Machining natural woods and man-made wood materials can be accomplished across a fairly wide range of moisture contents, if you know what the conditions are and can make provisions to deal with the unusual conditions. However, the 6 percent to 8 percent MC that our raw resource materials are dried to for use in the secondary industry is critical to a vast range of conditions and manufacturing processes needed to complete a finished product.
The 6 to 8 percent MC is the result of many decades of careful examination and research; it is not a random figure. At this MC natural woods as well as man-made wood-based panels are the most stable with respect to changes in physical dimension. That is, removing any more moisture will not greatly alter the piece's physical dimensions. Assuming that the kiln operations have been accomplished with care and precision, the natural stresses will also have been stabilized. This frees the sample from any great subsequent distortion of shape, if the moisture content is held close to that figure.
Cutting tools require normal moisture content to function well. The use of abrasives, stains, and sealers also rely on correct MC for their successful use. You only have to see once the results of putting a topcoat on wet wood to understand all of the costs involved.
As woodworkers, if we purchase from a reliable source and monitor the conditions within our own facility, we can avoid most of the problems relating to unusual moisture conditions. Changes of season cause the most problems - the first warm days of spring when someone opens all of the doors and windows, the warmest days of summer, the first weeks of the fall heating season. The problems are more pronounced if we do not quickly turn over our supply of raw materials.
It's amazing that so few of us go to the trouble of purchasing and using the needed instruments for measuring the moisture content of both the ambient air and raw materials that come into our plants. Even though the total cost of perhaps $200 could save us thousands of dollars, most of us still attempt to guess and hope for the best outcome.
Our responsibility in working with wood materials begins with at least maintaining the materials that we bring into our facility. For instance, purchasing kiln-dried wood materials and placing them on an open truck dock for two weeks in the middle of April doesn't make much sense. We cannot expect the wood to behave well at all - and it probably will not disappoint us.
The moisture content of the air in your facility changes constantly, though not dramatically. There are three distinctly different ways that the amount of water vapor suspended in the air is measured and described.
1. Relative humidity is the amount of water vapor present at a particular temperature. For instance, during the evening news, the local TV weatherman describes the relative humidity as 50 percent at 60 F. This means the air has 50 percent of the water vapor it can possibly hold at that temperature. If earlier that day, the temperature was 90 F, the relative humidity would have been 20 percent with the same amount of water vapor. By midnight, if the temperature drops to 40 F, the relative humidity will reach 100 percent - again, with the same amount of water vapor present.
2. Absolute humidity is the weight of water vapor present in a given volume of air. The water vapor is reported in grams and the air mass in cubic meters. The importance of these indices is that they relate the forecasting of localized weather by prescribing how much water is or can be transported to different locations. Here the importance is that cold air can supply only small amounts of moisture and warm air can supply huge amounts. The driest air is not in the equatorial deserts, but at polar locations.
3. Specific humidity is the ratio of the weight of water vapor to the weight of moist air, including the water vapor. This ratio is stated in grams of water to kilograms of air mass. This index describes the moisture present at different elevations - due to compression or expansion of the air mass (or different barometric pressures).
There are two basic choices of instruments for monitoring the moisture content of the air in a facility. A direct reading hygrometer indicates both relative humidity and temperature. This instrument should be placed in a location and position that is a representative to the larger manufacturing facility and open to good air flow.
A dual bulb or sling hygrometer has two liquid filled-thermometers; one is read directly (the dry bulb), the other has a cotton sleeve on the bulb that is saturated with water prior to reading (the wet bulb). Both instruments are either fanned or twirled about to hasten the evaporation of the wet bulb causing the temperature to drop. Comparing the two temperature readings on a psychometric chart or table will give very accurate relative humidity figures.
The cost of these instruments will vary from as little as $25 to several hundred dollars for an instrument that will record a constant flow chart of moisture conditions on a paper disc or continuous feeding roll chart.
Knowing what the conditions should be in our manufacturing facility to maintain the condition of raw materials is not the same as being prepared to monitor and make corrections as needed (by way of monitoring and adding or removing moisture from the air when it is called for).
To measure the moisture content in our raw materials, we have the choice between two types of instruments.
1. Direct current electrical resistance. This type of instrument involves driving small pin-type electrodes into the wood. This takes advantage of the fact that moisture is an excellent conductor, but dry wood is an effective insulator. The instrument is simply a specialized ohmmeter, which measures electrical resistance and converts the figures directly into moisture content. Pins are supplied in various lengths for measuring stock from veneer thickness up to 2 inches and more. A disadvantage of this type of instrument is that pin holes are unacceptable on a finished product.
2. Dielectric meters. This type of instrument generates a radio frequency that penetrates the wood and records an average moisture content. The two operating principles are either a power loss effect or electrical capacitance changes in the sample being read.
Various meters on the market can provide a wide range of abilities for any need. All moisture meters now in the marketplace are highly refined and very accurate.
The pitfalls of not monitoring moisture content
R ecently a former co-worker was relating an incident where he was setting up a CNC router to shape and size 1-1/4-inch-thick raw particleboard panels, which would later be laminated and have solid edge treatments. The tool for this machining process was a compound spiral solid carbide router bit. He set up the first panel and started the program running, though not at the feed speed that he would eventually ramp-up to.
After processing only 5 or 6 feet, he was concentrating on the information on the monitor and heard the router motor laboring. Looking at the cutting tool, he saw that it was glowing a dull cherry color and pushing a film of water ahead of itself.
Immediately killing the power, he returned the tool to its home position and proceeded to examine the tool and panel that was being cut. The cutting tool had just returned from being sharpened prior to this setup and still appeared in good condition, though possibly damaged by the enormous heat it had just endured.
He measured the moisture content of the panel and was amazed to find it between 20 and 22 percent. After examining the remainder of the bunk of panels, and nearly everything else in the plant that day, he could not find another panel with such a high moisture content. All measured between 6 and 9 percent.
To this day, he can find no explanation for how a single panel arrived at the plant with a moisture content nearly three times what it should have been.
The following day, my friend called to relate the experience and remind me that I had years earlier cautioned him against machining materials with high moisture content. Each 1 percent of elevated MC can raise the cutting temperature by as much as 70 F. He had gone on to look up the physical properties of the solid carbide router tool and concluded that if the normal tool-tip temperature was from 600 to 700 F, it must have reached nearly 1,500 F to have been glowing the dull cherry color he saw.
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