What is Geometric Dimension Ing

8/3/2019 What is Geometric Dimension Ing

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What Is Geometric Dimensioning & Tolerancing?

( GD&T )

Request Quote and Contact Form|Web-Based Fundamentals of GD&T Training

Geometric Dimensioning and Tolerancing (GD&T) is a language used on mechanicalengineering drawings composed of symbols that are used to efficiently and accurately

communicate geometry requirements for associated features on components and assemblies.

GD&T is, and has been, successfully used for many years in the automotive, aerospace,

electronic and the commercial design and manufacturing industries.In todays modern andtechnically advanced design, engineering and manufacturing world, effective and accurate

communication is required to ensure successful end products. Currently, ASME Y14.52009 is

the accepted geometric dimensioning and tolerancing standard superseding ANSI Y14.5M-1994

used within the USA and ISO 1101-2004 is used outside of the the USA.Success orientedindustries and organizations which, require accurate and common lines of communications

between engineering, design, manufacturing and quality should consider geometric dimensioningand tolerancing (GD&T) as their mechanical drawing standard. Some advantages of GD&T(geometric dimensioning and tolerancing) are;

1. Provides a clear and concise technique for defining a reference coordinate system(datum's) on a component or assembly to be used throughout the manufacturing and

inspection processes.

2. Proper application of geometric dimensioning closely dovetails accepted and logicalmechanical design process and design for manufacturing considerations.

3. Geometric dimensioning dramatically reduces the need for drawing notes to describecomplex geometry requirements on a component or assembly by the use of standard

symbology that accurately and quickly defines design, manufacturing and inspectionrequirements.

4. GD&T concepts such as MMC (maximum material condition) when applied properlywill facilitate and simplify the design of cost saving functional check gages,manufacturing fixtures and jigs.

The following ar common reasons one should apply GD&T principles:

1. Part features are critical to function or interchangeability.2. When functional gauging techniques are desired.3. When a common reference (origin) or datum is required to ensure communication is

consistent between design, manufacturing and inspection.

4. When a standard interpretation or tolerance is not already implied.5. Simplify tolerance analysis.6. Replace complex or long geometry requirement description notes with a single geometric

symbol.

Engineers Edge offers practical industry sensitive geometric dimensioning and tolerancing

(GD&T) training on-site and public seminars.
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Geometric Dimensioning and Tolerancing: Why ?

Geometric Dimensioning and Tolerancing (GDT) is a method for precisely defining the

geometry of mechanical parts. It introduces tools which allow mechanical designers, fabricators,and inspectors to effectively communicate complex geometrical descriptions which are not

otherwise able to be described in a defined language.

Figure 1.1

A good example of why GDT is needed is the automobile stamped chassis shown in Figure 1.1.

The rear quarter panel must fit snugly in order to allow spot welds and in this competitivebusiness, cosmetic appearance and noise abatement are critical. Without GDT, geometric

description of this assembly is difficult if not impossible. Computer modelling of these complex

surfaces continues to increase the complexity of interface shapes.

Illustrated in Figure 1.2 is an imprecise sand-cast sewer termination fitting. Its hole pattern must

mate with the corresponding pattern of the equally imprecise mating pipe. Flatness is also an

issue with these rough-surfaced parts. Inspectors must be able to pass as many parts as possiblewithout sacrificing fit. Tens of thousands of the parts are produced. GDT allows fabricators,

inspectors, and assemblers to match covers with equally imprecise pipes.

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dt/introduction.cfm%26hl%3Den%26client%3Dca-pub-4893968732705577%26adU%3Dwww.pcwi.com.au%26adT%3DPCWI%2BAustralia%26gl%3DMY&usg=AFQjCNGpNkRnrj0kv1eMq0ozEWt6GW9fKg


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Figure 1.2

Geometric Dimensioning and Tolerancing is a vast language of which there are many facets.However, what is commonly used is a small subset of the total. This subset is based on concepts

which MUST be learned in order to progress further. Without a solid understanding of these

fundamentals, one cannot gain a firm grasp of later topics. We will present the most essential(and often misinterpreted) topics in a step-by-step fashion, starting with a simple two-

dimensional case. After the 2D case has been understood, the full three-dimensional geometrywill be described. We also include common areas of confusion and a reference section, but at this

point the primary objective is to explain the fundamentals. Please select "2D DATUMS" from

the menu bar to the left to continue.

History of Geometric Dimensioning and Tolerancing

Geometric Dimensioning and Tolerancing symbols have been in use since at least the turn of thecentury. GDT was especially important during the Second World War in relation to extremely

high volume production of Liberty Ships, aircraft, and ground vehicles. The automotive industry,with its high volumes, has also benefited from GDT. The computer industry, in particular massstorage manufacturers, have used GDT extensively to increase their yields of high-volume and

low-margin hard disk drives. However, as with most engineering and scientific methodologies,

GDT was not rigorously established and documented until later in the twentieth century. The

American National Standards Institute publication in 1982 of ANSI Y14.5M-1982 was a turningpoint in the rigorous, unambiguous standardization of the methodology.

2D Datum ExamplesIt has

been found that the most powerful (yet often misunderstood) use for GDT is in the area of sizing

and positioning of holes which must mate with shafts. We will start with a two-dimensionalgeometry and progress to three dimensions.

Figure 2.1Pictured in Figure 2.1 is a two-dimensional part. The roughness

of its edges has been greatly exaggerated in order to clarify the

discussion. It might be thought of as a part with plane

dimensions large compared with its dimension into the page.
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For example, this might be a sheet metal part which is .125 inches thick with a length and width

of about ten inches.In this example we will determine the position of the hole and in the process, find out why

mutually-perpendicular ordered datums must be defined in order to define the position. Figure

2.1 shows the part with possible ways to define the location of the hole. As has been made

apparent, the linear dimensions originate from the sides of the part, but it is not clear from whereon the sides the dimensions should begin. (We consider where the center of the hole is later.) The

method of fixturing the part for measurement of its features is critical since many different

groups of people will be measuring the part: design, fabrication, fabrication inspection,purchasing inspection, etc. Without a common agreement as to how the part will be measured,

measurements become meaningless.

Figure 2.2One way to define where the dimensions should originate is shown in

Figure 2.2. A steel straight edge can be used to define a line for the two

edges. One problem with this method is that the two defined lines are not

necessarily perpendicular, as shown in the figure. Withoutperpendicularity, the part dimensions do not agree with print dimensions,

since print dimensions are assumed to be perpendicular.

We can force the two defined lines to be perpendicular to each other by using a right-angle

straight edge, as shown in Figure 2.3. Now when the part is pushed against these two edges sothat it cannot move (rock), the two edges can be used as mutually perpendicular datums.

However, as Figure 2.4 shows, the part position with respect to these two datums is ambiguous.

The final orientation of the part depends upon which side contacts first.

Figures 2.3 and 2.4

Figure 2.5 illustrates an ordered datum scheme that prevents confusion. The first side that is

pressed against one of the edges will contact at the two highest points of the part edge. The partnow only has one degree of freedom left: it can slide back and forth against the straight edge.

Figure 2.5

Once we butt the perpendicular side of the part against the

corresponding straight edge, we will have completely

constrained the position and orientation of the part in 2D
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space, as shown in Figure 2.5. This second side contacts its straight edge at one high point since

it is not able to rotate to contact more than one high point. Figure 2.5 labels the datums as A and

B, in the order of fixturing hierarchy. These datums are referred to as "functional datums" since

they contact the part and are physical hardware used, for example, on a factory floor.

With this ordering of datums, we have clearly positioned the part in 2D space. As we have seen,

this clear fixturing of the part is critical since it defines from where features are to be

dimensioned on the part. Please continue by selecting "2D Hole/Shaft" from the menu on the

left.

SIDE NOTE: Since this is a two-dimensional example, the part has three degrees of freedom:

two translational and one rotational. The reason for the first two high points of contact has to do

with the rotational degree of freedom of the part: one point allows the part to rotate, two does

not. The single point of contact for -B- is due to the fact that once the part can only slide along -

A- without rotating, a single high point will contact first on -B-. The part cannot rock, so asecond contact point does not occur.

2D Hole Positioning

In2D Datumswe have rigorously defined how we fixture the part for dimensioning features, we

can now define where the hole is.

Figure 2.6

Figure 2.6 shows two dimensions which show where the hole is with respect to datums A and B.

Plus/minus tolerances are also shown, as is a square tolerance zone within which the center ofthe circle must lie. Although it may seem that we are not using GDT here, without the GDT

datums, the two dimensions shown are ambiguous.

However, the plus/minus tolerance zone is not clearly defined since we do not know what is

meant by the "center of the circle". In addition, a square tolerance zone is typically not what is

needed for defining the position of a hole with respect to a mating shaft as shown below:
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Figure 2.7 illustrates the equivalent GDT tolerancing of the hole position. The tolerance zone

within which the center of the circle must lie is circular rather than rectangular.

Figure 2.7

We will later learn how the size and position of this circular zone is defined based upon the

symbols shown. In GDT, the "center of the circle" is defined as the center of the best-fit circle

determined by the actual hole. Coordinate Measuring Machine (CMM) equipment typically uses

at least three points on an actual hole to define the best-fit circle, as illustrated in Figure 2.7b.Figure 2.7c shows the CMM probe touching the side of the hole for a data point. The CMM

machine of course uses the A and B functional datum planes as its position and orientation

references.

Figure 2.7b Figure 2.7c

A big advantage of GDT is that the circular tolerance zone contains 57% more area than an

equivalent square tolerance zone. The largest deviation from true position occurs on the
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7/16

diagonals of a square, and the circle meets this, while providing 40% more possible deviation

along the vertical and horizontal. Therefore, more parts can be accepted by inspection. With thesquare tolerance zone, parts that can fit are rejected since typically only the vertical and

horizontal location deviations are checked.

The 7.5 and 3.0 dimensions in Figure 2.7 do not have attached tolerances for a reason. They arecalled basic dimensions and represent the exact position of the center of the circular tolerance

zone within which the center of the circle must lie. They can be recognized as basic dimensionsbecause they are box framed. The diameter of the circular tolerance zone comes from the feature

control frame which is below the 2.5 hole diameter dimension. The diameter of the tolerance

zone in the feature control frame is 0.7 inches. The first symbol in the frame designates thetolerance as apositional tolerance. The -A- and -B- are the GDT datums to which the zone

refers. For a two dimensional problem, the importance of -A- and -B- is not apparent, but we will

see in the 3D example how they come into play. For now, let us notice that the tolerance zone

location is located via -A- and -B-. The 0.2 tolerance on the 2.5 hole diameter allows thediameter of the hole to vary from 2.3 to 2.7, but the center of the hole must still lie within the

circular tolerance zone described above. The 0.2 tolerance will be discussed further underbonus tolerancing.

There is another way in which the circular tolerance zone for the hole can be interpreted. Figure

2.8 shows this tolerance zone as being the annular "racetrack" formed by two circles centered at7.5, 3.0, nominally 2.5 in diameter, and .2 above and below 2.5 in diameter.

Figure 2.8

If the actual hole profile is within this "racetrack", it is very similar to its center being within the0.7 diameter circular tolerance zone. This interpretation of hole positional tolerances is common

and probably originated before CMM machines allowed finding the center of the best fit circle.

This interpretation lends itself to inspection with calipers, gauge pins, and the like. However, incases where extreme precision and the utmost certainty of geometry are required, the true GDT

(ANSI Y14.5) definition should be used.

Now that we have observed how GDT can clearly define hole position, let us move on to bonustolerancing. If you would like to skip bonus tolerancing for now and move on to the 3D case of

what we have discussed, see3D Datums.
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2D Shaft Bonus ToleranceGDT

dimensioning of holes (and shafts) provides a powerful method for increasing inspection yield

without trial and error fitting or binning. This method is what is known as bonus tolerancing.

Figure 2.9 illustrates the possible size variation of the hole from the 2D Hole section. It does not,

however, show the variation in position of the hole that is shown in Figure 2.10, for a nominalhole size.

Figure 2.9 Figure 2.10

Combining the two types of variation, we get Figure 2.11, which shows the quite large envelope

which results. If the hole becomes larger, why not widen the tolerance on the hole position since

the mating part now has a larger target ? This is what bonus tolerancing provides.Ads by Google

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Figure 2.11

Figure 2.12 is the same hole that we have been discussing, but with an added symbol in thefeature control frame. The M with a circle around it stands for Maximum Material Condition

(MMC). For a hole, this is the smallest possible size, and for a shaft, it is the largest. In other

words, for mating holes and shafts, MMC is the tolerance condition where fit is most difficult
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9/16

Figure 2.12

If the hole increases in size from MMC, leeway in the position of the hole becomes available.

This addition to the positional tolerance zone diameter is defined as the amount that the holediameter increases over MMC.

If no MMC symbol is present, the tolerance is assumed to apply at Regardless of Feature Size(RFS) and there is no bonus tolerance.

For a continuation of this discussion concerning virtual condition, please see2D Bon TolExample.

2D Hole/Shaft Bonus Tolerance ExampleGDT

dimensioning of holes (and shafts) provides a powerful method for increasing inspection yield

without trial and error fitting or binning. This method is what is known as bonus tolerancing.

In the following series of figures, the captions suffice to explain the logic. What is being shown

is how increasing the size of a hole above MMC allows a bonus in the size of the positional

tolerance zone.

Figure 2.13

Figure 2.14
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10/16

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Figure 2.15

Figure 2.16
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Figure 2.17

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