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6.3.2 Choice of the principle of tolerance
When choosing the tolerance principle, should be based on the function of the measured element, give full play to on business functions and the feasibility of adopting this principle and economy. Independent principle, for example, even though it is the most basic position processing dimensional tolerance and shape tolerance of tolerance principle, and the most widely used. But there is a premise, is for parts with special function only can be used when required. But in actual design, in order to ensure that parts of nature, namely the guarantee with the limit of the gap and the limit interference, meet the design requirements, to cooperate with usually important to use inclusive. The matching between the bearing inner hole and shaft, for example, is to ensure minimum clearance.
For just need to ensure that the parts can be assembled, and to facilitate the processing and manufacturing of parts, can use maximum material requirement. Usually used in clearance fit, the applicable elements only axis or a center plane. Such as bearing end cover the hole position on the degree of tolerance.
The choice of 6.3.3 shape tolerance values or determine
In, when the choice of form tolerance value, several problems that should be considered and principles:
(1) shape tolerance, position tolerance and size tolerance
Determining shape tolerance value, should consider their coordination with the size tolerance, the general principle is:
Tolerance value is greater than the position tolerance value, shape and position tolerance value is greater than the size tolerance.
(2) to have cooperate with the requirements of form tolerance and size tolerance
Have cooperate and be strict to ensure the elements of its nature, should adopt tolerance requirement. In general, the shape tolerance is usually size tolerance of 25% to 65%. The roundness and cylindricity tolerance according to the selected at the same level in general.
(3) the shape tolerance and surface roughness
Usually, for the medium size and medium precision parts, the value of surface roughness can account for the shape tolerance of 20% to 25%.
(4) the structure characteristics of the parts need to be considered
For low rigidity parts such as the long and thin axis and has some structural characteristics of elements, because of its poor manufacturability, machining accuracy is affected, at this time, for the spindle, have to choose the larger shape tolerance values.
(5) the choice of benchmark
Main consideration when choosing a benchmark, according to the design and use requirements, and give attention to two or morethings benchmark unity and structural characteristics. Generally consider the following:
(1) should be based on the function of the design elements and the geometric relationships between the elements to select the benchmark. For example, the axis of rotation, usually with rolling bearing surface of the journal as a benchmark.
In terms of processing and measuring, should choose to locate in the jig, measuring tools to the corresponding benchmark to do benchmark.
(3) from the assembly relation, should choose the surface of the parts together, touch each other to do their own benchmarks, to ensure the correct assembly of parts. In the middle of the main shaft of the longest working period, in order to ensure the accuracy of his work, this journal journal of two a diameter of 95 public reference axis shows the radial circular run-out tolerance of 0.025 mm. Several of the main shaft shoulder function positioning, refer to the installation accuracy requirement of the shaft shoulder rolling bearing, gives two shoulder relative to the reference axis end circular run-out tolerance of 0.025 mm. Keyway symmetry tolerance of slot milling in order to ensure the center of the keyway as far as possible with the axis perpendicular to the plane. The tolerance value is 0.05 mm.
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