Electrical discharges and static charges occur when machining wood or wood composites such as medium density fiberboard. Occasionally, wood dust fires or explosions occur as a result of static discharges.
Where do the electrical charges originate? Electrical potentials between the workpiece and cutting tool have been shown to exist when cutting wood, particularly 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 wood were unclear. Other recent studies have shown tool wear to result from high-temperature corrosion/oxidation reactions when machining MDF. Consequently, to gain insight into tool wear, an investigation was undertaken to characterize the electrical potentials and discharges in a relatively simple wood-machining situation such as turning.
The initial observation and additional observations of electrical effects in the wear of wood-cutting tools have been previously reported for cutting green and dry wood. Several investigators have shown that a voltage occurs between the workpiece and the tool when machining green wood. In addition, when a negative potential was applied to the knife, tool wear was reduced. One researcher described small pits that formed on the rake face of the cutting tools in the area where the chips left the tool face. The chips produced when machining dry wood were shown to be negatively charged. Therefore pits on the tool rake face were attributed to electrical discharges where the charged chips left the tool face. However, the potentials reported in the literature indicated relatively constant negative voltages.
In one case, a nearly constant current was measured between the knife and the workpiece. In turning tests on green and air-dried incense cedar (Calocedrus decurrens), insulated tools wore less then uninsulated ones, and again, a relatively constant voltage was reported for the uninsulated cutters. These examples all indicate relatively steady negative potentials between the workpiece and the tool.
In drilling tests with green and air-dried western red cedar (Thuja plicata) the degree of corrosive wear in air-dried wood was lower than in green wood and was described as electrochemical in the green wood. Again, the voltage and current were described as constant and as negative from the workpiece to the tool when machining green wood.
Electrical source
Some gaps in the literature about electrical phenomena in wood machining are apparent. For example, the source of the electric potential has not been fully discussed. The source may be expected as the result of "static electricity" due to simple rubbing of two surfaces but, more probably, is the phenomenon known as the Kramer effect. Electrons (exoelectrons) are 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 and the voltage (potential) between the wooden workpiece and tool, particularly when machining dry wood and wood products which are virtually non-conductors.
A freshly generated surface of materials 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. Consequently, the high-temperature oxidation/corrosion described previously may be electrochemical in nature.
In addition to the current or potential from the workpiece to the tool, the possibility of eddy currents also exists. These circulatory currents could exist within or between the tool and workpiece. An accurate description of the electric current and potential between the tool and workpiece for wood machining has not been provided in the literature. These voltages and currents have been previously measured with relatively slow volt-ohmmeters and ampmeters. This study characterizes the possible voltage and amperage waveforms to provide additional insight into the electrical effects on tool wear and give new directions to research in wood machining, particularly in tool wear.
Experimental methods
A series of turning tests on MDF and voltage measurements were undertaken with M-2 high-speed steel tools at 550 and 330 rpm, 0.005-inch depth of cut, and 10 and 15 degree clearance and rake angles, respectively. During normal cutting conditions the HSS tool was grounded through the tool holder, Electrical activity at the tool-workpiece interface was indicated by electrical currents flowing between the tool and ground. To monitor these currents during the cutting tests described in this paper, the tool was electrically insulated from the tool holder, and grounded through a 1 ohm resistor, using very short connecting wires (see Figure 1). The transient voltage measured across this 1 ohm is then a direct measure of the tool current, in amperes.
Additional information about the electrical processes at the tool-workpiece interface is obtained by monitoring the transient tool voltage when the tool is insulated from ground by a very high (1 M ohm) resistance. The transient voltage and current signals were measured and recorded with a Tektronix TDS-510 dual-channel digital storage oscilloscope. The high sampling rate of this oscilloscope allows observation of the very short electrical events which are present during the cutting process. Voltages were recorded for the non-cutting (no contact between the tool and MDF-workpiece), sliding and cutting situation between the tool and MDF -workpiece.
Electrical characteristic voltage waveforms from the tool for non-cutting, sliding and cutting are shown in Figure 2. The non-cutting voltage with the lathe turning is almost coincidental with the zero axis. A more sensitive scale showed the voltage from non-cutting to oscillate randomly at about 1 mV. When the tool was just contacting and sliding on the workpiece a positive cyclic oscillating voltage was recorded. The voltage between the tool and ground when the tool is grounded through a 1 M ohm resistance may not represent the voltage between the tool and workpiece, because the voltage of the workpiece relative to ground is not zero. However, because the non-cutting voltage is near zero, the relative voltage would be almost as great as the recorded voltage transient for cutting (see Figure 2). The voltage cycles of the sliding tool, as might be expected, were equal to the rpm of the lathe at about 330. During cutting, an irregular voltage waveform, predominantly negative, showed discharges from the workpiece to the tool with occasional larger positive discharges from the tool to the workpiece. These were the common characteristic voltage waveforms repeatedly observed for the non-cutting, sliding, and cutting situations.
Many discharges
The waveform during cutting was apparently composed of many discharges, some of which were positive (see Figure 2). Previous research reported a relatively constant negative voltage between the workpiece and tool. The apparent visual average voltage of approximately 0.3-0.4 V is of the same order of magnitude reported for machining dry wood. The discharges ranged from more than -1.00 V to over +0.50 V.
The voltage drop across a 1 ohm resistance was also recorded (see Figure 3). Since current, i, is related to voltage, V, and resistance, R, by Ohm's law (i = V/R), the voltage is equal to the amperage at R = 1 ohm. The amperage is relatively constant but shows many small discharges as well as occasional large positive direction discharges relative to the neutral non-cutting condition. The current was also measured by the difference between the voltage before and after the 1 ohm resistance from each of the two channels. Both methods indicated currents of similar magnitude of 3-4 mA.
The voltage and current waveforms became positive for short durations. The voltage and current reversals partially satisfied the charge imbalance created by the cutting action when new surfaces were generated. Observation of the waveforms (see Figures 2-4) and other recordings allowed for a better description of the electrical phenomenon during the cutting of a dry wooden dielectric workpiece such as MDF. The voltage and current measurements indicated that numerous small discharges occurred from the workpiece surface to the knife at points where the knife contacted the freshly generated surface. The recorded waveform discharges may be the sum of many smaller discharges because a new surface area was formed simultaneously along the knife edge; many chemical bonds were severed mechanically at the same time. A high-voltage electrostatic field probably accumulated in the deforming workpiece in the cutting zone just ahead of the knife. The knife evidently only conducted electrons away from the workpiece/chip that were in contact with or close to the knife. The charge accumulated to the maximum dielectric strength of the workpiece and then apparently discharged back into the underformed workpiece. The large field discharge was apparently into the workpiece because of the voltage reversal of the waveform from the knife. The reversal indicates that for short periods of time a flow of electrons from the knife to the workpiece also occurred. Although the flow of electricity was predominantly from the wooden chip and workpiece to the tool, occasionally reversals did occur (see Figures 2-4).
Summary
Tool wear from machining dry wood or wood composites such as MDF is a complicated process. Voltages are generated and currents are discharged between the newly formed chip and workpiece surfaces in contact with or close to the tool edge. Although discharge reversals frequently occur, the discharges are mostly from the workpiece to the tool. The workpiece is generally the cathode (source of electrons) and the tool is the anode. The discharges are all composed of many smaller discharges at many points which may result in an EDM effect on the knife surface. Although the voltage and current observed in these tests seem small, the energy at a point contact could be quite high. The chips become negatively charged and the workpiece becomes positively charged as a result of a loss of electrons.
Figure 4: Waveform voltage discharge between the workpiece and tool when turning at 550 rpm and 0.005 inch depth of cut.
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