There are many established basic rules in mechanical design, and specifically, ASME Y14.5-2009 establishes sixteen essential rules.
These rules must be fully understood and applied when preparing, interpreting or reviewing technical drawings. Let's delve into each of these sixteen rules one by one.
Rule 1: All dimensions, with the exception of reference dimensions, maximum and minimum dimensions, or raw materials, must have tolerances.
Reference dimensions typically have no tolerances. Why is this? This is because reference dimensions are often repetitive or closed dimensions in drawings, which are used for reference information only. Reference dimensions do not guide production or inspection; therefore, when you see a reference dimension in a drawing, you can disregard it.
In drawings, we often see the notation MAX (maximum) or MIN (minimum). Do these dimensions have tolerances? The answer is yes. For MAX dimensions, the lower limit of their tolerance is zero, and for MIN dimensions, their upper limit of tolerance is infinity.
Therefore, when specifying the MAX or MIN dimensions, we must fully consider whether there is an impact on the function at the limit deviation. For example, if we mark a fillet as R1 MAX, then we must consider whether the absence of a fillet (i.e. when it is zero) will affect the function. In this case, an appropriate lower tolerance limit must be specified.
There are also many theoretical dimensions (i.e., basic dimensions) in the drawings. Do they have tolerances? Theoretical dimensions are defined as numerically correct sizes, shapes, profiles, directions, or positions used to define a body or target data.
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 and position tolerance of the body. When this theoretical dimension is used to define the size, shape, or position of a given target, its tolerance must be determined in accordance with ASME Y14.43 fixture and gauge tolerance guidelines. Therefore, theoretical dimensions also have tolerances.
The methods for indicating dimensional tolerances on drawings are as follows:
· Direct annotation of dimension limits or tolerance values on the dimension itself.
· Indication in the form of geometric dimensional tolerances.
· Definition of tolerances for dimensions specified in notes or tables.
· Defining tolerances for specific shapes or processes in other documents referenced in the drawing.
· Setting tolerances for all dimensions without tolerances specified in the general tolerance column.
Rule 2: Dimensions and tolerances must be fully defined to fully understand all characteristics of each shape.
The characteristics of a form include size, shape, direction, and position. You must define the size and tolerance of all features of each shape in the drawing. Sizes and tolerance values can be expressed through engineering drawings or defined by CAD product definition databases. It is not permitted to guess or determine dimensions by measuring the drawing.
Rule 3: Only indicate all dimensions necessary to describe the product.
The meaning of all necessary dimensions is that the dimensions in the drawing must be correct, neither too many nor too few, to fully express all the characteristics of all shapes. There should be no superfluous dimensions in the drawing, such as closed dimensions.
As discussed earlier, we can ignore any reference dimensions, so the drawing should minimize the use of reference dimensions. The reference dimensions, in addition to adding a feeling of disorder to the drawing, are useless.
Rule 4: Dimensions should be chosen based on the function and fit of the product and should not be open to multiple interpretations.
This emphasizes that the dimensions and tolerances we define during design must be based on meeting the functional and fit requirements of the product. The design process must consider manufacturability and inspectability, but not at the expense of functional requirements.
Rule 5: Product designs must not specify manufacturing methods
Product designs must indicate only the dimensions and performance requirements necessary for the product's functionality. The way it is manufactured is the responsibility of production engineers.
As designers, we must provide wide freedom to the production team. Our consideration should be to provide the widest possible tolerance range that meets the functionality of the product while allowing sufficient manufacturing capacity, rather than specifying the manufacturing method. For example, for a hole, we must mark only the diameter, without specifying whether it is drilled, punched, milled, turned, ground or made using other processes.
As long as the final product meets the diameter tolerance, the manufacturing process does not matter. However, when the manufacturing process is an inseparable part of the product characteristics, it must be specified in the design or reference documents. For example, if the functionality requires the hole to be free of spiral machining marks while meeting the diameter tolerance, the design may specify that the hole needs to be ground.
Rule 6: When providing final product dimensions, it is acceptable to mark non-mandatory process parameters such as machining tolerances
Typically, process parameters do not need to be marked in designs, but if they are, it should be clearly stated that they are not mandatory. As mentioned previously, this is the responsibility of production engineers and they should have broad freedom.
Rule 7: Dimensions must be organized logically for optimal readability. They must be placed on the actual contour and marked on visible contour lines
This is a basic requirement for writing, which we will not elaborate on here.
Rule 8: Raw materials such as wire, tube, plate, rod or other materials produced by measurement or mark number must be marked with linear dimensions such as diameter or thickness.
The measurement or brand number of the product must be marked in parentheses after the dimension
This rule is specific to raw materials, each with its own notation standard.
Rule 9: Centerlines and contours shown as right angles in the unmarked design are considered to be 90 degrees
There are many relationships in projects that are assumed to be 90 degrees. These assumed 90 degree tolerances must be controlled as unmarked angle tolerances.
Rule 10: Center lines or surfaces of bodies arranged or positioned by basic dimensions, if shown as right angles on the design without markings, are considered basic dimensions of 90 degrees
Arranged bodies refer to a group (two or more) of bodies with the same shape and size distributed in a regular pattern. When the centers of these bodies are defined or positioned by basic dimensions, the assumed 90-degree basic angle tolerance is controlled by the corresponding shape and position tolerance.
Rule 11: When the center line, center plane, or surface are displayed consistently on the diagram, it is assumed to be a zero-valued basic dimension and their relationship is defined by shape and position tolerances.
This is common knowledge. The tolerances of these basic dimensions, which are assumed to be zero, must be controlled by the corresponding shape and position tolerances. If shape and position tolerances are not specified, they shall be controlled by the shape and position tolerances not specified in the general technical requirements column.
Rule 12: Unless otherwise specified, all dimensions refer to an ambient temperature of 20°C (68°F). If measurements are carried out at other temperatures, dimension compensation must be considered.
Note that the ambient temperature here is 20 degrees, not 23 or 25 degrees. Therefore, we require all measurement rooms to control the temperature within 20 degrees to ensure that test results accurately reflect whether product requirements have been met.
If it is impossible to measure at an ambient temperature of 20 degrees, we must consider compensating for the effects of temperature on the measurement results, especially for parts with high temperature sensitivity.
Rule 13: Unless otherwise specified, all dimensions and tolerances apply to the free state condition.
All dimensions marked on the drawing refer to the dimensions of the parts in a stress-free state. For some non-rigid parts, we can mark the dimensions after the parts are constrained according to regulations, and the method of constraining the parts must be marked on the drawing.
If we want to mark some dimensions of the parts in their free state, we must mark them with the free state symbol F.
Rule 14: Unless otherwise specified, all geometric dimensional tolerances apply to the entire length, width, or depth of the form.
I believe everyone is familiar with this. What I want to remember is that due to the application of the inclusive principle, the length, width or depth of the form greatly affects the control of the form shape.
For a 3mm long round bar and a 30mm long round bar, the maximum allowable straightness under the same diameter tolerance is the same, but the actual bending situation is very different.
Rule 15: All dimensions and tolerances apply only to the product level described in the drawing. The dimensional tolerance of a shape described in one drawing level (such as a part diagram) does not necessarily apply to the dimensional tolerance of that shape in other drawing levels (such as assembly diagrams).
That is, the size on a part diagram may not necessarily apply to the assembly diagram. For example, if we weld a bracket with an opening of 10 +/- 0.5 to a platform, due to welding deformation, welding fixture attachment and other factors, it will be difficult to meet the size requirement of 10 +/- 0.5 on the welded part.
In other words, this size is no longer applicable to the design of the welded part. Therefore, we cannot require the same size in the assembly drawing based on the size of a part drawing. If it is necessary to control this shape on the assembly drawing, the size must be marked on the assembly drawing.
Rule 16: Unless otherwise noted, when a coordinate system appears in the drawing, it must be right-handed. Each coordinate axis must be marked and the positive direction indicated.
This point is rarely used, so no detailed explanation is needed, just follow it.
Above is an introduction to the 16 basic design guidelines stipulated by the ASME standard.
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