Consider tool wear mechanisms to improve life of router tooling:
Refractory tool materials should be selected
By Harold A. Stewart
Cutting tool life, including the life of router bits, can be improved with an operator’s understanding of high temperature phenomena and tool wear mechanisms. The machining of dry wood and reconstituted wood products such as medium-density fiberboard (MDF) involves high temperatures and pressures in the cutting zone at and near the tool edge. Wear mechanisms, other than abrasion, contribute significantly to wear of cutting tools during wood machining. High-temperature corrosion and oxidation have been found to contribute substantially to tool wear.
Hot- or high-temperature corrosion is generally associated with extraordinary reactions to form a salt deposit on a metal or oxide surface during the combustion of fossil fuels. A wedge-shaped tool slides through a series of oxidizing and reducing agents accompanied by exceptionally high temperatures and pressures. Temperatures have been measured and estimated to be 800° C at or near the tool edge when cutting wood. The rate of hot corrosion depends on many variables including material composition, microstructure, salt composition, atmosphere, temperature, extent of thermal cycling, salt or scale thickness, specimen geometry and erosive conditions. Consequently, the cutting of a reconstituted wood product such as MDF with a tungsten carbide tool is apparently an optimal combination for high-temperature oxidation/corrosion.
Hot corrosion occurs when cutting MDF and results from the major wear mechanisms and factors that occur.
The four major wear mechanisms are adhesion, abrasion, diffusion (tribochemical reactions) and fatigue. Each of the mechanisms may affect tool wear to a varying degree, depending upon the specific machining situation, and may interact or be part of the other mechanisms.
Mechanisms that
contribute to cutting wear |
1. adhesion
2. abrasion
3. diffusion
(tribochemical reactions)
4. fatigue |
Adhesion is the formation of “welds” between the tool and workpiece. The wear from adhesion results from the failure of the joints between the workpiece and tool. Abrasive wear, the stock removal by indentation, includes rubbing, plowing and cutting depending on the properties and geometry of the individual grits and workpiece surface. Abrasion depends on the indentation of one material to another. Diffusion is the atomic transfer of one material to the other at points of contact and is considered an integral part of the other mechanisms. Fatigue results from recycling of stresses or change of stress as the tool and workpiece interface as they pass by each other. Fatigue is probably the least apparent in woodworking processes. These mechanisms depend on time, temperature and pressure; they also require that the tool and workpiece surfaces be forced into close proximity with each other.
The chip and workpiece rub against the tool on the rake and clearance faces while conforming to the knife-edge. When machining wood and wood-based materials, three factors affect the mechanisms of tool wear:
• The tool rubs against the freshly cut, highly oxidizable surface of the chip or workpiece.
• Plastic deformation of the workpiece material results when forming the chip.
• The temperature and pressures at the sliding interface(s) are exceptionally high.
• The tool is brought into close contact with a highly chemically reactive surface of many ions and mechanically severed bonds at high temperatures and pressures.
The MDF cutting environment may be substantially more severe than a metal cutting one. Wood is technically a three-dimensional polymeric composite that is a relatively good thermal insulator. Because dry wood or wood-base products such as MDF have low thermal conductivity, the cutting temperatures generated are higher than previously thought. Unlike many metal-cutting situations, the workpiece, chip, tool and/or coolant cannot conduct heat away from the cutting zone. Most of the heat generated goes into the tool. Hence, the high temperatures and pressures and chemical components of a wooden workpiece are an adverse environment for cutting tools, particularly in a process such as routing.
Because wood machining is an adverse environment for machine tools, the selection of tool materials, coatings or treatments are important. Refractory tool materials should be selected. The complexity of the wood machining environment indicates the importance of limiting or carefully selecting adhesives, catalysts, additives, etc. for wood composites. By considering tool wear mechanisms, tool life can be improved.
Look for more exclusive contributions to Modern Woodworking magazine from Harold A. Stewart in the coming year. Stewart is a research scientist at the Mississippi State University Forest Products Laboratory and adjunct associate professor at North Carolina State University. He holds a B.S., B.S.F. and M.W.T. from the University of Michigan. Since 1966, Stewart has conducted many studies of abrasive and knife cutting to develop basic information and application techniques for wood and wood products surfacing as part of a comprehensive wood machining research program. He is a consultant for tool and machinery manufacturers and wood processing firms, has been published in technical and trade journals and lectures at universities, trade associations and individual firms. Stewart’s research has led to the development of the world’s first ceramic router bit, the development of new carbide grades for machining wood and other materials, and new resin formulations to reduce tool wear.
Please contact webmaster@modernwoodworking.com with your comments.
