Geometric Dimensioning and Tolerancing (GD&T) is a symbolic language used on mechanical drawings to communicate design requirements. Engineering, manufacturing, and inspection personnel must all be fluent in this system to make it effective. But it's not as simple as just learning the symbols. GD&T can also be thought of as a design philosophy that integrates the function of the part into the tolerances.

Below is a sample GD&T drawing:

The language of geometric dimensioning and tolerancing can be intimidating at first, but when used properly, it can yield many advantages:

Standardized, international system

More flexibility, particularly for complex shapes Allows cylindrical tolerance zones Datums are clearly defined Based on the fit and function of a part or assembly Eliminates the need for many notes Allows for more tolerance

This last benefit of GD&T is surprising to many people; they think that using GD&T will mean tighter tolerances. This concept is one of the first items covered in our GD&T Fundamentals class, using a simple, concrete example.

The following is a list of definitions for the most common terms used in GD&T. A more extensive glossary is distributed to each participant in our seminars.

Angularity — The geometric tolerance used to control a surface, axis, or center plane that is designed to be at a specified angle from a given datum.

ASME Y14.5M-1994 — Still the predominant standard for dimensioning and tolerancing in North America. It is published by the American Society of Mechanical Engineers, and is available in print book or PDF format.

ASME Y14.5-2009 — The new standard for dimensioning and tolerancing in North America, released in March of 2009. It features many changes, although most are minor changes in definitions or modifiers to replace previous notes. It is also available in print book or PDF format.

Bonus tolerance — An additional tolerance beyond what is given in the feature control frame. This is allowed if the tolerance number is followed by the MMC (or LMC) modifier. The amount of bonus tolerance is determined from the difference between a feature’s actual size and the MMC (or LMC).

Circularity — A geometric tolerance used to control all points on a surface, at any perpendicular cross-section, to be equidistant from the axis. Sometimes referred to as roundness.

Concentricity — A geometric tolerance used to control the median points of all diametrically opposed elements of a circular feature, in order to be congruent with a datum axis. It is commonly misused; usually runout or position can suffice.

Cylindricity — A geometric tolerance used to control a surface of revolution (inside or outside diameter) where all points are to be equidistant from a common axis. The cylindricity tolerance will control circularity, straightness, and taper.

Datum — A theoretically exact point, axis, or plane derived by contact a datum feature; it then becomes the origin from which geometric characteristics are measured.

Datum target — A specific point, line, or area on a datum feature that is used to establish a datum (rather than using the full feature to establish the datum).

Feature control frame — The rectangular box that is used to display a geometric tolerance. It is divided into compartments

which identify the geometric characteristic symbol, the tolerance value, and any datum references.

Flatness — A geometric tolerance used to control a surface which is designed to have all elements in the same plane. Flatness allows variation within two imaginary planes, separated by the given tolerance amount. With the 2009 standard, it may also be used on a centerplane of a feature of size.

Geometric dimensioning and tolerancing — Affectionately called GD&T, it is a symbolic system of controlling dimensional variation. In addition to providing standardized symbols and rules, it is a design philosophy that ensures that tolerances are based on fit and function.

Least material condition — A number that represents the condition where a feature of size has the least amount of material, while still being within the given size limits. Thus, LMC would be the smallest pin (for an external feature) or the largest hole (for an internal feature).

Maximum material condition — Often abbreviated as MMC, the size of a feature of size when it contains the most material allowed (while still being within the acceptable size limits).

Parallelism — The geometric tolerance used to control a surface, axis, or center plane that is designed to be parallel (zero degrees) in relation to a datum.

Perpendicularity — The geometric tolerance used to control a surface, axis, or center plane that is designed to be 90º to a datum.

Position — A geometric tolerance that controls the location of a feature of size. It defines a zone around the perfect location within which the axis or center plane of the feature is permitted to vary.

Profile of a line — A geometric tolerance used to control line elements (taken at any cross-section in the plane of the given view) as they relate to the true profile. It may or may not reference datums.

Profile of a surface — A geometric tolerance used to control a three-dimensional surface as compared to the true profile. It may or may not reference datums.

Projected tolerance zone — A concept sometimes used in GD&T to indicate that a tolerance zone does not exist within the part itself, but is projected above the part surface. This is usually done to ensure that a fastener will assemble through multiple parts without interference.

Regardless of feature size — Abbreviated RFS, it is a term used to indicate that a geometric tolerance is the same, no matter what size the feature is made at. It is the presumed condition, unless the symbol for MMC or LMC is shown.

Resultant condition — A number, similar to virtual condition, that represents the worst-case combination of size and

geometric tolerance. However, instead of representing the mating size, this shows how much material may be affected.

Rule #1 — The rule defined in the ASME standard stating that a size dimension, for a feature of size on a rigid part, not only controls the size, but also the form.

Rule #2 —The rule in the ASME standard stating that all geometric tolerances are assumed to be RFS unless the modifier for MMC or LMC is shown. This applies to the tolerance value and any datum references that are features of size.

Runout — The category of geometric tolerancing that controls a surface of revolution as compared to a datum axis other than its own. It can be used to control circularity, “wobble,” taper, and concentricity. The two types of runout are circular runout and total runout.

Shift tolerance — Also called datum shift, this refers to the looseness or “play” that is sometimes allowed on a datum feature. Similar to bonus tolerance, it is the result of using a modifier (usually MMC) after the datum reference in a feature control frame.

Straightness — The geometric tolerance applied to an axis or surface element which is designed to be perfectly straight. It may be applied to a surface (individual elements) or a feature of size (axis or centerplane).

Symmetry — The geometric tolerance used to control the median points of all opposed elements of two or more feature surfaces to be congruent with a datum axis or plane. It is commonly misused; usually position will suffice.

Virtual condition — A number that represents the worst-case mating size for an individual feature. It represents the combined effect of a feature’s size tolerance and the geometric tolerance.

Counterbored Holes Dimensions (metric)

METRIC SOCKET-HEAD CAP SCREWS

SCREW DIA

COUNTERBORE DIA

COUNTERBORE DEPTH

COUNTERSINK DIA

CLEARANCE DIA

(NORMAL FIT)

CLEARANCE DIA

(CLOSE FIT)

M1,6 3,50mm 1,6mm 2,0mm 1,95mm 1,80mm

M2 4,40mm 2mm 2,6mm 2,40mm 2,20mm

M2,5 5,40mm 2,5mm 3,1mm 3,00mm 2,70mm

M3 6,50mm 3mm 3,6mm 3,70mm 3,40mm

M4 8,25mm 4mm 4,7mm 4,80mm 4,40mm

M5 9,75mm 5mm 5,7mm 5,80mm 5,40mm

M6 11,20mm 6mm 6,8mm 6,80mm 6,40mm

M8 14,50mm 8mm 9,2mm 8,80mm 8,40mm

M10 17,50mm 10mm 11,2mm 10,80mm 10,50mm

M12 19,50mm 12mm 14,2mm 13,00mm 12,50mm

M14 22,50mm 14mm 16,2mm 15,00mm 14,50mm

M16 25,50mm 16mm 18,2mm 17,00mm 16,50mm

M20 31,50mm 20mm 22,4mm 21,00mm 20,50mm

M24 37,50mm 24mm 26,4mm 25,00mm 24,50mm

M30 47,50mm 30mm 33,4mm 31,50mm 31,00mm

M36 56,50mm 36mm 39,4mm 37,50mm 37,00mm

M42 66,00mm 42mm 45,6mm 44,00mm 43,00mm

M48 75,00mm 28mm 52,6mm 50,00mm 49,00mm

Counterbored Holes for Socket-Head

Cap Screws (USA)

SCREWDIA

ACOUNTERBORE

DIA

BCOUNTERBORE

DEPTH

CCOUNTERSINK

DIA

DCLEARANCE DIA

NORMAL FIT CLOSE FIT

#0 1/8 .060 .074 #49 #51

#2 3/16 .086 .102 #36 3/32

#4 7/32 .112 .130 #29 1/8

#5 1/4 .125 .145 #23 9/64

#6 9/32 .138 .158 #18 #23

#8 5/16 .164 .188 #10 #15

#10 3/8 .190 .218 #2 #5

1/4 7/16 .250 .278 9/32 17/64

5/16 17/32 .312 .346 11/32 21/64

3/8 5/8 .375 .415 13/32 25/64

7/16 23/32 .438 .483 15/32 29/64

1/2 13/16 .500 .552 17/32 33/64

5/8 1 .625 .689 21/32 41/64

3/4 1-3/16 .750 .828 25/32 49/64

7/8 1-3/8 .875 .963 29/32 57/64

1 1-5/8 1.000 1.100 1-1/32 1-1/64

1-1/4 2 1.250 1.370 1-5/16 1-9/32

1-1/2 2-3/8 1.500 1.640 1-9/16 1-17/32

1-3/4 2-3/4 1.750 1.910 1-13/16 1-25/32

2 3-1/8 2.000 2.180 2-1/16 2-1/32