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WHY USE GEOMETRIC DIMENSIONING AND TOLERANCING

(GD&T)

GD&T is a precise national and international symbolic language used on a mechanical

drawing to define the design intent based on the function of the part or assembly. It

describes the size, form, orientation, and location of a feature in relation to critical features

defined as datums. It establishes the requirements for the manufacturing process andinspection, at minimum cost. The old system of dimensioning and tolerancing is too

ambiguous with respect to design intent and is too costly.

Today, data is extracted from the drawing dataset and fed directly into the manufacturing

and inspection process without the need for viewing the drawing. The drawing is used toestablish the documentation that defines the dataset. This dataset includes the GD&T

symbolic language for the design intent. Figure 1 is an example.

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Figure 2 is a drawing showing all the parts in their perfect shape. However in the real world, the

individual parts may not assemble because the detail drawings did not communicate the design intent.

See Figure 3.

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Figure 4 is a drawing of one of the parts using general dimensioning, and Figure 5 is a drawing of the

same part using GD&T dimensioning. The problem with Figure 4 is that manufacturing and inspection

do not know which diameter or surface is critical for part function or interchangeability. The drawingcan be a legal document that may end up in litigation as to what the customer wanted verses what the

supplier interpreted.

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Why Use GD&T?

Note that every major corporation in the world uses this system.

Improves Communication

1. It allows engineering to more accurately define the design intent.

2. Drawings are easier to read.

3. It minimizes the need for ambiguous drawing notes. Notes can be misinterpreted because the writeror reader may have different technical backgrounds. GD&T symbology is a consistent standard.

4. GD&T is understood internationally.

5. With a basic GD&T education, the GD&T symbolic language is understood by engineering,

manufacturing, quality control, inspection, buyers, and programmers.

6. It protects suppliers from customers being over critical and asking for something that is not

specified on the drawing.

7. The supplier can give a more accurate bid because GD&T is very definitive.

8. It gives the company legal protection from drawing misinterpretation.

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Improves Company Yields

1. GD&T increases productivity because:

(a) Communication is improved;

(b) The tolerance zone becomes larger when manufactured at a size other than Maximum Material

Condition.

(c) Functional gauges can be used.

2. GD&T minimizes the rejection, rework, and scrappage of parts.

3. GD&T is more compatible with CAD/CAM/CIM.

4. GD&T helps improve schedules. Better communications leads to more accurate schedules.

5. Because of the bonus tolerance, manufacturing has the ability to produce the parts at a lower cost.With the bonus tolerance, the part may be able to be manufactured by a less costly method, such as

using a punch machine verses machining.

6. GD&T guarantees interchangeability, the old system does not.

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Improves Competitiveness

1. GD&T meets the requirements of ISO 9000.

2. Suppliers are able to lower their bids. I had a supplier say to me at the end of my GD&T seminar,

that because of the bonus tolerance he would now be able to win bids from competitors that didnt

understand GD&T.

3. GD&T lowers product cost.

* A drawing must be read, not interpreted, and the precision and unambiguity

of GD&T allows you to read the drawing *