Choose the right tool geometry to machine MDF

Determining optimum tool geometry and machining guidelines for reconstituted wood products such as medium-density fiberboard (MDF) is time-consuming and expensive. Optimum conditions require that a satisfactory surface and tool life must result from the selected machining combinations. In turn, knowledge of tool wear and surface quality as related to tool geometry and machining guidelines is required for a given raw material such as MDF.

Rake and clearance angles 

Tool force and edge recession data when cutting MDF show that the optimal range of rake angle is between (but not including) 10 and 30 degrees, with 10-degree clearance angle. These turning tests were conducted with an engine lathe. The turning test is similar to an orthogonal cutting situation, which may be considered as peripheral milling with an infinite cutter radius.

During conventional peripheral upmilling, the rake angle increases and the clearance angle decreases. Conversely, the rake angle decreases and the clearance angle increases during downmilling. A cutter with an infinite radius, e.g., a carpenter's plane or veneer slicer, would not have a change of rake angle. A cutter with a ¼-inch radius would have a small change.

For example, a 1/2-inch-diameter router bit, turning at 18,000 rpm and having a 550-inches-per-minute feed rate would only have a maximum rake angle increase and clearance angle decrease of 1.003 degrees. Consequently, because the rake and clearance angle changes are small, the results from the turning test should be applicable to a wide range of cutting conditions and provide guidelines for peripheral milling cutters.

Other orthogonal cutting results on MDF show that chip formation and surface quality improve as rake angle and depth of cut increase. Generally, the rake angle should be increased until tool life or surface quality decreases. Our initial results indicate that rake angle may be increased up to (but not including) 30-degrees with a 10-degree clearance angle. At a 30-degree or higher rake angle, the edge recession increases substantially. Likewise, at rake angles equal to or less than 10 degrees, edge recession increases and the tool forces are much greater.

Tests back findings 

Industrial tests performed by a tool manufacturer substantiate our results (Table 1). MDF was cut with 1/2-inch-diameter, tungsten-carbide, double-flute router bits at 20,000 rpm. Rake angles (15, 24, and 29 degrees) with a 15-degree clearance angle and clearance angles (12, 15, and 18 degrees) with a 24-degree rake angle were tested. Feed rates of 100, 300, and 550 inches per minute were also tested with 24- and 15-degree rake and clearance angles, respectively.

The results indicated only moderate wear as the rake angle ranged from 15 to 29 degrees, and slightly more wear as the clearance angle ranged from 15 to 18 degrees. Tool wear was substantially reduced as feed rate was increased from 100 to 550 inches per minute. Overall, these results confirm that rake angle should be increased, clearance angle should be decreased, and feed rate should be increased until tool life and/or surface quality deteriorate. Thus, moderate rake angles between 10 and 30 degrees are recommended for machining medium-density fiberboard.

Tool geometry guidelines 

Edge machinability is a primary advantage of MDF. Intricate contours can be machined along the edges of MDF panels by correspondingly contoured knives. The actual rake or clearance angle in a cutting situation may be substantially different from the desired angle as indicated previously.

The “velocity” rake and clearance angles are measured in a plane parallel to the tool velocity vector (instantaneous tool path) and perpendicular to the surface generated, i.e. from a tangent to the tool path. For example, the radial rake angle would be the tool velocity rake angle in peripheral milling with a helical cutter; the oblique or normal angle would be the tool velocity rake angle for complexly contoured cutting edges. Thus, intricately contoured cutters have many rake and clearance angles that may be substantially different than anticipated. Accelerated tool wear or poor surface quality may result from unsuspected adverse tool geometry along portions of the contoured cutting edge.

Frequently, adverse effects such as burns on the knife or workpiece and fuzziness on MDF coincide with segments along the cutting edges of router bits and shaper cutters. These edges often have low velocity rake and/or clearance angles. Some sawtooth designs also may have low velocity rake angles with respect to the tool edge and surface being cut. The low rake angles tend to produce smaller chips or dust and crush the surface.

Further, low rake angles push chips ahead of the tool in a plowing action and have higher cutting and frictional forces than high rake angles. The smaller chips and dust do not flow as easily as larger chips, and tend to fall or recirculate back into the cutting zone at the knife edge. Conditions-exist for accelerated tool wear along portions of the cutting edge as a result of actual tool geometry.

Effects of contoured edges 

The cutting edge situation may also be adversely affected by contoured or patterned cutting edges because of conflicting chip-flow directions. The chip-flow direction is a function of tool geometry and cutting direction. Because the tool geometry, such as rake angle, changes from point to point on a contoured knife edge, or at corners where two cutting-edge segments meet, the chip-flow direction of one point or edge may conflict with the chip-flow direction of another point or edge segment. As a result, more chip material or dust may be recirculated into the cutting zone.

First attempts at improving tool geometry may fail because chips and dust are not removed from the cutting zone. Increasing radial and axial rake angles, the hook and face shear angles, respectively, may improve the actual cutting but still have adverse effects upon tool wear. Increasing these angles may direct the chips into the cutting zone or cause them to pile up on the cutter axis. This would clog the cutter and cause more wear.

A number of methods can improve the velocity rake angle for wood machining. Multiple cutterheads or stacked cutters can be designed to optimize tool geometry. Each cutter or cutterhead could cut a different segment of the contour. The cutting situation could also be improved by orienting the cutterheads and workpiece at angles to each other. A combination of these methods could improve tool geometry of many woodworking situations.

Other published results from industrial tests substantiate that moderate rake angles of 15, 24 and 29 degrees have a small corresponding increase of wear for 1/2-inch-diameter router bits with a 15-degree clearance angle.


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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.