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A 250 mm x 250 mm surface plateA surface plate is a solid, flat plate used as the main horizontal reference plane for precision inspection, marking out (layout), and tooling setup.[1] The surface plate is often used as the baseline for all measurements to a workpiece, therefore one primary surface is finished extremely flat with tolerances below 11.5 μm or 0. mm per mm for a grade 0 plate.[2] Surface plates are a common tool in the manufacturing industry and are often fitted with mounting points so that it can be an integrated structural element of a machine such as a coordinate-measuring machine, precision optical assembly, or other high precision scientific & industrial machine. Plates are typically square or rectangular, although they may be cut to any shape.
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There are varying grades used to describe the accuracy of some metrology equipment such as: AA, A, B, and Workshop grade. While workshop grade is the least accurate, all grades of surface plates are held to a high degree of flatness.
Surface plates must be calibrated regularly to ensure that chipping, warping, or wear has not occurred. A common problem is wear to particular areas, such as that caused by the frequent use of a tool in one place (such as a height gauge), that causes an uneven surface and reduces overall accuracy of the plate, this may be greatly accelerated if abrasive dust is present. Tools and workpieces may also cause damage when dropped on the surface plate. Also, damage can be caused when swarf and other debris have not been removed. This will result in erroneous measurements. Damage to the plate can be corrected only by resurfacing, which requires specialised techniques and equipment depending on the grade of the plate.
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Unlike most mechanical precision instruments, surface plates do not derive their precision from more-precise standards. Instead they originate precision by application of the principle of "automatic generation of gages". In this process, three approximately flat surfaces are progressively refined to precise flatness by manually rubbing them against each other in pairs with colouring matter in between, and then hand-scraping the high points. Any errors of flatness are removed by this scraping, since the only stable, mutually conjugate surface shape is a plane.
The importance of the high-precision surface plate was first recognised by Henry Maudslay around . He originated the systems of scraping a cast-iron plate to flatness, rubbing marking blue between pairs of plates to highlight imperfections, and of working plates in sets of three to guarantee flatness by avoiding matching concave and convex pairs.
Joseph Whitworth, born in , had been an apprentice with Maudslay from but had left by the time he started his own business in . He described this process to the British Association in in his paper "On producing True Planes or Surfaces on Metals"&#;as he related during his chairman's address in at the inaugural meeting of the British Institute of Mechanical Engineers in Glasgow.[4][5] His paper, and his past work for Maudslay, has led to some writers claiming Whitworth as the originator of the surface plate scraping technique, not Maudslay.
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Before the Second World War, metal was the standard material used for surface plates, however, the war efforts of various countries put a strain on the availability of metal. A monument and metal shop owner (Wallace Herman) in Dayton, Ohio, along with his inventive employee Donald V. Porter, started using granite in place of metal for his surface plates. Today most surface plates continue to be made of black granite, more accurately referred to as black diabase, with the more wear-resistant surface plates being made of quartz-bearing granite. The quartz content of these granite surface plates increases the wear resistance of the plate as quartz is a harder stone. Black granite is dominantly used in machine bases, granite accessories, and custom applications for its superior stiffness, excellent vibration damping, and improved machinability. Quartz-bearing granite (usually pink, white, or grey) is often made thicker than black granite to provide equal load-bearing capabilities of the types of material used for surface plates, as it is not as stiff as black granite.
Damage to a granite surface plate will usually result in a chip but does not affect the accuracy of the overall plane. Even though it is chipped, another flat surface can still make contact with the undamaged portion of a chipped surface plate, whereas damage to a cast-iron plate often raises the surrounding material above the working plane causing inspected objects to no longer sit parallel to the surface plate.
Granite is also inherently stable, is non-magnetic, has excellent vibration damping characteristics, and will not rust.
On 3 August , Federal Specification GGG-P-463B was issued to provide requirements in United States customary units for igneous rock (granite) surface plates for use in precision locating layout, and inspection work. It encompassed new certification, recertification in the field, and recertification after resurfacing. GGG-P-463B was later revised and reissued on 12 September as GGG-P-463C, which provided common language and terms of classification for surface plate manufacturing and commerce. On 15 June an amendment was issued to the federal specification in order to include requirements in metric units.
Although GGG-P-463C was used extensively in American industry since its publication, the government did not issue any new revisions to keep up with advancements within industry. The American Society of Mechanical Engineers (ASME) decided to form a committee to revise the federal specification in accordance with modern technologies. Most notably, a more complete glossary was added with currently accepted definitions, and a new format was used that should be more familiar to current users of the Standard. ASME also recognised the need for updates to incorporate modern concepts such as traceability and measurement uncertainty that have undergone considerable development since . In June , ASME replaced Fed Spec GGG-P-463C with the American National Standard (ANS) ASME B89.3.7 &#; Granite Surface Plates.[7] Iso standard defines ISO-2 for granite surface plates, but it seems the current in use is still dating back .[8]
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A cast-iron surface plate, the handles aid in lifting the plate and using it for lapping operationsPrior to World War II, almost all surface plates were made from ribbed cast iron with the ribbing used to increase stiffness without incurring the weight of solid construction. The cast iron was aged to reduce stress in the metal in an effort to decrease the likelihood of the plate twisting or warping over time.
Cast-iron surface plates are now frequently used on production floors as a tool for lapping granite surface plates to achieve certain grades of accuracy. The metal allows itself to be impregnated with the lapping media over a large flat surface.
Despite a fall in popularity among machine shops, cast iron remains the most popular material for master surfaces (different use from a surface plates) among laboratory metrologists, machine builders, gauge makers, and other high-accuracy industries that have a requirement for gauging flatness. Cast iron that has been properly cast is more dimensionally and geometrically stable over time than granite or ceramics,[9] is more easily worked to a higher grade of flatness, and provides a better bearing surface to assist the creation of other master standards. These specialized surface plates are produced in sets of three, by the company that will be using them, so the plates may be regularly verified and refined, including by the Whitworth three plate method, without the need to send them out to be reconditioned. Despite its high stability, cast iron remains unsuitable for use as a normal surface plate in high-tolerance production applications because of thermal expansion. The nature and use of a master surface, by contrast, already necessitates expensive measures to control temperature regardless of material choice, and cast iron becomes preferable.
Cast iron, unlike granite, has very uniform optical properties and, unlike glass or ceramic, very small light penetration depth which makes it favorable for certain optical applications.[10]
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Glass is an alternative material and was used during World War II when material and manufacturing capacity were in short supply. Glass can be suitably ground and has the benefit that it chips rather than raising a burr, which is a problem when using gray cast iron.
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The surface plate is used in conjunction with accessories such as a square, straight edge, gauge blocks, sine bar, sine plate, dial indicator, parallels, angle plate, height gauge, etc.
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Granite surface plate calibration should be performed routinely to maintain proper flatness and ensure measurement accuracy over time. The intervals between calibrations depends on the environment where the surface plate is located. When needed, lapping or resurfacing is provided to bring measurements within grade guidelines. This procedure involves polishing the surface with an abrasive paste to remove all unwanted material. Granite surface plate calibration also includes cleaning and a light polishing.
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Advantages of granite in metrology is multi fold. Granite is the best material for providing an accurate surface reference in Metrology.
Our mission is to be the premier provider of precision granite and other mediums in this metrology field. By delivering superior value as measured by customer service and responsiveness, product quality, technical expertise and innovation.We do manufacturing of Surface plates, Straightedges, Master squares, Master angles, Cubes and other related items. The sizes will be based on Indian Standard and DIN standard, also we under take special sizes on customers request.
As a material for metrology aids granite, is a very hard natural igneous rock formation of visibly crystalline texture formed essentially of quartz and orthoclase / microcline. Due to its high quartz content provides greater resistance to wear and damage. Its anti-glare property results in less eyestrain for the person using the plates. Due to its hardness and wear qualities, non denting property, smoother action, greater precision, low maintenance cost, granite is used often for manufacturing of Surface plates, measuring prisms, guideways of precision instruments and other metrology aids.
When we go for the investigation of granite surface plates, it does not retain the surface topography with changes in atmospheric conditions like temperature. So, the behavior of the plate to be studied under varying conditions like:
First the surface plate is divided into square of 100X100mm. An electronic level with a sensitivity of 0./100mm placed on a bridge is used to measure the flatness of the surface. Readings are taken along and across the surface and the surface topography is computed to attain the contour. This bi-directional approach to every point gives accurate measurements of the surface.
Now, lets watch the behavior of the granite at various temperatures. We can see a significant change in the contour of the surface due to temperature rise. We cannot explain the magnitude of the change by the coefficient of thermal expansion of Dolerite rock, which is 2×10 -6 to 4X10 -6 per degree centigrade. At 26°C, we can notice the surface contour seems to be concave with overall flatness accuracy of 5.5 microns. When there is a rise in temperature to 40°C the surface contour changes to convex with overall flatness accuracy of 8.6 microns. This is the behavior of granite due to change in temperature.
When we study about the change in contour due to a change in atmospheric humidity is very insignificant. The change in contour of the surface with change in humidity of 20, 40 and 60% has been negligible.
When the surface was soaked with water and measurement taken within a short period the variation in surface contour is negligible. However a change was observed when the surface was allowed to be soaked with water for 12 hours. For a study, we take a granite plate of size 400X400X100. Before soaking in water the flatness was checked as 3.48 microns, when the top surface was soaked in water the flatness was recorded as 3.24 microns. Now the top surface was soaked in water for 12 hours, the reading of flatness was recorded as 2.34 microns and when all sides soaked in water for 3 hours, we got the flatness reading to be 1.91 micron. This was the effect of water on the accuracy of the surface plate.
However, we observe that for both, the moist condition and as well as for temperature change the surface returns to the original surface topography, once the surface plate returns to its original atmospheric condition. This is one of the advantages of granite in metrology. Studies on igneous rocks such as Granite and Dolerite showed that:
As stated above granite has pores and these can have with them entrapped moisture. The moisture content is maximum at the surface and decreases in the deeper layers. When you soak the stone in water and when the surface dries, break it into two halves. You will notice the newly exposed upper layers have distinctly higher content of water whereas it is less wet as the depth increases.
We know that the igneous rock like Granite, Dolerite, etc., consists of various substances such as Quartz, Hornblende, Calcite, and Orthoclause. Due to this various ingredients of stone have uneven volumetric or linear expansion within them. For example quartz expands four times more than the feldspar and twice as much as hornblende. Due to rise in temperature the quartz exerts a pressure against its surroundings and causes an expansion of the surface, which even if small affects the surface accuracy.
The variation of the surface topography thus is an inherent phenomenon due to the properties of a stone. Its value changes from stone to stone. It is more in stones with higher porosity and quartz content.
On moving still deeper in studies about the advantages of granite in metrology, we find the fact that, granite expands only about 0.036% at 25ºC whereas water expands more than 0.32%, at the same temperature. If the walls of the pores prevents water from expanding it can exert about 70 atmospheric pressure against the pore walls. Thus the entrapped moisture or water in the pores comes to an equilibrium pressure due to capillary action and there will be a pressure on the walls of the pores, which act as small pressure vessels causing the surrounding portion of the rock to expand due to this internal pressure. However this pressure will be maximum at top layer of the surface and reduces in the deeper portion. Thus the upper layers of the stone expand more than the lower causing the surface to swell.
The above points provide us required data regarding the advantages of granite in metrology. It also proves that fine-grained rocks have relatively higher density and hardness. They have higher resistance to scratch and are less porous. However, stability of granite under varying temperature and humidity conditions is a major limitation. Although the coefficient of thermal expansion is low when you compare to other engineering materials like cast iron and steel. And if any small impact or scratch made over the granite surface plate, it results in crushing of localized zone. The broken piece or powder comes out without harming the surface accuracy.
However, in the case of cast iron or steel surfaces, under similar circumstances a bulge would result. This affects the surface accuracy and often these are hard to notice by the naked eye.
Just the color alone is not an indication of the physical qualities of the stone. In general, the presence or absence of minerals determines the color of the granite. Which may have no bearing on the qualities that make good surface plate material. There are pink, gray and black granites that are excellent for surface plates.
The ageing qualities also adds to the advantages of granite in metrology. Measurement of a 1.6m x 1.0m surface plate kept in a metrology room immediately after its lapping over a period of four years, confirms this aspect. The variation in shape and the accuracy of this surface is negligible. Thus, the advantages of granite in metrology are many and provides an accurate surface for measurement.
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