Position - The zone within which the axis or center plane of a feature is permitted to vary from true theoretically exact position. Profile of a Line Control - A geometric tolerance that limits the amount of error for line elements relative to their true profile. Profile of a Surface Control - A geometric tolerance that limits the amount of error a surface can have relative to its true profile. Straightness Axis or Centerplane - The condition where an axis is a straight line or, in the case of a centerplane, each line element is a straight line.
Symmetry - The condition where the median points of all opposed elements of two or more feature surfaces are congruent with the axis or centerplane of a datum feature. Total Runout - A composite control affecting the form, orientation, and location of all surface elements of a diameter or surface relative to a datum axis.
All-Around Symbol - A circle placed on the bend of the leader line of a profile control. Datum Feature - A part feature that contacts a datum. Video Training. Web-Based Training. Digital Instructor Kits.
Reference Charts. Miscellaneous Products. Free Resources. ASME Y 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.
Axial straightness usually applies to the axis of a shaft or a hole, for example. We use this characteristic for features such as the face of a part. The symbol for flatness is a parallelogram tilted to the right. The difference between the highest and lowest point of a flat surface is its flatness. The flatness symbol does not require any datums, as it only shows a tolerance range within which the whole surface of a part must conform to in 3 dimensions.
A height gauge is used for checking the results. It touches upon different areas of the surface to determine that all the points are inside the tolerance zone.
It uses 2 concentric circles on a plane perpendicular to the part axis to define the suitable tolerance range in 2D. Each point of the final measurements has to fall between the circles.
Cylindricity is basically circularity in 3D, meaning that it runs along the whole length of the part. This feature also describes other cylindrical characteristics such as its taper, straightness, and roundness. This is why this feature is also expensive to inspect. We use profile controls to control the 3D tolerance zone around the feature.
This feature helps us to give tolerance limits to advanced curves and shapes. A feature that is good for advanced curves must naturally work well with simpler curves as well. This makes profile controls extremely versatile. This is why some designers recommend abandoning all other controls and working only with profile controls.
These controls form a tolerance limit around the advanced curve by mimicking it on both sides. It then prevents any point on the feature curve from going outside the tolerance limit. Profile controls are of two types. Line profile establishes a tolerance zone around varying 2D cross-sections of the part.
It controls individual line elements of a part feature. The line profile control traces the ideal curve at the tolerance limits on both sides. Surface profile control is more popular compared to line control. Instead of a two-dimensional tolerance zone, the surface profile control creates a 3D zone around a surface. Orientation tolerance controls the orientation of a feature with respect to a defined datum.
The angularity control keeps the angle of a feature with respect to the datum in check. We can use it to control a 2D line, but it is more popular with keeping surfaces 3D under control. This kind of tight control keeps the angle and the surface flatness in check and is recommended for part features that mate with other parts through the angled surface.
It is important to note that the angularity feature controls the angle indirectly, not being essentially the same as an angular tolerance e. We will be adding to this list periodically as we continue to build up our content. Position is always used with a feature of size. You can think of them as an anchor for the entire part; where the other features are referenced from. A datum feature is usually an important functional feature that needs to be controlled during measurement as well.
Profile of a surface describes a 3-Dimensional tolerance zone around a surface, usually which is an advanced curve or shape It comes in useful if a feature is to be defined on a drawing that needs to be uniformly flat without tightening any other dimensions on the drawing. The flatness tolerance references two parallel planes parallel to the surface that it is called out on that define a zone where the entire reference surface must lie.
Concentricity, sometimes called coaxially, is a tolerance that controls the central axis of the referenced feature, to a datum axis Parallelism is a fairly common symbol that describes a parallel orientation of one referenced feature to a datum surface or line
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