In
manufacturing and
mechanical engineering
Mechanical engineering is the study of physical machines that may involve force and movement. It is an engineering branch that combines engineering physics and mathematics principles with materials science, to design, analyze, manufacture, ...
, flatness is an important geometric condition for workpieces and tools.
In the manufacture of precision parts and assemblies, especially where parts will be required to be connected across a surface area in an air-tight or liquid-tight manner, flatness is a critical quality of the manufactured surfaces. Such surfaces are usually
machined
Machining is a process in which a material (often metal) is cut to a desired final shape and size by a controlled material-removal process. The processes that have this common theme are collectively called subtractive manufacturing, which utilizes ...
or
ground
Ground may refer to:
Geology
* Land, the surface of the Earth not covered by water
* Soil, a mixture of clay, sand and organic matter present on the surface of the Earth
Electricity
* Ground (electricity), the reference point in an electrical c ...
to achieve the required degree of flatness.
High-definition metrology
Metrology is the scientific study of measurement. It establishes a common understanding of units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to standardise units in Fran ...
, such as
digital holographic interferometry, of such a surface to confirm and ensure that the required degree of flatness has been achieved is a key step in such manufacturing processes. Flatness may be defined in terms of
least squares fit to a plane ("statistical flatness"), worst-case or overall flatness (the distance between the two closest parallel planes within).
Two parts that are flat to about 1
helium light band (HLB) can be "wrung" together, which means they will cling to each other when placed in contact. This phenomenon is commonly used with
gauge blocks.
Geometric dimensioning and tolerancing has provided geometrically defined, quantitative ways of
defining flatness operationally.
History
Joseph Whitworth popularized the first practical method of making accurate flat surfaces during the 1830s, using
engineer's blue
Engineer's blue (also known as scraping blue, Prussian blue, or simply bluing) is a highly pigmented paste used to assist in the mating of two or more components.
History
Joseph Whitworth popularized the first practical method of making accurat ...
and
scraping techniques on three
trial surfaces, in what is known as
Whitworth's three plates method.
By testing all three in pairs against each other, it is ensured that the surfaces become flat. Using two surfaces would result in a concave surface and a convex surface. Eventually a point is reached when many points of contact are visible within each square inch, at which time the three surfaces are uniformly flat to a very close tolerance.
Up until his introduction of the scraping technique, the same three plate method was employed using polishing techniques, giving less accurate results. This led to an explosion of development of precision
instruments using these flat surface generation techniques as a basis for further construction of precise shapes.
References
*
Wayne R. Moore, ''Foundations of Mechanical Accuracy'', Moore Special Tool Company, Bridgeport, CT (1970)
* Whitworth, J. 1858,
''Plane Metallic Surfaces'', Longman, Brown, and Co., London & Manchester.
{{reflist
External links
Flatness Overview - GD&T BasicsTwo surface plates made by WhitworthWhat is the right Flatness Tolerance for a Gasket Application
Metalworking terminology
ja:公差
pt:Tolerância (engenharia)