Generally, abrasion is assumed to be the major tool wear mechanism during machining of dry wood, wood products and wood composites such as medium density fiberboard. Much of the abrasion has been attributed to the reconstituted material, constituents such as adhesive and silica. However, the machining of dry wood and reconstituted wood products such as MDF also involves high temperatures and pressures in the cutting zone at and near the tool edge. High-temperature corrosion has been found to be a major contributor to the wear of wood cutting tools.
The heat and high temperatures during wood machining may be generated from several sources. These heat sources include friction, the distortion and/or fracture of chemical bonds, and the electrical discharge among the chip, workpiece and tool surfaces. Temperatures have been estimated to exceed 800C at or near the tool edge when wood machining. Because dry wood and wood materials are excellent heat and electrical insulators, most of the heat generated is conducted into the tool.
Importance of environment
The wood cutting environment may be substantially more severe than a metal cutting one. Dry wood is a three-dimensional polymeric composite which is a relatively good thermal insulator. Because dry wood or wood-based products, such as MDF, have low thermal conductivity, the cutting temperatures are generally higher than expected. Unlike many metal-cutting situations, the workpiece, chip and/or liquid coolant cannot conduct heat away from the cutting zone, and most of the heat generated goes into the tool.
Liquid coolants and lubricants have been applied to metal, plastics and green wood machining to reduce cutting temperatures or their effects on tool wear. Due to the hygroscopic nature of dry wood or wood composites, liquid coolants or lubricants are generally impractical for increasing tool life when machining wood or wood-based composite materials. However, cooling tools with compressed and/or refrigerated air may reduce tool wear.
Compressed air cooling
Cooled or refrigerated air can be produced for cooling a wood machining tool at down to -46C below the compressed air supply with no moving parts with a vortex tube. Compressed air, normally 80-100 psi, is forced tangentially into a vortex spin chamber. The air stream revolves in an outer vortex toward a hot end continually expanding where some hot air escapes. The remaining air, still spinning and expanding, is forced back through the center of the outer vortex and still cooling. The inner stream gives off more heat energy to the outer vortex. The inner vortex continues to cool and exits the vortex tube as cold air which can be directed at a cutting tool.
The vortex tube appears to be a low cost, reliable, relatively low maintenance solution to high-temperature wood machining problems. Therefore, a study was undertaken at Purdue University to apply vortex tube-cooled air to solid tungsten carbide router bits while machining MDF.
A concern of tool wear testing is obtaining meaningful results from minimum testing. Consequently, the tools need to be similar and represent the population. Likewise, the workpiece material such as MDF has to be relatively uniform. Randomization of the tools and a large sample of MDF can help the uniformity of the respective populations. If the tool and workpiece materials are similar, respectively, then other treatments such as refrigerated air should readily exhibit a difference or no difference in simple comparative tool wear tests. Three double-flute, solid tungsten carbide, 3/8-inch diameter router bits were randomly selected to cut 22 sheets (4 foot x 8 foot x 3/4 inch) at 16,000 rpm and at a 1/4 inch depth of cut per pass across the MDF on a CNC router. The total length of cut was more than 175,000 yards per flute.
One of three tools was cooled with ambient compressed air temperature of 70F. The other two-fluted tools were cooled with 40F and 20F air, respectively. The tool wear area for each flute (two samples) was determined with an image analysis-based method.
A digital camera attached to the microscope captured images of the clearance face of the tool. The images showed the area of tool material removed (tool void) and the adjacent worn area (wear scar) and an area that showed no wear. In order to compare wear among different tools and treatments, tool wear was expressed as a percentage of the same original measured clearance face area for each flute and averaged for each tool. The results are summarized in the table.
Statistical analysis showed that cooled air temperature significantly reduced total (wear void + wear scar) tool wear. The effect of temperature based on these tests showed that refrigerated air at both temperatures (40F and 20F) showed significantly less mean total tool wear (60 and 65 percent) compared to the tool cutting at ambient temperature (70F) with a total wear of 76.2 percent.
Cooling solid tungsten carbide tools with refrigerated air when machining MDF reduced tool wear. The results can probably be applied to other wood machining combinations or tool materials such as high-speed steel and solid wood and other wood composites. Additional elemental analysis of the clearance face showed the presence of elements which cause high-temperature wear phenomena such as oxidation and corrosion to occur. A panel of observers also determined that tools cooled with refrigerated air consistently produced a higher quality surface for a longer length of cut. Since the air-cooled tools would conduct less heat into the tool holder, arbor and bearing, CNC bearing life may also be extended by applying vortex tube cooled air. Accurate tool histories and controlled tests could show that cooling tools with refrigerated air could be widely applied throughout the woodworking industry.
The complete test results described are available in Wood and Fiber Science, 39(3), 2007, pp.443-449.
For more information contact Professor Rado Gazo, Wood Research Laboratory, Department of Forestry and Natural Resources, Purdue University, West Lafayette, IN 47907-1200. Phone 765.494.3634.
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