My work in inspection, and as a quality management system auditor, provides numerous opportunities to acquire knowledge. It also provides areas of risk for ensuring valid and reliable results. One risk that keeps repeating itself in my audit findings is the treatment of surface plates. Instead of being treated as a precision device, they are treated as an area to stack tools, parts and other debris. The root cause of this treatment is the lack of knowledge about the surface plate and its mistaken identity as just a table. Let&#;s take a journey of the history and recommended care for the surface plate.
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The surface plate was designed and developed as a precision tool by Henry Maudslay and Joseph Whitworth in order to ensure the production of identical tool parts. The first surface plate was composed of cast iron. The first surface plates were sold in by the Crown Windley company. During World War II, metal was in high demand which resulted in the introduction of using granite, a non-corroding metal that is extremely hard and non-magnetic.
A flatness check of a surface plate. Source: ASQ
Engineering and manufacturing specifications are demanding tighter tolerances, which elevate the importance of accurate measurements. To deliver measurements, each piece of metrology equipment requires a base plate.
Granite is the most common material used for surface plates. The surface plate does require maintenance. The following are recommendations for maintaining these precision instruments:
A surface plate on a CMM (r) and other surface plate devices (l). Source: ASQ
Along with checking the flatness of parts, surface plates can identify part warpage. The surface plate is the &#;0&#; reference point. If a height gage is used on the plate, the inspector can place the gage anywhere on the plate and zero the gage. This can be completed in conjunction with a gage block. The gage block is removed, and small parts can be accurately measured. This is also a technique to verify small parts for flatness.
The surface plate is an underrated metrology tool. It has been more than 80 years since granite was identified as an alternative to cast iron. Granite surface plates have become foundation for many standards in metrology. The coordinate measuring machine (CMM) does represent the acceptance of granite as the only material used in this vital area of metrology. Hopefully, you were able to obtain knowledge and insight on why I feel it is critical to properly maintain surface plates. Valid and reliable results must be acquired in order to make proper decisions on the disposition of parts.
A 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.
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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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For more information, please visit Precision Surface Plate.