True Position is one of the most widely used concepts in Geometric Dimensioning and Tolerancing (GD&T). It offers a more complete way to define the location of features on a part, especially holes and slots. Instead of standard X and Y dimensions with individual tolerances, True Position controls the overall deviation from a perfect location using a circular or cylindrical tolerance zone.
By using True Position, engineers can control how much a feature can shift in any direction from its ideal location. This approach is much more effective than separating ± tolerances for each coordinate. It also works better for features aligning with mating parts or assemblies. Let’s break it down from the start.
What Is the True Position in GD&T?
True Position is the exact location of a feature, such as a hole, pin, or slot, as defined concerning datums. Simply, it tells us where a feature should be placed on a part. Instead of focusing on the exact X and Y values, it defines a zone—usually a circle or cylinder—within which the center of the feature must fall.
This zone is centered on the theoretical location, called the “nominal” location. The part is considered acceptable if the actual center of the feature falls within the zone. The size of the zone depends on the allowed tolerance, which is specified in the feature control frame.
For example, if a hole’s true position tolerance is 0.2 mm, the center of that hole must fall within a 0.2 mm diameter circle around the exact target location. Whether the hole shifts left, right, up, or down, it’s still okay if it stays inside that circular area.
True Position is often used with other GD&T symbols, such as datums, material condition modifiers like MMC (Maximum Material Condition), and basic dimensions. These elements work together to describe a feature’s location, size, and limits more clearly than traditional tolerancing.
Key Symbols and Terminology
To work with True Position, it’s important to understand the key symbols and terms used in a GD&T feature control frame. These elements define how the tolerance is applied and what measurements matter.
- ⭘ Position Symbol: This is the GD&T symbol used to represent True Position. It looks like a circle with a crosshair inside and is placed at the start of the feature control frame.
- Feature Control Frame: This rectangular box contains the GD&T instructions. It tells you the type of control (like position), the tolerance value, material condition modifiers, and the reference datums.
- Basic Dimensions: These are the ideal measurements, usually in a box, that define the exact location of the feature. They do not have tolerances. The tolerance is defined by the feature control frame instead.
- Datums: These are reference features used to measure the true position of other features. They can be surfaces, edges, or holes labeled A, B, C, etc.
- Tolerance Zone: This is the area around the true position where the feature can exist. This is usually a circular area (2D) or a cylindrical area (3D) for holes or pins.
- MMC and LMC Modifiers: MMC (Maximum Material Condition) and LMC (Least Material Condition) allow the position tolerance to change based on the size of the feature. You get a bonus tolerance if a hole is larger than its smallest allowed size.
Core Concepts Behind True Position
True Position is part of a larger system called GD&T. This system was developed to give engineers a clearer and more functional way to describe parts. You need to understand the key ideas before applying for a True Position.
The Basic GD&T Framework
GD&T stands for Geometric Dimensioning and Tolerancing. It uses a standardized set of symbols to describe features’ size, form, orientation, and location. These rules are based on the ASME Y14.5 standard.
The GD&T system defines a “perfect” version of a part. It then limits how much each feature can vary from that perfect version. Instead of checking only distances or angles, GD&T checks how a feature relates to other features. This helps ensure function.
At the heart of GD&T are several control types:
- Form controls (like flatness or roundness) manage the shape of features.
- Orientation controls (like parallelism or perpendicularity) manage angles.
- Location controls (like position) manage placement.
- Profile controls define complex surfaces.
True Position is a type of location control. It tells you how close a feature’s center must be to its location.
GD&T also uses datums, reference points, lines, or planes. Datums help establish a common frame for measurements. For example, a hole’s position is measured from edges or surfaces defined as datums.
True Position vs. Linear Tolerancing
A hole might be shown as 50.00 ± 0.10 mm from an edge in traditional linear tolerancing. That means it can be placed between 49.90 mm and 50.10 mm along one axis. The same applies to the other axis. This creates a square box of tolerance.
The problem? The corners of that box are farther from the center than the sides. That creates uneven tolerance zones and unexpected results. Some parts might technically pass inspection, but still not fit.
True Position fixes this. It replaces the square box with a circle. If the tolerance is 0.20 mm, the feature’s center must fall inside a 0.20 mm diameter circle. This circle is centered on the basic (perfect) location.
This change creates a more realistic and uniform way to measure. It matches how parts behave in real assemblies. It also makes the tolerances easier to control and check, especially with coordinate measuring machines (CMMs).
In short:
- Linear tolerancing allows uneven variation.
- Prawdziwa pozycja gives a uniform and round zone that reflects real-world fit.
Understanding the Feature Control Frame
The feature control frame is the box that carries the GD&T instructions. For True Position, this frame tells you everything you need to know about how a feature is controlled.
A basic feature control frame has three parts:
- The symbol – This is usually the position symbol ⭘.
- The tolerance – This shows the diameter of the allowed zone. It might include a symbol like MMC (Maximum Material Condition).
- The datum references – These are the features used as measuring points.
Here’s an example:
⭘ | 0.2 | A B C
This means:
- The feature must lie within a 0.2 mm diameter zone.
- That zone is measured about datums A, B, and C.
If you add a material condition modifier, like MMC, it looks like this:
⭘ | 0.2 M | A B C
This allows for bonus tolerance when the feature is not at its worst-case size.
Basic dimensions—numbers boxed on the print—define the ideal location. These are not measured with plus/minus tolerances. The feature control frame defines the allowable variation.
How is the True Position Calculated?
Calculating True Position helps determine whether a feature’s location is within the allowed tolerance zone. Let’s explore how it works, step by step.
Theoretical Exact Dimensions (TEDs)
Theoretical Exact Dimensions, or TEDs, are the basic dimensions shown on a drawing. These are boxed values that define the perfect location of a feature.
Unlike standard dimensions, TEDs do not have any tolerance. Instead, the feature control frame provides the tolerance. This helps separate the ideal placement from the allowable variation.
Na przykład:
- A hole may have TEDs of 50.00 mm from the left edge and 30.00 mm from the bottom edge.
- These values represent the exact center point of the hole on the part.
- The hole’s True Position is then checked relative to this center.
TEDs must always be used with datum references. This creates a clear and repeatable measuring system.
When calculating True Position, you measure the actual feature’s center and compare it to the TED-based location. The difference is what the formula captures.
Material Condition Modifiers: MMC, LMC, and RFS
Material condition modifiers change how much positional variation is allowed based on the size of the feature. These modifiers give manufacturers more flexibility without affecting part function.
There are three common conditions:
MMC (Maximum Material Condition):
- This is the condition where the feature contains the most material.
- For holes, it means the smallest hole size.
- When the hole gets larger than this, you gain extra tolerance—this is called bonus tolerance.
LMC (Least Material Condition):
- This is the opposite.
- For holes, it’s the largest hole size.
- It’s used less often but is useful in cases where part strength depends on material presence.
RFS (Regardless of Feature Size):
- This means the position tolerance stays fixed, no matter the feature size.
- It’s the default condition if no modifier is given.
Bonus tolerance (with MMC or LMC) is simple in principle:
- You subtract the actual hole size from the MMC hole size.
- That value is added to the geometric tolerance.
The Formula for True Position (2D and 3D)
The True Position formula calculates the distance from a feature’s actual measured location to its theoretical location.
For a 2D position (flat part, like a hole on a plate), the formula is:
True Position = 2 × √[(X_measured − X_theoretical)² + (Y_measured − Y_theoretical)²]
- X and Y are the actual and nominal (theoretical) coordinates.
- The factor of 2 accounts for the full diameter of the circular tolerance zone.
Przykład:
If a hole is measured at X = 49.95 mm and Y = 30.05 mm, but the TEDs are X = 50.00 mm and Y = 30.00 mm:
True Position = 2 × √[(−0.05)² + (0.05)²]
= 2 × √[0.0025 + 0.0025]
= 2 × √0.005
= 2 × 0.0707
= 0.1414 mm
If the allowed position tolerance is 0.2 mm, this feature passes.
For a 3D position, you add the Z axis:
True Position = 2 × √[(XΔ)² + (YΔ)² + (ZΔ)²]
This applies to features that must be located in 3D space, such as pins or shafts in cast or milled parts.
CMM machines or optical scanners usually perform this calculation during inspection. But knowing the math behind it helps you read reports and adjust processes.
Datums and Reference Frames
Datums create a fixed coordinate system from which to measure. In GD&T, they act like anchors on the part. So, understanding datums is key to applying True Position correctly.
What Are Datums?
A datum is a physical feature on a part—such as a flat surface, a hole, or an edge—that serves as a reference. It is used to build a measurement framework.
Each datum establishes one or more axes or planes:
- A surface sets a flat reference plane.
- A slot or hole can set the center of a feature, defining an axis or point.
- Multiple datums together define a 3D coordinate system.
In drawings, datums are identified with capital letters (A, B, C) and connected to the feature by a leader line or symbol.
Datums must be:
- Repeatable for inspection
- Stable during manufacturing
- Functional to the part’s end use
They help ensure measurements are accurate and consistent across production and inspection setups.
How do Datums Affect True Position?
True Position is always measured relative to one or more datums. These datums define the part’s orientation and location in space.
When a feature control frame lists datums (like A | B | C), each one locks in a specific direction:
- Datum A sets the primary plane (usually the flat base).
- Datum B sets a secondary direction (like left/right alignment).
- Datum C defines the third direction (depth or rotation control).
The order of datums matters. It affects how the part is clamped, measured, and inspected.
Once datums are established:
- The basic dimensions (TEDs) define the ideal feature location from those datums.
- The measured position of the feature is then compared to that theoretical location.
- The True Position tolerance defines the acceptable zone.
If the part isn’t aligned to the correct datums during inspection, the results will be wrong—even if the feature is made correctly.
Datum Order and Selection Strategy
The order of datums in a feature control frame defines how the part is set up during measurement. The order tells you:
- What surface is held first (primary)
- What surface is held second (secondary)
- What surface is held third (tertiary)
Each step further locks the part in space.
Here’s how to choose datums:
- Primary Datum (A): Choose the largest, most stable, and functional surface. It should support the part during use or assembly.
- Secondary Datum (B): Pick a surface perpendicular to A. It helps orient the part left to right or front to back.
- Tertiary Datum (C): This usually locks the part in the final direction, such as rotation or height.
Good datum selection reflects how the part is assembled or used in real life. If the wrong datums are picked, features may be measured incorrectly—even if they were carefully made.
For example, if a mounting hole needs to line up with a wspornik, the bracket-facing surface should be one of the datums. That way, the hole’s location is measured based on how the part will be installed—not just how it looks on paper.
Inspection and Measurement Techniques
Once a part is manufactured, the next step is to check if the features meet the drawing requirements. For features with True Position, this means verifying that they fall within the allowed tolerance zone. There are several tools and methods available to do this.
CMM (Coordinate Measuring Machine) Methods
A CMM is one of the most accurate and widely used tools for checking True Position. It works by probing the part surface and recording the coordinates of each feature.
Here’s how it works:
- The part is mounted in a fixture.
- The machine aligns it using the defined datums (A, B, C).
- A probe touches the feature, such as a hole’s wall or surface.
- The software compares the actual center to the TED-based location.
- It calculates the True Position using the standard formula.
Benefits of using a CMM:
- Wysoka precyzja i powtarzalność
- 2D and 3D measurements
- Automatic data capture and reporting
- Easily handles multiple features and patterns
CMMs are ideal for parts with tight tolerances, complex geometry, or large feature sets. However, they require setup time and are usually used in quality control labs rather than on the shop floor.
Dial Indicator and Gage Pin Approaches
Dial indicators I gauge pins provide fast, manual ways to check True Position for simpler parts or shop-floor checks.
Gage Pin Method:
- Use a pin that matches the feature’s size.
- Please insert it into the hole or slot.
- Use the dial indicator to sweep the pin and measure how far its center is from the TED.
Dial Indicator Sweep:
- Mount the part on a rotary table or fixture.
- Use a dial indicator on a height stand or test fixture.
- Sweep across the feature’s surface or edge.
- Check deviation from the expected position.
This method works well for large holes or features with generous tolerances. It’s quick and doesn’t need software. However, it’s less precise than a CMM and more prone to user error. It’s best used for basic parts or quick in-process checks.
Wnioski
True Position in GD&T precisely controls feature location using a circular or cylindrical tolerance zone. It offers better accuracy and flexibility than traditional ± tolerances. Engineers can clearly define how far a feature can deviate from its exact position by using basic dimensions, reference datums, and modifiers like MMC.
Need help applying GD&T principles like True Position to your custom metal parts? Skontaktuj się z nami już dziś for expert support and fast manufacturing solutions tailored to your needs.
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Kevin Lee
Mam ponad dziesięcioletnie doświadczenie zawodowe w produkcji blach, specjalizując się w cięciu laserowym, gięciu, spawaniu i technikach obróbki powierzchni. Jako dyrektor techniczny w Shengen, jestem zaangażowany w rozwiązywanie złożonych wyzwań produkcyjnych i napędzanie innowacji i jakości w każdym projekcie.
