November 14, 2023
41．What areas of special attention need to be paid attention to when adopting the maximum entity requirement?
A: the deviation of the datum element is considered based on its external acting size rather than the deviation of the actual size from the control boundary.
If the measured element is a group element, the compensation amount obtained from the deviation of the control boundary from the datum element's action size can only be compensated to the group element, that is, the geometric frame, rather than to each measured element.
The maximum solid requirement is mainly used to ensure assembly parts and should be used on parts produced in batches to facilitate inspection with gauges. The maximum solid requirement must be applied to the central feature, either to the measured feature or to the datum feature, or both. When a three-basis system is adopted, the central element may adopt the maximum solid requirement. The maximum solid requirement is mainly used for position tolerance, common used for coaxiality, symmetry, position, etc. For form tolerances only axis straightness can be used. Canada, the United States and other countries also use plain line straightness, cylindricity, etc, which will cause inconsistencies in understanding. Neither ISO nor our country adopts them.
42. What are the minimum entity requirements?
A: the minimum entity requirement is a method by which the geometric tolerance can obtain a compensation value when the measured element or datum element deviates from the minimum entity state. Tolerance requirements. The measured elements can adopt the minimum entity requirements at the same time as the datum elements. In this case, the geometric tolerance can be obtained from two aspects compensation value.
43．What areas of special attention need to be paid attention to when adopting minimum entity requirements?
A: the actual minimum physical state of the measured element is controlled by the minimum physical effective boundary formed by the minimum physical size and the geometric tolerance value. When the measured element is in this effective state, its actual outline should be within the minimum physical effect boundary, such as the actual when the size deviates from the minimum physical size, the allowable geometric error value exceeds the given tolerance value. This ensures that the actual contour of the part is not it will exceed the boundaries defined in the drawing design to ensure the strength and minimum wall thickness of the part. When the given geometric tolerance value is zero
When , the geometric tolerance is zero. At this time, the minimum entity effective boundary of the measured element is equal to the minimum entity boundary, and the minimum entity effective boundary the size is equal to the minimum physical size.
When the minimum entity requirement is applied to a datum feature, the corresponding boundaries that the datum feature should adhere to are determined by the requirements of the datum feature itself. If the benchmark feature itself adopts the minimum entity requirement, the corresponding boundary is the minimum entity effective boundary. If the benchmark feature itself does not adopt the minimum entity requirement, the corresponding boundary is the minimum entity boundary. If the actual contour of the reference feature deviates from the corresponding boundary, that is, its internal action size deviates from the corresponding boundary size, then the reference feature is allowed to float within a certain range.
44. How to label thread annotations?
A: in general, the thread axis, as the measured or reference element, is the center diameter axis. If a large diameter axis is used, it should be represented by "DM", and if a small diameter axis is used, it should be represented by "LD".
45. How to label gears and splines?
A: when the gear and spline axis are used as the measured or reference elements, the pitch diameter axis is represented by "PD", the major diameter (the top circle diameter for external gears and the root circle diameter for internal gears) axis is represented by "MD", and the minor diameter (the root circle diameter for external gears and the top circle diameter for internal gears) axis is represented by "LD".
46. How to choose geometric tolerances?
A: the geometric features of the part, for example, the processed cylindrical part will produce cylindricity error, the conical part will produce roundness error, and the processed flat part will produce flatness error; There will be coaxiality errors in stepped shafts and holes.
The functional requirements of the parts, such as cylindricity error, will affect the rotational accuracy. Therefore, the diameter of the machine tool spindle should specify cylindricity tolerance and coaxiality tolerance. If the axes of the two pairs of holes in the gearbox are not parallel, it will affect the correct meshing of the gears.
For example, the diameter of the transmission shaft in a gearbox should be specified with cylindricity and coaxiality tolerances based on its geometric characteristics and functional requirements. But for the convenience of detection, radial circular runout (or full runout) tolerance can be used instead.
The project features of geometric tolerance, such as cylindricity tolerance, can control roundness error, orientation tolerance can control related shape errors, positioning tolerance can control related orientation errors, and runout tolerance can control related shape, orientation, and positioning errors. Therefore, when cylindricity tolerance is specified for the same measured feature, roundness tolerance is generally no longer specified, positioning tolerance is specified, and orientation tolerance related to it is usually no longer specified.
47. How to determine the value of geometric tolerance?
A: the basic principle for selecting geometric tolerance values is to meet the functional requirements of the parts, while also considering economic efficiency and convenient testing. If allowed, lower tolerance levels should be chosen as much as possible.
48. How to choose a benchmark?
A: the functional requirements of each element of the part should generally be based on the main mating surfaces such as the journal of the rotating shaft, bearing holes, installation positioning surfaces, important support surfaces, and guide surfaces. These surfaces themselves require high dimensional and shape accuracy, which meets the conditions as benchmarks.
Considering the installation, positioning, and measurement of parts, using the process reference as the reference for positional tolerance is beneficial for ensuring positional accuracy during processing. For the convenience of measurement, if it can be unified with the measurement reference, it should be made as uniform as possible. A single datum is generally used for parts with relatively single orientation or positioning requirements, using only one plane or a straight line as the datum feature, such as parallelism tolerance, perpendicularity tolerance, symmetry tolerance, etc. The combination datum is generally used for cylindrical parts supported by two holes or two journals, and requires a given coaxiality or runout tolerance. Multiple datums are mostly used to give positioning tolerance, The annotation used to determine the positional accuracy of the hole system.
49. How to specify unmarked tolerances?
A: if the required form and position tolerances of the parts can be guaranteed by ordinary machine tool processing, it is not necessary to indicate them on the drawing and usually do not need to be checked. However, not indicating the form and position tolerances does not mean that there are no form and position tolerance requirements. GB/T1184-1996 provides provisions for unmarked tolerances. The values of unmarked tolerances are divided into four levels: A, B, C, and D, which actually correspond to the 9th to 12th levels of geometric tolerance.
50. What is a geometric tolerance zone?
A: geometric tolerance zones are areas that limit the variation of actual shape or position elements. The points, lines, and surfaces that make up the actual features must be within this area, and the tolerance zone is the maximum allowable value of error, which is determined by four factors: size, shape, direction, and position. These four factors are determined by the function of the part and the characteristics of the elements.