Society of Mechanical Engineers ASME Y14.5M-1994 Geometric Dimensioning and Tolerancing (GD&T) standards. Note that this version is a revision of the ANSI Y14.5M-1982 standards.

#### ABTech MicroForm Gage Software

- Definitions
- Form Tolerances
- Orientation Tolerances
- Location Tolerances
- Runout Tolerances
- Further Explanation

## Definitions

Selected standards relevant to the MicroForm software are listed below using the ASME numbering system.

**1.3.3 Datum:** A theoretically exact point, axis or plane derived from the true geometric counterpart of a specified datum feature. A datum is the origin from which the location or geometric characteristics of features of a part are established1.

**1.3.18 Full Indicator Movement (FIM):** The total movement of an indicator where appropriately applied to a surface to measure its variations1.

*ABTech’s note: The term FIM has replaced TIR (Total Indicator Reading) in standards terminology. As FIM is the actual movement of the indicator itself, not just what is represented on a mechanical dial or digital display, the errors in the display (i.e., mechanical limitations, signal responsiveness, resolution, etc) are theoretically absent in FIM. As there is no means to truly measure FIM, TIR is still the accepted norm practiced in general industry.*

**5.1 Tolerances of Location:** This section establishes the principles of tolerances of location. Included are position, concentricity, and symmetry used to control the following relationships:

- center distance between such features as holes, slots, bosses and tabs;
- location of features [such as in (a) above] as a group, from datum features, such as a plane and cylindrical surfaces;
- coaxiality of features;
- concentricity or symmetry of features- center distances of correspondingly-located feature elements equally disposed about a datum axis or plane1.

*NOTE: Definition (d) is the method used as the standard in the MicroForm gage software.*

**5.12 Concentricity: ** Is that condition where the median points of all diametrically opposed elements of a figure of resolution (or correspondingly-located elements of two or more radially disposed features) are congruent with the axis (or center point) of a datum feature1.

**5.12.1 Concentricity Tolerancing:** A concentricity tolerance is a cylindrical tolerance zone whose axis (or center point) coincides with the axis (or center point) of the datum feature(s). The median points of all correspondingly-located elements of the feature(s) being controlled, regardless of feature size, must lie within the cylindrical tolerance zones. The specified tolerance and the datum reference can only apply on an RFS basis. A concentricity tolerance requires the establishment and verification of the feature’s median points1.

**6.2 Form and Orientation Control:** Form Tolerances control straightness, flatness, circularity (roundness) and cylindricity. Orientation tolerances control angularity, parallelism and perpendicularity (squareness)1.

**6.3.1.3 Identifying Datum References:** It is necessary to identify features on a part to establish datums from which dimensions control orientation, runout and when necessary, profile1.

**6.4 Form Tolerances:** Form Tolerances are applicable to single features or elements of single features; therefore, form tolerances are not related to datums1.

**6.4.2 Flatness:** Flatness is the condition of a surface having all elements in one plane1.

**6.4.2.1 Flatness Tolerance:** A flatness tolerance specifies a tolerance zone defined by two parallel planes within which the surface must lie. When a flatness tolerance is specified the feature control frame is attached to a leader directed to the surface or to an extension line of the surface. It is placed in a view where the surface elements to be controlled are represented by a line1.

**6.4.3 Circularity (Roundness):** Circularity is a condition of a surface where:

- For a feature other than a sphere, all points of the surface intersected by any plane perpendicular to an axis are equidistant from that axis.
- For a sphere all points of the surface intersected by any plane passing through a common center are equidistant from that center1.

**6.4.3.1 Circularity Tolerance:** A circularity tolerance specifies a tolerance zone bounded by two concentric circles within which each circular element of the surface must lie, and applies independently at any plane1.

**6.6.1 Specifying Orientation Tolerances in Relation to Datum Features:** In specifying orientation tolerances to control angularity, parallelism, perpendicularity and profile, the considered feature is related to one or more datum features. Relation to more than one datum feature is specified to stabilize the tolerance zone in more than one direction. Note that angularity, perpendicularity, and parallelism, when applied to plane surfaces, control flatness if a flatness tolerance is not specified1.

**6.6.3 Parallelism:** Is the condition of a surface or center plane, equidistant at all points from a datum plane; or an axis, equidistant along its length from one or more datum planes or a datum axis1.

**6.6.3.1 Parallelism Tolerance:** A parallelism tolerance specifies one of the following:

- a tolerance zone defined by two parallel planes, parallel to a datum plane or axis within which the surface or center plane of the considered feature must lie1.
*NOTE: This is the method used as the standard in the MicroForm gage software.* - a tolerance zone defined by two parallel planes parallel to a datum plane or axis within which the axis of the considered feature must lie.
- a cylindrical tolerance zone parallel to one or more datum planes or a datum axis, within which the axis of the feature must lie.
- a tolerance zone defined by two parallel lines parallel to a datum plane or axis within which the line element of the surface must lie.

**6.6.4 Perpendicularity:** Perpendicularity is the condition of a surface, center plane, or axis at a right angle to a datum plane or axis1.

**6.6.4.1 Perpendicularity Tolerance:** A perpendicularity tolerance specifies one of the following:

- A tolerance zone defined by two parallel planes perpendicular to a datum plane or axis, within which the surface or center plane of the considered feature must lie1.
*NOTE: This is the method used as the standard in the MicroForm gage software.* - A tolerance zone defined by two parallel planes perpendicular to a datum axis, within which the axis of the considered feature must lie.
- A cylindrical tolerance zone perpendicular to a datum plane, within which the axis of the considered feature must lie.
- A tolerance zone defined by two parallel lines perpendicular to a datum plane or axis, within which the line element of this surface must lie.

**6.7 Runout:** Runout is a composite tolerance used to control the functional relationship of one or more features of a part to a datum axis1.

**6.7.1 Runout Tolerance:** The types of features controlled by runout tolerances include those surfaces constructed around a datum axis and those constructed at right angles to a datum axis1.

**6.7.1.2 Types of Runout Control:** There are two types of runout control, circular runout and total runout. The type used is dependent upon design requirements and manufacturing considerations. Circular runout is normally a less complex requirement than total runout1.

**6.7.1.2.1 Control of Circular Elements:** Circular runout provides control of circular elements of a surface. The tolerance is applied independently at each circular measuring position as the part is rotated 360 degrees. Where applied to surfaces constructed around a datum axis, circular runout may be used to control the cumulative variations of circularity and coaxiality. Where applied to surfaces constructed at right angles to the datum axis, circular runout controls circular elements of a plane surface (wobble)1.

*1 – Source: The American Society of Mechanical Engineers. 1994. ASME Y14.5M-1994 Dimensioning and Tolerancing. New York, New York.*

## Form Tolerances

Form Tolerances are applicable to single features or elements of single features; therefore, form tolerances are not related to datums.

**Flatness:** calculated by the minimum separation of parallel lines that encloses all points of the measured surface. The flatness result displayed is the distance between the two parallel lines.

**Circularity (Roundness):** defines how “out of round” the surface is; as in how much deviation from a perfect circle. This is defined by two concentric circles that encompass all points of the measured surface. The difference in radius of the two circles is the roundness error.

## Orientation Tolerances

Angularity, parallelism, and perpendicularity are orientation tolerances applicable to related features. These tolerances control the orientation of features to one another.

**Parallelism:** defined as the amount of deviation between the measured surface and the best fit plane of the datum surface. This is found by two parallel planes with minimal separation, parallel to the datum plane that encompass all points of the measured surface. The distance between the planes is the parallelism error (see figure 3). Note that the parallelism result includes the flatness error for the measured surface.

**Perpendicularity:** defined by two minimally separated parallel planes, perpendicular to a datum axis that encompass all points of the measured surface. The distance between the parallel planes is the perpendicularity error (see figure 4). Note that this result includes the flatness error of the measured surface.

## Location Tolerances

MicroForm gages measure two types of Concentricity: “in plane” and “out of plane”.

**Concentricity:** In order to calculate concentricity, the eccentricity (or distance between centers) must first be calculated. The center points are found using the Least Squares method to calculate the best fit circle for each surface. The centers of the best fit circles are used to find eccentricity. Then the concentricity result is simply twice the eccentricity of surface 2 relative to surface 1 (see figure 5).

Figure 6 shows the evaluation of concentricity relative to a reference axis, this is known as Out of Plane Concentricity.

## Runout Tolerances

MicroForm gages calculate two types of Runout: Circular Runout and Plane Runout.

Circular Runout: defined as the filtered probe reading of the measured surface when rotated about a datum axis or datum surface (see figure 7). Note that in order to measure the Circular Runout according to the ASME standard the filter can be set to “0” in the configuration screen. This reports TIR of the surface relative to the datum axis or datum surface. This includes the roundness error of the second surface.

Plane Runout: defined as the best fit plane of the measured surface relative to the best fit plane of the datum surface or datum axis (see figure 8). This can also be expressed by subtracting the datum plane or datum axis from the best fit plane of the measured surface. This does not include the flatness error of either surfaces. Another term that is used is called “Squareness.” The MicroForm gage does not display this result as it is simply 1/2 of Plane Runout.

Note: as there is no ASME or ANSI standard for plane runout this definition is ABTech’s interpretation.

## Further Explanation

Understanding the definitions and results of the multi-surface tolerances can be challenging. The following page is a closer look into ABTech’s interpretation of these tolerances. These explanations follow the standards and/or definitions described above and will hopefully provide some insight.

### Quick Tips:

- When the term
**Surface**is used in a definition or explanation, the measurement*includes*the Form of the part at the measurement location. - When the term
**Best Fit Plane**is used in a definition or explanation, ABTech is referring to the plane that is calculated from the surface of the part. The best fit plane*excludes*the Form of the part at the measurement location. - The result of
**High Point**always refers to the measurement result that is*inclusive*of the Form of the part at the measurement location. These measurements are Perpendicularity, Parallelism and Circular Runout. - The result of
**Vector Direction**always refers to the result that uses a calculated best fit plane of the measurement location (*exclusive*of Form). These results are Concentricity and Plane Runout.

### Perpendicularity and its High Point:

- The
**Perpendicularity**result is the measured surface relative to a pre-defined reference axis.*This result includes the Form (Flatness) of the measured surface.*The**High Point**of this result is the angular location (in degrees) of the maximum displacement between the measured surface and the reference axis. - The term
**Plane Runout**is a sub-result of Perpendicularity. From the perpendicularity measurement a best fit plane is calculated. The Plane Runout result is the difference between this best fit plane and the reference axis. The**Vector Direction**is the angular location (in degrees) of the maximum displacement between the best fit plane and the reference axis. - The Term
**Squareness**is also a sub-result of Perpendicularity and is simply half of Plane Runout. This is not a displayed result on the MicroForm system.

### Parallelism and its High Point:

The **Parallelism** result is the measured surface relative to a datum plane. This result includes the Form (Flatness) of the measured surface. The datum plane is the best fit plane of the first surface. The **High Point** of this result is the angular location (in degrees) of the maximum displacement between the measured surface and the datum plane.

- The term
**Plane Runout**is a sub-result of Parallelism. From the Parallelism measurement a best fit plane of the second surface is calculated. The Plane Runout result is the difference between this best fit plane and the datum plane. The**Vector Direction**is the angular location (in degrees) of the maximum displacement between the best fit plane and the datum plane.

### Circular Runout and its High Point:

The **Circular Runout** result is the measured surface relative to a pre-defined Reference Axis. This result includes the Form (Roundness) of the measured surface. The **High Point** of this result is the angular location (in degrees) of the maximum displacement between the measured surface and the reference axis.

*Note: The difference between an In Plane and Out of Plane measurement is the result for In Plane is relative to a Datum Center Point, whereas the result for Out of Plane is relative to a Reference Axis.*

- The term
**Concentricity**is a sub-result of Circular Runout. From the Circular Runout measurement a best fit center point of the measurement surface is calculated. The Concentricity result is twice the displacement between this best fit center and the reference axis. The**Vector Direction**is the angular location (in degrees) of the maximum displacement between the best fit center point and the reference axis.