Are there shape tolerances for precision parts machining?

Positional tolerances for precision parts machining are the actual total variation with respect to a reference position. Positional tolerances are used to limit the difference between the actual position of a measuring element on a machine part and the ideal position, which is determined by a reference dimension or a theoretically correct dimension. As with form tolerances, each of the positional tolerances is used to limit the variation of the actual element of a part over a specific area by means of a positional tolerance band. Due to the concept of tolerance bands used in machining, the positional tolerance band limits not only the positional error of the element under test, but also the shape error of the element. Unlike shape tolerance zones, positional tolerance zones must have a correspondence with the datum. The datum plays a role in orienting and positioning the positional tolerance band of the measured element.

Precision parts processing positional tolerance can be divided into three categories: orientation tolerance, positioning tolerance and runout tolerance. Orientation tolerance is the amount of variation allowed from the actual element measured to the ideal element in a particular direction. The direction of the ideal element is determined by the reference angle and the theoretical correct angle. Orientation tolerance is divided into three items: parallelism, perpendicularity and inclination. Positioning tolerance is the total amount of change allowed from the actual element measured to the ideal position element. The position of the ideal element depends on the reference value and the theoretically correct size. Positional tolerances are divided into three parts: positional coaxiality and symmetry.

The runout tolerance is the maximum allowable runout when the measured actual element is rotated without axial movement or when it is rotated continuously about the reference axis. The maximum runout referred to is the difference between the maximum and minimum readings measured by the indicator in a given direction. The runout is categorized into circular runout and complete runout.

Circular runout: refers to the actual surface to be measured around the reference axis of rotation without axial movement, in the specified direction of the indicator measured the maximum reading difference.

Full runout: refers to the actual surface being measured around the reference axis of rotation without axial movement, at the same time the indicator for parallel or perpendicular to the reference axis of movement, in the whole process of the indicator measured by the maximum difference between the readings.

For precision parts processing machining industry, the development of tolerance is to determine the geometric dimensions of the product, and make it within a certain range of variation to meet the requirements of interchangeability and use.

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