← The Bench · Sheet C-07
GD&T reference
Fourteen geometric characteristics, what each tolerance zone is actually shaped like, and whether it needs a datum. Then a frame builder, so you can see the callout you are describing.
Feature control frame builder
Every control, and the shape of the zone it creates.
The single most useful habit in GD&T is to stop reading a callout as a rule and start seeing it as a solid region in space that the feature has to sit inside. This column tells you what that region is.
| Symbol | Characteristic | Tolerance zone | Datum | Notes |
|---|---|---|---|---|
| Form — controls a feature against itself | ||||
| ⏤ | Straightness | Two parallel lines in the viewing plane, or a cylinder when applied to an axis | Never | Applied to a derived median line it becomes a size-envelope control and can override Rule #1. |
| ⏥ | Flatness | Two parallel planes | Never | Says nothing about orientation or where the surface sits — only that it is flat. |
| ○ | Circularity | Two concentric circles in any cross-section normal to the axis | Never | Checked section by section. A tapered shaft can be perfectly circular everywhere. |
| ⊭ | Cylindricity | Two coaxial cylinders | Never | Circularity, straightness and taper in one control. Expensive to inspect — use deliberately. |
| Orientation — controls attitude relative to a datum | ||||
| ∠ | Angularity | Two parallel planes at the basic angle, or a cylinder about an axis | Required | The angle itself is basic. Only the deviation from it is toleranced. |
| ⊥ | Perpendicularity | Two parallel planes 90° to the datum, or a cylinder | Required | Angularity at 90°. Nothing more. |
| ∥ | Parallelism | Two parallel planes at the basic distance, or a cylinder | Required | Angularity at 0°. Does not control the distance — size or position does that. |
| Location — controls where the feature is | ||||
| ⌖ | Position | Usually a cylinder about the true position; two parallel planes for a width | Usually | The workhorse. With Ⓜ it releases bonus tolerance as the feature departs from MMC, which is free and routinely forgotten. |
| ◎ | Concentricity | Cylinder about the datum axis containing the derived median points | Required | Removed in Y14.5-2018. It controlled median points, not the surface, and was almost never what the designer meant. Use runout or position. |
| ⌯ | Symmetry | Two parallel planes about the datum centre plane | Required | Also removed in Y14.5-2018, for the same reason. Use position on the width. |
| Profile — controls the whole shape at once | ||||
| ⌒ | Profile of a line | A two-dimensional band following the true profile, at each cross-section | Optional | Use where the section matters but the surface as a whole does not, such as an extrusion. |
| ⌓ | Profile of a surface | A three-dimensional band following the true profile | Optional | The most capable control in the standard. It can hold form, orientation, location and size simultaneously. |
| Runout — controls the surface as the part turns | ||||
| ↗ | Circular runout | Full indicator movement at one fixed cross-section per revolution | Required | Cheap and quick. Catches circularity and coaxiality together at each section. |
| ⋰ | Total runout | Full indicator movement across the whole surface while it rotates | Required | Adds taper and profile along the length. The right control for a bearing journal. |
Modifiers you will meet
Rule #1 is doing more than you think
For a feature of size, the size limits alone control form. At maximum material condition the feature must sit inside a perfect-form envelope. A great many drawings are covered with flatness and straightness callouts that Rule #1 already enforced for free.
Datums are physical, not conceptual
A datum is not a line on a model. It is derived from a real datum feature through real contact, and the order — primary, secondary, tertiary — is the order in which the part touches the fixture. Get the order wrong and you have specified a different part while changing nothing on the screen.
Bonus tolerance is real money
Position at Ⓜ on a clearance hole releases tolerance as the hole grows. On a typical bolted flange that is often a third more usable tolerance than the frame appears to give, at no cost to function. Leaving it off is a decision to make the part harder to produce than it needs to be.
Which edition applies
Y14.5-2009 and Y14.5-2018 are both in wide use, and they differ in ways that matter — concentricity and symmetry are gone from 2018, and the treatment of profile changed. Put the edition in the drawing notes. Every drawing, every time.
Welding symbol decoder
Reading the arrow, the reference line and the tail.
Open C-10The circularity bench
Draw a circle. Get a real Y14.5 number back.
Open C-01ASME VIII-1 pressure vessel
Shell and head thickness, MAWP, joint efficiency.
Open C-05Bolted joint & torque
Preload, torque and residual clamp.
Open