There are many customary basic principles in mechanical drawing, and ASME Y14.5-2009 stipulates sixteen basic principles, which must be well understood by everyone in drawing, reading or reviewing drawings. The following is a brief introduction to the 16 basic principles.
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Rule 1: All dimensions must have tolerances except reference dimensions, maximum and minimum dimensions or raw materials
Reference dimensions generally have no tolerances. why? Since a reference dimension is generally a repeated or closed dimension on a drawing, it is used only as reference information. The reference size is neither used to guide production nor to guide inspection, so when you see the reference size on the drawing, you can ignore it directly.
On drawings, we often see the marking method of maximum MAX or minimum MIN. Are there tolerances for these dimensions? The answer is yes. For MAX size, its lower tolerance limit is 0, while for MIN size, its upper tolerance limit is infinity. Therefore, when we specify the MAX or MIN size, we must fully consider whether it will affect the function when the limit deviation occurs. For example, we mark a rounded corner as R1
MAX, then we must consider whether the function will be affected when the fillet is 0 (that is, there is no fillet), and if so, we must specify the lower limit of the appropriate tolerance.
There are still many theoretical dimensions (that is, basic dimensions) on the drawings, so do they have tolerances? The so-called theoretical size refers to a numerical value used to define the theoretically correct size, shape, outline, direction or position of a body or target reference. When this theoretical dimension is used to define the size, shape, profile, direction or position of a body, its tolerance is defined by the corresponding shape tolerance of the body; when this theoretical size is used to define the size, shape or position of the target datum , its tolerance should be in accordance with ASME
Y14.43 Gauge and fixture tolerance criteria to determine. Therefore, the theoretical dimensions also have tolerances.
There are several ways to mark dimensional tolerances on drawings:
· Dimension limit or dimension tolerance value marked directly on the dimension
· Marked in the form of geometric tolerances
· Define tolerances on specified dimensions in notes or tables
· Define tolerances on specified features or processes in other documents referenced in drawings
· Tolerances are defined for all untolerated dimensions in the General Tolerances column
Rule 2: Dimensions and tolerances must be defined comprehensively so that all properties of each form can be fully understood
Properties of a body include size, shape, orientation, and position. The dimensions and tolerances of each feature must be defined on the drawing for all properties. Dimensions and tolerance values can be expressed by engineering drawings or defined by CAD product definition databases. Measuring drawings or making assumptions to determine dimensions is not permitted.
Article 3: Only mark all the necessary dimensions used to describe the product
All necessary dimensions mean that the dimensions on the drawing should be no more, 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 drawing should minimize the use of reference dimensions. Reference dimensions serve no purpose except to add a sense of clutter to the drawing.
Article 4: The size should be selected according to the function and fit of the product, and there should be no multiple interpretations
What is emphasized here is that the dimensions and tolerances we define during design should meet the functional requirements and matching requirements of the product. Manufacturability and testability requirements 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
Only the size and performance requirements that meet the product functions should be marked on the product drawings. As for how to process and manufacture, it is the work of manufacturing engineering. As a designer, the manufacturer should be given full freedom. What we should consider is the maximum tolerance range on the premise of meeting the functional requirements of the product, so that the manufacturing has sufficient manufacturing capacity, rather than specifying the manufacturing method. For example, for a hole, we only need to mark the diameter, not to indicate whether it is drilling, punching, milling, turning, grinding or other processes. Regardless of the process method used, as long as the finished product can meet the diameter tolerance requirements. Only when the manufacturing process is an integral part of the product characteristics should it be stated on drawings or reference documents. For example, due to functional requirements, it is required that the hole should not be processed in a spiral shape while meeting the diameter tolerance, and the hole can be required to be ground on the drawing.
Article 6: It is allowed to mark the non-mandatory process parameter size that provides processing allowance and other information while giving the final product size, and these sizes should be marked as non-mandatory
Generally speaking, there is no need to mark the process parameters on the drawings, and if necessary, it must also be marked as non-mandatory. As mentioned earlier, this is the job of manufacturing engineering and they should be given full freedom.
Rule 7: Dimensions should be properly laid out for optimal readability. Dimensions should be laid out on the true outline drawing and marked on the visible outline
This is a basic requirement for drawing, so it will not be expanded here.
Article 8: Wires, pipes, plates, bars or other raw materials produced according to measuring tools or grades should be marked with linear dimensions such as diameter or thickness. The measuring tool or product grade should be marked in brackets after the size
This article is for raw materials, and each raw material has its corresponding standard to specify the labeling method.
Article 9: The center line and the contour line of the shape are displayed as right angles on the drawing, but the default is 90 degrees if there is no mark
There are many default 90-degree relationships on the drawings, and these default 90-degree tolerances should be controlled according to the unspecified angle tolerances.
Article 10: If the centerline or surface of the array shape positioned or defined by the basic dimensions is displayed as a right angle on the drawing without marking, it will default to the basic dimension of 90 degrees
An array shape refers to a group (two or more) of size shapes that have the same shape and size and are regularly distributed. When the centers of these shapes are defined or located by basic dimensions, the default 90-degree basic angle tolerance is controlled by the corresponding geometric tolerance.
Article 11: When the central axis, central plane or surface are consistent on the drawing, the default is the basic dimension with a value of 0, and their mutual relationship is defined by the geometric tolerance
This is also a basic common sense. The tolerances of these basic dimensions, which are defaulted to 0, should be controlled by the corresponding geometrical tolerances. If no geometrical tolerance is specified, it will be controlled by the uninjected geometrical tolerances in the general technical requirements column.
Article 12: Unless otherwise noted, all dimensions are at room temperature 20°C (68°F). Compensation for dimensions should be considered if measured at other temperatures
Note that the room temperature mentioned here is 20 degrees, not 23 degrees or 25 degrees. Therefore, we require the room temperature to be controlled at 20 degrees in the measurement room to ensure that the test results actually meet the product requirements. If there is really no condition to measure at room temperature of 20 degrees, we should consider compensating the measurement results for temperature effects, especially for parts with high temperature sensitivity.
Article 13: Unless otherwise specified, all dimensions and tolerances apply to free state conditions
All dimensions indicated on drawings refer to the dimensions of the part in the free state in which all stresses have been relieved. For some non-rigid parts, we can mark the size of the part after the part is constrained according to the regulations. The method of part constraint must be marked on the drawing. Status symbol circle F.
Article 14: Unless otherwise specified, all geometric tolerances apply to the entire length, width or depth of the shape
I believe everyone is familiar with this point. What I want to remind everyone is that due to the application of the principle of tolerance, the length, width or depth of the body has a great relationship with the control of the shape of the body. A 3mm long round rod and a 30mm long round rod have the same maximum straightness allowed under the same diameter tolerance, but the actual bending situation is very different.
Article 15: All dimensions and tolerances are only applicable to the product level expressed in this drawing. The dimensional tolerance of a certain shape expressed on one drawing level (such as a part drawing) does not absolutely apply to the dimensional tolerance of the shape on another drawing level (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, we weld a bracket with an opening of 10+/-0.5 to a platform. Due to the influence of factors such as welding deformation and welding fixture clamping, it is difficult for this opening to meet the 10+/- on the weldment. 0.5 size is required. This means that this dimension is no longer applicable to weldment drawings. Therefore, we cannot use the size on a part drawing to require the size of the same shape on the assembly drawing. If the shape needs to be controlled on the assembly drawing, the dimension must be marked on the assembly drawing.
Article 16: Unless otherwise specified, when the coordinate system appears on the drawing, it must be placed on the right. Each coordinate axis must be labeled and indicate the positive direction
There are relatively few applications of this point, so I won't go into details, just follow it.
The above is an introduction to the 16 basic guidelines for drawings stipulated in ASME standards.
