How to Specify True Position and Flatness on Machine Drawings?

Specify true position using a feature control frame with the position symbol (⌛), tolerance value (e.g., ⌀0.008), and datum references (A, B, C). For flatness, use the flatness symbol (􀀁) pointing directly to the surface, with a tolerance zone (e.g., 0.05) between two parallel planes. Avoid over-specifying—apply GD&T only to functional features to prevent unnecessary cost increases.

What Is GD&T and Why Use It on Engineering Drawings?

GD&T (Geometric Dimensioning and Tolerancing) is a universal engineering language that defines geometric relationships in a standardized way, controlling form, orientation, location, and profile based on how parts actually function together rather than just prescribing allowable size deviations.

Traditional tolerancing only specifies allowable size deviations. GD&T goes further, controlling:

GD&T provides clarity for manufacturing by explicitly defining allowable variation in how parts function together. This system ensures correct geometry for functionality, interchangeability, and performance.

A feature control frame communicates how a feature is measured and what variation is acceptable. It contains:

• A geometric characteristic symbol (e.g., position, flatness)

• The tolerance value

• References to relevant datums

At 6CProto, our ISO 9001:2015-certified team uses advanced CMM inspections to verify GD&T callouts, ensuring every component meets exact tolerances.

How Do You Specify True Position on a Machine Drawing?

True position is specified using a feature control frame with the position symbol (⌛), a diameter symbol (⌀) before the tolerance value, the tolerance (e.g., ⌀0.008), and datum references (A, B, C) to define the exact coordinate or location.

True Position is the exact coordinate or location defined by basic dimensions or other means that represents the nominal value. The position tolerance defines the allowable deviation from this true position.

Step-by-step specification:

1. Identify the feature: Typically a hole, pin, or cylindrical feature requiring precise location

2. Add a feature control frame: Place it below or beside the dimension

3. Include the position symbol: ⌛ (circle with crosshairs)

4. Add diameter symbol: ⌀ precedes the tolerance value for diametric features

5. ** Specify tolerance value:** e.g., ⌀0.008 (the tolerance zone is a cylinder)

6. Reference datums: A, B, C define the coordinate system for measurement

Example feature control frame:

This means: The hole’s axis must be within a ⌀0.008 cylindrical tolerance zone, measured relative to datums A, B, and C.

When to use diameter symbol: 99.9% of the time, a missing diameter symbol before the tolerance value is a typo. For cylindrical features, the tolerance zone is also a cylinder, so the diameter symbol is required.

Calculating true position deviation: Use the Pythagorean theorem: diametric deviation = 2 × √(X² + Y²). For X-deviation = 0.004 and Y-deviation = 0.003, diametric deviation = 2 × √(0.004² + 0.003²) = 0.010.

GD&T Basics offers a True Position Calculator to convert X and Y deviation measurements into actual diametric values and indicate whether a feature meets drawing requirements.

Which Datum References Are Required for True Position?

Datum references (A, B, C) are required for true position to establish the coordinate system for measurement. Without datums, position tolerance has no reference frame, making inspection impossible. Typically, three datums define a 3D coordinate system.

How datums work:

• Datum A: Primary reference (usually the largest, most stable surface)

• Datum B: Secondary reference (perpendicular to A)

• Datum C: Third reference (perpendicular to A and B)

Step 4 in implementing GD&T: Establish datum references by designating a datum feature. Orientation and location controls (like position) rely on datum references for accurate evaluation.

Datum selection best practices:

1. Functional priority: Choose datums that represent how the part functions in assembly

2. Stability: Select the largest, most accessible surfaces as primary datums

3. Accessibility: Ensure datums are accessible for CMM inspection

4. Repeatability: Avoid datums on features with high variation (e.g., cast surfaces)

Example: For a machined bracket with four holes, Datum A = bottom surface, Datum B = left edge, Datum C = front edge. The position tolerance is measured relative to this coordinate system.

At 6CProto, our CMM inspection system uses your specified datums to verify true position, ensuring parts meet assembly requirements.

When Should You Apply Flatness Instead of General Tolerances?

Apply flatness when a surface must be purely planar regardless of other dimensions—such as sealing surfaces, mounting faces, or optical surfaces. Flatness controls how flat a surface is regardless of any other datums or features, making it ideal for functional surfaces that require precise contact.

Flatness vs. general tolerances:

• General tolerances: Control size only (e.g., 10mm ±0.1)

• Flatness: Controls form independently (e.g., surface must be within 0.05 between parallel planes)

When flatness is necessary:

1. Sealing surfaces: Fluid/gas barriers requiring zero leakage

2. Mounting faces: Components requiring uniform contact (e.g., motor mounts)

3. Optical surfaces: Lenses or mirrors requiring precise planarity

4. Precision assemblies: Parts where cumulative variation affects function

Flatness of a surface is the condition of being purely planar. The flatness tolerance zone is a 3D tolerance zone, meaning you check variation up and down the Y-axis over the entire plane.

Flatness placement on drawings:

• Surface flatness: Arrow points directly to the surface (not in line with dimension)

• Flatness of Feature of Size (FOS): Callout is directly in line with the size dimension

Noticing the flatness symbol’s placement is critical for determining whether the requirement is for a surface or a feature of size.

How Do You Specify Flatness Without Over-Specifying?

Specify flatness by placing the flatness symbol (􀀁) pointing to the surface with a leader arrow, followed by a tolerance value (e.g., 0.05). Avoid over-specifying by applying flatness only to functional features—unnecessary flatness callouts on non-critical surfaces increase inspection time and cost without improving function.

Step-by-step flatness specification:

1. Identify the functional surface: Only apply flatness where functionally required

2. Add the flatness symbol: 􀀁 (two parallel lines)

3. Point to the surface: Leader arrow touches the surface directly

4. Specify tolerance: e.g., 0.05 (between two parallel planes 0.05 apart)

Example:

Over-specification pitfalls:

Step 3 in implementing GD&T: Define tolerances with precision. Explore how good or bad a part can be while still functioning optimally—avoid overly constrained values.

Recommended flatness tolerances by application:

For non-critical surfaces, rely on general tolerances (e.g., ISO 2768-mK) rather than adding individual flatness callouts—this reduces cost and simplifies inspection.

At 6CProto, our free DFM analysis flags over-specified GD&T and recommends cost-effective tolerances based on your functional requirements.

Could You Over-Specify GD&T and Raise Prices?

Yes—over-specifying GD&T by applying callouts to non-functional features or using unnecessarily tight tolerances raises prices by 30–50% due to increased inspection time, special equipment requirements, and higher rejection rates.

How over-specification increases cost:

Rule of thumb: Apply GD&T only to features that impact function. For a bracket with four mounting holes, specify position tolerance on the holes but not flatness on non-sealing surfaces.

Step 2 in implementing GD&T: Select appropriate controls aligned with functional intent. Avoid controls that overly complicate validation, as this impedes functionality evaluation.

Strategies to avoid over-specification:

1. Functional analysis: Identify which features affect assembly/performance

2. Tolerance hierarchy: Use general tolerances for non-critical features

3. Benchmark against capability: Match tolerances to standard machining (±0.1 mm general)

4. DFM review: Get supplier feedback before finalizing drawings

At 6CProto, our DFM analysis identifies over-specified GD&T and suggests cost-effective alternatives, reducing unit costs by 30%+ without compromising function.

Why Is the Diameter Symbol Required Before Position Tolerance?

The diameter symbol (⌀) is required before position tolerance for cylindrical features because the tolerance zone is a cylinder, not a circle. Without it, the tolerance is interpreted as radial (half the intended value), causing inspection failures and part rejection.

Technical explanation:

• For cylindrical features (holes, pins), the position tolerance zone is a cylinder

• The diameter symbol indicates the tolerance value is diametric (full diameter)

• Without ⌀, the tolerance is interpreted as radial (radius only), which is half the intended value

Example:

• Correct: ⌛ ⌀0.008 (tolerance zone = ⌀0.008 cylinder)

• Incorrect: ⌛ 0.008 (tolerance zone interpreted as 0.004 radius)

99.9% of missing diameter symbols are typos. Very few niche sections of the standard allow dropping the diameter symbol, and they use a different dimensioning scheme.

Consequences of missing diameter symbol:

1. Inspection failure: CMM interprets tolerance as radial, rejecting合格 parts

2. Cost increase: Suppliers assume tighter tolerance, raising prices

3. Part rejection: Functional parts fail inspection due to specification error

Always include ⌀ before position tolerance for holes, pins, and cylindrical features.

6CProto Expert Views

The most common GD&T mistake we see on machine drawings isn’t wrong symbols—it’s missing datum references for position tolerances. A position callout without A, B, C datums is like giving coordinates without a map. Inspectors can’t measure it, so they reject the part or inflate the quote to cover uncertainty. Another frequent error: flatness on every surface. A machined aluminum enclosure doesn’t need 0.02 mm flatness on non-sealing surfaces—that’s optical-grade precision. Match tolerances to function: sealing surfaces get 0.02–0.05 mm, mounting faces get 0.05–0.10 mm, and general surfaces rely on ISO 2768-mK. At 6CProto, our free DFM analysis flags these issues before production, saving clients 30–50% on unit costs without compromising function. GD&T isn’t about adding more callouts—it’s about adding the right ones.”
— 6CProto Engineering Team, ISO 9001:2015 Certified CNC Manufacturer

Conclusion

Specifying true position and flatness on machine drawings requires understanding GD&T’s purpose: controlling form, orientation, location, and profile based on functional intent. For true position, use a feature control frame with the position symbol (⌛), diameter symbol (⌀), tolerance value (e.g., ⌀0.008), and datum references (A, B, C). For flatness, place the flatness symbol (􀀁) pointing to the surface with a tolerance zone (e.g., 0.05 between parallel planes).

Avoid over-specification by applying GD&T only to functional features—unnecessary callouts on non-critical surfaces increase costs by 30–50%. Match tolerances to function: sealing surfaces (0.02–0.05 mm), mounting faces (0.05–0.10 mm), and general surfaces (ISO 2768-mK). Always include the diameter symbol before position tolerance for cylindrical features to prevent inspection failures.

At 6CProto, our free DFM analysis identifies GD&T optimization opportunities, reducing unit costs while maintaining functional precision. From initial concept to market-ready production, we’re your trusted partner in bringing manufacturing innovation to life .

FAQs

What is the difference between true position and position tolerance?True position is the exact nominal location (the ideal coordinate). Position tolerance is the allowable deviation from that true position, defined by a feature control frame.

Do I need datum references for flatness callouts?No. Flatness is a form control that doesn’t require datum references. It defines how flat a surface is regardless of other datums or features.

Why did my position tolerance fail inspection even though dimensions looked correct?Most likely the diameter symbol (⌀) was missing before the tolerance value. Without it, CMM interprets the tolerance as radial (half the intended value), rejecting certified parts.

How tight should my flatness tolerance be for a sealing surface?For sealing surfaces requiring zero leakage, use 0.02–0.05 mm flatness. For mounting faces, 0.05–0.10 mm is sufficient.

Can I use general tolerances instead of GD&T for most features?Yes. Apply general tolerances (e.g., ISO 2768-mK) to non-critical features and use GD&T only for functional features affecting assembly or performance—this reduces cost by 30–50%.