FCF Builder Guide

Overview

The FCF Builder walks through frame creation one decision at a time. Instead of memorizing symbol order, you choose design intent first and DatumPilot narrows to valid options with live rule checks.

This guide summarizes common GD&T practice. Always defer to the applicable edition of the governing standard for authoritative requirements.

Key Benefits

• Live Preview: See your FCF render as you build it
• Real-time Validation: Catch errors before they reach drawings
• Context-Aware Options: Only valid choices appear based on your selections
• Rule-Aware Guidance: Uses DatumPilot's deterministic rule checks and feedback

Characteristic Selection

Select a geometric characteristic to begin building your FCF. Characteristics are grouped by control intent so setup stays predictable.

Form Controls

Form controls limit feature shape without referencing other features.

Note: Form controls never reference datums. The builder disables datum selection when these are chosen.

Orientation Controls

Orientation controls relate feature direction to at least one datum.

Note: Orientation controls require at least one datum reference.

Location Controls

Location controls place features from a datum setup.

Note: Location controls require datum references. Position is the most common option for practical coaxial or symmetry-style intent.

Profile Controls

Profile controls compare real geometry to intended contour.

Note: In DatumPilot, Profile semantics come from your Target:

• Choose a surface target (faces, cylinders, cones, etc.) for Profile of a Surface use cases.
• Choose Linear edge / line element only when your CAD/reference can be represented as a true line element for Profile of a Line use cases.

Runout Controls

Runout controls evaluate rotating surfaces about a datum axis.

Note: Runout controls always require a datum axis.

Tolerance Zone Definition

After selecting a characteristic, define the tolerance zone that controls feature variation.

Diameter Symbol (⌀)

The diameter symbol (⌀) indicates a cylindrical tolerance zone for FoS-derived axes (e.g., holes, shafts/pins, bosses) when applicable.

DatumPilot applies ⌀ automatically based on the selected Control Symbol and Target semantics:

• Surface targets never expose MMC/LMC and never force ⌀.
• FoS-derived axis targets (holes/shafts/bosses) will show ⌀ for characteristics where the tolerance zone is diametral.

Tolerance Value

Enter the total tolerance zone size in your selected unit (mm or inch).

• Numeric values only (e.g., 0.25, 0.010)
• The builder validates against reasonable ranges
• Unit follows your account settings (Settings > Display)

Material Condition Modifiers

Material condition modifiers change how tolerance applies based on feature size.

When Modifiers Apply

Note: MMC and LMC only apply to features of size (holes, pins, slots, tabs). The builder enables these options only when valid.

Bonus Tolerance (MMC/LMC)

When MMC or LMC is specified, the actual tolerance available depends on the produced feature size:

MMC Example (hole at ⌀10.0 ±0.1, position tolerance ⌀0.25 at MMC):

• At MMC (⌀9.9): Position tolerance = 0.25
• At ⌀10.0: Position tolerance = 0.25 + 0.10 = 0.35
• At LMC (⌀10.1): Position tolerance = 0.25 + 0.20 = 0.45

The DatumPilot calculator computes bonus tolerance when you provide feature size data.

Datum Reference Frame

Datum references establish the coordinate system for measuring the controlled feature.

Datum Order

Datums are referenced in order of precedence:

Important: Datum sequence changes setup behavior. |A|B|C| and |B|A|C| are not equivalent because each reference constrains motion in a different order.

Selecting Datums

1. Primary Datum (required for most controls): Select from available datum letters (A-Z)
2. Secondary Datum (optional): Select a different letter
3. Tertiary Datum (optional): Select a third unique letter

The builder prevents selecting the same datum twice.

Datum Material Boundary Modifiers

Like tolerance modifiers, datums can have material boundary modifiers:

When to Use: MMB is common when using datum features that are also features of size (e.g., a datum hole). This allows the part to shift within the datum feature simulator.

Validation Rules

The builder enforces these datum rules:

• No duplicates: Each datum letter can appear only once
• Minimum datums: Position requires at least one; form controls require none
• Modifier restrictions: MMB/LMB only on datum features of size

Target Selection

Select a Target after choosing the Control Symbol. The Target dropdown is filtered to only show options supported by DatumPilot's rule set for that control.

Targets are stored internally as semantic types (Surface vs Feature of Size), with CAD-friendly labels such as:

• Surfaces: Planar face, General surface, Cylindrical/Conical/Spherical surfaces
• FoS (Feature of Size): Hole, Shaft/Pin, Slot, Tab, Boss
• Line: Linear edge / line element (primarily for Profile use cases)
• Patterns: Hole pattern, Feature pattern
• Macros: Pocket (resolved by control to Pocket surfaces vs Pocket width (FoS))

Pocket Resolution (Macro)

Pocket is not a single semantic target. DatumPilot resolves it based on the selected control:

• Profile → Pocket surfaces (bottom + walls)
• Position / Orientation → Pocket width (FoS) (between opposing walls)

Multiple Selection and <CF> (Continuous Feature)

If you’re controlling multiple interrupted but intended-to-function-as-one entities (e.g., interrupted faces or interrupted slots), DatumPilot can store a <CF> flag to treat the selection as one continuous feature for interpretation and validation.

For pattern features, specify the quantity:

• Enter the count (e.g., 4 for a 4-hole bolt circle)
• The FCF displays as "4X" prefix
• Calculations apply to each feature in the pattern

Advanced Options

Projected Tolerance Zone

A projected tolerance zone extends the position tolerance zone above the surface of the part.

When to Use

Use projected tolerance zones when:

• Fasteners (bolts, studs) extend through the part
• Mating part alignment depends on the fastener angle, not hole position
• Interference could occur outside the controlled feature

Setting Up

1. Enable Projected Tolerance Zone toggle
2. Enter Projection Height (typically mating part thickness + engagement)
3. The FCF displays with the ⓟ symbol and height value

Composite FCF

Live Preview

The preview panel renders your FCF in real-time as you make selections.

Reading the Preview

The preview displays:

• Characteristic Symbol: First compartment
• Tolerance Zone: Second compartment (diameter symbol if applicable, value, modifier)
• Datum References: Subsequent compartments in order

Validation Indicators

Inline Messages

Validation messages appear below the preview:

• Errors: Must be resolved (e.g., "Position requires at least one datum reference")
• Warnings: Advisory only (e.g., "RFS is implicit and typically not shown")

Validation Rules

DatumPilot validates FCFs against its deterministic rule set. Here are common errors and solutions.

Error: Datum Required

Message: "Datum required for [characteristic]"

Cause: Location and orientation controls need datum references.

Solution: Add at least one datum reference in the Datum Reference Frame panel.

Error: Modifier Not Allowed

Message: "MMC not allowed for [characteristic]"

Cause: Form controls (Flatness, Straightness, Circularity, Cylindricity) always apply RFS.

Solution: Remove the material condition modifier or select a different characteristic.

Error: Duplicate Datum

Message: "Duplicate datum reference"

Cause: Same datum letter appears more than once.

Solution: Select unique datum letters for primary, secondary, and tertiary positions.

Error: Invalid Tolerance Value

Message: "Tolerance must be a positive number"

Cause: Zero, negative, or non-numeric tolerance entered.

Solution: Enter a positive decimal value (e.g., 0.25).

Warning: Implicit RFS

Message: "RFS is implicit per ASME Y14.5-2018"

Cause: Explicitly showing RFS modifier when it's the default.

Solution: This is valid but unnecessary. Modern practice omits RFS symbol.

Understanding Error Codes

Each validation message includes a code for reference:

Saving and Exporting

Save to Library

1. Click Save FCF when validation passes
2. Save the frame to your Library
3. Enter a descriptive name (e.g., "Bore Position to A-B-C")
4. Click Save

Tip: Use consistent naming conventions across your Library for easy reference.

Export Options

Copy to Clipboard

The Copy JSON button copies the FCF data structure. Use this for:

• Integration with other tools
• Documentation
• Backup

AI Interpretation (Pro Feature)

The AI interpreter provides plain-language explanations of your FCF.

What You Get

1. Tolerance Zone Description: Physical shape and size of the acceptable zone
2. Datum Precedence: How each datum constrains degrees of freedom
3. Material Condition Effects: How MMC/LMC affects actual tolerance
4. Measurement Setup: Guidance for inspection planning

Example Interpretation

For a Position FCF ⊕ | ⌀0.25 Ⓜ | A | B | C:

"This Feature Control Frame specifies the position of a feature of size relative to datum reference frame A-B-C. The tolerance zone is a cylinder of diameter 0.25mm at Maximum Material Condition.

This frame checks feature location from datum setup A-B-C with a ⌀0.25 tolerance at MMC.

As actual size moves away from MMC, extra positional allowance becomes available.

Datum A anchors setup first, B refines orientation, and C removes the remaining freedom."

Tips and Best Practices

Start with the Characteristic

Begin every FCF by selecting the geometric characteristic. This:

• Enables appropriate options automatically
• Prevents invalid modifier selections
• Sets context for datum requirements

Use Accurate Targets

Select the target that matches your geometry:

• Enables correct MMC/LMC calculations
• Improves AI interpretation accuracy
• Produces better documentation

Review Warnings Before Export

Warnings don't prevent saving, but review them:

• Some indicate potential drawing review issues
• Others suggest standard practice improvements

Legal and Standards Notice

• Educational guidance only. For authoritative requirements, consult the applicable standard.
• DatumPilot is not affiliated with or endorsed by ASME, ISO, or ANSI.

Name FCFs Descriptively

Good names keep your Library manageable:

• ✓ "Mounting Hole Position to A-B-C"
• ✓ "Bore Perpendicularity to Datum A"
• ✗ "FCF 1"
• ✗ "Position"

Use the Library Consistently

Organize FCFs logically:

• Use one naming pattern per part or assembly
• Group related FCFs together
• Include revision identifiers when needed

Next Steps

• Learn tolerance stack-up analysis in the Stack-up Guide
• Return to Getting Started for basic navigation
• Contact support@datumpilot.com for additional help