How Temperature Impacts Tool Wear
Harold Stewart has recently published an article that demonstrates the great affect temperature can have on tool wear. The information from this article was taken from the experiments and articles published by Harold A. Stewart from Mississippi Forest Products Lab.
Abrasion has been assumed to be the main culprit when evaluating tool wear for cutting MDF. Many believed that the materials making up MDF such as the adhesives and silica were the major causes for abrasion and tool wear. However, MDF as well as dry wood products and reconstituted wood products such as Mali involve a high temperature and pressure in the cutting zone and at or near the tool edge. It was found that high-temperature corrosion was found to be a major contributor to the wear of tungsten carbide when cutting MDF. The high temperatures that result at the cutting zone may actually contribute greatly to the wear of the cutting tools.
A little background on high-temperature corrosion:
High-temperature corrosion is most commonly associated with extraordinary reactions to form a salt deposit on a metal or oxide surface during combustion of fossil fuels.
In our case, high-temperature corrosion refers to a wedge-shaped tool sliding through a series of oxidizing and reducing agents accompanied by exceptionally high temperatures and pressures.
These temperatures have been measured and have been estimated to reach up to or be near 800 degrees Celsius at the tool edge when cutting wood. The rate of hot corrosion depends on many different things including the material being cut, the position, the microstructure, salt composition, atmosphere, temperature, extent of thermal cycling, salt or scale thickness, geometry of the material, and erosive conditions.
Due to the properties of MDF, The combination of cutting MDF wood with Tungsten carbide becomes a perfect recipe for high-temperature oxidation/corrosion.
Hot corrosion occurs when cutting MDF and results from the major wear mechanisms and factors which occur.
The four major causes of wear are adhesion, abrasion, diffusion (tribochemical reactions), and fatigue. Each of the causes may affect tool wear to a different degree, depending on the specific machining situation and may interact or be part of other causes.
1. Adhesion is the formation of “welds” between the work piece and tool. The wear from adhesion results from the breaking of the “welds” or joints between the work-piece and tool.
2. Abrasive wear includes rubbing, plowing and cutting depending on the properties and geometry of the individual grits and work piece surface.
3. Diffusion is the atomic transfer of one material to the other at points of contact and. is considered an integral part of the other causes of wear.
4. Fatigue results from cycling of stresses or change of stress as the tool and work piece interface as they pass by each other. Fatigue is probably the least apparent in woodworking.
The four causes of wear listed above depend on time, temperature and pressure. They also require that the tool and work piece surfaces are forced into close proximity with each other.
When machining wood and wood-based materials, three factors affect the causes of tool wear:
1. The tool rubs against the freshly cut, highly oxidizable surface of the chip or work piece. Plastic deformation of the work piece material results when forming the chip.
2. The temperature and pressures at the sliding interface(s) are exceptionally high.
3. 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.
When looking at these three factors, it seems that the MDF cutting environment may be substantially more severe than a metal cutting one.
Wood is technically a three dimensional polymeric composite which 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 was believed in the past. Unlike many metal-cutting situations, the work piece, chip, tool and/or coolant cannot conduct heat away from the cutting zone. Most of the heat generated goes into the tool. Therefore, the high temperatures and pressures and chemical components of a wooden work piece are a bad environment for cutting tools.
This complex combination of reactions indicates the importance of limiting or carefully selecting adhesives, catalysts, additives, etc. for wood composites such as MDF.
These results also indicate the importance of selecting refractory tool materials for wood machining.
By considering tool wear causes with contributing factors such as high-temperature corrosion/oxidation, wood products and tool materials can be improved and selected to decrease tool wear.
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