1 Selection of Rough Benchmarks
The principle of choice is:
(1) Non-machined surface principle.
(2) The principle of minimum machining allowance.
(3) Important superficial principles.
(4) The principle of non-reuse.
(5) The principle of facilitating clamping.
2 Selection of Precise Benchmarks
Design datum is the datum used in the drawings.For example, the axis line on the part drawing is the design basis for each outer circle and hole.From the process analysis of the part, it can be seen that the design datum of the part is the axis of 50k6mm and 35mm, and the principle of selection according to the precise datum is:
(1) The principle of datum overlap.
(2) The principle of unification of benchmarks.
(3) The principle of taking itself as a benchmark.
(4) Mutual benchmarking principle.
(5) The principle of facilitating clamping.
According to the above selection principle, when processing: also from the aspect of product use, 65mm and 50mm of Journal of active spiral bevel gear are mating surfaces, and 65mm of journal end face is assembly datum according to installation distance (118mm); from the aspect of product design, 65mm of journal end face is design datum, and its axis is the datum of geometry and position tolerance.Therefore, the locating datum for toothed machining can be determined as the end face of 65mm and 50mm journals and 65mm journals.