21 September 2023

GEOMETRIC DIMENSIONING AND TOLERANCING - PART 2

This is the second and last part of a blog series. Read about introduction and fundamentals of Geometric Dimensioning and Tolerancing here.

We have covered the practical aspects of deploying Geometric Dimensioning and Tolerancing in this part.

Geometric design

8. Understanding tolerance zones
 

8.1 Rectangular tolerance zone 

A rectangular tolerance zone is a two-dimensional tolerance zone that is defined by a rectangle. It is used to specify the acceptable limits of variation for a particular feature, such as size, position, or form. The rectangular tolerance zone is defined by two parallel lines and two perpendicular lines, and it represents the maximum allowable deviation from the specified feature. 
In a rectangular tolerance zone, the feature must be contained within the tolerance zone, meaning that the feature must lie entirely within the rectangle. The width and height of the rectangle determine the maximum allowable deviation in each direction, and the position of the rectangle determines the maximum allowable deviation from the specified position of the feature. 

8.2 Cylindrical tolerance zone:


A cylindrical tolerance zone is a three-dimensional tolerance zone defined by a cylinder. It is used to specify the acceptable limits of variation for a particular feature, such as size, position, or form. The cylindrical tolerance zone is defined by a central axis and a radius, and it represents the maximum allowable deviation from the specified feature. 
In a cylindrical tolerance zone, the feature must be contained within the tolerance zone, meaning that the feature must lie entirely within the cylinder. The radius of the cylinder determines the maximum allowable deviation from the specified size of the feature, and the position of the cylinder determines the maximum allowable deviation from the specified position of the feature. 

9. Datums 

Datum is a reference point, line, or plane used as a basis for geometric dimensioning and tolerancing (GD&T). The datum is used to establish a coordinate system for measuring the dimensional features of an object and to define the allowable variations in the dimensions of the object.
There are three types of datums in GD&T: 

  • Primary Datum - The primary datum is the first datum plane, line, or point that is established in a drawing. It is the reference feature that is used to establish the X, Y, and Z axis of the coordinate system. All other datums are referenced to this datum. 
  • Secondary Datum - The secondary datum is a datum feature that is used in conjunction with the primary datum to establish the orientation of the part. It is perpendicular to the primary datum and is used to control the angularity of the part. 
  • Tertiary Datum - The tertiary datum is a datum feature that is used to establish the location of the part. It is perpendicular to both the primary and secondary data and is used to control the position of the part. 

Each datum is identified by a capital letter (A, B, C, etc.) and is shown on the drawing with a symbol that indicates its type (plane, line, or point). The datums are used to establish the GD&T tolerances for the dimensions of the part, and they provide a common reference for measuring and inspecting the part to ensure that it meets the required specification.

10. Bonus tolerance and virtual condition

10.1 Bonus tolerance 

Bonus tolerance is an additional tolerance that is provided beyond the basic size tolerance. It allows for variations in the dimensions of a part without affecting the functional requirements of the part. Bonus tolerance is sometimes provided for features that are not critical to the performance of the part or for features that have a low probability of being manufactured to the exact size required. Bonus tolerance is usually shown as a minus tolerance value, and it is subtracted from the basic size tolerance. 

10.2 Virtual condition 

A virtual condition is a hypothetical condition that is used to establish the allowable variations in the dimensions of a part. It is the ideal size and shape of a feature that is used as a reference for determining the allowable variations in the feature's dimensions. The virtual condition is used to establish the maximum material condition (MMC) and the least material condition (LMC) of the feature, which are the conditions that represent the extremes of the allowable variation in the feature's dimensions. The virtual condition is used to calculate the position and profile tolerances of the feature. 
Both bonus tolerance and virtual condition are important concepts in GD&T because they allow for greater flexibility in the manufacturing process without sacrificing the functional requirements of the part. By providing additional tolerance and using virtual conditions to establish allowable variations, manufacturers can more easily produce parts that meet the required specifications, while reducing the risk of producing parts that are out of tolerance and do not function properly. 

11. Feature control frame 
 

The feature control frame consists of all the relevant symbols defining the feature-specific controls. Feature control frame can be used to convey the geometric tolerance of a feature or part as intended. The below image shows what a general feature control frame looks like. 

Figure 3: Feature Control Frame

Figure 3: Feature Control Frame

12. GD&T Symbols Overview 

12.1 Form Controls 
In GD&T, form controls are used to specify the allowable variation in the shape of a feature. These controls are used to ensure that a feature meets the required form or shape, even when the feature is not in its perfect position or orientation. The three main form controls used in GD&T are: 

Geometric design

By using form controls in GD&T, designers and engineers can specify the required shape of a feature with a high degree of precision, allowing manufacturers to produce parts that meet the required specifications. This can lead to improved product quality, reduced errors and rework, and increased productivity in manufacturing.

12.2 Orientation Controls

Orientation controls are used to specify the allowable variation in the orientation of a feature. The three main orientation controls used in GD&T are:

Geometric design

By using orientation controls in GD&T, designers and engineers can ensure that the features of a part are properly aligned and oriented according to the design requirements. This can lead to improved product performance, reduced errors and rework, and increased productivity in manufacturing.

12.3.    Profile Controls

Profile controls are used to specify the allowable variation in the shape of a feature in relation to when compared to its true profile. These controls ensure that the feature is within a specified tolerance zone that is defined by a series of points or a curve that represents the true profile of the feature. The two main profile controls used in GD&T are:

Geometric design

By using profile controls in GD&T, designers and engineers can ensure that the features of a part have the correct shape and are within the specified tolerance zone. This can lead to improved product performance, reduced errors and rework, and increased productivity in manufacturing.

12.4.    Location Controls

Location controls are used to specify the allowable variation in the location of a feature relative to a set of datum features. These controls ensure that the feature is in the correct position and orientation relative to the design requirements. The three main location controls used in GD&T are:

Geometric design

By using location controls in GD&T, designers and engineers can ensure that the features of a part are located and oriented correctly relative to the design requirements. This can lead to improved product performance, reduced errors and rework, and increased productivity in manufacturing.

12.5.    Runout Controls

Runout controls are used to specify the allowable variation in the location and orientation of a feature as it rotates or moves relative to a datum feature. These controls ensure that the feature remains within a specified tolerance zone as it rotates or moves, which is important for parts that require precise rotational or translational movement. The two main types of runout controls used in GD&T are:

Geometric design

By using runout controls in GD&T, designers and engineers can ensure that parts with rotating or moving features have the correct location and orientation, which is important for proper functioning and long-term reliability. This can lead to improved product performance, reduced errors and rework, and increased productivity in manufacturing.

12.6.1.    Maximum Material Condition

In GD&T, the maximum material condition (MMC) feature control frame modifier is used to denote a feature's biggest size or maximum material condition. In other words, it indicates the maximum possible quantity of material that could be present within the specified feature tolerance zone.

12.6.2.    Least Material Condition

In GD&T, the term Minimum or Least Material Condition (LMC ) is used to describe the least size or quantity of material that is allowed for a specific aspect of a part. Maximum Material Condition (MMC), which is the highest size or quantity of material that is allowed for a certain characteristic, is the opposite of MMC.

12.6.3.    Regardless of Feature Size

RFS is the default condition when there are no material modifiers are mentioned. As per RFS, the features should be ideal as per the nominal dimension and the tolerance.
 

13.    Rules in GD&T

13.1.    Rule-1 (Taylor Principle or Envelope Principle)

Rule-1 of the GD&T states that a regular feature's actual surface is limited to the area specified by the feature in perfect form at MMC. This implies that for a feature to measure at MMC, its form must be ideal, which is not possible to achieve in practice.

13.2.    Rule-2 (Regardless of Feature Size)

Under rule #2 of GD&T, regardless of feature size (RFS) is the default condition of all geometric tolerances and doesn't need to be called explicitly. Any GD&T callout you produce is managed independently of the size dimension of the part, regardless of what feature size simply signifies.

14.    Example – Beta Shaft

15.    Example – Gear

Geometric design

Figure 4: Geometric Dimensioning and Tolerancing on Beta Shaft Part

16. Conclusion
 

In conclusion, geometric dimensioning and tolerancing is a crucial step in the design and production of medical devices. Components and assemblies are built to precise specifications and meet the necessary functional requirements thanks to the usage of standardized symbols, phrases, and concepts.

In the manufacturing of medical devices, geometric tolerancing and dimensioning serve to cut costs, increase efficiency, and maintain patient safety. Engineers and manufacturers can create the requisite levels of precision and accuracy for the design and production of medical devices by employing this process.

Furthermore, designers, manufacturers, and inspectors can communicate with one another using geometric tolerancing and dimensioning. As a result, the development and production processes are streamlined, and more effective communication and collaboration are encouraged.

Geometric dimensioning and tolerancing are used to help guarantee that products meet regulatory standards and specifications. Meeting regulatory criteria, such as ISO 13485 and the FDA's Quality System Regulation (QSR), is crucial for the medical device sector. Manufacturers can offer simple and clear proof of compliance with these standards by utilizing standardized symbols and wording.

Overall, the application of geometric tolerancing and dimensioning in the medical device industry is essential for the development and production of safe and effective devices. It promotes precise manufacturing and reduces errors, resulting in better patient outcomes. As technology continues to advance, the use of these concepts and methodologies will become increasingly important in the design and production of new medical devices.

Medical device engineers and manufacturers need to stay up to date on the latest standards and regulations related to geometric tolerancing and dimensioning. By doing so, they can ensure that their devices are produced to the highest levels of quality and safety, ultimately benefiting patients and healthcare providers alike.

17. Our Services
 

Our design services focus on optimizing product designs for manufacturability and inspection using GD&T principles. This includes translating GD&T specifications, helping with tolerance stack-up analysis, and making suggestions for the best dimensional control strategies. By collaborating closely with clients' design teams, we ensure that the designs align with GD&T's requirements. Our teams provide valuable insights and recommendations during the design phase to enhance manufacturability, reduce costs, and minimize errors. This collaborative approach results in improved product quality, streamlined manufacturing processes, and faster time-to-market for medical devices.

This blog is written by Basana Gouda, Senior Mechanical Engineer at Decos. 

Decos is a cutting-edge technology services partner ready to meet your software needs in the medical domain. If you have a question on one of our projects or would like advice on your project or a POC, just contact Devesh Agarwal. We’d love to get in touch with you!
 

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