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Shape and position tolerances questions and answers

November 8, 2023

1. What are the main contents of the national standards for shape and position tolerances?

A: There is GB/T1182-96 General principles, definitions, symbols and graphic representations of shape and position tolerances.
    GB/T4249-96 Tolerance principles.
    GB/T13319-03 Geometric Tolerances Positional Tolerance Notation Method.
    GB/T16671-96 Shape and position tolerances Maximum physical requirements, minimum physical requirements and reversible requirements.
    GB/T16892-97 Shape and position tolerances. Injection method for non-rigid parts.


2. How do shape and position errors occur?

A: due to the positioning error of the workpiece on the machine tool during the machining process, incorrect relative motion between the tool and the workpiece, deformation of the workpiece caused by clamping force and cutting force, release of the internal stress of the workpiece, etc, the finished workpiece will have various shapes. and position error.


3. What do shape and position errors affect?

A: various shape and position errors will have varying degrees of impact on the assembly and performance of parts. Therefore, in addition to the appropriate dimensional tolerance and surface roughness requirements, the geometric accuracy of mechanical parts must also specify reasonable shape and position tolerances for the parts.


4. What is the research object of geometric tolerance?

A: the research object of geometric tolerance is the elements, namely points, lines and surfaces.


5. What are elements?

A: elements refer to the characteristic parts on the part: points, lines, and surfaces. Any part, regardless of its complexity, is composed of many elements.


6. What are actual factors?

A: the factors that actually exist after the parts are processed (errors exist). The actual elements are represented by the approximate substitute elements (measured actual elements) of the actual elements obtained by measuring a limited number of points on the actual elements according to the prescribed method.


Each actual element can have several alternative elements due to different measurement methods. The smaller the measurement error, the closer the measured actual elements are to the actual elements.


7. What are called ideal elements?

A: elements of theoretical correctness (no error). In technical drawing the elements we draw are ideal elements. The ideal outline elements are represented by solid lines (visible) or dashed lines (invisible), and the ideal center elements are represented by dotted lines.


8. What are the outline (real) elements?

A: points, lines or surfaces on the surface of the part.


9. What is a central (derived) feature?

A: the center point (center of a circle or sphere), center line (axis) or center plane obtained from one or several contour (composition) elements.


10. What is a measured element?

A: the elements required for geometric tolerance are given on the drawing, which are the objects of measurement.