Inspection and GD&T
Blueprint and GD&T symbols
The 14 geometric characteristic symbols in five groups, then modifiers and dimensioning symbols. ASME Y14.5.
Technical review pending. Formulas and values are cited. This notice comes down after a machinist review.
| Symbol | Name | Group | Controls | Datum reference | Shop meaning |
|---|---|---|---|---|---|
| Straightness | Form | Line elements of a surface, or the axis of a feature of size | Never | Lay a straightedge along it, or indicate along one line; the line may not bow more than the tolerance. | |
| Flatness | Form | A whole surface, between two parallel planes | Never | Every point of the face sits between two parallel planes the tolerance apart. No datum: it says nothing about how the face sits relative to other faces. | |
| Circularity (roundness) | Form | Each circular cross section, between two concentric circles | Never | Checked one slice at a time; lobing on a centerless-ground part shows up here. | |
| Cylindricity | Form | The whole cylindrical surface, between two coaxial cylinders | Never | Roundness, straightness and taper of the full length at once. | |
| Profile of a line | Profile | Each line element of a surface, against a true profile set by basic dimensions | Optional | Cross sections of the contour stay inside a band around the drawn shape. | |
| Profile of a surface | Profile | A whole surface, against a true profile set by basic dimensions | Optional | The whole surface stays inside a band around the CAD shape; with datums it also controls location and orientation. | |
| Angularity | Orientation | A surface or axis at a basic angle to a datum | Always | The angle is basic; the tolerance is a zone width, not degrees. | |
| Perpendicularity | Orientation | A surface or axis at 90° to a datum | Always | Square to the datum face within the zone; a surface or a hole axis. | |
| Parallelism | Orientation | A surface or axis parallel to a datum | Always | Set the datum face on the surface plate and indicate across the top; the total reading is the error. | |
| Position | Location | Location of a feature of size (axis or center plane) from its true position | Usually | Hole and pin locations from the datums. A Ø zone is diametral: twice the radial offset. | |
| Concentricity (removed in 2018) | Location | Median points of a feature, about a datum axis | Always | Still on older drawings. Hard to inspect; the 2018 edition uses position, profile or runout instead. | |
| Symmetry (removed in 2018) | Location | Median points of a feature, about a datum center plane | Always | Still on older drawings for slots and tabs; the 2018 edition uses position or profile instead. | |
| Circular runout | Runout | Each circular element, as the part turns 360° on the datum axis | Always | Indicator in one spot, turn the part one revolution; total indicator reading (TIR) at each spot. | |
| Total runout | Runout | The whole surface, as the part turns and the indicator travels along it | Always | Sweep the indicator the full length while the part turns; one TIR for the whole surface. |
Concentricity and symmetry were removed in ASME Y14.5-2018. They still appear on drawings made to the 2009 and earlier editions.
| Symbol | Name | Where it appears | Meaning |
|---|---|---|---|
| Maximum material condition (MMC) | Feature control frame, after a tolerance or datum letter | Tolerance applies at the smallest hole or largest pin. Bonus tolerance grows as the feature departs from MMC. | |
| Least material condition (LMC) | Feature control frame, after a tolerance or datum letter | Tolerance applies at the largest hole or smallest pin. Used to protect wall thickness. | |
| Projected tolerance zone | Feature control frame, after the tolerance, with the projected height | The zone extends above the part by the stated height, so a bolt or press-fit pin clears the mating part. | |
| Free state | Feature control frame, after the tolerance | Tolerance applies with the part unrestrained. Used on parts that flex, such as thin rings. | |
| Tangent plane | Feature control frame, with an orientation tolerance | Only the plane resting on the high points must fall in the zone, not every point of the surface. | |
| Unequally disposed profile | Feature control frame, profile tolerance | The profile zone is shifted off center. The value after the symbol is the part of the zone on the side that adds material. | |
| Independency | Next to a size dimension | Turns off the envelope rule (Rule #1) for that size, so size no longer limits form. | |
| Continuous feature | Next to a size dimension or on a leader | Treats interrupted features (a bore broken by a groove) as one feature. The 2018 edition extends it to surfaces. | |
| Statistical tolerance | After a size tolerance or in a feature control frame | The tolerance was set by statistical stack-up. A drawing note gives the process control requirements. | |
| Between | Under a profile feature control frame, between two labeled points | Profile applies from one labeled point to the other along the outline. | |
| All around | Circle at the bend of a profile leader | Profile applies all the way around the outline shown in that view. | |
| All over | Two circles at the bend of a profile leader | Profile applies to every surface of the part. | |
| Datum feature | On a surface, an extension line or a size dimension | Names the physical feature that sets up a datum. On a size dimension, the datum is the axis or center plane. | |
| Datum target | Leader to a point, line or area on the part | A contact point, line or area used to set up a datum. Lower half: letter and number. Upper half: target area size, when there is one. | |
| Basic dimension | Any dimension | Theoretically true size, location or angle. Its tolerance comes from a feature control frame, not the title block. | |
| Reference dimension | Any dimension | Information only. Not used for inspection or manufacture. | |
| Diameter | Before a size; in a feature control frame, before the tolerance | A diameter. In a feature control frame, the tolerance zone is a cylinder. | |
| Spherical diameter | Before a size; in a feature control frame, before the tolerance | Diameter of a sphere. In a feature control frame, a spherical tolerance zone. | |
| Radius | Before a radius value | The arc must fall between two arcs at the size limits. Flats and reversals inside that zone are allowed. | |
| Controlled radius | Before a radius value | A smooth curve with no flats or reversals, and the radius at every point within the limits. | |
| Spherical radius | Before a radius value | Radius of a sphere, such as a ball end. | |
| Counterbore (spotface) | Hole callout, before the counterbore diameter | Counterbore diameter follows, then its depth. Since the 2009 edition, a spotface is this symbol with SF inside. | |
| Countersink | Hole callout, before the countersink diameter | Countersink diameter at the surface follows, then the included angle. | |
| Depth (deep) | Hole callout, before a depth | Depth of the hole, counterbore or feature that comes before it. | |
| Square | Before a size | The feature is square; one size covers both sides. | |
| Arc length | Over a dimension value | Measured along the arc, not straight across the chord. | |
| Slope | Before a ratio, on a flat taper | Rise per unit length of a flat, sloped surface, given as a ratio. | |
| Conical taper | Before a ratio, on a cone | Change in diameter per unit length of a cone, given as a ratio. | |
| Dimension origin | Circle at one end of a dimension line | Measure from the end with the circle. That surface is the origin; the other end must fall within the tolerance. | |
| Envelope requirement (ISO drawings) | After a size tolerance, ISO drawings only | The feature must fit a boundary of true form at MMC size. ASME Y14.5 applies this by default (Rule #1), so ASME drawings carry no symbol for it. |
X.XXX stands for any dimension value. The envelope symbol is ISO practice; ASME drawings apply the envelope to features of size by default.
Source: ASME Y14.5-2018 (dimensioning and tolerancing); concentricity and symmetry per ASME Y14.5-2009; envelope symbol per ISO 14405-1. Symbols redrawn and meanings summarized, not reproduced.
Feature control frame anatomy
| Compartment | What goes in it | In the example |
|---|---|---|
| 1. Geometric characteristic | One of the 14 symbols above | Position |
| 2. Tolerance | Ø when the zone is a cylinder, the tolerance value, then any modifier (MMC, LMC, projected zone, free state, tangent plane) | Ø 0.010 in at MMC |
| 3. Primary datum | The datum the part sets on first (most contact) | A |
| 4. Secondary datum | The datum the part is pushed against next | B |
| 5. Tertiary datum | The last datum, which stops the remaining motion | C |
How to read the tables
The group tells you what kind of control it is. Form controls shape only and never takes a datum. Orientation and runout always point to a datum. Location sets where a feature sits from the datums.
Datum order in the frame is order of setup, not alphabetical order. A frame reading C, A, B means the part sets on C first.
MMC and LMC modifiers only apply to features of size: holes, pins, slots and tabs. A flat face has no size, so it cannot be at MMC.
Independency, unequally disposed profile, continuous feature and the all over symbol came in with the 2009 edition. Drawings made to older editions will not show them.
FAQ
What are the 14 GD&T symbols?
Form: straightness, flatness, circularity and cylindricity. Profile: profile of a line and profile of a surface. Orientation: angularity, perpendicularity and parallelism. Location: position, concentricity and symmetry. Runout: circular runout and total runout. Concentricity and symmetry were removed in ASME Y14.5-2018, which leaves 12 in the current edition.
What does the M in a circle mean on a blueprint?
Maximum material condition (MMC): the tolerance applies when the feature is at its smallest hole or largest pin size. As the hole gets bigger (or the pin smaller) within its size limits, the part gains bonus tolerance equal to that departure from MMC.
Which GD&T symbols need a datum?
Orientation (angularity, perpendicularity, parallelism) and runout (circular and total) always reference a datum. Form (straightness, flatness, circularity, cylindricity) never does. Profile may or may not. Position usually does.
Were concentricity and symmetry removed from GD&T?
Yes. ASME Y14.5-2018 removed both. They still appear on drawings made to the 2009 and earlier editions, so shops still see them. The 2018 edition covers those needs with position, profile or runout.
What is the difference between a basic dimension and a reference dimension?
A basic dimension, shown in a box, is the theoretically true value; its tolerance comes from a feature control frame. A reference dimension, shown in parentheses, is information only and is not inspected.
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