11. SURFACES
· No surface is perfectly smooth, but the better the surface quality, the longer a product generally lasts, and the better is performs.
· Surface texture can be difficult to analyze quantitatively. Two surfaces may be entirely different, yet still provide the same CLA (Ra) value.
· Recent developments in production technique, and metrology equipment have made it possible to specify and measure surface quality.
· There are standards, such as the CSA B95 1962.
· Surface Quality can be important when dealing with,
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- lubrication - small indentations can hold lubricant
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- resistant to wear - smoother surfaces wear less
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- tool life - rough surfaces will correlate to shorter tool life
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- fatigue/stress raisers -
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- corrosion - smoother surfaces easier to clean, less surface area to erode
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- noise reduction - smooth surfaces make less noise when rubbing, for example meshing gears.
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- fit - pressure seals could leak through pits
· Surface geometry can be quantified a few different ways.
· Real surfaces are rarely so flat, or smooth, but most commonly a combination of the two.
· Some other terms of interest in surface measurement,
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- Surface texture - all of the details that make up a surface, including roughness, waviness, scratches, etc.
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- Lay - the direction of the roughness on a newly manufactured surface. The roughest profile will be perpendicular to the lay.
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- Flaws - small scratches, cracks, inclusions, etc.
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- Cutoff - a value selected to be less than the waviness, but greater than the roughness length. This is controlled using electrical or digital filters. Typical values might be; 0.010", 0.030", 0.100"
