Geometrical Tolerances and Errors in Engineering

Geometrical Tolerances and Errors

• Geometrical tolerancing involves specifying the maximum permissible variation of form, profile, orientation, location, and runout of a feature.
• The tolerance value represents the width or diameter of the tolerance zone within which the point, line, or surface of the feature shall lie.

Straightness Tolerance and Error

• Straightness refers to a condition where an element of a surface or centerline is a straight line.
• A straightness tolerance specifies a tolerance zone within which the considered element of the surface or centerline must lie.
• Straightness tolerance can be applied to:
  • Surface elements (e.g., lines, cylindrical surfaces, conical surfaces, flat surfaces)
  • Controlling surface elements only (e.g., bending or wavy condition of the surface, but not the straightness of the centerline or conicity of the cylinder)

Circularity (Roundness) Tolerance and Error

• Circularity refers to a condition where all points on a circular line or the surface of a circular feature are the same distance from a common axis or center point.
• Errors of circularity can occur due to ovality, lobing, or random irregularities from a true circle.
• A circularity tolerance is measured radially and specifies the width between two circular rings for a particular cross section within which the circular line or the circumference of the feature shall lie.
• Circularity tolerance must be less than half the size tolerance since it controls surface elements only.

Assessing Circularity

• The circularity error is the minimum radial separation between two concentric circles within which all points on the measured surface shall lie.
• Measuring principle involves establishing the relationship of the perimeter of a feature with a perfectly round form, regardless of its size or the exact position of its center.
• Measurement can be done using a polar chart, where readings are entered and evaluated for circularity errors.

Measuring Principle

• The measurement of circularity presents some problems, as it does not lend itself to direct measurement.
• Indirect measurement involves establishing the relationship of the perimeter of a feature with a perfectly round form.
• Measurement can be done using a revolving table and a fixed indicator or by revolving the indicator around the part.

Alternative Measuring Procedures

• Alternative methods include checking parts for circularity by revolving them in suitable vee-blocks while measuring the upper surface with an indicator gage.
• When using this method, the full indicator reading is approximately equal to a measurement over a diameter instead of over a radius.