wedge-shaped tool. Reducing the split ahead of the knife reduces the severity (depth) of chipped grain.

The number of knife marks per inch can be easily calculated from the following equation:

KM/in =(rpm)N 

(fpm) 12 in/ft 

KM/in = Knife Marks per inch 

rpm = Cutterhead revolutions per minute 

N = Number of knives (cutting edges or teeth per revolution) 

fpm = Feed rate in feet per minute 

12 = Conversion factor for feet to inches 

Knife mark study methods

A simple demonstration was carried out to show the benefit of increasing knife marks per inch. Hard maple lumber, including all sound growth characteristics such as knots, was planed with the top front head of a moulding machine.

All growth characteristics were included because the severest machining defects generally occur in the areas of sloped grain associated with the growth characteristics. The lumber was conditioned at 8 percent moisture content.

Machine parameters except for feed rate and rake angle were not varied. The cutterhead diameter was 6 inches and rpm was 4,700. The rake angle was varied from 10 to 45 degrees in increments of 5 degrees. The 40-degree rake angle was omitted. The feed rate was varied to nominally give 10, 20 and 30 knife marks per inch.

The depth of cut was constant at 1/16 inch, a common setting for finish knife planing. The depth of cut does not affect the average or maximum chip thickness per knife as much as feed rate; hence, the feed rate and rake angle interaction was studied. The constant depth of cut (1/16 inch) also served as a basis for comparing depth of defect results.

Sufficient material was planed at each combination of test variables to give many examples of chipped grain representing the severest conditions likely to occur in most practical situations. The defect depth determines how much further processing is necessary; therefore, the defect depth should be measured, and be the criterion for selecting the rake angle and number of knife marks per inch.

Defect study results

The following results were observed:

1. Moderate rake angles of 20 and 25 degrees produced the shallowest chipped grain per inch (see Figure 1).
2. At rake angles equal to or less than 25 degrees, the depth of defect increased as the number of knife marks per inch decreased.
3. At rake angles equal to or greater than 30 degrees, the depth of defect was approximately the same at all numbers of knife marks per inch (see Figure 1).
4. At 10 knife marks per inch the depth of chipped grain was approximately the same for all rake angles (see Figure 1).
   Figure 1:  Maximum depth of defect 
5. The most severe chipped grain defect occurred when planing against approximately a 10 to 15-degree slope of grain.

The depth of defect in a prescribed machining situation depends primarily on the slope of grain. Further, these results with a 25-degree rake angle indicate that the most severe chipped grain occurs at approximately a 10 to 15-degree slope of grain, which is found frequently near knots and cross grain.

When planing with rake angles of 20 and 25 degrees at 30 knife marks per inch, approximately 1/32 inch would have to be sanded away. If the knife marks were decreased to 10 or less by increasing feed rate, 1/16 inch or more would have to be sanded to remove the chipped grain from boards planed with any rake angles.

At fast feed rates (10 knife marks per inch), the depth of defect is not reduced greatly when using 20-25 degree rake angles, as it was at slower feed rates. As feed rate was increased to 10 knife marks per inch or less, the depth of defect became approximately the same for all rake angles tested. Hence, hard maple and all wood should be knife planed with a 20 or 25-degree rake angle at slower feed rates.

Rake angles of 20 and 25 degrees agree favorably with past recommendations for knife planing hard maple and other hardwoods.

Previous work has also shown that surface quality (when planing relatively straight-grained material) increases (frequency of defect decreases) as feed rate decreases to give 20 knife marks per inch. This study showed that for widely varied slope of grain, surface quality (depth of defect) substantially improves as feed rate decreases to 30 knife marks per inch or greater.

Further, the surface quality, based on depth of chipped grain, would probably continue to improve if feed rates were decreased further. However, production rate may become impractically slow at feed rates resulting in more than 30 marks per inch.

Burnishing, glazing and surface damage may result from reducing feed rates below the 30 knife marks per inch.

Although moderate rake angles of 20 and 25 degrees and slower feed rates reduce the depth of chipped grain, it follows that proper machine maintenance, adjustment and knife sharpening must also be carried out to minimize waste from knife planing parallel to the grain.