Selection Of Geometric Tolerance

1. Selection Of Geometric Tolerance Items

The function of comprehensive control items shall be fully exerted to reduce the geometric tolerance items given in the drawings and the corresponding geometric error detection items.

On the premise of meeting the functional requirements, the items with simple measurement shall be selected. For example, coaxiality tolerance is often replaced by radial circular runout tolerance or radial circular runout tolerance. However, it should be noted that the radial circular runout is a combination of coaxiality error and cylindrical surface shape error, so when replacing, the runout tolerance value given should be slightly greater than the coaxiality tolerance value, otherwise it will be too strict.

2. Selection Of Tolerance Principles

The function of tolerance and the feasibility and economy of adopting the tolerance principle shall be fully exerted according to the functional requirements of the measured elements.

The principle of independence is used in situations where the requirements for dimensional accuracy and form and position accuracy differ greatly and need to be met respectively, or the two are not related to each other to ensure the movement accuracy, sealing, and unspecified tolerance.

Inclusion requirements are mainly used in situations where the matching nature needs to be strictly guaranteed.

The maximum entity requirement is used for the central element, and is generally used when the fitting requirements are for assembly (no matching property requirements).

The minimum material requirement is mainly used to ensure the strength and minimum wall thickness of parts.

The combination of the reversible requirement and the maximum (minimum) entity requirement can make full use of the tolerance zone, expand the range of the actual dimensions of the measured elements, and improve the efficiency. It can be selected without affecting the service performance.

3. Selection Of Datum Features

1) Selection of reference position

(1) The joint surface of the part positioned in the machine is selected as the reference position. For example, the bottom plane and side face of the box, the axis of disc parts, the supporting journal or supporting hole of rotating parts, etc.

(2) The datum elements shall have sufficient size and stiffness to ensure stable and reliable positioning. For example, using two or more axes that are far apart to form a common datum axis is more stable than one datum axis.

(3) Select the surface with relatively accurate machining as the reference part.

(4) The assembly, processing and inspection datum shall be unified as far as possible. In this way, the error caused by the disunity of datum can be eliminated; It can also simplify the design and manufacturing of fixtures and measuring tools, making measurement convenient.

2) Determination of benchmark quantity

In general, the number of datums shall be determined according to the geometric functional requirements for orientation and positioning of tolerance items. Most orientation tolerances require only one datum, while positioning tolerances require one or more datums. For example, for parallelism, perpendicularity and coaxiality tolerance projects, generally only one plane or one axis is used as the datum feature; For positional tolerance items, two or three datum features may be used to determine the positional accuracy of the hole system.

3) Arrangement of datum sequence

When more than two datum features are selected, the order of datum features shall be specified and written in the tolerance frame in the order of first, second and third. The first datum feature is primary, and the second datum feature is secondary.geometric tolerances

4. Selection Of Geometric Tolerance Value

General principle: select the most economical tolerance value on the premise of meeting part functions.

According to the functional requirements of parts, considering the economy of processing and the structure and rigidity of parts, determine the tolerance value of elements according to the table. Consider the following factors:

  • The shape tolerance given by the same element should be less than the position tolerance value;
  • The shape tolerance value of cylindrical parts (except the straightness of the axis) shall be less than its size tolerance value; On the same plane, the flatness tolerance value shall be less than the parallelism tolerance value of the plane to the datum.
  • The parallelism tolerance value shall be less than its corresponding distance tolerance value.
  • Approximate proportional relationship between surface roughness and shape tolerance: Generally, Ra value of surface roughness can be taken as (20%~25%) of shape tolerance value.
  • In the following cases, considering the difficulty of processing and the influence of other factors other than the main parameters, under the condition of meeting the requirements of part functions, reduce the level 1 to 2 appropriately for selection:
  • The hole is relative to the shaft;
  • Shafts and holes with large slenderness ratio; Large distance shafts and holes;
  • Surfaces of parts with large width (more than 1/2 length);
  • Parallelism and perpendicularity tolerance of line to line and line to face.

5. Stipulation Of Unspecified Tolerance In Shape And Position

In order to simplify the drawing, it is not necessary to mark the form and position tolerance on the drawing for the form and position accuracy that can be guaranteed by ordinary machine tool processing. The provisions of GB/T1184-1996 shall be followed if the form and position tolerance is not marked. The general contents are as follows:

(1) Three tolerance classes, H, K and L, are specified for the unspecified straightness, flatness, perpendicularity, symmetry and circular runout

(2) The unmarked roundness tolerance value is equal to the diameter tolerance value, but cannot be greater than the unmarked tolerance value of radial circular runout.

(3) Undeclared cylindricity tolerance values are not specified, but are controlled by the roundness tolerance of features, straightness of isolines and parallelism of relative isolines.

(4) The unmarked parallelism tolerance value is equal to the larger of the dimension tolerance between the measured feature and the datum feature and the unmarked tolerance value of the shape tolerance (straightness or flatness) of the measured feature, and the longer of the two features shall be taken as the datum.

(5) Undeclared coaxiality tolerance value is not specified. If necessary, the unmarked tolerance value of coaxiality can be taken as equal to the unmarked tolerance value of circular runout.

(6) The tolerance values of unmarked line profile, surface profile, inclination and position are controlled by the marked or unmarked linear dimension tolerance or angle tolerance of each feature.

(7) Undeclared full run out tolerance value is not specified.

6. Drawing Representation of Undeclared Tolerance Values in Form and Position

If the unspecified tolerance value specified in GB/T1184-1996 is adopted, the standard and grade code shall be indicated in the title block or technical requirements:”GB/T1184—K”.

The working tolerance not marked with “Tolerance principle shall be in accordance with GB/T 4249” on the drawing, which shall be implemented in accordance with the requirements of “GB/T 1800.2-1998”.

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