Machinery's Handbook, 31st Edition
Surface Plates
725
Table 1. Granite Rock Types, Physical Properties, and Mineral Components GGG-P-463c
Modulus of Elasticity 10 6 psi GPa
Natural Color Bluish gray
Mineral Constituents, Descending Order Of Abundance Orthoclase, smokey quartz, a oligoclase, albite, biotite, muscovite, magnetite and zircon Oligoclase, orthoclase and microcline, quartz, biotite, apatite and zircon Orthoclase with a small amount of microcline, plagioclase, quartz a , biotite, magnetite, and garnet Orthoclase and microcline, quartz a , hornblende, biotite, plagioclase and magnetite Microcline, quartz, plagioclase, biotite, muscovite and magnetite Plagioclase, pyroxene and magnetite Plagioclase, pryoxene, hornblende, magnetite and biotite
Rock Type
Texture Fine- grained
3.5–7.0 24.1–48.2
Light gray
Biotite granite
3.5–7.0 24.1–48.2
Medium- grained
5.0–9.0 34.4–62.0
Pink
Biotite hornblende granite Biotite- muscovite Diabase Hypersthene Gabbro Muscovite- biotite granite-gneiss
Reddish brown
Fine- grained
6.0–9.0 41.3–62.0
Light gray Dark gray Light gray
Medium- to fine-grained
5.0–7.0 34.4–48.2 9.0–12.0 62.0–82.7 10.0–12.0 68.9–82.7
Fine- grained
Medium- grained Microcline and orthoclase, oligoclase, quartz, rutile, muscovite 3.5–8.0 24.1–55.1 a 28 to 32% quartz by volume. In certain conditions, high quartz content tends to increase wear life. As indicated in Table 1 , fine-grained pink granite containing a small amount of quartz has a lower modulus of elasticity and therefore a lower load bearing per square foot capac ity. The presence of large quartz crystals, however, results in a high degree of wearability by providing an ultra-smooth surface finish with increased surface hardness that resists wear on the granite and the precision ground and lapped instruments used on it. Although the load-bearing properties are lower than those of black or dark gray granite, this can be compensated for by increasing the thickness of the plate. As a result of the increased wearability of pink granite, the interval between lapping compared to fine-grained black granite can be as much as five times as long. This is an important consideration when planning the maintenance costs and downtime involved in maintaining a production schedule. The precision lapped and calibrated granite surface plate is a high-precision piece of equipment and must be maintained as such. Great care should be taken at all times to protect the surface, and attention to cleanliness is critical in the life span of the surface plate. It is essential that the surface be protected from the buildup of dirt, grease, airborne grime and oils. The plate should be covered when not in use to avoid accidentally dropping objects on the surface and chipping or cracking the precision finished surface. All surface plates should be installed and supported according to the manufacturer’s design and recommendations. Plates up to and including 6–12 feet are supported in a three-point non-distortable support system of hard rubber pads that are installed during manufacturing and remain in place during lapping and finishing. These pads are critical in the correct support of the surface plate and must never be removed or repositioned. Always make sure the surface plate is resting on these pads, and never support the plate by its ledges or under the four corners as this will cause deformation of the surface and introduce errors beyond the tolerance limits. Surface plates made to meet standard guidelines and accuracy parameters established over the last fifty years and published in documents such as the Federal Specification GGG-P-463c will be manufactured in certain pre-designed sizes. These designs have been analyzed for dimensional stability and dependability and will, with proper care, provide dependable measurement assurance for many years. There are manufacturers that will special order surface plates in a wide variety of sizes and configurations to meet the needs
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