Chip prints appear initially as shallow depressions or dents in wood surfaces after knife-cutting processes such as knife planing (see photo). Chip prints are zones of crushed wood below the surface and are generally produced by wood chips being compressed between the wooden workpiece surface and a harder knife, feed roll and/or pressure bar. Chip prints form when chips have fallen back into the cutting circle and have been pressed into the wood surface by a knife in a revolving cutterhead or by wood chips falling back onto the wood surface and being pressed into the wood surface by feed rolls or pressure bars.
Expansion, spring back
Chip prints expand or spring back when exposed to moisture or some finishing materials. Usually the chip prints (or little dents) expand higher than the surrounding surface. Even after sanding the surface, little bumps may appear where chip prints were visible. Although sanding may remove the visible little dents, some crushed wood may remain undetected under the surface and subsequently reappear as little bumps from later processing or in service at higher moisture contents. Surface sanding should remove wood beyond just removing the visible chip prints. Wiping the surface with a damp cloth or sponge may be one method of examining for residual crushed wood.
Chip prints are more prevalent when machining wood at 6-8 percent moisture content than wood over 12 percent or higher moisture content.
Higher moisture content and green wood are more resilient from compressive stresses and spring back more. Chip prints generally do not occur from machining green wood.
Chip prints are also more prevalent in winter. Generally, chips are negatively charged and freshly machined workpiece surfaces are positively charged. These opposite and attractive charges dissipate slower in winter when the heated air is usually drier than other seasons of the year. Consequently, if wood chips fall onto the surface they tend to stick to the surface through subsequent processing. Conditions for chip prints to occur are generally more favorable during winter months.
Exhaust system necessary
An adequate exhaust system for removing wood chips and dust at each cutterhead or sanding head is paramount for eliminating or minimizing chip prints. Chip and dust collecting systems need satisfactory air flow and proper direction around or near the cutterheads and freshly machined surfaces. All too often chip and dust collecting systems have satisfactory air flow, but the air is misdirected around the cutterhead and too far away from the machined surface under the exhaust hood. The exhaust system needs to be properly designed and placed to remove the chips and dust from the cutting circle as well as to prevent the chips and dust from falling back onto the freshly machined surface or the machinery itself.
Wood machining parameters also need to be selected and coordinated to minimize or eliminate chip prints. The depth of cut, feed rate, cutterhead revolutions and number of knives can all affect chip prints. The chips can be too heavy or too light for the dust and chip removal system to properly remove the chips. For example, large depths of cut at deep parts of a moulding pattern may produce chips too large to clear the cutting circle or be so heavy they fall on the freshly machined wooden surface.
Additionally, the air flow within a deep cove may be insufficient to remove the chips. Roughing and finishing cuts at the optimal machine setting of knife marks per inch and depth of cut can reduce chip prints as well as other common wood machining defects such as fuzzy grain ( FDM, September 2006), raised grain ( FDM, May 2006) and chipped grain ( FDM, January 2006).
Tool geometry is another factor that can affect the frequency and severity of chip prints. Low rake and clearance angles tend to increase the severity of chip prints because the compressive forces toward the machined surface are greater and grow as a tool wears. Therefore, selecting the proper tool geometry and maintaining it will reduce chip prints. In fact, selecting proper and moderate machining parameters will eliminate or minimize most common wood machining defects.
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