FDM - April 2008

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 tan-

gentially 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.

Testing challenges

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. Randomiza-tion of the tools and a large sample

Wear and temperature

Temperature Wear Wear Total degrees (F) Void (%) Scar (%) Wear (%)

70 32.07 44.09 76.16

40 25. 39 34. 64 60.03

20 25. 68 40. 63 66. 31

Wear void, wear scar, and total wear as a percentage of the original clearance face area for the average of two flutes.

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