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.
You bought the system and now it doesn't work. Experts advise on how to design and maintain dust collection systems.
Learn the desirable and not-so-desirable special
properties of some woods
A variety of wood glues are used to assemble the 175 pieces in a six-string guitar. Matching the proper adhesive with the application is important for Taylor Guitars.
Proper heat treatment is as important as the selection of HSS for tooling.
Select the correct teeth for your cutting
Q: We have a gang rip saw that we have trouble with. When the lumber is about halfway through, the piece travels away from the fence. The infeed has rollers that are at an angle so that the lumber is pushed against the fence. That seems to work well for the beginning of the pieces, but then it moves away from the fence. When we increase the feed roll pressure it seems like it might get worse. Any ideas?
Q: I have read every Wood Doctor column you have written and have gained a lot of practical information. But here is a question I have not seen addressed before. As background, our fairly large company has really gone into JIT, just-in-time, manufacturing, which means in-process materials cannot sit around very long at all. Well, this has recently translated into machining our glued up panels (edge-glued on a clamp carrier) within 24 hours after they are glued, or sometimes less. Of course, you know what the problem is: sunken glue joints that are obvious after finishing. My suggestion of waiting three days after gluing, as we have always done, has not been well-received. I am hoping that you have some help for us.
The companies in this directory are making investments to make sure that you have the tools and products necessary to develop successful products with lightweight panels.
Q: We are having a problem with raised glue joints in
our solid wood (mahogany) panels and we would like to know what
your recommendation is for the amount of moisture content that we
could get by with, without causing this problem? This applies
also to high-frequency gluing.
Fascinating article. In addition to the "spark" as mentioned in the article, all to often the spark can come from foreign object sucked into the system from either a nail or screw, or in the worse cases someone disposing of a cigarette butt.
Figure 3: Waveform voltage discharge between the MDF workpiece and tool when turning at 550 rpm.
Figure 4: The voltage between the MDF workpiece and tool for the noncutting, sliding, and cutting conditions at 330 rpm. The depth of cut was 0.005 in for cutting. The non cutting condition is almost coincidental with the zero axis. The voltage for cutting is less then at 550 rpm in Figure 3.
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