There are many agreed-upon basic principles in mechanical drawing, and ASME Y14.5-2009 stipulates sixteen basic principles. These basic principles must be understood by everyone when drawing, reading or reviewing drawings. Below, I will introduce these 16 basic principles to you one by one.
Article 1: All dimensions must have tolerances except reference dimensions, maximum and minimum dimensions or raw materials.
Reference dimensions generally have no tolerances.
Why? Because reference dimensions are generally repeated dimensions or closed dimensions on drawings, they are only used as reference information.
Reference dimensions are neither used to guide production nor inspection, so you can ignore them when you see reference dimensions on drawings.
We often see the maximum MAX or minimum MIN marking methods on drawings. Are there any tolerances for these dimensions? The answer is yes.
For MAX size, its lower limit of tolerance is 0, while for MIN size, its upper limit of tolerance is infinity. Therefore, when we specify the MAX or MIN size, we must fully consider whether it will affect the function at the extreme 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 an appropriate lower limit of tolerance.
There are many theoretical dimensions (i.e. basic dimensions) on the drawings, so do they have tolerances?
The so-called theoretical size refers to a numerical value that is used to define the theoretically correct size, shape, outline, direction or position of a form or target datum.
When this theoretical dimension is used to define the size, shape, outline, direction or position of a body, its tolerance is defined by the corresponding geometric tolerance of the body;
When this theoretical dimension is used to define the size, shape, or location of a target datum, its tolerances should be determined in accordance with the ASMEY14.43 Gauge and Fixture Tolerance Guidelines.
Therefore, theoretical dimensions also have tolerances.
The following methods are used to mark dimensional tolerances on drawings:
· Mark dimensional limits or dimensional tolerance values directly on dimensions
· Marked in the form of geometric dimensional tolerances
· Define tolerances for specified dimensions in notes or tables
· Define tolerances for specified features or processes in other documents referenced by drawings
· Define tolerances for all untoleranced dimensions 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 body include size, shape, orientation and position.
Dimensions and tolerances must be defined on the drawing for all features of each feature.
Dimensions and tolerance values can be expressed by engineering drawings or defined by a CAD product definition database.
Determining dimensions by measuring drawings or making guesswork is not allowed.
Article 3: Only mark all dimensions necessary to describe 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 extraneous dimensions on the drawing, such as closed dimensions.
As mentioned before, we can ignore any reference size, so drawings should use as few reference sizes as possible. Reference dimensions have no meaning other than adding a messy feel to the drawing.
Article 4: Size should be selected based on the function and fit of the product, and there should not be multiple interpretations.
What is emphasized here is that the dimensions and their tolerances defined during our design should meet the functional requirements and coordination 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 dimensions and performance requirements that meet the product's functions should be marked on the product drawings. How to process and manufacture them is the work of manufacturing engineering.
As designers, manufacturing personnel should be given full freedom.
What we should consider is the maximum tolerance range that meets the functional requirements of the product so that manufacturing has sufficient manufacturing capabilities, rather than stipulating manufacturing methods.
For example, for a hole, we only need to mark the diameter, but not the drilling, punching, milling, turning, grinding or other processes.
No matter what process method is used, as long as the finished product can meet the diameter tolerance requirements.
Only when the manufacturing process is an integral part of the characteristics of the product should it be described on the drawing or reference document.
For example, due to functional requirements, the hole must meet the diameter tolerance and must not have spiral processing marks. You can indicate on the drawing that the hole is required to be ground.
Article 6: When giving the dimensions of the final product, it is allowed to indicate the dimensions of non-mandatory process parameters that provide information such as machining allowances. These dimensions should be marked as non-mandatory.
Generally speaking, process parameters do not need to be marked on the drawings. If they are required, they must be noted as non-mandatory.
As mentioned earlier, this is the job of manufacturing engineering and they should be given full freedom.
Article 7: Dimensions should be rationally arranged to achieve optimal readability. Dimensions should be laid out on the actual outline drawing and marked on the visible outline.
This is a basic requirement for drawing and will not be expanded upon 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 parentheses after the size.
This article is for raw materials, and each raw material has its corresponding standards to stipulate the labeling method.
Article 9: The center line and the outline of the shape are displayed as right angles on the drawing and the default is 90 degrees if not marked.
There are many default 90-degree relationships on the drawings. These default 90-degree tolerances should be controlled according to the unnoted angle tolerances.
Article 10: If the centerline or surface of an array shape positioned or defined by basic dimensions is shown as a right angle on the drawing without annotation, it defaults to the basic dimension of 90 degrees.
An array of shapes refers to a group (two or more) of shapes with the same shape and size and regularly distributed.
When the centers of these shapes are defined or positioned 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 appears consistent on the drawing, it defaults to the basic dimension with a value of 0, and their mutual relationship is defined by the geometric tolerance.
This is also basic common sense. The tolerances of these basic dimensions, which default to 0, should be controlled by the corresponding geometric tolerances. If no geometric tolerance is specified, it will be controlled by the unindicated geometric tolerance in the general technical requirements column.
Article 12: Unless otherwise noted, all dimensions refer to room temperature 20°C (68°F). If measurements are made at other temperatures, compensation for the dimensions should be considered.
Note that the room temperature mentioned here is 20 degrees, not 23 degrees or 25 degrees. Therefore, we require the room temperature in the measurement room to be controlled at 20 degrees to ensure that the true response of the test results meets product requirements.
If there is really no condition to measure at room temperature of 20 degrees, we should consider compensating the temperature effect on the measurement results, especially for parts with high temperature sensitivity.
Article 13: Unless otherwise noted, all dimensions and tolerances apply to free state conditions.
All dimensions marked on the drawings refer to the dimensions of the parts in the free state where all stress is released.
For some non-rigid parts, we can mark the dimensions of the parts after they are constrained according to regulations. The method of constraining the parts must be marked on the drawing. At this time, if we also want to mark part of the part's dimensions in its free state, we must mark the free state symbol circle F.
Article 14: Unless otherwise noted, all geometric dimensional 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 principle of inclusion, the length, width or depth of a body has a great relationship with the shape control of the body. A 3mm long round rod and a 30mm long round rod have the same maximum straightness under the same diameter tolerance, but the actual bending conditions are vastly different.
Article 15: All dimensions and tolerances only apply to the product level represented by the drawing. The dimensional tolerance of a feature expressed on one drawing level (such as part drawing) does not absolutely apply to the dimensional tolerance of the feature on other drawing levels (such as assembly drawing).
In other words, the dimensions on a parts 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 welding deformation, clamping of the welding fixture and other factors, it is difficult for this opening to meet the 10+/-0.5 size requirement on the welded parts.
This means that this dimension is no longer applicable to weldment drawings. Therefore, we cannot use the dimensions on a part drawing to require the dimensions 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 right-centered. Each axis must be labeled and indicate the positive direction.
This point has relatively few applications and will not be explained in detail. Just follow it.
The above is an introduction to the 16 basic drawing guidelines stipulated in the ASME standard.
