GD&T
Geometric Dimensioning and Tolerancing is a system for defining and communicating engineering tolerances. It uses a symbolic language on engineering drawings and computer-generated three-dimensional solid models that explicitly describe nominal geometry and its allowable variation.
It tells the manufacturing staff and machines what degree of accuracy and precision is needed on each controlled feature of the part
ISO defines GD&T as “geometrical product specifications (GPS)—Geometrical tolerancing—Tolerancing of form, orientation, location and run-out.” In short, “geometrical product specifications” refer to the shape, size, and positional relationship of a product, while “tolerance” means the allowable error. “Geometric tolerance” is characterized by a definition that includes the allowable errors for the form and position in addition to size.
Applications
➢ Provides a precise and consistent method for communicating design intent
➢ Establishes a coordinate system (that references the datum features) for inspection and manufacturing
➢ Reduces the need to explain complex requirements
➢ Facilitates and simplifies gaging requirements
➢ Identifies critical-to-function features
➢ Simplifies tolerance analysis
Course Highlights:
➢ Introduction
➢ Advantage of GD&T
➢ Feature And Rules in GD&T
➢ Datum Controls
➢ Fundamental of Routing
➢ Creating Routing Components
➢ Using P & ID files
➢ Electrical Ducting, Cable Tray, and HVAC Routes
➢ Tolerances in GD&T
➢ Adding GD&T To a Drawing/design
Duration
- 25 Hours Theory
- 25 Hours Practical
- 20 Hours Project work
Technical Feature
Geometric Dimensioning and Tolerancing is a system for defining and communicating engineering tolerances. It uses a symbolic language on engineering drawings and computer-generated three-dimensional solid models that explicitly describe nominal geometry and its allowable variation.
➢ Why is geometric tolerance necessary?
For example, when the designer orders sheet parts, the size tolerance-based instructions will be as follows:
These parts are non-conforming or defective products.
These parts are produced because there is no mention of parallelism in the drawing.
The fault lies with the tolerance instruction by the designer, and not with the manufacturer.
The drawing for the same part will be as follows using geometric tolerance. Here, the geometric characteristics “parallelism” and “flatness” are used in addition to size. This can prevent errors like those we saw above with size tolerance.
Geometric tolerance has the advantage of accurately and efficiently communicating the designer’s intended design in a way that cannot be expressed using size tolerance alone
➢ Rule #1
Rule #1 states that where only a tolerance of size is specified, the limits of size of an individual feature of size prescribe the extent to which variations in its geometric form, as well as its size, are allowed. No element of a feature shall extend beyond the MMC boundary of perfect form. The form tolerance increases as the actual size of the feature departs from MMC toward LMC. There is no perfect form boundary requirement at LMC
Let us now discuss rule #1 in detail. The simplest meaning of rule #1 is at MMC the feature is in perfect form. For example consider a shaft with diameter Ø5.000-Ø5.040. Then its MMC is Ø5.040. Now assume we have this part produced at Ø5.040 then according to rule #1 the part should be in perfect form. That is, it should have perfect straightness, circularity and cylindricity.
➢ Rule #2
Rule #2 states that RFS automatically applies, in a feature control frame, to individual tolerances of size features and to datum features of size. MMC and LMC must be specified when these conditions are required.
Now examine rule #2, here also lets assume a stepped shaft with Ø5.000 and Ø3.000-Ø3.004. The bigger diameter is assigned as datum (Say C) and the smaller diameter Ø3.000 is controlled by a concentric geometric tolerance and this tolerance is applied without any dimensional size constraints ie regardless of feature size (RFS)
✓Datum Control:
A datum is the theoretical exact plane, axis or point location that GD&T or dimensional tolerances are referenced to. You can think of them as an anchor for the entire part; where the other features are referenced from. A datum feature is usually an important functional feature that needs to be controlled during measurement as well.
As stated before, datums can be located on points, axes,edges, and surfaces. It is important though that they are called out correctly on the drawing to control the right type of feature. Here is how different types of features are called out on engineering drawings.
➢ Form Tolerances
➢ Orientation Tolerances
➢ Profile Tolerances
➢ Location Tolerances
➢ Run out Tolerances
Step 1: Define Part features that would serve as origin with specific directions for measurement
➢ This step relates to Datums. Datums need to be selected based on the following criteria:
➢ Representative of Mating Features
➢ Reflect Functional Assembly
➢ Stable
➢ Repeatable
➢ Accessible
➢ All of the above criteria are equally important. The choice and order of Datums Selection need to be in alignment with the Quality objectives we intend to protect. In other words, Design For Assembly (DFA), that influences Datum Selection and Precedence (order), needs to be in alignment with Design For Quality (DFQ) objectives.
➢ In this case, the datums would be as shown in the picture below.
Step 2: Specify Nominal (Basic) Dimensions for Location and/or Orientation of Features from Datums
In our example, since the Cylindrical and Conical features are located concentric to Datum A, they are at Zero Basic Dimension and hence not shown.
Step 3: Specify Tolerance Zone Boundaries for Part Features in terms of shape and size along with specific rules for compliance
This step requires knowledge of process capability existing (with the organization or suppliers). Additionally the dimensioning schema should have the least number of dimensions between the datum reference and features that form a part of the measurable Design for Quality Objective. This is essential to identify the correct Critical To Quality (CTQ) or Key Characteristics for the part that would help in inspection and reporting. In comparison to the Plus/ Minus Tolerancing Schema, the GD&T approach ensures clarity in setup during manufacture & inspection, alignment with Quality objectives while keeping cost as a driver for change.
Step 4: Allow Dynamic Interaction of Tolerance Zones between Features in a Part, and across parts, simulating Assembly possibilities for Maximizing Tolerances
This step differentiates GD&T from Plus / Minus Tolerances in enforcing a methodology that would help reduce cost while meeting Quality objectives. Drawing is incomplete without this step. Even companies that incorporate GD&T, fall short of this requirement, sometimes leading to an erroneous conclusion that GD&T is expensive and its impact on quality is not significant.
Step 5: Cost of Precision Vs Cost of Poor Quality Analysis for Optimal Tolerances
By choosing alternate Process Capability, providing costs incurred to maintain specified tolerances and performing a risk assessment based on Risk Priority Number (RPN) of Design Failure Modes & Effects Analysis (DFMEA), a designer can arrive at optimal tolerances and provide KC’s (Key Characteristics) on drawings. This step ensures that management is provided with alternatives in terms of costs, choice of suppliers and investment in machinery and manufacturing lines to achieve desired levels of quality in an objective manner.
