GEOMETRICAL DIMENSIONING & TOLERANCING (GD&T)

Reference Datum

Referencing Datums in Feature Control Frames

Once the designer specifies the datums, he/she should also communicate through drawing when and how the datums must be used. For this the feature control frames are normally used. When feature control frames are stated, they reference the datums to be used for measurement. In the above example, the bolt holes are toleranced relative to …

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Planar datum

PLANAR DATUM, DATUM FEATURE SYMBOL & DATUM SELECTION.

DEFINITION A theoretically exact plane, point or axis from which a measurement is made is known as a datum. A part feature which comes in contact with a datum is known as a datum feature. A planar datum is the true geometric counterpart of a planar datum feature. The theoretical perfect boundary of a specified …

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Implied datums

IMPLIED DATUMS: Definition, Shortcomings & Consequences.

DEFINITION An assumed plane, axis or point used to measure a dimension is called as an implied datums. This method is an old way of coordinate tolerancing. The above image shows an example of implied datum. The bottom and left sides of the blocks are considered implied datums. SHORTCOMINGS OF IMPLIED DATUMS There are 2 …

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Cylindricity

6 Concepts About Cylindricity That’ll Brighten Your Day.

DEFINITION When all the points of a surface are equidistant from a common axis, then this condition is known as cylindricity. A cylindricity control helps in limiting the cylindricity error on a part surface. This control creates a tolerance zone of two coaxial cylinders with which all the points of the surface must lie. The …

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Circularity

CIRCULARITY: Application, Legal Specification & Inspection.

DEFINITION When all the points of a circular surface are perpendicular and equidistant from a common axis, this condition is known as circularity.This control is applicable to any diametrical cross section.A geometric tolerance capable of limiting the amount of circularity on a part surface is known as circularity control.As per this control, the elements of …

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Straightness

STRAIGHTNESS CONTROL (AXIS OR CENTER PLANE)

How to determine when a straightness control applies to a FOS? Only straightness control has the ability to be applicable on either a surface or a feature of size.There is a difference between straightness control applied to a FOS and to a surface.The following conditions are applicable when a straightness control is used on a …

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Straightness

STRAIGHTNESS (SURFACE ELEMENTS)

DEFINITION A straightness control is a geometric tolerance which helps in limiting the amount of straightness error allowed in a particular surface.The tolerance zone for a straightness control are two parallel planes which is equal to the straightness tolerance value.The first line element of the tolerance zone is produced by the two high points of …

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Bonus tolerance

How to Explain Bonus Tolerance to Your Boss

BONUS TOLERANCE Bonus tolerance is a vital concept in manufacturing industry which helps to dimension the parts.Manufacturing costs can be reduced significantly with the use of this tolerance.An additional tolerance for a geometric control is known as Bonus tolerance.It is applicable when a modifier such as MMC or LMC is present in the feature control …

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Basic dimension

BASIC DIMENSION, VIRTUAL AND BOUNDARY CONDITIONS.

BASIC DIMENSION A numerical value which conveys us the exact value, form, orientation or location of a feature is called as Basic dimension.There are uses of basic dimensions on engineering drawings. To define accurately the exact location, form, orientation or true profile of a part feature. Define information of gage (e.g datum targets).Basic dimensions helps …

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