The furniture and cabinet manufacturing industries commonly substitute one or more modern wood composites for plywood. These composites include particleboard (PB), wafer board (WB), medium-density fiberboard (MDF) oriented strand board (OSB), and laminated veneer lumber (LVL). Both the plywood and wood composites are wood-adhesive combinations.
Because grooves were sometimes observed where the glue lines contacted the tooling, the adhesives were thought to be abrasive. Consequently, many wood composites such as wood-adhesive combinations were considered to be abrasive. Subsequent research has shown factors added to the resin based adhesives may have caused the accelerated tool wear as well as contaminants to the glue line of plywood or wood-glue combinations of wood composites. A simple ash content test may reveal some of the tool wear causes. These ash test results usually include total ash and partial elemental mineral content when done by an independent testing laboratory.
American native wood species generally contain about 0.2 to 0.4 percent total ash content. These ash totals are for the woody material only which does not include bark or other debris. Generally, tool wear from machining wood products has been shown to increase when ash content increases above 0.5 percent. The additional ash content above 0.2-0.4 percent is usually from additives to the adhesives or “dirt” included in or contaminating the raw material during manufacturing.
Causes of wear
The four major causes of wear are adhesion, abrasion, diffusion, and fatigue. Each may affect tool wear to a varying degree.
Adhesion is the formation of welds between the tool material and workpiece. Wear from adhesion results from failure of the joints between the workpiece and tool. The “welds” may include various bonds being formed and broken which could remove material from the workpiece and/or tool.
Abrasive wear and stock removal by indentation usually form a scratch, including rubbing, plowing, and cutting. The amount of these factors depends upon the properties and geometries of the individual grits and workpiece surface and components. Abrasion depends upon the indentation of a harder material into another.
Diffusion is the atomic or molecular transfer of one material to the other at points of contact and is considered an integral part of the other wear mechanism. The transfer of material frequently results in tribo-chemical reactions among elements of the tool, workpiece, and immediate atmosphere adjacent to the sliding surfaces. The results are often compounds and/or scales different from the parent tool and workpiece materials.
Fatigue is probably the least apparent wear mechanism from wood machining. Fatigue is the recycling of stresses at the interfaces of the tool and workpiece. Fatigue results from the repeated impacts of two objects. The impacts can result from the interrupted cutting of a multi-toothed cutter or saw. Another source of repeated impacts could be between the cutting edges and particular matter in the workpiece as the cutting proceeds through fresh material.
These mechanisms depend on time, pressure, and temperature; they also require that the tool and workpiece surfaces are forced into close proximity with each other. A wedge-shaped tool slides through a series of oxidizing and reducing agents at very high temperatures and pressures. Hard particles of dirt or grit may be distributed throughout the workpiece, particularly if it’s a wood composite.
Plywood is a relatively clean, low-ash content. As a veneer bolt is peeled, it is first rounded up; thus removing the outer wood which may contain more bark which may contain more dirt. MDF, PB, WB, and OSB are all manufactured from whole tree chips which may include the outer wood and some bark. Modern MDF plants usually have chip washers which remove part of the debris.
Ash in composite products
The other composite products (PB, WB, and OSB) may not have chip washers. Plywood usually has less than 0.5 percent ash content. Table 1 shows ash content for MDF from a plant with a chip washer to be about 0.5 percent. Ash content for PB from a plant without a chip washer to be about 1.0 percent. Analyses (Table 1) further shows that the PB has approximately twice the silicon content of MDF. Because of the silicon content and some other factors, the PB caused substantially more tool wear in my tests.
Although silicon probably in the form of silica “sand” may cause some abrasion during machining, the silica may also cause fatigue at the tool surface. Because of the relatively high velocity of the tool to the workpiece, the silica or any other hard particulate matter may cause a sand blasting effect. The fatigue or repeated impacts on a surface for cleaning scale or oxides from a surface, such as prior to welding, provides a more chemically reactive surface. Such cleaning action would allow other wear mechanisms such as adhesion or tribo-chemical reactions to occur. No matter, if silica is present as silica or part of “dirt”; tool wear will generally increase as silicon content increases.
Ash analyses can help determine increased tool wear causes. These analyses can be conducted in house following ASTM standards or by private laboratories such as:
Midwest Laboratories, Inc.,
13611 B Street
Omaha, NE 68144
Inclusion of this lab contact does not constitute endorsement, but is included for the convenience of the reader.
This article has described some of the benefits of ash analyses to help determine the causes of increased tool wear from machining wood composites. A subsequent article will describe some of the benefits from more complete chemical analyses.
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