What is the most critical "datum" when reading and drawing drawings?

Drawings are an indispensable vehicle for making parts and products. Since the quality of the drawings determines the quality of the final parts and final products, I think many businesses want to improve the quality of the drawings. How do you make a good drawing, that is, a drawing that can make the same product even if you look at it, and that is easy to check after manufacturing? The most important thing here is the awareness of the "benchmark". In this article, we will explain the most important "datum" when reading and drawing drawings.

A "datum" of a drawing is a line (polygon) that has a lot of dimension lines (lines representing lengths) that are annotated in the drawing. In other words, the most important starting point for dimensioning in a drawing is the "datum" of the drawing. The datum of the drawing is also the datum at the time of machining and inspection. For example, in the case of machining from the datum part, or if all the holes that are dimensioned by the datum are stamped with the same die, or if the datum part is fixed to measure the length. Therefore, designers need to set the benchmark for drawings based on the operability of processing and inspection. It can be seen that the benchmark of the drawing is important in the entire process of design, processing, and inspection. The key is that when drawing a drawing, you need to first decide where to set the datum, and when reading the drawing, you need to first understand where the datum is.

How to decide on a benchmark

The basis of the drawing is usually determined according to the use method of the part (product) to be made. For example, if one side of the part is pressed against the wall and fixed, the face in contact with the wall is used as the reference. In addition, in the case of stoppers, the shaft of the handle is placed into the hole to rotate, so the datum is the center of the hole in the center of rotation. The benchmark of the drawing is the key part of the part (product) itself. In order to set the datum correctly, it is essential to accurately understand the function of the part (product).

What happens if the drawing doesn't have a datum?

What exactly happens when you don't set a datum drawing? The disadvantages of drawings without datums are shown below.

• The drawings are not uniform, and the drawings are difficult to read, and the drawings do not have datums, which means that the dimension lines are marked from the various lines (surfaces) within the drawings. As a result, drawings become incoherent and difficult to read. Drawings of parts (products) with complex shapes will be more difficult to read. Drawings don't make sense if only those who draw them can understand them. In order to create drawings that are understandable and easy for anyone to read, the most important thing is to set a benchmark.

• Difficult to process and inspectAs explained above, the datum of the drawing is also the datum at the time of machining and inspection. Therefore, when there is no reference in the drawing, the processing personnel and surveyors cannot determine which part should be used as the basis for processing and inspection. As a result, it is necessary to reconsider the preparation for machining and inspection, which incurs additional man-hours. The most important thing for designers is to create drawings that can improve the efficiency of the subsequent process.

• Tolerances accumulate within the drawingWhen dimensions are marked from various lines (surfaces) without determining the datum of the drawing, the tolerances of each dimension are accumulated, and as a result, the tolerances in other parts sometimes become larger. For example, let's consider the case of making 2 holes in a metal plate. If you set the hole positions to be based on the dimensions at different ends of the plate, the dimensional tolerance between the holes becomes larger than the predetermined general tolerance. In this case, if you set it from the center of any of the holes, you can prevent the accumulation of dimensional tolerances between the holes. In order to prevent parts from being finished but not assembled, it is important to be aware of tolerances when designing.

In order to guarantee the quality of the drawings, it is important to set the benchmark correctly. When reading and drawing, it is necessary to consider the function of the part (product) and where to set the base. In this way, the design ability will be continuously improved, and the drawings will be useful to other designers and subsequent processes.