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  • October 29, 2021
  • Networking

We first need to understand that parts are not really useful by themselves instead part are typically put together with other parts to create a useful assembly. Traditional dimensioning and tolerancing does not guarantee that your parts will fit and function at the assembly level. By using GD and T we can dimension and tolerance parts in a way which guarantees that they will fit and function at the assembly level.

Traditional Approaches to Dimensioning and Tolerancing –

In the 1940s and 1950s the importance of assigning and controlling tolerances that would consistently produce interchangeable parts and a quality product increased in importance.

Genichi Taguchi and W. Edwards Deming began to tell industries worldwide the importance of quality product and the quality should be checked before a product was released to production.

The space race and cold war of the 1960s plays an important role on modern engineering education. In this period of time, the trend in engineering education is shifted toward a more theoretical and mathematical approach from a design- oriented curriculum in the united states.

Several different product design philosophies were developed during this period of time for e.g. Engineering Driven, Process Driven and an Inspection Driven Design.

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Engineering driven design –

It is based on the thesis that to ensure the perceived functional requirements of a product the design engineer can specify any tolerance values deemed necessary.

Traditionally, the design engineer assigns tolerance values based on past experience, guess or build-test-fix methods during product development.

After determination of tolerances there is no communication between the design engineer and the manufacturer.

The drawback is that errors are always found after the part processing has begun. So this increases manufacturing cycle times, engineering change orders and overall costs.

Process Driven Design – 

This is completely based on the capability of the manufacturing process.

The inspection process can verify the tolerance specifications for the manufacturing process only.

Inspection Driven Design – 

The designer should have the knowledge of metrology, then only the method is effective.

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What is GD and T – 

  • GD and T is a System of dimensioning and tolerancing mechanical parts. 
  • It’s a system that is more powerful than traditional +/- dimensioning and tolerancing.
  • It’s a system which allows you to know with 100% certainty your parts will fit and function at the assembly level.
  • It’s a system where we learn vocabulary, Definitions, Rules, and then apply Symbols.
  • It’s a system which only works if Designer/Engineer/ Manufacturer all agree to use the same system.

Where do we get this system we call “GD and T”?

The American Society of Mechanical Engineers (ASME)

What is this System officially called?

“ASME Y 14.5 Dimensioning and Tolerancing” standard.

Benefits of using GD and T. – 

  1. You can be 100% certain that your parts will fit and function properly at the assembly level
  2. Save time and save money by reducing the number of design-manufacturing-test fit cycles.
  3. Conveys your design intend to the
  • Machinists
  • Inspectors
  • Future Designers inheriting your work



                                MODIFYING SYMBOLS

                                                               At  maximum material condition           M
At least material condition            L        
Projected tolerance zone           P      
Free state           F    
Tangent plane          T 
Diameter          ø     
Spherical Diameter          Sø
Radius         R
Spherical Radius         SR
Controlled Radius         CR
Reference         (  )
Arc Length          ͡
Statistical Tolerance        ST
Between         ↔

Why do we use GD and T?

When a number of people work with a part, its important they all consider part dimensions the same. For example, the designer specifies the distance to a hole’s nominal location; the manufacturer measures off this distance and drills a hole; then an inspector measures the actual distance to that hole. In this everyone must be in perfect agreement about following things:

1.From where to start the measurement

2.Direction to go

3.Where the measurement ends.

GD and T provides the rules to assure that.

Hidden costs that GD and T reduces are as follows:

1.Programmers time for trying to interpret drawings and questioning the designers.

2.Rework of manufactured parts due to misunderstandings

3.Handling and documentation of functional parts that are rejected

4.Assemblies failing to operate, failure analysis, quality problems, customer complaints, loss of market share and customer loyalty.

It all adds up to an excessive, yet unaccounted cost.

We use GD and T because it is the right thing to do, it’s what people all over the world understands, and it saves money.

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When do we use GD and T?

Other than GD and T stipulations, a part ability to satisfy design requirements depends on the following things:

1.Pride in workmanship – Most workers strive to achieve the industry unwritten customary standards of product quality.

2.Common Sense -This is a fairly reliable sense developed by experienced manufacturers for what a part is supposed to do

3. ProbabilityThe execution of a process can usually be predicted statistically.

4.Title block, workmanship, or contractual standards. Sometimes these provide clarification.

Depending on these things carries risks. Where a designer considers the risks too high, specifications should be correctly spelled out with GD and T.


The goal of GD and T is to guide all parties toward understanding part dimensions the same, including the origin, direction, and destination for each measurement.

1.Identify element surfaces to function origins and offer particular policies explaining how those surfaces set up the place to begin and path for measurements.

2.Convey the ideal distances and orientations from origins to different surfaces.

3.Establish limitations and/or tolerance zones for particular attributes of every floor in conjunction with particular policies for conformance.

4.Allow dynamic interplay among tolerances (simulating real meeting possibilities) in which suitable to maximize tolerances.


Feature control frame is a rectangular sign used to express each geometric control for a feature on the drawing. The feature control frame is divided into cells expressing the following, sequentially from left to right. Only the basic dimensions are shown out of the feature control frame.

The 1st cell represents a geometric characteristic symbol specifying the type of geometric control.

The 2nd cell represents the geometric tolerance value.

The 3rd, 4th, and 5th cells are each added as needed to contain the primary, secondary, and tertiary datum references sequentially, each of which may be followed by a material condition modifier symbol as appropriate.

Vaishnavi Shinde

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