The first compartment starting from the left contains the geometric characteristic symbol. This is where we specify the geometric characteristic. In total, there are 14 types of geometric tolerances based on the number of symbols, and 15 when classified.
The different types of geometric characteristics are form control, profile control, location control, orientation control, etc. We will dive deeper into each category later on. The first symbol shows the type of tolerance zone. In the absence of a symbol, we assume a total wide zone. The second symbol in the 2nd block gives the value of the tolerance zone in mm. This value must always be present in the 2nd block. The third symbol in this block is the modifier for tolerance.
This material modifier is only present when the feature has a size, for instance, a hole. More information on the various modifiers available will be discussed further in the article. The third block gives information about the datum s with a minimum of 1 and a maximum of 3 datums. In the case of 3 datums, they are referred to as primary, secondary, and tertiary datums with each being shown enclosed in a different box the image above has 2, for example.
This block may also contain a material modifier. All the information above helps us to understand how to read a feature control frame.
In order to better understand how tolerance can be adjusted using material condition modifiers, we need to be familiar with the different options. Material condition modifiers convey the intent when tolerance applies to a feature at a specific feature size. Whenever we give tolerances to any feature, it establishes two material conditions.
Consider a shaft of diameter mm. This is what we mean by the maximum material condition. On the other hand, the same shaft, when manufactured with a diameter of These limits are called material conditions. The feature contains the maximum material at this feature size. For internal FOS, such as the diameter of a hole, MMC represents the smallest possible size within stated tolerance limits because a smaller hole means that more material will be left. When we need to apply geometric tolerance at the maximum material condition, we mention this condition in the feature control frame.
For external FOS, it will be the smallest possible size within stated limits. And for internal FOS, it will be the largest possible size. This feature removes excessive material and thus weight. To apply geometric tolerances at this condition, we use its symbol in the feature control frame. Datums are reference points for measuring dimensional tolerances.
It could be a point, a line, or a plane. There are 6 degrees of freedom 3 translational and 3 rotational that we need to control to manufacture and inspect parts effectively. We use DRF to establish these degrees of freedom. Datum features are the actual part features such as holes and slots.
They can show variation from desired positions. Among all the datum features, we give the highest preference to those that mate with other parts in the assembly. We can mention more than one datum in our feature control frame. As per the sequence of the DRFs in the feature control frame, the parts are mated to the DRFs in decreasing order of importance.
Up to 14 GD and T symbols are available to represent different geometric characteristics of features. These symbols help us to specify these characteristics as requirements for the final product. We place these symbols in the first compartment of the feature control frame. We shall look at these types of tolerance control. As the name suggests, form control relates to the final form or shape of the feature. We define form controls to limit the deviation of the geometric tolerance from its ideal form.
Some popular form control characteristics are as follows. To indicate the straightness characteristic of a feature such as an axis or a surface , we use its symbol a straight horizontal line in the first compartment. Surface straightness can apply to flat surfaces like a side of a block or curved surfaces like a side of a cylinder along the direction of the axis.
It defines the allowable variation of a line 2 dimensions on the surface within a specified tolerance. Ahluwalia and A. Fainguelernt, R. Weill and P. Irani, R. Mittal and E. Whybrew, G. Britton, D. Robinson and Y. Ngoi and C. Ngoi and S. He and P. Download references. You can also search for this author in PubMed Google Scholar. The engineer or designer should strive to keep tolerances as large as possible while preserving the function of the part. Small tolerances can increase cost in the manufacturing, inspection, and tooling of parts.
In order to design, manufacture, and verify parts, the necessary DOF must be constrained. Parts are mated to the DRF so measurements, processing, and calculations can be made. Datums are points, axes lines , and planes, or some combination of these components, that make up the DRF. Datum features are the actual, physical features holes, faces, slots, etc.
The illustrations below are provided to emphasize that Datums left are theoretical perfect and datum features right are real imperfect. In defining a part, an engineer will identify the datum features on a part that are most important to the functional requirements of the design—usually the features that mount the part in the assembly.
Datum features referenced in the end compartments of a feature control frame see Feature Control Frame , in an order of precedence, will mate the part to the datum reference frame. These symbols are placed in the first compartment of a feature control frame and define the type of tolerance that is to be applied to the feature. The characteristics are grouped together into types of tolerance: form, orientation, location, runout, and location of derived median points.
The primary use and description of each characteristic is also shown. Geometric tolerances are applied to features by feature control frames. If applied to surfaces, orientation tolerances also control form.
Location tolerances control location and are always associated with basic linear dimensions. Position locates and orients the median plane or axis of features of size. Profile locates feature surfaces.
Profile is the most powerful characteristic of all, and also controls orientation and form. The feature control frame states the requirements or instructions for the feature to which it is attached. Simply put, the feature control frame controls features. Each feature control frame contains only one message requirement ; if two messages for a feature are necessary, two feature control frames are required. The first compartment of a feature control frame contains one of the fourteen geometric characteristic symbols.
Only one of the symbols can be placed in a feature control frame; if there are two requirements for a feature, there must be two feature control frames or a composite tolerance.
The second compartment of a feature control frame contains the total tolerance for the feature. If there is no symbol preceding the tolerance, the default tolerance zone shape is parallel planes or a total wide zone, as in the position of a slot or profile of a surface. Following the feature tolerance in the feature control frame, a material condition modifier, such as MMC or LMC see Material Condition Modifiers may be specified if the feature has size, such as a hole.
If the feature has size, and no modifier is specified, the default modifier is RFS. If the feature has no size, such as a plane surface, then the modifier is not applicable.
The third and following compartments of a feature control frame contain the datum feature reference s if they are required. For example, if a form tolerance, such as flatness or straightness, is specified, then no datum feature reference is allowed. However, if a location tolerance like position is specified, the datum feature references are usually specified.
If two parts do not fit together during assembly, this method will not give you the answer as to why. This helps include additional contributors, and can give a better understanding of outputs. This method is good for surface areas, like product skins, or simple structures. Con — Lack of influencers, poor root cause analysis — This method lacks 3D dimensional contributors. This can mean the difference between good and bad parts in many products.
Any product with more than a few parts should be analyzed in 3D to account for how all of the parts influence the final geometry. In addition, because it lacks so many influencers, it is a poor method for root cause analysis. When issues arise on the plant floor, this may leave you with more questions than answers. This also allows for in-depth root cause analysis. In addition to these benefits, many tolerance analysis software systems allow the user to model the assembly process. This gives a thorough understanding of how your manufacturing and assembly process will affect and be affected by variation.
A user needs to be trained and understand engineering practices in order to effectively model both parts and process. DCS also offers mentor based training and services to help you build your models. Why do Tolerance Analysis? Improve Quality - Improve both visual and mechanical quality of your product. Determine Gap and Flush - Determine optimum Gap and Flush conditions based on your manufacturing processes.
Optimize Processes - Simulate your manufacturing and assembly processes to determine optimum methods and order.
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