{"id":13134,"date":"2022-02-22T17:25:23","date_gmt":"2022-02-22T15:25:23","guid":{"rendered":"https:\/\/fractory.com\/?p=13134"},"modified":"2024-01-26T13:52:17","modified_gmt":"2024-01-26T11:52:17","slug":"symmetry-gdt-explained","status":"publish","type":"post","link":"https:\/\/fractory.com\/symmetry-gdt-explained\/","title":{"rendered":"Symmetry (GD&T) Explained"},"content":{"rendered":"

The 2009 edition of ASME Section Y14.5 defines 14 geometric tolerances in GD&T<\/a>. The manufacturing industry uses these tolerances to convey manufacturing intent from the designers to the manufacturers. GD&T helps us to inspect, control and measure the various features of a machine part.<\/p>\n

The 14 geometric tolerances are classified into 5 main groups – form, location, profile, orientation and layout. Symmetry is one of the three tolerances under location control (the other two being true position and concentricity).<\/p>\n

As the name suggests, it controls the symmetry of part features such as tapers, holes, chamfers, curves, etc. This may not be required in general applications. However, in special applications where balance and equidistant loading is of great concern (high-speed applications), symmetry becomes increasingly important.<\/p>\n

What is Symmetry?<\/h2>\n

GD&T symmetry is a 3D tolerance that ensures that part features are symmetrical about a datum plane. The callout defines a central plane and creates a tolerance zone around it.<\/p>\n

The GD&T symmetry callout ensures symmetry control by checking the distance between any two corresponding points on either side of the datum plane and calculating their median points. These median points must lie near the datum plane and be within the symmetry tolerance zone specified in the feature control frame.<\/p>\n

Theoretically, the inspector must check all the median points and find them within the tolerance zone. However, for practical purposes, fewer points at different cross-sections are inspected. The remaining median positions are interpolated to achieve the median plane.<\/p>\n

Symmetry Tolerance Zone<\/h2>\n

\"symmetryThe symmetry tolerance consists of two parallel planes, one on each side of the datum center plane. The distance between the two parallel surfaces is the tolerance limit for the callout. For instance, if the tolerance limit is set at 0.03 mm, the two planes will be at a distance of 0.015 mm on either side of the datum plane. This type of zone is the default tolerance zone type in GD&T. It is also sometimes known as the total wide tolerance zone.<\/p>\n

All the points on the median plane must lie in the volume between the two planes of the tolerance zone for approval.<\/p>\n

Symmetry vs Other Callouts<\/h2>\n

The GD&T symmetry callout is a type of location control. It ensures that two features are at their proper locations when checked against the datum plane. Other location controls can also perform the same job, although using a different method and tolerance zone type. The symmetry tolerance is comparable to concentricity and true position in terms of what they can achieve.<\/p>\n

Symmetry and Concentricity<\/h3>\n

The concentricity callout<\/a> controls the concentricity of cylindrical surfaces whereas symmetry controls are typically applied to any non-cylindrical surface. Many refer to concentricity as the circular version of symmetry. ASME Y14.5M-1994, 5.14 states that: “…symmetry and concentricity controls are the same concept, except as applied to different part configurations.”<\/p>\n

GD&T symmetry controls the location of two features by establishing a datum plane. The concentricity symbol, on the other hand, checks the concentricity by establishing a central datum axis. It then measures the spread of actual centers of cylindrical cross-sections; and if they are within the cylindrical tolerance zone around the ideal datum axis. Concentricity derives an actual central axis instead of a median plane.<\/p>\n

Both symmetry and concentricity are incredibly difficult to measure. For accurate measurements, a coordinate measuring machine (CMM) is a must.<\/p>\n

Symmetry and True Position<\/h3>\n

Symmetry and true position can both be used to define the ideal location for a part feature. They may even be used interchangeably in some situations. However, true position is far more versatile compared to symmetry. It can do everything symmetry can do but the opposite is not true.<\/p>\n

The true position callout<\/a> can establish a total wide tolerance zone as well as a circular zone. This increases the range of features that can be controlled by it. True position allows for bonus tolerances, whereas symmetry does not. Symmetry also does not allow datum feature shift and projected tolerance zone, both of which are possible with true position.<\/p>\n

Another difference is that true position can be called Relative to Feature Size (RFS), or with Least\/Maximum Material Condition (LMC\/MMC). Symmetry is always applied RFS.<\/p>\n

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