Engineers and designers use true position GD&T to explain part features on a technical drawing. A callout defines the feature along with its tolerance zone and is always found in the feature control frame (frame containing material modifier, geometric symbol and values).
The feature control frame, along with the true position GD&T callout, specifies a feature location.
In this article, you’ll learn how to use GD&T, what callout features are, and position symbols. You’ll also learn the differences between position and true position and how you can optimize the feature control frame for your drawings.
Technical Drawing Basics
Technical drawings allow engineers and designers to effectively communicate technical ideas graphically. There are many technical drawing formats and the most common one is the ISO, which has a pre-defined layout.
Technical drawing shows multiple views of an object, with many projections and most importantly size and location dimensions in mm or any other unit.
Understanding Feature Control Frame
A feature control frame, found with position and true position symbols, is a key element of Geometric Dimensioning and Tolerancing (GD&T) used in engineering drawings to show variations in form, orientation, location and profile of a feature.
The example above shows a feature control frame that specifies a true position tolerance for a hole on a part.
First box: the geometric tolerance symbol of true position (⌖) controls the location of a feature.
Second box: the diameter symbol shows the tolerance applies to a circular tolerance zone and the 0.02 value denotes the allowable variation of position.
Material condition modifiers (M): the maximum material condition (MMC) tells you that the given tolerance applies when the feature is at its smallest allowable size.
Datum feature: A, B, and C are called datum, which show the feature’s location control about the given datum planes.
What is True Position?
In the ASME standards, there’s no “True Position” symbol. In fact, true position is simply part of “Position”.
On a drawing, the position symbol marks the exact location of a feature like a hole with reference to the datum surface and the datum axis. These datums can be thought of as fixed reference points and datum planes can be thought of as fixed reference planes.
The location is fixed using basic dimensions, and this location is called the true position. If a part to sketch ratio of 1:1 is used, the blueprint of the feature aligns perfectly with the feature on the part.
True position GD&T is usually accompanied by modifiers and tolerance zones. These modifiers are based on material conditions and directly affect the positional tolerance zones.
True Position Tolerance Zone
Another way to think of “True Position” (GD&T) is to imagine a datum axis where the cylindrical tolerance zone is anchored. So, the exact position is the axis, but allowable cylindrical area around it suffices as position without any error in machining.
This brings the concept of tolerance zone which defines the allowable variation from the nominal position (datum axis). Let’s look at cylindrical tolerance zone using the bolt example above.
In this case, the position of the hole is defined using basic dimensions and the upper feature control frame gives the cylindrical tolerance zone (radial direction) as 0.2 mm. You can interpret this as the center of the hole must align within a cylinder of 0.2 mm diameter.
Formula for True Position
You can calculate the variation in true position using
TP=2 sqrt((Xactual-Xbasic)2-(Ymeasured-Ynominal)2)
True Position VS Position (in manufacturing)
In all popular engineering standards, the term “True Position” is non-existent. True position is a term used on the shop floor to understand the deviation of the physical feature on the part from the actual position.
The symbol for true position GD&T and position is the same. True position is a way to geometrically control a feature using basic dimensions.
True Position VS Linear Toleranceosition (in manufacturing)
There are many other ways to define a location and position. This also means there are different types of GD&T symbols with different tolerances.
Linear Tolerancing
Linear tolerances use +/- or ± with values to show how far a feature can deviate from nominal dimensions. For example, 20.00 mm ± 0.2 mm for a hole feature gives location tolerance between 9.8 mm and 10.2 mm.
This means the hole can shift laterally in either direction. It uses a square tolerance zone in both the X and Y directions.
Square tolerance zone has limitations when used with cylindrical and round features. There is a common error with diameter position tolerance which says if a hole is at its extreme position, + 0.2 mm in X and + 0.2 mm in Y, the actual center can be 0.28 mm from the nominal dimension and not 0.2 mm!
True Position GD&T
The true position defines a tolerance zone using a circular tolerance zone rather than separate X and Y limits. For the first hole, the center must lie within a circular tolerance zone of 0.2 mm. This ensures the position of the hole is equal in all directions.
Types of Tolerance Zone
A tolerance zone is the allowable region within which a feature must be located. In Geometric Dimensioning and Tolerancing, different types of tolerance zone are used for geometric control.
Cylindrical Tolerance Zone
True position uses cylindrical tolerance zone for location tolerance and nominal position. Cylindrical tolerance zone ensures feature’s axis stays within a circular boundary.
The callout for cylindrical tolerance zone uses a diameter symbol and position symbol. Datums help fix the location of the tolerance zone using basic dimensions from reference datums. Cylindrical tolerance zone is a 3D tolerance zone that virtually extends around the hole axis.
Circular Tolerance Zone
This is used for 2D features and works similarly to a cylindrical tolerance zone. The tolerance zone is ring shaped in a 2D plane or cross section. This controls the roundness of a feature. Gears and bearing drawings commonly show circular tolerances.
Planar Tolerance Zone
Flat surfaces use planar tolerance zone framework. This consists of two parallel planes controlling the variation in a surface’s height. Any surface bumps exceeding the plane require smoothing or grinding.
Square Tolerance Zone
In linear tolerancing, the allowable variation in a feature’s location is controlled separately in a 2D plane along the X and Y axes. It creates a box of allowable area around the feature.
Square tolerance zone can be defined using ± symbol and allowable variation value.
Tips On Using True Position GD&T
Whether a designer, engineer, or a shop floor manager, you need to understand how to use true position in feature callouts. True position is defined using feature control frame and sometimes a composite feature control frame which gives the location tolerance and orientation of the feature.
A feature control frame (FCF) uses
True position symbol (⊕)
Diameter symbol (⌀)
Tolerance value
Material Condition Modifier
Datum reference
This means to correctly use the true position tolerance, you should know how to use basic dimensions and datums.
How To Select Datum
A datum is simply a reference point or plane used for measuring. When selecting a datum use a hierarchy by assigning primary datum, secondary, and tertiary datum labels based on feature importance.
First Reference: primary datum is the main reference point. When selecting datums, always constrain degrees of movement.
The above example shows a composite tolerance with position control.
In this case, cylindricity is tested. After producing a shaft for a piston, you can use a V Block which acts as a datum aid by fixing multiple degrees of freedom and the cylindricity is measured using a gauge.
On a technical drawing, the datum feature would be represented with the shaft’s axis.
Using The Correct Tolerance Zone
The correct tolerance zone in GD&T allows for manufacturability, functioning parts, and quality control. Using the correct tolerance zone is important for assembling parts and avoiding running failures.
| Feature | Tolerance Zone | What It Does |
| Cylindrical Surface | ⨁ Ø | Ensures perfectly round surface along the cylinder length |
| Axis of feature | True Position (⨁ Ø) | Datum |
| Acetal | 180-210℃ | 80–100℃ |
| Flat Surface | ⏤ | Ensures surface is within acceptable parallel plane variation |
| Round features | ○ | Checks roundness value |
For true positional tolerance, always use cylindrical tolerance zone with positional tolerance value. Remember, square tolerance zone (±) is ideal for linear dimensions.
Modifiers
When reading true position tolerance GD&T callout you’ll come across various encircled letters. These letters represent information and are called modifiers.
M: Allows for additional tolerance with the condition that the feature is at its maximum material size. This modifier is called Maximum Material Condition (MMC)
L: Allows for additional tolerance with the condition that the feature is at its least material size. Also called Least Material Condition (LMC)
S: Regardless of Feature Size (RFS) is used when exact dimension is required and no change in tolerance.
Calculating True Position (for Square/Rectangle Tolerance Zone)
True position is given using cylindrical tolerance zone. In cases where true position is defined using square tolerance zone, you can find the actual positional tolerance using a simple formula.
True Position = 2 x Square root [(actual X – True X)2 -(actual Y – True Y)2]
Conclusion
True position in geometric dimensioning and tolerancing is a tool that ensures manufacturing quality and feature position. Using a datum reference point and cylindrical axis, you can essentially eliminate the risk of variation in a single direction.
Understanding and applying these principles requires knowledge and technical insight discussed in this article. This will be useful for working with gauges, CMM machines, and help you achieve precision from the design process to the end manufacturing.