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GD&T

This article covers CNC turning, detailing its process, benefits, limitations, and essential information for making parts. 

Table of Contents

Overview   |   Datums & Features   |   Feature Control Frame   |   MCM   |   Symbols

Overview

GD&T provides a different method for defining dimensions and tolerances compared to traditional plus/minus tolerancing. While engineers create parts with ideal geometry in CAD, the reality is that manufactured parts are never flawless.

Using GD&T correctly enhances quality and cuts down on delivery time and costs. It does this by offering a universal language to clearly convey design intentions and by concentrating on functional interfaces for part tolerancing.

These are the main benefits of using Geometric Dimensioning and Tolerancing (GD&T):

  • Standardized design language
  • Clear, precise, and consistent communication between customers, suppliers, and production teams
  • Method for calculating the worst-case mating limits
  • Repeatable production and inspection processes
  • Assembly is assured from qualified production parts

A thorough understanding of GD&T  will significantly improve communication with your manufacturing and quality control teams or supplier, increase the quality of your parts, and reduce costs.

Datums and Features

A datum is a theoretical exact plane, axis, or point location that serves as a reference point for measurements on machined components. Datums ensure that parts are manufactured consistently and accurately.

Primary datum: This is the first datum feature used to establish the datum reference frame. It is the most critical datum and is usually chosen based on the function of the part in the assembly. The primary datum provides the main reference for all other measurements 

Secondary datum: This datum feature is used in conjunction with the primary datum to further define the datum reference frame. It provides additional orientation and location references for the part 

Tertiary datum: This is the third datum feature used to complete the datum reference frame. It provides the final orientation and location references, ensuring that the part is fully constrained in all degrees of freedom.

Datum Reference

Features, also known as datum features, are the actual physical features on the part that are used to establish the datums. They can be points, lines, planes, or a combination.

These features can include surfaces, edges, holes, slots, and other geometric elements that define the shape and functionality of the part.

Surfaces

Flat or curved areas on a part.

Edges

The lines where two surfaces meet.

Holes

Circular openings in a part.

Slots

Rectangular or other types of shaped openings.

Bosses

Raised cylindrical areas on a part.

Grooves

Indentations or channels on a part.

Feature Control Frame


Feature Control Frame (1)

The first section of a feature control frame includes a geometric characteristic symbol. Each frame can hold only one symbol. If a feature has two requirements, use either two separate frames or a composite tolerance. The symbol indicates the type of control applied to the feature.

The second part of a feature control frame shows the complete tolerance for the feature. This tolerance is always a total value, not a plus/minus range.

If the tolerance has a diameter symbol (⌀) before it, imagine the tolerance zone as a circular or cylindrical area—commonly used for hole positioning. Without this symbol, the tolerance zone defaults to parallel planes, typically used for positioning slots or surface profiles.

After the feature tolerance in the feature control frame, you might see a material condition modifier like Max Material Condition (MMC) or Least Material Condition (LMC) for features of size, such as holes. If no modifier is specified, the default is RFS (Regardless of Feature Size), though it's not shown in the frame. For non-size features, like plane surfaces, these modifiers aren't used.

The remaining sections of the feature control frame will include datum feature references if needed. For instance, if a form tolerance like flatness or straightness is specified, no datum reference is used. Conversely, if a location tolerance, such as position, is specified, datum references are typically included.

The order of datum references is based on their importance, not the alphabet. They are read from left to right as primary, secondary, and tertiary. Typically, Datum A is the primary, followed by B and C.

Datum Reference

The primary feature is the first to make contact, requiring at least three points of contact. The secondary feature is the next, needing at least two points, and the tertiary feature is the last, with at least one point of contact. When all three datum features are contacted simultaneously, they form three perpendicular datum planes, known as the Datum Reference Frame (DRF). The DRF is established using Datum Simulators, which include manufacturing, processing, and inspection tools like a surface plate, collet, three-jaw chuck, or gage pin.

Material Condition Modifiers

When defining geometric controls, it's important to specify that a tolerance is linked to a feature's size. Engineers use Maximum Material Condition (MMC) and Least Material Condition (LMC) to express this requirement.

These material condition modifiers are placed in a feature control frame after the feature tolerance. Using MMC and LMC modifiers allows for extra geometric tolerance, known as "bonus" tolerance, when features move away from the specified condition.

Maximum Material Condition (MMC)

The condition where a feature holds the most material possible within specified size limits, such as the largest pin or the smallest hole.

Least Material Condition (LMC)

The condition where the feature contains the least material within the stated limits of size. (ex: smallest pin and/or largest hole).

GD&T Symbols

GD&T is a system where all engineered parts are made up of features. Geometric tolerances are set on these features using feature control frames, which use symbols to show the allowed tolerance.

These characteristics and their symbols fall into four main categories (or characteristics of features): form, orientation, location, and runout.

Form tolerances control the “shape” of features and are often used as a refinement of size, which means they do not require a datum reference.

Orientation tolerances control the "tilt" of features, link to basic angle dimensions, and refine location. Because orientation GD&T is relative, these feature control frames always reference a datum. When applied to surfaces, orientation tolerances manage form.

Location tolerances control the location and are linked to basic linear dimensions. Location GD&T can position a feature or its size based on the feature itself or a set of derived median points. These characteristics are highly versatile and powerful, allowing control over size, form, and orientation within a single feature control frame.

Runout tolerances control the functional and rotational accuracy of a part feature, usually cylindrical or rotational parts, by limiting how much a surface or feature can deviate as it spins around a datum axis.

StraightnessForm Tolerance

Straightness

Controls the deviation of a line on a surface or an axis within a tolerance zone that is defined by two parallel lines a distance apart; no datum required.

FlatnessForm Tolerance

Flatness

Holds a plane within a given tolerance zone defined by two parallel lines; no datum required.

circularityForm Tolerance

Circularity

Also known as roundness, it controls deviation from a true circle; the tolerance zone is the space between two concentric circles.

CylindricityForm Tolerance

Cylindricity

Controls deviation from a cylinder; while similar to circularity, the tolerance zone is between two concentric cylinders.

AngularityOrientation Tolerance

Angularity

Controls the angle between two surfaces, with two parallel planes (both at the required angle to the datum feature) acting as the tolerance zone.

ParallelismOrientation Tolerance

Parallelism

Controls parallelism between two parallel surfaces, with two parallel planes (both parallel to the datum feature) acting as the tolerance zone.

PerpendicularityOrientation Tolerance

Perpendicularity

Controls perpendicularity between two 90-degree surfaces, with two parallel planes (both perpendicular to the datum feature) serving as the tolerance zone.

PositionLocation Tolerance

Position

Also known as True Position, it controls how far a feature of size can deviate from where it should be; the acceptable area is generally defined by a circular or cylindrical tolerance zone.

Surface ProfileLocation Tolerance

Surface Profile

A 3D tolerance zone that defines where the surface needs to be located; it can also be applied to curved surfaces.

Line ProfileLocation Tolerance

Line Profile

A 2D tolerance zone that defines the profile along a 2D cross section of a surface.

Total RunoutRunout Tolerance

Total Runout

Controls the acceptable variation in a surface when it’s rotated 360 degrees around a central axis (the datum feature); the tolerance zone is defined by two concentric cylinders.

Circular RunoutRunout Tolerance

Circular Runout

The 2D version of Total Runout.

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