FDM - January 2009

FDM
TOOLING

fdmonline.com

Kni fe marks per inch
Increasing knife marks per inch can be beneficial.
By Harold Stewart

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” (www. fdmonline.com/chippedgrain.aspx) 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 lum-

0.100

Figure 1.

.090

.080

{ 10 Knife marks per inch 20 30

.070

Depth of defect (inches)

.060

.050

.040

.030

.020

.010

.000

ber, 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-de- gree 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.

10° 15° 20° 25° 30° 35° 45°

Rake Angle

Maximum depth of defect measured after knife planing at 10, 20 and 30 knife marks per inch with each rake angle. Maximum depth of chipped grain is at about a 10-degree slope of grain.