Surface Plate

A surface plate is a solid, flat plate used as the main horizontal reference plane for precision inspection, marking out (layout), and tooling setup. The surface plate is often used as the baseline for all measurements to the workpiece, therefore one primary surface is finished extremely flat with accuracy up to .00001″/.00025 mm for a grade AA or AAA plate. Surface plates are a very common tool in the manufacturing industry and are often permanently attached to robotic type inspection devices such as a coordinate-measuring machine.

Accuracy and grade

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, the plates are often held to a high degree of flatness.

Unlike most instruments of mechanical precision, 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 manual rubbing against each other in pairs with a fine abrasive. Any errors of flatness tend to wear off with this abrasion, since the only stable, mutually conjugate surface shape is a plane.

Surface plates must be calibrated on a regular basis to ensure that chipping, warping or wear has not occurred. A common problem with surface plates are specific areas or a section that is frequently used by another tool (such as a height gauge) that will cause wear to a specific point resulting in an uneven surface and reduced overall accuracy to the plate. Tools and workpieces may also cause damage when dropped on the surface plate or when material chips have not been removed. This will result in erroneous measurements and can only be fixed by resurfacing the plate.

Plate material

Granite

Before the Second World War, metal was the standard material used for surface plates, however, after 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 started using granite in place of metal for his surface plates. Today most surface plates continue to be made of black granite with less common surface plates being made of pink granite or diabase. The quartz content of surface plate determines the wear ability of the granite surface plate with higher quartz content being more resistant to wear. Pink granite contains much higher levels of quartz and is often thinner than black granite, which reduces the load-bearing capabilities of the surface plate.

Damage to a granite surface plate will usually result in a chip but does not affect the accuracy of the overall plane. Even though chipped, another flat surface can still make contact with the undamaged portion of a chipped surface plate where as 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, non-magnetic, and will not rust.

Cast iron

A cast iron surface plate

Prior to World War II, most all surface plates were made from ribbed cast iron with the ribbing used to increase strength while reducing the weight. 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 fallout in popularity among machine shops, cast iron remains the most popular material for surface masters (different usage from a surface plate) among laboratory metrologists, machine builders, gage 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, 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 without the need to send the plates out for external rework. Despite the very stable structure, cast iron remains unsuitable even in high tolerance production applications for use as a normal surface plate due to thermal expansion encountered with regular use as an inspection tool. The nature and use of a surface master, by contrast, already necessitates expensive measures to control temperature regardless of material choice, and cast iron becomes preferable.

Cast iron unlike granite has also very uniform optical properties, and unlike glass or ceramic material very small light penetration depth which makes it a favorable material for certain optical applications ^[1].

Glass

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 cast iron.

Accessories

The surface plate is used in conjunction with accessories such as a square, straight edge, gage blocks, sine bar, sine plate, dial indicator, parallels, angle plate, height gage, etc.

References

Further reading

• Society of Manufacturing Engineers, (1991). Fundamentals of Tool Design. Dearborn, Michigan: Society of Manufacturing Engineers. ISBN 0-87263-412-4
• Wayne R. Moore (1970), Foundations of Mechanical Accuracy, The Moore Tool Company 

External links

• Two surface plates made by Whitworth
• “Granite Surface Plates”. L.S. Starrett Company. http://www.starrett.com/pages/860_granite_surface_plates.cfm. Retrieved 2007-02-15. 
• “Frequently Asked Questions”. L.S. Starrett Company. http://www.tru-stone.com/pages/faq.asp. Retrieved 2007-02-15. 

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