Where does the spark originate?
January 31, 2012 | 6:00 pm CST

The status of new regulations and standards that would affect handling combustible dust hazards was aptly described in CabinetMaker+FDM, January, 2010. Reference is widely placed on “static” electricity and dust as a highly combustible combination. Very little is discussed about how the dust becomes so highly combustible or where does the “static” electricity originate. Electrical potentials between the workpiece and cutting tool have been shown to exist, particularly when machining green wood. The voltages have been reported as relatively constant. In addition, electrical effects contribute to tool wear by galvanic type corrosion when cutting green wood, but their influences when machining air-dried or drier wood were unclear. Small pits were formed on the rake face of the cutting tools in the area where the chips left the tool face indicates an electric discharge machining effect. The chips produced when machining dry wood were shown to be negatively charged.

 Electrical potentials between the workpiece and tool have been shown to be generated when cutting green and dry wood or wood products such as medium density fiberboard (MDF). A series of turning tests on MDF were undertaken to characterize the electrical potentials in wood machining (Figure 1). The results indicated numerous discharges generally from the MDF workpiece (cathode) to the tool (anode) (Figure 2). However, both voltage and current reversals or discharges occurred from the tool to the MDF-workpiece (Figure 3 and Figure 4). These electrical discharge characteristics from wood machining could provide insight for the wear mechanisms for dry wood and wood products as well as for combustible wood dust problems. Faster machining speeds generate higher voltages as shown by the comparison of Figures 3 & 4.
Some voids are apparent in the literature about electrical phenomena in wood machining. For example, the source of the electric potential has not been fully discussed, if at all. The source may be expected as the result of “static electricity” due to simple rubbing of two surfaces, but; more probably, is the result from the Kramer effect. All materials contain free electrons (exoelectrons). Good electrical conductors such as metals have more exoelectrons than poor electrical conductors such as dry wood and wood products. The Kramer effect describes free electrons as being emitted from a freshly scratched, cut, or fractured surface as a result of greater atom spacing at the surface than in the internal regions. The Kramer effect would explain the negatively charged wooden chips or dust and the voltage (potential) between the wooden workpiece and tool, particularly when machining dry wood and wood products which are virtually electrically non-conductors. A freshly generated surface of wooden materials, workpiece, or chip, would be in a highly active chemical state, and electric potentials could have an important influence on bond formation at points of contact as well as on surface chemical reactions. Chips or dust such as from wood composites are formed by mechanically destroying chemical bonds which exposes numerous free radicals waiting to react i.e. oxidize , burn, or explode. The smaller the chip or finer the dust, the more reactive free radicals available for burning or exploding.
The chips or dust from wood machining are negatively charged reactive particles. For safety, the machinery, exhaust duct, and chip dust collectors should all be grounded. Static charges of thousands of volts can accumulate and need to be dissipated as well as ignitions prevented when so much reactive surface area is present.

Have something to say? Share your thoughts with us in the comments below.

About the author

Harold A. Stewart wrote a tooling column for FDM magazine, looking at ways to improve the cutting process. He was previously an educator and consultant to wood products companies.