Fundamentals of GD&T This Courseware Belongs to …………………………………………
Fundamentals of GD&T
Introduction Manufacturing Process began with the industrial revolution in 1800s. The drawings used were very different from the ones which are used these days. A typical drawing of those days was a neatly inked, multi viewed artistic master piece that portrayed the part with almost pictorial precision. Dimensions were generally considered unnecessary because the manufacturing process was different. Manufacturing was a cottage industry employing artisans who did it all from parts fabrication to final assembly. These craftsmen passed their hard won skills down from generation to generation. There were no assembly lines, no widely dispersed departments or corporate units scattered across the nation. To them there was no such thing as variation. Nothing less than perfection was good enough. When misfits and assembly problems occurred, the craftsmen would simply “CUT – AND – TRY” “FILE – AND – IT” until the assembly worked perfectly. Manufacturing was a quality process, at the same time slow, laborious and as a result very expensive. The advent of the assembly line and other improved technologies revolutionized manufacturing. In today’s industrial revolution, the designer often starts by creating an ideal assembly, where all parts fit together with optimal tightness and clearances. He will have to convey to each part’s manufacturer that ideal sizes and shapes, or nominal dimensions of all the part’s surfaces. If multiple copies of the part are to be manufactured, the designer must understand that it is impossible to make them all identical. Every manufacturing process has unavoidable variations that impart corresponding variations to the manufactured parts. The designer must analyze his entire assembly and make a decision of how much variation can be allowed in size, form, orientation and location. Then in addition to the ideal part geometry, he must communicate to the manufacturer the calculated magnitude of variation (Tolerance) each feature can have and still contribute to the efficient functionality of the part in the assembly. For all this needed communication, words are usually inadequate. Throughout the twentieth century, a specialized language based on graphical representations and math has evolved to improve communication. In its current form, the language is recognized throughout the world as Geometric Dimensioning and Tolerancing which in this text is referred as GD&T. Thus GD&T is an international language that is used on engineering drawings to accurately discrete a part. The GD&T language consist of a well defined set of symbols, rules, definitions and conventions. GD&T is a precise mathematical language that can be 1
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Fundamentals of GD&T used to describe the size, form, orientation and location of a part feature. It is also a design philosophy on how to design and dimension a part.
GD&T Standards GD&T standards are based on ASME Y14.5M-1994. This standard is considered as the national standard for dimensioning and tolerancing in the United States. ISO (International Standards Organization) and ANSI (American National Standard Institute) also have published geometric dimensioning and tolerancing standards which is 90% similar to ASME Y14.5M 1994 standard. This text is based on ASME Y14.5M-1994 and hence forth will be referred to as Y14.5M.
Dimension Units The millimetre is the common unit of measurement used on engineering drawings made to the metric system. The conventions used when specifying dimensions in metric units are discussed below. 1. When a metric dimension is a whole number the decimal point and zero are omitted. (eg.)
“12”
2. When a metric dimension is less than one millimeter, a zero precedes the decimal point. (eg.)
“0.3”
3. When a metric dimension is greater than a whole number by a fraction of a millimeter, the last digit to the right of the decimal point is not followed by a zero. (eg.)
”13.2”
4. When using unilateral tolerances, a single ‘0’is used without a ‘+’ or ‘-‘sign for the zero part of the value.
(eg.) 5. Angular Dimensions are established in degrees and decimal degrees, or in degrees, minutes and seconds. (eg.) 2
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In Industry a general note would be given on the engineering drawing to involve the metric system. (eg.) “UNLESS OTHERWISE SPECIFIED, ALL DIMENSIONS ARE IN MILLIMETERS�
Fundamental Dimensioning Rules 1. Each dimension should have a tolerance, except for those dimension specifically identified as reference, maximum, minimum or stock. The tolerance may be applied directly to the dimension, indicated by a general note or located in the title block of the drawing. 2. Dimensioning and tolerancing must be complete to the extent that there is full understanding of the characteristics of each feature. Neither measuring the drawing or assumption of a dimension is permitted. 3. Each necessary dimension of an end product must be shown. Only dimensions needed for complete definition should be given. Reference dimension should be kept to a minimum. 4. Dimensions should be selected and arranged to suit the function and mating relationship of a part. Dimensions must not be subject to more than one interpretation. 5. The drawing should define the part without specifying the manufacturing process. 6. It is allowed to identify certain processing dimension that provide for finish allowance, shrink allowance, and other requirements, provided the final dimensions are given on the drawing. 7. Dimensions should be arranged to provide required information arranged for optimum readability. Dimensions should be shown in true profile views and should refer to visible outlines. 8. A 900 angle is implied where center lines and displaying features are shown on a drawing at right angles and no angle is specified. The tolerance for these 90 0 angles is same as the general angular tolerance specified in the title block or in a general notes. 3
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Fundamentals of GD&T 9. A 900 basic angle applies where center lines of features are located by basic dimensions and no angle specified. 10. Unless otherwise specified, all dimensions are measured at 20 0C (680F). Compensation may be made for measurements taken at other temperature. 11. All dimensions and tolerance apply in a free state condition except for non rigid parts. Free State condition describes distortion of the part after removal of forces applied, during manufacturing. Non rigid parts are those that may have dimensional change due to thin wall characteristics. 12. Unless otherwise specified, all geometric tolerances apply for full depth, length and width of the feature. 13. Dimensions and tolerances apply only at the drawing level where they are specified. A dimension specified on a detail drawing is not mandatory for that feature on the assembly drawing.
Dimensional Tolerance Dimensional Tolerance is used to define the acceptable variation in the size of the part. This tolerance may be expressed as direct limits or as tolerance values applied directly to a dimension.
Geometric Tolerance Geometric Tolerance is used to define the acceptable variation in the shape of the part.
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Co-ordinate Tolerancing System Co-ordinate tolerancing is a dimensioning system where a part feature is located (or defined) by means of rectangular dimensions with given tolerances.
Figure 1
Co-ordinate tolerancing simply does not have the completeness to precisely communicate the part requirements. Co-ordinate tolerancing consist three major short comings.   
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Square or rectangular tolerance zone Fixed size tolerance zone Ambiguous instructions for inspection.
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Square or Rectangular Tolerance Zone In the above figure the hole location tolerance zone is formed by the maximum and minimum of the vertical and horizontal location dimensions. Due to this a 0.5 square tolerance would be formed. The illogical aspect of a square tolerance zone is that the hole can be off its nominal location in the diagonal directions a greater distance than in the vertical and horizontal directions. A more logical and functional approach is to allow the same tolerance for a hole location in all directions, creating a cylindrical tolerance zone.
Figure 2
Fixed Size Tolerance Zone The print specification requires the centre of the hole to be within a 0.5 square tolerance zone whether the hole is at its smallest size limit or its largest size limit. When the important function of the hole is assembly, the location of the hole is most critical when the hole is at its minimum limit of size. If the actual size of the hole is larger than its minimum size limit, its location tolerance can be correspondingly larger without affecting the part function.
Ambiguous Instructions for Inspections Square and fixed tolerance zones can cause functional parts to be scrapped. Since coordinate tolerancing does not allow for cylindrical tolerance zones or tolerance zones that increase with the hole size. Figure 3 shows two logical methods an inspector could use to set up the part from figure 1 for inspecting the holes .The inspector could rest the part on the face first, long side second and short side third, or the inspector could rest the part on the face first, the short side second and long side third. Because there are three different ways to hold the part for 6
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Fundamentals of GD&T inspection, two inspectors could get different measurements from the same part. This can result in two problems: good parts may be rejected, or worse yet, bad parts could be accepted as good parts. The problem is that drawing does not communicate to the inspector which surfaces should touch the gauging equipment first, second and third.
Figure 3
Geometric Dimensioning and Tolerancing System Geometric Dimensioning and Tolerancing (per ASME Y14.5M-1994) is an international language that is used on engineering drawings to accurately describe the size, form, orientation, and location of part features. It is also a design-dimensioning philosophy that encourages designers to define a part based on how it functions in the final product or assembly. Since GD&T is expressed using line drawings, symbols, and Arabic numerals, people everywhere can read, write, and understand it regardless of their native tongues.
Figure 4
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Fundamentals of GD&T GD&T is an exact language that enables design engineers to "say what they mean" on a drawing, thus improving product designs and lowering cost. Process engineers and manufacturing use the language to interpret the design intent and to determine the best manufacturing approach. Quality control and inspection use the GD&T language to determine proper set-up and part verification.
Figure 5
Understanding how to apply and interpret GD&T correctly will help you:
Create clear, concise drawings Improve product design Create drawings that reduce controversy, guesswork, and assumptions throughout the manufacturing process Effectively communicate or interpret design requirements for suppliers and manufacturing
Comparison between Co ordinate Tolerance and Geometrical Tolerance The difference between coordinate tolerancing and geometric tolerancing are summarized below.When comparing these tolerancing methods, it is easy to understand why geometric tolerancing is replacing coordinate tolerancing. • • • • •
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Co ordinate Tolerance Square or Rectangular Tolerance Zone Higher Manufacturing Cost Fixed Tolerance Zone Multiple Inspection Results Defect parts accepted
• • • • •
Geometrical Tolerance Diameter Tolerance Zone Lesser Manufacturing Cost Flexible Tolerance Zone Clear Inspection instructions Eliminates Disputes
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Fundamentals of GD&T
GD&T BENEFITS 1. Improves Communication GD&T can provide uniformity in drawing specifications and interpretation, thereby reducing controversy, guesswork and assumptions. Design, production and inspection all work in same language. 2. Provides Better Product Design The use of GD&T can improve your product designs by providing designers with the tools to “say what they mean” and by following the functional dimensioning philosophy. 3. Increases Production Tolerances There are two ways tolerances are increased by the use of GD&T. First, under certain conditions GD&T provides”bonus” – or extra - tolerance for manufacturing. This additional tolerance can make significant saving in the additional costs. Second, by the use functional dimensioning, the tolerances are assigned to the part based upon its functional requirements. This often results in a larger tolerance for manufacturing. It eliminates the problems that result when designers copy existing tolerances, or assign tight tolerances, because they don’t know how to determine a reasonable (functional) tolerance.
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Definitions Features and Features of Size A feature is a general term applied to a physical portion of a part, such as a surface, hole or slot. An easy way to remember this term is to think of a feature as a part surface. The part in figure 6 contains seven features: the top and bottom, the left and right sides, the front and back, and the whole surface.
Figure 6
A feature of size (FOS) is one cylindrical or spherical surface, or a set of two opposed elements, or opposed parallel surfaces, associated with a size dimension. A key part of FOS definition is that the surface or elements must be opposed. An axis, median plane or centre point can be derived from a feature of size. Figure 7 shows several examples of features of size.
Figure 7
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Internal and External Feature of Size There are two types of features of size – external and internal. External features of size comprised of part surfaces (or elements) that are external surfaces, like a shaft diameter or the overall width or height of a planar part. An internal FOS is comprised of part surfaces (or elements) that are internal part surfaces, such as a hole diameter or the width of a slot. A feature of size dimension is a dimension that is associated with a feature of size. A nonfeature of size dimension is a dimension that is not associated with a feature of size. Actual local size is the value of any individual distance at any cross section of a FOS. The actual local size is checked at a point along the cross section of the part. A part FOS may have several different values of actual local size. The actual mating envelope (AME) of an external feature of size is a similar perfect feature counterpart of the smallest size that can be circumscribed about the feature so it just contacts the surfaces at the highest points. The actual mating envelope (AME) of an internal feature of size is a similar perfect feature counterpart of the largest size that can be inscribed within the feature so that it just contacts the surfaces at their highest points. Modifiers communicate the additional information about the drawing or tolerancing of a part. The table below shows the list of modifiers commonly used.
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Fundamentals of GD&T Maximum Material Condition (MMC): Maximum material condition is the condition in which a feature of size contains the maximum amount of material everywhere within the stated limits of size- for example, the largest shaft diameter or smallest hole diameter. Least Material Condition (LMC): Least Material Condition is the condition in which a feature of size contains the least amount of material everywhere within the stated limits of size- for example, the smallest shaft diameter or the largest hole diameter. Regardless of feature size is the term that indicates a geometric tolerance applies at any increment of size of the feature within its size tolerance. An-other way to visualize RFS is that the geometric tolerance applies at whatever size the part is produced. Every feature of size has a maximum and least material condition. . Limit directions directly specify the maximum and least material condition of feature of size. The projected tolerance zone modifier changes the location of the tolerance zone on the part. It projects the tolerance so that it exists above the part. The tangent plane modifier denotes that only the tangent plane of the toleranced surfaces needs to be within this tolerance zone. The diameter symbol is used two ways: inside a feature control frame as a modifier to denote the shape of the tolerance zone, or outside the feature control frame to simply replace the word “diameter”. The modifier for reference is simply the method of denoting that information is for reference only. The information is not to be used for manufacturing or inspection. To designate a dimension or other information as reference, the reference information is enclosed in parentheses. A radius is a straight line extending from the center of an arc or a circle to its surface. The symbol for a radius is ‘R’. When the “R” symbol is used on the drawing, it creates a zone defined by two arcs (the minimum and maximum radii). The part surface must lie within this zone. Figure 8 shows a radius tolerance zone. The part surface may have flats or reversals within the tolerance zone.
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Figure 8
A controlled radius is a radius with no flats or reversals allowed. The symbol for a controlled radius is “CR”. When “CR” symbol is used it creates a tolerance zone defined by two arcs (the minimum and maximum radii). The part surfaces must be within the crescent – shaped tolerance zone and be an arc without flats or reversals. Figure 9 shows a controlled radius tolerance zone.
Figure 9
A Feature Control Frameis a rectangular box that is dived into compartments within which geometric characteristic symbol, tolerance value, modifiers, and datum references are placed. The compartments of a feature control are shown in figure 10.
Figure 10
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Fundamentals of GD&T The first compartment of the feature control frame is called the geometric characteristic portion. It contains one of the fourteen characteristic symbols. The second compartment of the feature control frame is referred to as the tolerance portion. The tolerance portion of a feature control frame may contain several bits of a information. For example, if the tolerance value is preceded by a diameter symbol, the shape of the tolerance zone is a cylinder. If a diameter symbol is not shown in front of the tolerance value, the shape of the tolerance zone is parallel planes, parallel lines or a uniform boundary in the case of profile. The tolerance value specified is always a total value. When specifying a non datum related control, the feature control frame will have two compartments. When specifying a datum related control, the feature control frame may have up to five compartments: the first for a geometric characteristic symbol, one for tolerance information, and up to three additional compartments for datum references. The third, fourth and fifth compartments of the feature control frame are referred to as the datum reference portion of the feature control frame.
Rule # 1 (Individual Feature of Size or Envelope Rule) Where only a tolerance of size is specified, the limits of size of an individual feature prescribe the extent to which variations in its form as well as in its size are allowed. In industry this rule is often termed as “Perfect form at MMC”. In Rule #1, the word “perfect form” means perfect flatness, straightness, circularity and cylindricity. If a FOS is produced at MMC, it is required to have perfect form. If a FOS is not at MMC, then a form error is allowed. The is explained in figure 11.
Figure 11
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Fundamentals of GD&T Rule #1 applies whenever a FOS is specified on a drawing. There are 2 ways Rule #1 can be overridden:
If a straightness control is applied to a feature of size, Rule #1 is overridden If a note such as “Perfect form at MMC not required” is specified next to a FOS dimension, its exempts the FOS dimension from Rule#1
Limitation of Rule #1 A part often contains multiple FOS. Rule #1 does not affect the location, orientation or relationship between features of size. FOS shown perpendicular, symmetrical, or coaxial must be controlled for location or orientation to avoid incomplete drawing specifications. There are 2 exceptions to Rule #1:
Rule #1 does not apply to flexible parts that are not restrained. Rule #1 does not apply to stock sizes such as bar stock, tubing, sheet metal or structural shapes.
Rule # 2 (All Applicable Geometric Tolerances Rule) RFS applies, with respect to the individual tolerance, datum reference, or both, where no modifying symbol is specified. Modifier must be specified on the drawing where required. Modifiers cannot be used to certain geometric tolerance always. Where a geometric tolerance is applied on an RFS basis, the tolerance is limited to the specified value regardless of the actual size of the feature.
Rule # 2a (Alternate practice of Rule #2) For a tolerance of Position, RFS may be specified in Feature Control Frames if desired and applicable. In this case, the RFS symbol would be the symbol from the 1982 version of Y14.5. Figure 12 shows example of Rule #2 and Rule #2a
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Figure 12
Basic Dimension: A Basic Dimension is a numeric value used to describe the theoretically size, true profile, orientation, or location of a feature or gage information (i.e., datum targets). In simple terms a basic dimension locates a geometric tolerance zone or defines gage information (example: datum targets). When basic dimension are used to describe part features, they must be accompanied by a geometric tolerance to specify how much tolerance the part feature may have. A good way to look at this is that the basic dimension specifies only the half the requirement. To complete the specification, a geometric tolerance must be added to the feature involved with the basic dimension. Title block tolerances do not apply to Basic dimension. Basic dimensions must get their tolerances from a geometric tolerance or from a special note. Virtual condition (VC) is a worst-case boundary generated collective effects of a feature of a size at MMC or at LMC and the geometric tolerance for that material condition. The VC of a FOS includes effects of the size, orientation, and location for the FOS. The Virtual condition boundary is related to the datums that are referred in the geometric tolerance used to determine the VC. Inner Boundary (IB) is a worst-case boundary generated by the smallest feature of size minus the stated geometric tolerance (and any additional tolerance if applicable). Outer boundary (OB) is a worst-case boundary generated by the largest feature of size plus the stated geometric tolerance (and any additional tolerance if applicable). Worst-Case boundary (WCB) is a general term to refer to the extreme boundary of a FOS that is the worst case for assembly. Depending upon the part dimensioning, worst-case boundary can be a virtual condition, inner boundary or outer boundary. Bonus Tolerance is an additional tolerance for a geometric control. Whenever a geometric tolerance is applied to a FOS, and it contains an LMC (or MMC) modifier in the tolerance of the feature control frame, a bonus tolerance is permissible.
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