Question

how to select perpendicular gd&t tolerance value using IT tolerance. and gave example

Answer 1

GD&T Symbol:

Relative to Datum: Yes

MMC or LMC applicable: Yes

GD&T Drawing Callout:

Surface Perpendicularity:

Axis Perpendicularity:

Description:

Surface:

Perpendicularity is a fairly common symbol that requires the referenced surface or line to be perpendicular or 90° from a datum surface or line. Perpendicularity can reference a 2D line, but more commonly it describes the orientation of one surface plane perpendicular to another datum plane. The tolerance of the perpendicularity callout indirectly controls the 90° angle between the parts by controlling the location where the surfaces have to lie. See the tolerance zone below for more details.

Note: Perpendicularity does not control the angle of the referenced feature –the tolerance is in distance units. (mm/in)

Axis:

Axis control can also be called out for Perpendicularity and is one of the more common forms of axes call outs. When it is referenced for a circular feature, the feature control frame will contain the diameter (Ø) symbol. Axis Perpendicularity can be applied to a positive feature (pin/boss) or to a negative feature (a hole). When Perpendicularity is referenced for axial control of a feature, the symbol now specifies a cylindrical boundary where the axis of the referenced feature must lie. This cylindrical boundary is formed by taking a line that is directly perpendicular to the datum feature. When this version of Perpendicularity is called out it is to be used with maximum material condition to enable easy gauging of the part. See example 2 below for how these particular parts are gauged.

GD&T Tolerance Zone:

Surface:

Two parallel planes or lines which are oriented perpendicular to the datum feature or surface. The planes are held perpendicular to the datum, but only ensure that the entire feature falls into the tolerance zone.

Remember: Perpendicularity does not directly control the angle of the referenced surface; it controls the envelope (like flatness) where the surface needs to be.

Axis:

A cylinder surrounding a referenced theoretical axis which is directly perpendicular to the datum feature. The tolerance zone is the diameter of this symbol in which the central axis of the measured feature must lie.

Gauging / Measurement:

Surface:

Perpendicularity is measured using a height gauge, similar to flatness, however, the gauge (or part) is locked to a 90° datum to measure how perpendicular the surface is. The entire surface has to be measured if it is a flat feature.

Axis:

To ensure that a part or feature is axially perpendicular, Maximum material condition is most often called out on axis perpendicularity to allow easy measurement with a gauge. This allows it to be designed for either a negative (hole) or positive (pin) feature and can take into account a bonus tolerance.*

Gauge size for an internal feature (like a hole):

Gauge Ø (pin gauge)= Min Ø of hole (MMC) – Perpendicularity Tolerance

Gauge size for an external feature (like a pin):

Gauge Ø (hole gauge) = Max Ø of pin (MMC) + Perpendicularity Tolerance

See Example #2 below for a good example Axis Perpendicularity using MMC.