Surfaces
In theory, every physical feature on a part is a surface. Surfaces can be flat, cylindrical, round, conical – any conceivable shape. The controls in this half of the map are defined by Y14.5 to apply to surfaces, each control delineating a tolerance zone to control (i.e., contain) the points that comprise the surface.
In theory, every physical feature on a part is a surface. Surfaces can be flat, cylindrical, round, conical – any conceivable shape. The controls in this half of the map are defined by Y14.5 to apply to surfaces, each control delineating a tolerance zone to control (i.e., contain) the points that comprise the surface.
- more... Some older GD&T books use the terms “feature” and “feature of size” in place of “surface” and “feature of size.” The map is a venn diagram of all features, with surfaces being represented in the top half and derived features in the bottom. It’s conceptually important to note that there is no physical axis, for example, on a part – there is only the cylindrical surface that gives rise to a theoretical axis, only the physical surface from which a CMM or inspector may derive, calculate, or find.
What is GD&T?
- more... GD&T (Geometric Dimensioning and Tolerancing) is a particular way of communicating dimensions and tolerances for size, location, orientation, and form. In North America, the current standard is ASME Y14.5-2009, which is very similar to ISO, DIN, JIS, and other standards used outside of the US.
Geometric: A way of thinking about and communicating dimensions and tolerances for a product.
Dimensioning: Calculating, recording, and communicating desired values for size, location, orientation, and form.
Tolerancing: Calculating, recording, and communicating acceptable ranges for size, location, orientation, and form.
What’s ASME Y14.5-2009?
- more... American Society of Mechanical Engineers publishes various engineering standards relating to dimensioning and tolerancing. The standard for defining a geometric approach has their rather cryptic index “Y14.5” (don’t ask why, just remember Y14.5) and was last updated in 2009.
Features of Size
Any surface or set of surfaces from which a plane, line, or point may be derived.
Any surface or set of surfaces from which a plane, line, or point may be derived.
- more... Distinctions between different features of size can be made in several ways, including internal vs. external or round vs. square, but an important distinction is the difference between regular and irregular.
Orientation
Category of control symbols that control (in this case) surface orientation.
More...Orientation controls (applied to surfaces) create zones similar to flatness (two parallel planes separated by the linear tolerance value), except that the tolerance zones are required to be at a theoretical angle to a datum reference.
In other words, if you want the side of an object to be perpendicular to the bottom, the bottom is identified as a datum feature using the datum feature identification symbol, and then the side of the object is controlled with respect to the bottom. The part would be set on the bottom, while the side was checked, not vice-versa. Flowchart
Category of control symbols that control (in this case) surface orientation.
More...Orientation controls (applied to surfaces) create zones similar to flatness (two parallel planes separated by the linear tolerance value), except that the tolerance zones are required to be at a theoretical angle to a datum reference.
In other words, if you want the side of an object to be perpendicular to the bottom, the bottom is identified as a datum feature using the datum feature identification symbol, and then the side of the object is controlled with respect to the bottom. The part would be set on the bottom, while the side was checked, not vice-versa. Flowchart
Angularity
Symbol used to control the angular relationship, typically defined by a theoretically exact basic dimensionBasic dimension: A theoretically exact dimension (indicated by a rectangular box around the dimension) used to dimension a true position, true profile, true angle, or gage/fixture dimension. Title block tolerances do not apply to basic dimensions. (i.e., the true angle desired). May be used to control any angle, including perpendicular or parallel relationships.
Example
Things to RememberAbout angularity applied to a surface:
1. It must reference a datum
2. The tolerance is in millimeters (or inches)
3. The specified angle must be a basic dimension
4. It does not affect the size dimensions
5. When applied to a surface, the default interpretation is that it also controls flatness
6. It can be applied to EACH ELEMENT
7. No MMC or LMC modifiers are to be used on the tolerance value (however, MMB or LMB may be used on datum references)
Symbol used to control the angular relationship, typically defined by a theoretically exact basic dimensionBasic dimension: A theoretically exact dimension (indicated by a rectangular box around the dimension) used to dimension a true position, true profile, true angle, or gage/fixture dimension. Title block tolerances do not apply to basic dimensions. (i.e., the true angle desired). May be used to control any angle, including perpendicular or parallel relationships.
Example
Things to RememberAbout angularity applied to a surface:1. It must reference a datum
2. The tolerance is in millimeters (or inches)
3. The specified angle must be a basic dimension
4. It does not affect the size dimensions
5. When applied to a surface, the default interpretation is that it also controls flatness
6. It can be applied to EACH ELEMENT
7. No MMC or LMC modifiers are to be used on the tolerance value (however, MMB or LMB may be used on datum references)
Perpendicularity
Symbol used to control a 90° angular relationship. There is no need to specify the 90° angle as basic, as it is implied by fundamental rule j.fundamental rule j: "A 90° basic angle applies where center lines of features in a pattern or surfaces shown at right angles on a 2D orthographic drawing are located or defined by basic dimensions and no angle is specified."
Example
Things to RememberAbout perpendicularity applied to a surface:
1. It must reference a datum
2. The tolerance is in millimeters (or inches)
3. The 90º angle is understood to be basic, so any general tolerance for angles (title block, for example) do not apply
4. Perpendicularity does not affect the size dimensions
5. When applied to a surface, the default interpretation is that it also controls flatness
6. It can be applied to EACH ELEMENT
7. No MMC or LMC modifiers are to be used on the tolerance value (however, MMB or LMB may be used on datum references)
8. Alternate practice allows the use of the angularity symbol to indicate perpendicularity
Symbol used to control a 90° angular relationship. There is no need to specify the 90° angle as basic, as it is implied by fundamental rule j.fundamental rule j: "A 90° basic angle applies where center lines of features in a pattern or surfaces shown at right angles on a 2D orthographic drawing are located or defined by basic dimensions and no angle is specified."
Example
Things to RememberAbout perpendicularity applied to a surface:
1. It must reference a datum
2. The tolerance is in millimeters (or inches)
3. The 90º angle is understood to be basic, so any general tolerance for angles (title block, for example) do not apply
4. Perpendicularity does not affect the size dimensions
5. When applied to a surface, the default interpretation is that it also controls flatness
6. It can be applied to EACH ELEMENT
7. No MMC or LMC modifiers are to be used on the tolerance value (however, MMB or LMB may be used on datum references)
8. Alternate practice allows the use of the angularity symbol to indicate perpendicularity
Parallelism
Symbol used to control a parallel relationship.
Example
Things to RememberAbout parallelism applied to a surface:
1. It must reference a datum
2. The tolerance is in millimeters (or inches)
3. The tolerance value must be less than any size tolerance that may indirectly be controlling the surface through rule #1
4. When applied to a surface, the default interpretation is that it also controls flatness
5. It can be applied to EACH ELEMENT
6. No MMC or LMC modifiers are to be used on the tolerance value (however, MMB or LMB may be used on datum references)
7. Alternate practice allows the use of the angularity symbol to indicate parallelism
Symbol used to control a parallel relationship.
Example
Things to RememberAbout parallelism applied to a surface:1. It must reference a datum
2. The tolerance is in millimeters (or inches)
3. The tolerance value must be less than any size tolerance that may indirectly be controlling the surface through rule #1
4. When applied to a surface, the default interpretation is that it also controls flatness
5. It can be applied to EACH ELEMENT
6. No MMC or LMC modifiers are to be used on the tolerance value (however, MMB or LMB may be used on datum references)
7. Alternate practice allows the use of the angularity symbol to indicate parallelism
When applied to a surface, each orientation control applies to the entire surface, with one 3-D zone, like flatness (except, of course, it relates the zone to a datum which flatness doesn’t do). There are no orientation symbols that imply that only 2-D zones apply to any given cross-sectional line element, so the words "EACH ELEMENT" can be added to create a 2-D tolerance zone.
Example
Example
When applied to a surface, each orientation control applies to the entire surface, creating a 3-D tolerance zone consisting of two parallel planes, which is the same size and shape tolerance zone as flatness. So, flatnessIf the words “Each Element” are used, then it’s only straightness.
is included by default when any orientation control is applied to any surface. The tangent plane only modifier modifies the tolerance so that it only applies to a plane that is tangent to the surface, in short, to a plane containing the three highest points of the surface.
Sometimes this is necessary to avoid rejecting a part that overall has the proper orientation but may not have good enough form.
Example
Sometimes this is necessary to avoid rejecting a part that overall has the proper orientation but may not have good enough form.
Example
Form
Category of control symbols that control (in this case) surface form.
More...Form controls (applied to surfaces) create zones within which all points of the controlled surface or line section must lay. Flatness and cylindricity create 3-D zones that apply to the entire surface, while straightness and circularity create 2-D zones that apply to any given cross-sectional line element. None of the four form controls ever reference datums.
Category of control symbols that control (in this case) surface form.
More...Form controls (applied to surfaces) create zones within which all points of the controlled surface or line section must lay. Flatness and cylindricity create 3-D zones that apply to the entire surface, while straightness and circularity create 2-D zones that apply to any given cross-sectional line element. None of the four form controls ever reference datums.
Flatness is defined as the condition in which all points comprising a surface are coplanar. The control defines a tolerance zone consisting of two parallel opposed planes, separated by the distance indicated in the feature control frame. It must be applied to one individual surface and not related to a datum.
Example
Flowchart
Things to RememberAbout flatness applied to a surface:
1. It never references a datum; it is intended to control an individual surface.
2. The tolerance is not plus/minus, it’s the total distance between two parallel planes comprising a 3-dimensional zone within which the surface points must fall.
3. The tolerance value must be less than the size tolerance, unless the independency symbol has been applied to the size tolerance.
4. No size-related modifiers (MMC, LMC, the diameter symbol) are allowed.
5. It is different than surface roughness/surface finish: it does not require any machine pattern or differentiate between roughness or waviness. It’s simply a 3-D zone that all surface points must fall within.
6. It may be applied on a per unit basis.
Example
Flowchart
Things to RememberAbout flatness applied to a surface:1. It never references a datum; it is intended to control an individual surface.
2. The tolerance is not plus/minus, it’s the total distance between two parallel planes comprising a 3-dimensional zone within which the surface points must fall.
3. The tolerance value must be less than the size tolerance, unless the independency symbol has been applied to the size tolerance.
4. No size-related modifiers (MMC, LMC, the diameter symbol) are allowed.
5. It is different than surface roughness/surface finish: it does not require any machine pattern or differentiate between roughness or waviness. It’s simply a 3-D zone that all surface points must fall within.
6. It may be applied on a per unit basis.
Straightness is defined as the condition in which all points comprising a straight line segment are colinear. The control defines a tolerance zone consisting of two parallel opposed lines, separated by the distance indicated in the feature control frame. It must be applied to one individual surface and not related to a datum.
Example
Flowchart
Things to RememberAbout straightness applied to surface line elements:
1. It never references a datum; it is intended to control individual line elements
2. The tolerance is not a plus/minus; but the total deviation
3. In a drawing, the reader can tell that it is applied to the surface when it points to the surface
4. The tolerance value must be less than the size tolerance, unless the independency symbol has been applied to the size tolerance
5. No size-related modifiers (MMC, LMC, the diameter symbol) are allowed
6. It can be applied on a unit basis
Example
Flowchart
Things to RememberAbout straightness applied to surface line elements:1. It never references a datum; it is intended to control individual line elements
2. The tolerance is not a plus/minus; but the total deviation
3. In a drawing, the reader can tell that it is applied to the surface when it points to the surface
4. The tolerance value must be less than the size tolerance, unless the independency symbol has been applied to the size tolerance
5. No size-related modifiers (MMC, LMC, the diameter symbol) are allowed
6. It can be applied on a unit basis
Cylindricity is defined as the condition in which all points comprising a surface are cylindrical. The control defines a tolerance zone consisting of two coaxial cylinders, radially separated by the distance indicated in the feature control frame. It must be applied to one individual surface and not related to a datum.
Example
Flowchart
Things to RememberAbout cylindricity:
1. It never references a datum or uses MMC or LMC
2. It must be applied to a cylindrical surface
3. The tolerance is not plus/minus, it is the radial (total) distance between two coaxial cylinders
4. It covers the entire length, so surface straightness and taper are controlled
5. Rule #1 still applies to control the axis straightness
6. The tolerance value must be less than the diameter tolerance.
Example
Flowchart
Things to RememberAbout cylindricity:1. It never references a datum or uses MMC or LMC
2. It must be applied to a cylindrical surface
3. The tolerance is not plus/minus, it is the radial (total) distance between two coaxial cylinders
4. It covers the entire length, so surface straightness and taper are controlled
5. Rule #1 still applies to control the axis straightness
6. The tolerance value must be less than the diameter tolerance.
Circularity is defined as the condition in which all points comprising a line element are circular. The control defines a tolerance zone consisting of two coaxial circles, radially separated by the distance indicated in the feature control frame. It must be applied to one individual cross-sectional line element and not related to a datum.
Example
Flowchart
Things to RememberAbout circularity:
1. It never references a datum or uses MMC or LMC
2. It must be applied to a surface of revolution (e.g., cylinder, cone, sphere, or other shape with 2-D circular cross sections)
3. The tolerance is not plus/minus, it is the radial (total) distance between two coaxial circles
4. It applies individual cross-sections
5. Rule #1 still applies to control the straightness
6. The tolerance value must be less than the diameter tolerance
Example
Flowchart
Things to RememberAbout circularity:1. It never references a datum or uses MMC or LMC
2. It must be applied to a surface of revolution (e.g., cylinder, cone, sphere, or other shape with 2-D circular cross sections)
3. The tolerance is not plus/minus, it is the radial (total) distance between two coaxial circles
4. It applies individual cross-sections
5. Rule #1 still applies to control the straightness
6. The tolerance value must be less than the diameter tolerance
Profile
Category of control symbols that control surface profile. They are similar to form controls when they do not reference datums, and they are similar to orientation controls when they do reference datums.
More...Profile controls, which may only be applied to surfaces, create zones within which all points of a controlled surface or line section must lay. Like the form category controls, they can create 3-D zones or 2-D: profile of a surface creates a 3-D zone that applies to a designated surface and profile of a line creates a 2-D zone that applies to any given line profile. Unlike form category controls, profile controls can reference datums.
Category of control symbols that control surface profile. They are similar to form controls when they do not reference datums, and they are similar to orientation controls when they do reference datums.
More...Profile controls, which may only be applied to surfaces, create zones within which all points of a controlled surface or line section must lay. Like the form category controls, they can create 3-D zones or 2-D: profile of a surface creates a 3-D zone that applies to a designated surface and profile of a line creates a 2-D zone that applies to any given line profile. Unlike form category controls, profile controls can reference datums.
The tolerance zone for profile of a line is a 2-D zone (an area) extending along the entire length of the controlled line element. It may or may not be related to a datum reference frame.
Examples
Drawing
Interpretation
Flowchart
Things to RememberAbout profile of a line:
1. It only applies to 2-D cross-sections; each is measured independently
2. The desired profile must be dimensioned with basic dimensions
3. The tolerance is assumed to be equal bilateral, but may be unequal bilateral or even unilateral if indicated by phantom lines on the drawing or by using the U (unequal) symbol
4. It may or may not reference a datum, depending on the degree of control needed
5. No MMC or LMC modifiers are to be used on the tolerance value (however, MMB or LMB may be used on datum references)
Examples
Drawing
Interpretation Flowchart
Things to RememberAbout profile of a line:
1. It only applies to 2-D cross-sections; each is measured independently
2. The desired profile must be dimensioned with basic dimensions
3. The tolerance is assumed to be equal bilateral, but may be unequal bilateral or even unilateral if indicated by phantom lines on the drawing or by using the U (unequal) symbol
4. It may or may not reference a datum, depending on the degree of control needed
5. No MMC or LMC modifiers are to be used on the tolerance value (however, MMB or LMB may be used on datum references)
The tolerance zone for profile of a surface is a 3-D zone (a volume) extending along the entire shape of the controlled surface. It may or may not be related to a datum reference frame.
Examples
Drawing
Interpretation
Flowchart
Examples
Drawing
Interpretation Flowchart
A modifier for describing an unequally disposed profile tolerance. The default for profile tolerances is equal bilateral, unless otherwise specified. These two examples create the same unequal bilateral tolerance zone, one using the symbol, one using phantom lines.
Examples
Format
Example Using Symbol
Example Using Phantom Lines
Examples
Format
Example Using SymbolExample Using Phantom Lines
A circle at the knee of a leader line indicates that the profile tolerance applies all around the true profile of the designated features of the part in the view where it is specified.
Examples
Drawing
Interpretation
Examples
Drawing
Interpretation
A double circle at the knee of a leader line indicates that the profile tolerance applies all over the 3-D true profile of the designated part. (An alternate method is to place the words “ALL OVER” below the feature control frame.)
Examples
Drawing
Interpretation
Examples
Drawing
Interpretation
Runout
Runout tolerances control surfaces constructed around a datum axis or at right angles to a datum axis. They control location, orientation, and form, and imply an inspection method using an indicator.
Runout tolerances control surfaces constructed around a datum axis or at right angles to a datum axis. They control location, orientation, and form, and imply an inspection method using an indicator.
The tolerance zone for circular runout is a 2-D circular zone (an area) centered on a datum axis, thereby controlling circularity and coaxiality of a cylindrical, conical, or other surface of rotation. It can also be applied to circular elements of a plane that is at a right angle to a datum axis, thereby controlling wobble.
Examples
Drawing
Interpretation
Flowchart
Things to RememberAbout circular runout:
1. It must reference a datum axis RMB
2. It must be used on surfaces that are constructed around or intersect the datum axis at 90°
3. It does not control longitudinal variation (i.e., does not control straightness)
4. No modifiers are allowed
5. It controls orientation and form (wobble) when used on surfaces that intersect the datum axis at 90°
6. It controls location, orientation, and form (circularity) when used on surfaces constructed around the datum axis
7. Although circular runout is a surface control, it indirectly controls the axis and can be placed on a drawing under the size dimension (as if it applied to a feature of size)
Examples
Drawing
Interpretation Flowchart
Things to RememberAbout circular runout:
1. It must reference a datum axis RMB
2. It must be used on surfaces that are constructed around or intersect the datum axis at 90°
3. It does not control longitudinal variation (i.e., does not control straightness)
4. No modifiers are allowed
5. It controls orientation and form (wobble) when used on surfaces that intersect the datum axis at 90°
6. It controls location, orientation, and form (circularity) when used on surfaces constructed around the datum axis
7. Although circular runout is a surface control, it indirectly controls the axis and can be placed on a drawing under the size dimension (as if it applied to a feature of size)
The tolerance zone for total runout is a 3-D cylindrical zone (a volume) centered on a datum axis, thereby controlling cylindricity and coaxiality of a cylindrical surface. It can also be applied to all elements of a plane that is at a right angle to a datum axis, thereby controlling perpendicularity.
Examples
Drawing
Interpretation
Flowchart
Things to RememberAbout total runout:
1. It must reference a datum axis RMB
2. Must be used on surfaces that are constructed around or intersect the datum axis at 90°
3. It does control longitudinal variation (i.e., does control straightness)
4. No modifiers are allowed
5. It controls orientation and form (perpendicularity and flatness) when used on surfaces that intersect the datum axis at 90°
6. It controls location, orientation, and form (cylindricity) when used on surfaces constructed around the datum axis
7. Unlike profile of a surface, it does not control size
8. Although total runout is a surface control, it indirectly controls the axis and can be placed on a drawing under the size dimension (as if it applied to a feature of size)
Examples
Drawing
Interpretation Flowchart
Things to RememberAbout total runout:
1. It must reference a datum axis RMB
2. Must be used on surfaces that are constructed around or intersect the datum axis at 90°
3. It does control longitudinal variation (i.e., does control straightness)
4. No modifiers are allowed
5. It controls orientation and form (perpendicularity and flatness) when used on surfaces that intersect the datum axis at 90°
6. It controls location, orientation, and form (cylindricity) when used on surfaces constructed around the datum axis
7. Unlike profile of a surface, it does not control size
8. Although total runout is a surface control, it indirectly controls the axis and can be placed on a drawing under the size dimension (as if it applied to a feature of size)
For “nonrigid” parts, typically a part that experiences elastic deformation under its own weight, such as large sheet metal parts, there can be confusion about the conditions under which drawing requirements apply to the part. For example, does a profile tolerance apply to a sheet metal part after the part is restrained, or does it apply to the part in its free state?
The default condition is that unless otherwise specified, all drawing requirements apply when the part is in its free state. (See paragraph 5.5 of Y14.5.)
Quite often, parts are restrained using a general note that reads something like, “All requirements apply to the part in the restrained state.” The drawing then goes on to list how the part is to be restrained.
In the case of a drawing with a general restraint note, a part may still need to meet some requirement in the free state, perhaps to ensure that the part is reasonably close to meeting the requirement in the free state and to avoid inducing residual stresses into a part during the restraining process.
In such cases, the free-state modifier may be placed in the feature control frame after the tolerance and any other modifers.
Example
Note: The flatness control must be checked prior to restraining the part and checking the other requirements.
The default condition is that unless otherwise specified, all drawing requirements apply when the part is in its free state. (See paragraph 5.5 of Y14.5.)
Quite often, parts are restrained using a general note that reads something like, “All requirements apply to the part in the restrained state.” The drawing then goes on to list how the part is to be restrained.
In the case of a drawing with a general restraint note, a part may still need to meet some requirement in the free state, perhaps to ensure that the part is reasonably close to meeting the requirement in the free state and to avoid inducing residual stresses into a part during the restraining process.
In such cases, the free-state modifier may be placed in the feature control frame after the tolerance and any other modifers.
Example
Note: The flatness control must be checked prior to restraining the part and checking the other requirements.
The virtual condition of an internal feature of size is calculated as: VC = MMC – geometric tolerance
Example
VC = MMC – geometric tolerance.
The virtual condition of the hole is 23.8 mm.
Example
VC = MMC – geometric tolerance.
The virtual condition of the hole is 23.8 mm.
The virtual condition of an external feature of size is calculated as: VC = MMC + geometric tolerance
Example
VC = MMC + geometric tolerance.
The virtual condition of the pin is 9.3 mm.
Example
VC = MMC + geometric tolerance.
The virtual condition of the pin is 9.3 mm.
Virtual Condition
Defined as a constant boundary generated by the collective effects of the “worst case” size and geometric tolerance for a considered feature of size. For example, the inner boundary created by the smallest sized hole and the maximum location tolerance.
Defined as a constant boundary generated by the collective effects of the “worst case” size and geometric tolerance for a considered feature of size. For example, the inner boundary created by the smallest sized hole and the maximum location tolerance.
Square
"Square" is a term referring to features of size consisting of two parallel opposed planes, including keys, keyways, slots, tabs, and widths. Like square pizzas, a “square” feature of size is usually rectangular.
"Square" is a term referring to features of size consisting of two parallel opposed planes, including keys, keyways, slots, tabs, and widths. Like square pizzas, a “square” feature of size is usually rectangular.
Round
"Round" is a term referring to features of size consisting of a single cylindrical (or spherical) surface, including holes, dowels, shafts, etc. Diametral and diametrical are terms sometimes used to describe round features of size. The diameter symbol precedes all diametral values.
"Round" is a term referring to features of size consisting of a single cylindrical (or spherical) surface, including holes, dowels, shafts, etc. Diametral and diametrical are terms sometimes used to describe round features of size. The diameter symbol precedes all diametral values.
Internal/External
Internal and external are terms used to indicate female and male, respectively. Used to denote the relative positioning of two parts or features of parts that assemble. Holes and IDs (internal diameters) are internal, while shafts and ODs (outer diameters) are external.
Internal and external are terms used to indicate female and male, respectively. Used to denote the relative positioning of two parts or features of parts that assemble. Holes and IDs (internal diameters) are internal, while shafts and ODs (outer diameters) are external.
MMC
MMC (maximum material condition) is the condition in which a feature of size contains the maximum amount of material within the stated limits of size. For example, the minimum allowable hole diameter or keyway width, or the maximum allowable shaft diameter or tab width.
More...
MMC is a useful concept that refers to the size limit (small hole, large shaft) most closely associated with fit/assembly issues.
MMC (maximum material condition) is the condition in which a feature of size contains the maximum amount of material within the stated limits of size. For example, the minimum allowable hole diameter or keyway width, or the maximum allowable shaft diameter or tab width.
More...
MMC is a useful concept that refers to the size limit (small hole, large shaft) most closely associated with fit/assembly issues.
LMC
LMC (least material condition) is the condition in which a feature of size contains the least amount of material within the stated limits of size. For example, the maximum allowable hole diameter or keyway width, or the minimum allowable shaft diameter or tab width.
More...
LMC is a useful concept that refers to the size limit (large hole, small shaft) most closely associated with wall thickness issues.
LMC (least material condition) is the condition in which a feature of size contains the least amount of material within the stated limits of size. For example, the maximum allowable hole diameter or keyway width, or the minimum allowable shaft diameter or tab width.
More...
LMC is a useful concept that refers to the size limit (large hole, small shaft) most closely associated with wall thickness issues.
Location
Category of control symbols that control the locations of features of size, such as the center distances between holes, or slots; the location of a pattern of holes from datum planes; coaxiality between two diametral features; symmetry; or the concentricity of features distributed about a datum plane.
More...Position controls (applied to features of size) usually create tolerance zones consisting of two parallel planes (for square features of size), cylinders (for round features of size), or spheres (for spherical features of size) within which the center plane, axis, or center (respectively) of the controlled feature must lay.
Concentricity creates a tolerance zone consisting of a cylinder within which the median points of all diametrically opposed elements of a surface of revolution must lay.
Symmetry creates a tolerance zone consisting of two parallel planes within which the median points of all opposed elements of two or more feature surfaces must lay.
Category of control symbols that control the locations of features of size, such as the center distances between holes, or slots; the location of a pattern of holes from datum planes; coaxiality between two diametral features; symmetry; or the concentricity of features distributed about a datum plane.
More...Position controls (applied to features of size) usually create tolerance zones consisting of two parallel planes (for square features of size), cylinders (for round features of size), or spheres (for spherical features of size) within which the center plane, axis, or center (respectively) of the controlled feature must lay.
Concentricity creates a tolerance zone consisting of a cylinder within which the median points of all diametrically opposed elements of a surface of revolution must lay.
Symmetry creates a tolerance zone consisting of two parallel planes within which the median points of all opposed elements of two or more feature surfaces must lay.
Position
Position is one of the most useful geometric control symbols, and may be applied to internal, external, round, square, spherical, or bounded feature of size. Position may also use the MMC or LMC modifiers or the projected tolerance zone modifier.
When using position, the location of the controlled feature to any referenced datums must be defined using stated or implied basic dimensionsBasic dimension: A theoretically exact dimension (indicated by a rectangular box around the dimension) used to dimension a true position, true profile, true angle, or gage/fixture dimension. Title block tolerances do not apply to basic dimensions. . Any right-angle (90°) relationship (for angles), or centered relationship (for coaxial diameters or coplanar center planes) are implied as basic for purposes of locating features.
Example
In this example, the basic dimensions define the theoretically exact location of the pattern of four holes. The axis of each of the four holes must lay within one of the four 0.4 mm cylinders. The center of each of the 0.4 mm tolerance zone cylinders is precisely located by the basic dimensions, and the tolerance zone allows each actual hole is to vary up to 0.2 mm away from the true position in any direction (radially).
The term true position refers to the location (32, 24, and 10 mm) of the tolerance zones; the symbol is most properly referred to as a position tolerance. Flowchart
Position is one of the most useful geometric control symbols, and may be applied to internal, external, round, square, spherical, or bounded feature of size. Position may also use the MMC or LMC modifiers or the projected tolerance zone modifier.
When using position, the location of the controlled feature to any referenced datums must be defined using stated or implied basic dimensionsBasic dimension: A theoretically exact dimension (indicated by a rectangular box around the dimension) used to dimension a true position, true profile, true angle, or gage/fixture dimension. Title block tolerances do not apply to basic dimensions. . Any right-angle (90°) relationship (for angles), or centered relationship (for coaxial diameters or coplanar center planes) are implied as basic for purposes of locating features.
Example
In this example, the basic dimensions define the theoretically exact location of the pattern of four holes. The axis of each of the four holes must lay within one of the four 0.4 mm cylinders. The center of each of the 0.4 mm tolerance zone cylinders is precisely located by the basic dimensions, and the tolerance zone allows each actual hole is to vary up to 0.2 mm away from the true position in any direction (radially).
The term true position refers to the location (32, 24, and 10 mm) of the tolerance zones; the symbol is most properly referred to as a position tolerance. Flowchart
Symmetry
Symmetry can only be applied to square features of size. Symmetry is similar to position, but unlike position, it doesn’t apply to the center plane of the controlled feature. Instead, symmetry controls the median points of all opposed elements of two feature surfaces.
Example
In this example, the median points of the opposed sides of the slot are what must lay in the tolerance zone, not the slot’s center plane. Note: Symmetry can not use the MMC or LMC modifiers. Flowchart
Things to RememberAbout symmetry:
1. It must always reference a planar feature of size (square) datum RMB
2. It is only used on planar (square) features of size that are centered to the datum
3. It never uses a diameter symbol
4. No MMC/LMC modifiers are allowed on the tolerance
5. No MMB/LMB modifiers are allowed on the datum reference
6. It is difficult to measure
7. Symmetry includes position control; the reverse is not true
8. Most applications will use position instead
9. It is defined differently in ISO, JIS, DIN, and in older versions of the Y14.5 standard
Symmetry can only be applied to square features of size. Symmetry is similar to position, but unlike position, it doesn’t apply to the center plane of the controlled feature. Instead, symmetry controls the median points of all opposed elements of two feature surfaces.
Example
In this example, the median points of the opposed sides of the slot are what must lay in the tolerance zone, not the slot’s center plane. Note: Symmetry can not use the MMC or LMC modifiers. Flowchart
Things to RememberAbout symmetry:1. It must always reference a planar feature of size (square) datum RMB
2. It is only used on planar (square) features of size that are centered to the datum
3. It never uses a diameter symbol
4. No MMC/LMC modifiers are allowed on the tolerance
5. No MMB/LMB modifiers are allowed on the datum reference
6. It is difficult to measure
7. Symmetry includes position control; the reverse is not true
8. Most applications will use position instead
9. It is defined differently in ISO, JIS, DIN, and in older versions of the Y14.5 standard
Concentricity
Concentricity can only be applied to round (diametral) features of size. Concentricity is similar to position, but unlike position, it doesn’t apply to the axis of the controlled feature. Instead, concentricity controls the median points of all diametrically opposed elements of a surface of revolution
Example
In this example, the median points of the diametrically opposed points of the small diameter’s surface are what must lay in the tolerance zone, not the small diameter's axis. Note: Concentricity cannot use the MMC or LMC modifiers. Concentricity also requires a datum axis. Flowchart
Things to RememberAbout concentricity:
1. It must always reference a planar feature of size (square) datum RMB
2. It must always reference a diametral (round) datum
3. It is only used on diametral (round) features of size that are coaxial to the datum
4. It always requires a diameter symbol
5. No MMC/LMC modifiers are allowed on the tolerance
6. No MMB/LMB modifiers are allowed on the datum reference
7. It is difficult to measure, but it is included in both circular and total runout
8. Concentricity includes position control; the reverse is not true
9. Most applications will use runout or position instead
10. It is defined differently in ISO, JIS, DIN, and in older versions of the Y14.5 standard
Concentricity can only be applied to round (diametral) features of size. Concentricity is similar to position, but unlike position, it doesn’t apply to the axis of the controlled feature. Instead, concentricity controls the median points of all diametrically opposed elements of a surface of revolution
Example
In this example, the median points of the diametrically opposed points of the small diameter’s surface are what must lay in the tolerance zone, not the small diameter's axis. Note: Concentricity cannot use the MMC or LMC modifiers. Concentricity also requires a datum axis. Flowchart
Things to RememberAbout concentricity:1. It must always reference a planar feature of size (square) datum RMB
2. It must always reference a diametral (round) datum
3. It is only used on diametral (round) features of size that are coaxial to the datum
4. It always requires a diameter symbol
5. No MMC/LMC modifiers are allowed on the tolerance
6. No MMB/LMB modifiers are allowed on the datum reference
7. It is difficult to measure, but it is included in both circular and total runout
8. Concentricity includes position control; the reverse is not true
9. Most applications will use runout or position instead
10. It is defined differently in ISO, JIS, DIN, and in older versions of the Y14.5 standard
Form
You may notice that only two of the four Form controls show up here. That’s because only straightness and flatness can be correctly applied to a feature of size. Cylindricity and circularity are defined as applying only to the surface elements.
More...It’s important to be able to make the distinctions between controlling surface straightness and axial straightness, and between controlling surface flatness and center plane flatness. (This box of Form controls is in the bottom part of the map to indicate that the straightness and flatness symbols are for controlling axis or center plane form.)
You may notice that only two of the four Form controls show up here. That’s because only straightness and flatness can be correctly applied to a feature of size. Cylindricity and circularity are defined as applying only to the surface elements.
More...It’s important to be able to make the distinctions between controlling surface straightness and axial straightness, and between controlling surface flatness and center plane flatness. (This box of Form controls is in the bottom part of the map to indicate that the straightness and flatness symbols are for controlling axis or center plane form.)
Straightness
Straightness applied to a feature of size creates a cylindrical tolerance zone within which the feature’s axis (derived median line) must lie.
Examples
Example
The shaft now has an axial straightness allowance of 0.1 in addition to the size requirement of 12.4 to 13.0. You can tell that the straightness control applies to the derived median line because it is placed on the drawing directly under the size. (Contrast this to the way straightness would be applied to a surface in the top part of the map. Also, this diameter symbol before the 0.1 indicates the tolerance zone is a cylinder, not two parallel lines.)Flowchart
Things to RememberAbout straightness applied to a feature of size:
1. Straightness never references a datum; it is intended to control individual features of size
2. The tolerance is not a plus/minus; but a total allowable deviation
3. It is only correctly applied to an axis, and the tolerance zone is a cylinder (and requires the diameter symbol)
4. It needs to be associated with the feature’s size dimension
5. When straightness is applied to an FOS, size-related modifiers (MMC, LMC, the diameter symbol) are allowed
6. It can be applied on a unit basis
Straightness applied to a feature of size creates a cylindrical tolerance zone within which the feature’s axis (derived median line) must lie.
Examples
Example
The shaft now has an axial straightness allowance of 0.1 in addition to the size requirement of 12.4 to 13.0. You can tell that the straightness control applies to the derived median line because it is placed on the drawing directly under the size. (Contrast this to the way straightness would be applied to a surface in the top part of the map. Also, this diameter symbol before the 0.1 indicates the tolerance zone is a cylinder, not two parallel lines.)Flowchart
Things to RememberAbout straightness applied to a feature of size:1. Straightness never references a datum; it is intended to control individual features of size
2. The tolerance is not a plus/minus; but a total allowable deviation
3. It is only correctly applied to an axis, and the tolerance zone is a cylinder (and requires the diameter symbol)
4. It needs to be associated with the feature’s size dimension
5. When straightness is applied to an FOS, size-related modifiers (MMC, LMC, the diameter symbol) are allowed
6. It can be applied on a unit basis
Flatness
Flatness applied to a feature of size creates a tolerance zone consisting of two parallel planes within which the feature’s center plane (derived median plane) must lie.
More...Prior to the 2009 revision, Y14.5 used straightness to create the same tolerance zone for center plane control, which was often a source of confusion: Why use straightness to control flatness? With the 2009 update, the practice of using straightness as a center plane control has been discontinued.
Drawing
The flatness control produces a tolerance zone of two parallel planes, but it does not apply to the surface elements. Instead, it applies to the center plane (derived median plane). Interpretation
The flatness tolerance zone (shown in yellow) must contain the derived median plane (shown in red), which is derived from the midpoints of the top and bottom actual surfaces (shown in green).
Flowchart
Things to RememberAbout flatness applied to a feature of size:
1. Flatness never references a datum; it is intended to control individual features of size
2. Flatness can be applied to a center plane3; the tolerance zone is two parallel planes
3. Flatness needs to be associated with the feature’s size dimension
4. When flatness is applied to an FOS, size-related modifiers (MMC, LMC) are allowed
5. Flatness of a center plane may be applied on a unit basis
Flatness applied to a feature of size creates a tolerance zone consisting of two parallel planes within which the feature’s center plane (derived median plane) must lie.
More...Prior to the 2009 revision, Y14.5 used straightness to create the same tolerance zone for center plane control, which was often a source of confusion: Why use straightness to control flatness? With the 2009 update, the practice of using straightness as a center plane control has been discontinued.
Drawing
The flatness control produces a tolerance zone of two parallel planes, but it does not apply to the surface elements. Instead, it applies to the center plane (derived median plane). Interpretation
The flatness tolerance zone (shown in yellow) must contain the derived median plane (shown in red), which is derived from the midpoints of the top and bottom actual surfaces (shown in green).
Flowchart
Things to RememberAbout flatness applied to a feature of size:1. Flatness never references a datum; it is intended to control individual features of size
2. Flatness can be applied to a center plane3; the tolerance zone is two parallel planes
3. Flatness needs to be associated with the feature’s size dimension
4. When flatness is applied to an FOS, size-related modifiers (MMC, LMC) are allowed
5. Flatness of a center plane may be applied on a unit basis
Whenever straightness or flatness are applied to a feature of size, Rule #1 no longer applies to that feature.
Rule #1Rule #1, sometimes called the envelope principle or Taylor principle, is a idea that says size tolerances automatically limit the shape (form) of the toleranced feature. It is essentially a formal description of the widely recognized GO/NO-GO gage concept for checking sizes.
Rule #1 officially states, “The form of an individual regular feature of size is controlled by its limits of size,” but can be simply stated in just a few words: “size controls form.”
Exceptions to Rule #1Rule #1 does NOT automatically apply to flexible parts, like rubber hoses and the like.
Also, it does NOT automatically apply to stock sizes, which are typically governed by industry or manufacturers standard tolerances.
The independency symbol (circled I) can exempt a feature from the form control implied by Rule #1. However, using the independency symbol without a supplementary form control (such as flatness or straightness) will leave the feature’s form uncontrolled. Rule #1 states that size controls form...
Rule #1: Example
The above part, when interpreted according to Y14.5, has a worst-case outer boundary of 13.0 mm. In addition to the fact that every 2-D cross sectional measurement must be between 12.4 and 13.0 mm, the part must also fit into a perfect boundary of 13.0 mm (a “GO” gage).
Note: ISO does not make this assumption with size dimensions, and requires a note (such as “Perfect form is required for features of size at MMC”) or a “Envelope” modifier (a circled E) after the size to establish the same boundary of form.
Rule #1 Overridden: Example
The above part, when interpreted according to Y14.5 or ISO, has a worst-case outer boundary of 13.1. In addition to the fact that every 2-D cross sectional measurement must be between 12.4 and 13.0, the axis must also be verified, regardless of its size. So if a part were made at 13.0 mm (size) and had an axial variation of 0.1 (straightness), the effective envelope the part would have is 13.1 mm.
Therefore, the size limit (12.4-13.0) is no longer controlling the form, and we say that “the straightness control overrides Rule #1.”
Rule #1Rule #1, sometimes called the envelope principle or Taylor principle, is a idea that says size tolerances automatically limit the shape (form) of the toleranced feature. It is essentially a formal description of the widely recognized GO/NO-GO gage concept for checking sizes.
Rule #1 officially states, “The form of an individual regular feature of size is controlled by its limits of size,” but can be simply stated in just a few words: “size controls form.”
Exceptions to Rule #1Rule #1 does NOT automatically apply to flexible parts, like rubber hoses and the like.
Also, it does NOT automatically apply to stock sizes, which are typically governed by industry or manufacturers standard tolerances.
The independency symbol (circled I) can exempt a feature from the form control implied by Rule #1. However, using the independency symbol without a supplementary form control (such as flatness or straightness) will leave the feature’s form uncontrolled. Rule #1 states that size controls form...
Rule #1: Example
The above part, when interpreted according to Y14.5, has a worst-case outer boundary of 13.0 mm. In addition to the fact that every 2-D cross sectional measurement must be between 12.4 and 13.0 mm, the part must also fit into a perfect boundary of 13.0 mm (a “GO” gage).
Note: ISO does not make this assumption with size dimensions, and requires a note (such as “Perfect form is required for features of size at MMC”) or a “Envelope” modifier (a circled E) after the size to establish the same boundary of form.
Rule #1 Overridden: Example
The above part, when interpreted according to Y14.5 or ISO, has a worst-case outer boundary of 13.1. In addition to the fact that every 2-D cross sectional measurement must be between 12.4 and 13.0, the axis must also be verified, regardless of its size. So if a part were made at 13.0 mm (size) and had an axial variation of 0.1 (straightness), the effective envelope the part would have is 13.1 mm.
Therefore, the size limit (12.4-13.0) is no longer controlling the form, and we say that “the straightness control overrides Rule #1.”
Orientation
Category of control symbols that control (in this case), features of size.
More...Orientation controls (applied to features of size) create zones similar to straightness and flatness applied to features of size (cylinders and two parallel planes separated by the linear tolerance value), except that the orientation tolerance zones are required to be related to a datum reference at a theoretically exact (basic) angle.
In other words, if you want a hole through an object to be perpendicular to the bottom, the bottom is identified as a datum feature using the datum feature identification symbol, and then the hole is controlled with respect to the bottom. The part is set on the bottom, while the hole is checked.Flowchart
Category of control symbols that control (in this case), features of size.
More...Orientation controls (applied to features of size) create zones similar to straightness and flatness applied to features of size (cylinders and two parallel planes separated by the linear tolerance value), except that the orientation tolerance zones are required to be related to a datum reference at a theoretically exact (basic) angle.
In other words, if you want a hole through an object to be perpendicular to the bottom, the bottom is identified as a datum feature using the datum feature identification symbol, and then the hole is controlled with respect to the bottom. The part is set on the bottom, while the hole is checked.Flowchart
Angularity
When applied to a feature of size, the angularity control applies to the axis or center plane of the feature being controlled.
Things to RememberAbout angularity applied to a feature of size:
1. It must reference a datum
2. The tolerance is in millimeters (or inches)
3. When applied to an axis, the diameter symbol is required to describe a cylindrical tolerance zone
4. Tangent plane modifier is not allowed
5. Material condition modifiers (MMC & LMC) are allowed
6. The specified angle must be a basic dimension
7. It does not affect the size dimensions
When applied to a feature of size, the angularity control applies to the axis or center plane of the feature being controlled.
Things to RememberAbout angularity applied to a feature of size:
1. It must reference a datum
2. The tolerance is in millimeters (or inches)
3. When applied to an axis, the diameter symbol is required to describe a cylindrical tolerance zone
4. Tangent plane modifier is not allowed
5. Material condition modifiers (MMC & LMC) are allowed
6. The specified angle must be a basic dimension
7. It does not affect the size dimensions
Parallelism
When parallelism is applied to a feature of size, it is the axis (or center plane) that is being controlled.
Example
Here the parallelism is applied to the axis of the top hole. The axis must lie within a tolerance zone of diameter 0.1 mm, and that tolerance zone must be parallel to the datum axis E. Things to RememberAbout parallelism applied to a feature of size:
1. It must reference a datum
2. The tolerance is in millimeters (or inches)
3. When applied to an axis, the diameter symbol is required to describe a cylindrical tolerance zone
4. Tangent plane modifier is not allowed
5. Material condition modifiers (MMC & LMC) are allowed
6. The controlled feature must be parallel to the referenced datum
7. It does not affect the size dimensions
8. Alternate practice allows the use of the angularity symbol to indicate perpendicularity (added 2009)
When parallelism is applied to a feature of size, it is the axis (or center plane) that is being controlled.
Example
Here the parallelism is applied to the axis of the top hole. The axis must lie within a tolerance zone of diameter 0.1 mm, and that tolerance zone must be parallel to the datum axis E. Things to RememberAbout parallelism applied to a feature of size:
1. It must reference a datum
2. The tolerance is in millimeters (or inches)
3. When applied to an axis, the diameter symbol is required to describe a cylindrical tolerance zone
4. Tangent plane modifier is not allowed
5. Material condition modifiers (MMC & LMC) are allowed
6. The controlled feature must be parallel to the referenced datum
7. It does not affect the size dimensions
8. Alternate practice allows the use of the angularity symbol to indicate perpendicularity (added 2009)
Perpendicularity
When perpendicularity is applied to a feature of size, it is the axis (or center plane) that is being controlled.
Example
Here the perpendicularity is applied to the center plane of the slot, which must fall within two parallel planes 0.25 mm apart that are also perpendicular to datum plane C. Things to RememberAbout perpendicularity applied to a feature of size:
1. It must reference a datum
2. The tolerance is in millimeters (or inches)
3. The 90º angle is understood to be basic, so any general tolerance for angles (title block tolerances, for example) do not apply
4. Perpendicularity does not affect the size dimensions
5. When applied to an axis, the diameter symbol is required when indicating a cylindrical tolerance zone
6. Tangent plane modifier is not allowed
7. Material condition modifiers (MMC & LMC) are allowed
8. Alternate practice allows the use of the angularity symbol to indicate perpendicularity (added 2009)
When perpendicularity is applied to a feature of size, it is the axis (or center plane) that is being controlled.
Example
Here the perpendicularity is applied to the center plane of the slot, which must fall within two parallel planes 0.25 mm apart that are also perpendicular to datum plane C. Things to RememberAbout perpendicularity applied to a feature of size:
1. It must reference a datum
2. The tolerance is in millimeters (or inches)
3. The 90º angle is understood to be basic, so any general tolerance for angles (title block tolerances, for example) do not apply
4. Perpendicularity does not affect the size dimensions
5. When applied to an axis, the diameter symbol is required when indicating a cylindrical tolerance zone
6. Tangent plane modifier is not allowed
7. Material condition modifiers (MMC & LMC) are allowed
8. Alternate practice allows the use of the angularity symbol to indicate perpendicularity (added 2009)
These modifiers are used with features of size. They either follow the specified geometric tolerance to indicate that the specified tolerance applies at a particular feature size, or they follow a datum reference to indicate a particular material boundary interpretation for simulating the datum.
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The ASME standard assumes that geometric tolerances apply to the full length (or depth) of the toleranced feature; the projected tolerance zone modifier clarifies that the tolerance zone applies a specified length outside of the toleranced feature.
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Concentricity and symmetry may correctly be applied to only round and square features of size, respectively.
When a geometric tolerance is applied to a feature of size without any size modifier (i.e., without an MMC or LMC in the feature control frame), it applies regardless of feature size (RFS) meaning that the geometric tolerance does not vary regardless of the size the feature happens to be; the geometric tolerance is constant and applies equally at all sizes, from LMC to MMC.
Every control symbol inside this dashed line – and only those inside – can have the MMC or LMC symbols applied to the geometric tolerance. Under no other circumstances is it “legal” per Y14.5 for a geometric control to have an MMC or LMC modifier follow the tolerance value.
Every control symbol inside this dashed line – and only those inside – can have the MMC or LMC symbols applied to the geometric tolerance. Under no other circumstances is it “legal” per Y14.5 for a geometric control to have an MMC or LMC modifier follow the tolerance value.
Every control symbol inside this dashed line – and only those inside – can have the MMC or LMC symbols applied to the geometric tolerance. Under no other circumstances is it “legal” per Y14.5 for a geometric control to have an MMC or LMC modifier follow the tolerance value.
When a tolerance applied to a feature of size has no MMC or LMC modifier, it is interpreted as applying to the feature regardless of the feature's size (RFS, Rule #2).
More...The idea is to better control symmetrical relationships, alignments, etc., and inspection typically requires variable measurements, NOT pass/fail or go/no-go. It is usually better to think of the tolerance zone, which is fixed, or static, than it is to focus on the outer or inner boundary created by the geometric tolerance, since the boundary (actual mating envelope) will vary with the part’s size.
There is no additional “bonus” tolerance available from the size variation, and if a datum feature of size is referenced without an MMB or LMB, then there is no datum shift and the datum is referenced RMB, regardless of material boundary (this is part of Rule #2).
More...The idea is to better control symmetrical relationships, alignments, etc., and inspection typically requires variable measurements, NOT pass/fail or go/no-go. It is usually better to think of the tolerance zone, which is fixed, or static, than it is to focus on the outer or inner boundary created by the geometric tolerance, since the boundary (actual mating envelope) will vary with the part’s size.
There is no additional “bonus” tolerance available from the size variation, and if a datum feature of size is referenced without an MMB or LMB, then there is no datum shift and the datum is referenced RMB, regardless of material boundary (this is part of Rule #2).
When a tolerance applied to a feature of size has an LMC modifier, it is interpreted as applying to the feature at LMC only. As a feature’s actual size increases from LMC (for an external feature) or decreases from LMC (for an internal feature), the difference between the actual size and LMC is used as additional geometric tolerance.
More...The idea of using an LMC modifier is to protect wall thickness and not to ensure assembly. If a hole’s function is to provide mass reduction, not accept a mating part, then the location of the hole is most critical when the hole is produced at it’s largest diameter: that’s when it is most likely to remove too much material near an edge, for example.
When placed on a datum reference, an actual part’s axis may shift relative to a datum simulator and the circled M is called an LMB modifier, for least material boundary.
More...The idea of using an LMC modifier is to protect wall thickness and not to ensure assembly. If a hole’s function is to provide mass reduction, not accept a mating part, then the location of the hole is most critical when the hole is produced at it’s largest diameter: that’s when it is most likely to remove too much material near an edge, for example.
When placed on a datum reference, an actual part’s axis may shift relative to a datum simulator and the circled M is called an LMB modifier, for least material boundary.
When a tolerance applied to a feature of size has an MMC modifier, it is interpreted as applying to the feature at MMC only. As a feature’s actual size increases from MMC (for an internal feature) or decreases from MMC (for an external feature), the difference between the actual size and MMC is used as additional geometric tolerance.
More...The idea of using an MMC modifier is to make sure that parts assemble, without over controlling geometric attributes. Inspection can be made with a fixed attribute gage, a pass/fail or go/no-go gage. It is usually better to think of the boundary created by the additive effects of size and geometric tolerance, which together create a fixed, or static, boundary than it is to focus too much on the tolerance zone, which changes in size as the feature’s size changes. The amount of geometric tolerance that is allowed as the size varies is called “bonus” tolerance: the maximum amount of bonus is always the difference between the feature’s MMC and LMC limits.
When placed on a datum reference, an actual part may shift on a fixed-sized datum simulator and the circled M is called an MMB modifier. Virtual condition is simply the term used to refer to the fixed-sized boundary that results from the accumulation of size and geometric tolerance.
More...The idea of using an MMC modifier is to make sure that parts assemble, without over controlling geometric attributes. Inspection can be made with a fixed attribute gage, a pass/fail or go/no-go gage. It is usually better to think of the boundary created by the additive effects of size and geometric tolerance, which together create a fixed, or static, boundary than it is to focus too much on the tolerance zone, which changes in size as the feature’s size changes. The amount of geometric tolerance that is allowed as the size varies is called “bonus” tolerance: the maximum amount of bonus is always the difference between the feature’s MMC and LMC limits.
When placed on a datum reference, an actual part may shift on a fixed-sized datum simulator and the circled M is called an MMB modifier. Virtual condition is simply the term used to refer to the fixed-sized boundary that results from the accumulation of size and geometric tolerance.
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john.stolter@gd-t.com
(586) 693-0219
john.stolter@gd-t.com
(586) 693-0219