The vortex tube supplied with compressed air delivers hot air from one end and cool air up to -50F lower than the ambient temperature from the other end. The one-time cryogenic treatment tempering changes the material's micro structure by slowly cooling the tooling to about -300F. These technologies are applied to wood machining situations. Read more.

The previous articles describe how machining dry wood or wood products could produce a substantially more severe environment than machining other materials. The heat and high temperatures during wood machining may be generated from several sources including friction, the distortion and/or fracture of chemical bonds, and the electrical discharges among the chip, workpiece, and tool surfaces. High-temperature corrosion/oxidation has been determined to be a major contributor to the wear of wood cutting tools. These high-temperature phenomena are a major tool wear mechanism when machining dry wood products such as MDF and are similar to reactions from the combustion of fossil fuels or high temperature reactors.

Selecting technologies to enhance tool life for wood machining requires the technologies have easy applicability, be relatively inexpensive, and resist the conditions already mentioned. Both the vortex tube and the cryogenic treatment appeared to meet the requirements. Generally, cooling cutting tools with liquid coolants and lubricants is impractical when machining dry wood or wood composites. Further, a selected technology to enhance tool life must change the adverse wood machining environment or change the tool sufficiently to resist the effects of such an environment. Both the vortex tube and cryogenic treatment appeared to meet the criteria. A series of studies were undertaken at Purdue University, Department of Forestry and Natural Resources to determine the affects upon tool life by applying a vortex tube and a cryogenic treatment to tooling in a common woodworking situation.

The technologies were applied separately and combined. The results of just the vortex tube were briefly described in FDM April 2008. These results will briefly describe the effects of the cryogenic treatment alone and in combination with the vortex tube.

Router bit study

For this study, four double-flute, solid, tungsten carbide router bits from the randomized tool population (12) with ½ in. diameter were selected. The tools contained 10 percent cobalt. Three of the tools were cryogenically treated to -300F. Wood machining was conducted on a CNC router at 35 fpm feed rate and 16,000 rpm. One of the three cryogenically treated tools as well as an untreated tool cut MDF at an ambient temperature of 70F without the application of cool air. The two remaining cryogenically treated tools had cooled air of 40F and 20F, respectively, applied to them during cutting. Twenty-two sheets (4 ft. x 8 ft. x ¾ in.) at ¼ in depth of cut per pass across the MDF on the CNC router. The total length of cut was more than 175,000 yards per flute.

Tool wear was quantified with image and measurement software in conjunction with a stereo light microscope. Tool wear was measured on the clearance face of the tool. The total tool wear was defined as a ratio of the sum of the wear void and wear scar area to the original clearance face area. The wear void was the material completely worn away.
The area remaining that was beginning to show wear through scratches and rubbing was the wear scar. Tool wear was expressed as a percentage of a fixed original clearance face area.

Tooling wear results

The overall results for percentage wear void, wear scar, and total wear are shown in Tables 1 and 2.

The cryogenic treatment significantly reduced the total wear from 76.2 to 46 percent of the original clearance face area (Table 1). Cooling air from a vortex tube also significantly reduced total tool wear from 46 to 41.3 percent and 35.1 percent, respectively (Table 2). Both treatments individually reduced tool wear and together they further reduced tool wear.

The similar tool wear results for each of two flutes for each combination of milling similar MDF with similar two-fluted cryogenically treated and untreated solid carbide router bits with and without refrigerated (cooled) air are valid. The consistent tool wear results indicate the tooling and MDF samples were representative of their respective populations.

Since high-temperature phenomena are considered a major wear mechanism, energy-dispersive spectroscopy (EDS) analysis was performed to simply verify the elemental agents were available for the reactions. Tungsten and cobalt as well as nitrogen, oxygen, sodium, phosphorous, sulfur, chlorine, potassium, and calcium were all present on the tools. Sulfur and chlorine are known to be among the most corrosive contaminates in high-temperature environments. The cobalt binder is removed through reactions with MDF decomposition products, although the exact mechanisms have not been identified.

The cryogenically treated tool which was cooled with refrigerated air showed a reduction of contaminating elements from the MDF.

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