16 basic rules of mechanical drawing
There are many basic rules in mechanical drawing, and ASME Y14.5-2009 stipulates 16 basic rules, which everyone must know when drawing, reading or reviewing drawings. The following will introduce these 16 basic rules one by one.
Article 1: Except for reference dimensions, maximum and minimum dimensions or raw materials, all dimensions must have tolerances. Reference dimensions generally do not have tolerances. Why? Because reference dimensions are generally repeated dimensions or closed dimensions on drawings, and they are only used as reference information. Reference dimensions are neither used to guide production nor to guide inspection, so when you see reference dimensions on drawings, you can ignore them directly. We often see the maximum MAX or minimum MIN marking method on drawings. Do these dimensions have tolerances? The answer is yes. For the MAX dimension, its lower tolerance limit is 0, and for the MIN dimension, its upper tolerance limit is infinity. Therefore, when we specify the MAX or MIN dimension, we must fully consider whether it will affect the function at the limit deviation. For example, if we mark a fillet as R1MAX, then we must consider whether it will affect the function when the fillet is 0 (that is, there is no fillet). If so, we must specify the appropriate lower tolerance limit. There are also many theoretical dimensions (i.e. basic dimensions) on the drawings, so do they have tolerances? The so-called theoretical dimension refers to a value that is used to define the theoretically correct size, shape, contour, direction or position of a shape or target reference. When this theoretical dimension is used to define the size, shape, outline, direction or position of a shape, its tolerance is defined by the corresponding geometric tolerance of the shape; when this theoretical dimension is used to define the size, shape or position of the target datum, its tolerance should be determined in accordance with the ASMEY14.43 gauge and fixture tolerance criteria. Therefore, theoretical dimensions also have tolerances. There are several ways to mark dimension tolerances on drawings:
· Mark the dimension limit or dimension tolerance value directly on the dimension
· Mark in the form of geometric dimension tolerance
· Define tolerances for specified dimensions in notes or tables
· Define tolerances for specified shapes or processes in other files referenced by the drawing
· Define tolerances for all dimensions without tolerances in the general tolerance column
Article 2: Dimensions and tolerances must be fully defined so that all characteristics of each shape can be fully understood. The characteristics of a shape include size, shape, direction and position. The dimensions and tolerances of all characteristics of each shape must be defined on the drawing. Dimensions and tolerance values can be expressed by engineering drawings or defined by CAD product definition databases. It is not allowed to determine the size by measuring the drawing or guessing.
Article 3: Only mark all necessary dimensions used to express the product. All necessary dimensions mean that the dimensions on the drawing should be no more and no less, just enough to fully express all the characteristics of all shapes. There should be no redundant dimensions on the drawing, such as closed dimensions. As mentioned earlier, we can ignore any reference dimension, so the reference dimensions should be used as little as possible in the drawing. Reference dimensions have no meaning except adding a sense of clutter to the drawing.
Article 4: The dimensions should be selected according to the function and matching of the product, and there should not be multiple interpretations. What is emphasized here is that the dimensions and tolerances defined during the design should be based on the premise of meeting the functional requirements and matching requirements of the product. The requirements of manufacturability and detectability should be considered during the design process, but not at the expense of functional requirements.
Article 5: The processing method should not be marked on the product drawing. The product drawing should only mark the dimensions and performance requirements that meet the product function. As for how to process and manufacture, it is the work of manufacturing engineering. As a designer, you should give the manufacturing personnel full freedom. What we should consider is the maximum tolerance range under the premise of meeting the functional requirements of the product, so that there is enough manufacturing capacity for manufacturing, rather than specifying the manufacturing method. For example, for a hole, we only need to mark the diameter, without indicating whether it is drilled, punched, milled, turned, ground or other processes. No matter what process method is used, as long as the finished product can meet the diameter tolerance requirements, it will be fine. Only when the manufacturing process is an indispensable part of the product characteristics, it should be described on the drawing or reference document. For example, due to functional needs, the hole is required to meet the diameter tolerance and not have spiral processing marks. It can be indicated on the drawing that the hole is required to be ground.
Article 6: When giving the final product size, it is allowed to mark non-mandatory process parameter dimensions that provide information such as processing allowance. These dimensions should be marked as non-mandatory. Generally speaking, process parameters do not need to be marked on the drawing. If they need to be marked, they must be marked as non-mandatory. As mentioned earlier, this is the work of manufacturing engineering, and they should be given full freedom.
Article 7: Dimensions should be arranged reasonably to achieve optimal readability. Dimensions should be arranged on the real contour drawing and marked on the visible contour line. This is a basic requirement for drawing, which will not be expanded here.
Article 8: Wires, tubes, plates, bars or other raw materials produced by gauges or brands should be marked with linear dimensions such as diameter or thickness. The gauge or product brand should be marked in brackets after the size. This article is for raw materials. Each raw material has its own corresponding standard to specify the marking method.
Article 9: The center line and the contour line of the shape are displayed as right angles on the drawing without marking. The default is 90 degrees. There are many default 90-degree relationships on the drawing. The default 90-degree tolerance should be controlled according to the unmarked angle tolerance.
Article 10: The center line or surface of the array shape located or defined by the basic dimension, if it is displayed as a right angle on the drawing without marking, is defaulted to the basic dimension of 90 degrees. Array shape refers to a group (two or more) of dimensional shapes with the same shape and size and distributed in a regular pattern. When the center of these shapes is defined or located by the basic dimension, the default 90-degree basic angle tolerance is controlled by the corresponding geometric tolerance.
Article 11: When the center axis, center plane or surface are shown consistent on the drawing, the basic dimension with a value of 0 is assumed, and its relationship is defined by the geometric tolerance. This is also a basic common sense. The tolerance of these basic dimensions that are assumed to be 0 should be controlled by the corresponding geometric tolerance. If no geometric tolerance is specified, it shall be controlled by the unmarked geometric tolerance in the general technical requirements column.
Article 12: Unless otherwise specified, all dimensions refer to room temperature of 20°C (68°F). If measured at other temperatures, compensation for the dimensions should be considered. Note that the room temperature here is 20 degrees, not 23 degrees or 25 degrees. Therefore, we require the room temperature in the measuring room to be controlled at 20 degrees to ensure that the test results truly reflect whether the product requirements are met. If there is really no condition to measure at a room temperature of 20 degrees, we should consider compensating the measurement results for the influence of temperature, especially for parts with high temperature sensitivity.
Article 13: Unless otherwise specified, all dimensions and tolerances are applicable to free state conditions. All dimensions marked on the drawings refer to the dimensions of the parts in the free state with all stresses released. For some non-rigid parts, we can mark the dimensions of the parts after being constrained according to regulations. The method of constraining the parts must be marked on the drawing. At this time, if we still want to mark the dimensions of some of the parts in the free state, we must mark the free state symbol circle F.
Article 14: Unless otherwise specified, all geometric dimension tolerances apply to the entire length, width or depth of the shape. I believe everyone is familiar with this. I would like to remind everyone that due to the application of the inclusion principle, the length, width or depth of the shape has a great relationship with the shape control of the shape. A 3mm long round bar and a 30mm long round bar have the same maximum straightness allowed under the same diameter tolerance, but the actual bending conditions are very different.
Article 15: All dimensions and tolerances are only applicable to the product level described in the drawing. The dimensional tolerance of a certain shape described on a drawing level (such as a part drawing) is not absolutely applicable to the dimensional tolerance of the shape on other drawing levels (such as an assembly drawing). That is to say, the dimensions on a part drawing are not absolutely applicable on the assembly drawing. For example, if we weld a bracket with an opening of 10+/-0.5 to a platform, due to the deformation of welding, the clamping of the welding fixture and other factors, it is difficult for this opening to meet the size requirement of 10+/-0.5 on the welded part. In other words, this size is no longer applicable to the welded part drawing. Therefore, we cannot use the size on a part drawing to require the size of the same shape on the assembly drawing. If it is necessary to control the shape on the assembly drawing, the size must be marked on the assembly drawing.
Article 16: Unless otherwise specified, when a coordinate system appears on the drawing, it must be right-placed. Each coordinate axis must be marked and indicate the positive direction.