Why are marking tables made of cast iron?

Flat surfaces serve as vital reference planes in various industrial applications.

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A surface plate plays a crucial role as a solid, flat reference plane for tasks such as precision inspection, tooling setup, and layout marking. It is regarded as the baseline for all measurements pertaining to a workpiece, with one primary surface meticulously finished for exceptional flatness, boasting tolerances below 11.5 µm or 0. mm per mm for grade 0 plates. Commonly found in the manufacturing sector, surface plates are frequently equipped with mounting points, enabling their integration as structural components within machines, including coordinate-measuring machines and high-precision optical assemblies. Typically, these plates are square or rectangular in shape, but can be customized as needed.

Accuracy and Grade

Surface plates are classified into various grades based on their accuracy, including AA, A, B, and Workshop grade. Among these, Workshop grade is the least accurate; however, all surface plate grades are required to maintain a high standard of flatness.

Regular calibration of surface plates is essential to identify any chip, warp, or wear impacts. A common issue involves localized wear due to repeated tool usage, resulting in an uneven surface that compromises overall accuracy, further exacerbated by the presence of abrasive dust. Any droppage of tools or workpieces can also lead to damage, which might yield erroneous measurement results. Corrections to the plate can only be achieved through specialized resurfacing techniques and equipment, determined by the plate's grade.

History of Surface Plates

Unlike other precision mechanical instruments, surface plates achieve their accuracy through the application of the "automatic generation of gages" principle. This entails refining three approximately flat surfaces progressively by manually rubbing them together, interspersed with coloring materials, followed by hand-scraping to eliminate high points. This scraping method effectively corrects flatness errors, as a plane remains the only stable conjugate surface shape.

The significant role of high-precision surface plates was first acknowledged by Henry Maudslay, who pioneered scraping techniques for cast-iron plates, employing marking blue to identify imperfections and working with sets of three plates to ensure flatness. Joseph Whitworth, who trained under Maudslay, also contributed to this practice, sharing insights during his address to the British Association in 1850.

Surface Plate Materials

Granite

Before World War II, metal was the standard material for surface plates, but the war led to metal shortages. Wallace Herman and his innovative employee, Donald V. Porter, began using granite in place of metal for their surface plates. Nowadays, the majority of surface plates are still crafted from black granite, also known as black diabase, with quartz-bearing granite utilized for more wear-resistant options. The addition of quartz enhances the wear resistance of these plates, while black granite is preferred for machine bases due to its rigidity and superior vibration-damping properties. When damaged, granite surface plates might chip but not compromise the overall plane's accuracy; unlike cast-iron plates, which can distort if damaged.

Granite's essential stability, non-magnetic nature, and rust resistance make it an attractive choice for surface plates. In 1971, the Federal Specification GGG-P-463B was established in the United States, outlining requirements for granite surface plates used in precision applications, later revised in 2006 as GGG-P-463C to incorporate modern terminology and practices.

Cast Iron

Prior to World War II, the predominant material for surface plates was ribbed cast iron, which increased stiffness without adding substantial weight. This cast iron undergoes aging processes to minimize internal stress, reducing the risk of warping. Presently, cast-iron surface plates are commonly used on production floors to lap granite surfaces for improved accuracy.

Despite declining popularity, cast iron remains favored in laboratory settings and other high-accuracy domains for gauging flatness. Properly cast iron displays greater dimensional stability over time compared to granite or ceramics and can be crafted to a superior grade of flatness, serving as a robust bearing surface for master standards. Although cast iron is unsuitable for typical high-tolerance production applications due to thermal expansion, it excels in controlled environments typical for master surfaces.

Unlike granite, cast iron features uniform optical properties and limited light penetration, advantageous for certain optical applications.

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Glass as an Alternative

During wartime material shortages, glass surfaced as an alternative for surface plates. It can be accurately ground and notably chips instead of creating a rough burr, which is a concern with gray cast iron.

Accessories for Surface Plates

Surface plates work best when combined with various accessories, such as squares, straight edges, gauge blocks, sine bars, sine plates, dial indicators, parallels, angle plates, and height gauges.

Calibration of Surface Plates

Routine calibration of granite surface plates is essential for maintaining accurate flatness and measurement precision over time. The calibration frequency largely depends on the operating environment of the surface plate. When necessary, a lapping or resurfacing process is performed to restore measurements within specified guidelines, usually involving polishing the surface with abrasive pastes to remove contaminants. Calibration also includes thorough cleaning and light polishing.

[Solved] Why Are Surface Plates Used for Marking?