Precision Machining Process – Positioning and benchmarking of parts

First, the benchmark
A baseline is a point, line, or face that is used to determine the geometric relationship between the geometric features of a production object. For a mechanical part, a benchmark is those points, lines, and faces on which the position of the other points, lines, and faces on the part is determined.
In the design and processing process of machine parts, it is one of the main factors that directly affect the process of part processing and the size and position accuracy of the parts according to different requirements. According to the function and application, the benchmark can be divided into design benchmark, process benchmark two categories.
(i) Design benchmarks
A design baseline is a reference used to dimension a part diagram. On a machine part map, there can be one or more design benchmarks. In Figure 1-1a, A and B are design benchmarks, in Figure 1-1b, the 40mm outer circle is the design reference for the 60mm outer circle, in Figure 1-1c, plane 1 is the design reference for plane 2 and hole 3, hole 3 is the design reference for holes 4 and 5, and in Figure 1-1d, the center line is the design reference for the inner hole 30mm, gear division circle 48mm and top circle 50h8mm.
(2) Process benchmark
A process reference is a reference used in the process of a part. Depending on the purpose, the process reference can also be a sub-process baseline, positioning reference, measurement reference and assembly reference.
1, operation benchmark
In an operation diagram, the baseline used to determine the size, shape, and position of the processed surface is called an operation baseline. , the machining surface is a D-hole, requiring its centerline to be perpendicular to the A-side and keeping distance from the C-side and B-faces in dimensions L1 and L2, then A, B and C are the operation benchmarks for this operation.
2, positioning benchmark
When machining, the reference used to determine the correct position of the work piece on the machine or in the fixture is called the positioning reference.
3, the measurement benchmark
The reference used to measure the shape, position, and dimensional error of the work piece, called the measuring reference, during or after processing. As shown in Figure 1-3, dimension L1 can be measured using a depth caliper, and face T is the measurement reference for end face A.
4, assembly benchmark
When assembling, the reference used to determine the relative position of a part or component on a product is called an assembly reference. For example, the inner hole of the gear is the assembly reference for the gear. In general, the design baseline is given on the part drawings, and the process baseline is determined by the process personnel according to the specific process selection.
When analyzing benchmarking issues, you must pay attention to two points:
1) Points, lines, and faces that serve as references do not necessarily exist specifically on the work piece (e.g. hole centerlines, pivot lines, etc.), but are represented by specific surfaces, which are called base surfaces. For example, the centerline of the inner hole is represented by the inner hole surface, the centerline of the inner hole is the base, and the inner hole surface is the base surface. Therefore, the question of choosing a baseline is the question of choosing the right base. In particular, benchmarks and substrates can sometimes be collectively referred to as benchmarks for the sake of narrative convenience.
2) As a base, there may be points and lines or very small faces without area, but there is always a certain area representing the base of such a baseline. For example, install a long shaft with a top edge on the bed, the base is the axis, it has no area, the base surface is the top cone, it is small but does have a certain area.