Hello! Welcome back.
In our previous lessons, we've systematically built up the toolkit for specifying every critical aspect of a component. We've covered how to define datums, apply various GD&T controls, select appropriate tolerances for manufacturing, and specify surface finishes and material treatments.
Today, we bring all those pieces together. This lesson addresses the culminating outcome for this module: Create detailed drawings for critical components including dimensions, tolerances, and GD&T. A detailed drawing is arguably the most important document a design engineer produces. It is the official instruction set that translates your design intent into a physical part. We'll cover the "why" of advanced tolerancing and the "how" of implementing it within Onshape.
1. From Dimensions to Geometric Control
In traditional drafting, we often rely on simple plus/minus tolerances on linear dimensions. While this works for simple parts, it has limitations. It defines a square or rectangular tolerance zone, which may not accurately represent the part's functional needs, and it doesn't explicitly control the orientation or form of features.
Geometric Dimensioning and Tolerancing (GD&T) is a symbolic language that gives you, the designer, precise control over a feature's form, orientation, and location. This allows you to specify tolerances that directly relate to a part's function, ensuring parts fit and work as intended while often allowing for larger, more manufacturable tolerances.
To build a strong foundation, let's start with a clear, concise overview of what GD&T is and why it's so powerful.
The video "Understanding GD&T" by The Efficient Engineer is an excellent primer on the subject. It explains the core concepts of GD&T, including the structure of a feature control frame and the crucial role of datums.
Please watch from 1:02 to 4:05 and from 9:28 to 12:25. The first segment introduces the 14 geometric characteristics and, most importantly, breaks down the feature control frame. The second segment explains what datums are and how they create a datum reference frame (DRF), which is the foundation for inspection.
As the video explained, the two core components of any GD&T callout are:
- Feature Control Frame: The rectangular box that contains all the tolerance information: the geometric characteristic symbol, the tolerance value and zone shape, any material modifiers, and the datum references.
- Datums: Theoretically exact points, axes, or planes derived from physical features on your part. They form a coordinate system (the DRF) against which all measurements are made, mimicking how the part is located in its final assembly.
2. The Anatomy of a Feature Control Frame
The feature control frame is the "sentence" of the GD&T language. Understanding how to read it is essential. Let's get a more detailed, text-based breakdown.
Our Guide to Geometric Dimensioning and Tolerancing
The article "Our Guide to Geometric Dimensioning and Tolerancing" from Fictiv provides a detailed, industry-focused explanation. We'll focus on the section that dissects the feature control frame.
Please read the section titled "GD&T Feature Control Frames". Focus on understanding the role of each compartment: the geometric characteristic, the tolerance value, and the datum references, including their order of precedence.
Let's look at a practical example.
In the drawing above, find the callout for the two holes. It reads:Ø0.3 M | A | B | C
Here's how to interpret it, reading from left to right:
- Geometric Characteristic: The symbol is for Position, a location control.
- Tolerance Zone: The axis of the hole must lie within a cylindrical tolerance zone of Ø0.3mm.
- Material Modifier: The M in a circle stands for Maximum Material Condition (MMC). This means the 0.3mm tolerance applies when the hole is at its smallest size. As the hole gets larger (departs from MMC), a "bonus tolerance" is added, making the part easier to manufacture.
- Datums: The tolerance zone is located and oriented relative to a datum reference frame established by Datum A (primary), Datum B (secondary), and Datum C (tertiary).
Test your understanding!
Looking at the GD&T Drawing Example image again, find the feature control frame pointing to the top surface. It contains the symbol for Perpendicularity, a value of 0.1, and datum references A and B.
How would you describe this requirement in plain English?
Show answer
The top surface must be perpendicular to Datum A within a tolerance zone of 0.1mm. Datum B is also referenced, which helps lock the orientation of the tolerance zone. In simpler terms, the entire top surface must lie between two perfectly parallel planes that are 0.1mm apart and are oriented at a perfect 90-degree angle to Datum A.
3. Creating Detailed Drawings in Onshape
Now that we've covered the "what" and "why," let's move to the "how." Onshape has a dedicated Drawings environment with all the tools needed to create professional engineering drawings.
The following video gives a fantastic overview of the entire process, from creating the first view to adding dimensions and annotations.
Onshape - Drawings Overview - How to Create New Drawings, Views, and Dimensions
The video "Onshape - Drawings Overview" is a practical, step-by-step guide to the Onshape drawing environment. It will show you how to create views, add dimensions, and place annotations.
Please watch the entire video (up to 15:22). It covers three main stages that you will perform every time you create a drawing: Creating the Drawing and Views (0:00 - 5:47): Starting a drawing, choosing a template, and placing orthographic, isometric, and section views. Adding Supplemental Geometry and Dimensions (5:47 - 13:11): Placing centerlines, centermarks, and various types of dimensions (linear, ordinate, chamfer, radial). Adding Notes and Properties (13:11 - 15:22): Creating general notes and using parametric properties to automatically populate the title block.
4. Applying GD&T Symbols in Onshape
The video gave a brief glimpse of dimensioning but didn't go deep into GD&T. Onshape has a specific set of tools for adding datum identifiers and feature control frames.
For a thorough guide on these specific tools, the official Onshape learning materials are the best resource.
Onshape's self-paced course on "Detailed Drawings" has a specific lesson on GD&T. It will show you exactly which buttons to click to add all the symbols we've been discussing.
In the course outline, navigate to section 03. Annotations and find the lesson titled "Geometric Dimensioning and Tolerancing". Please go through this short interactive lesson. It will demonstrate how to: Create a Datum symbol. Construct a Feature Control Frame, including selecting the characteristic symbol, entering a tolerance, and adding datum references.
When you combine all these elements—multiple views, dimensions with tolerances, GD&T callouts, surface finish symbols, and material notes—you get a complete, professional drawing like the one below.
Take a moment to study this drawing. You can see how the designer has used:
- Orthographic and Section Views to show internal and external features clearly.
- Dimensions with bilateral tolerances (e.g.,
35 +0.01/-0) for size control. - GD&T callouts (like the runout symbol) to control features critical for rotation.
- Surface Finish symbols on bearing journals.
- A note specifying the material and heat treatment.
This drawing leaves no room for ambiguity. A manufacturer has all the information needed to produce and inspect the part correctly.
Conclusion
Today, we've transitioned from understanding individual specifications to integrating them into a complete engineering drawing—the primary deliverable for a design engineer.
Key Takeaways:
- A detailed drawing is a comprehensive set of instructions that uses views, dimensions, tolerances, notes, and symbols to fully define a part.
- GD&T provides a precise language to control a feature's form, orientation, and location, ensuring functional requirements are met.
- The Feature Control Frame is the core of a GD&T callout, specifying the characteristic, tolerance, and datum references.
- Onshape provides a full suite of tools for creating views, adding dimensions, and applying all necessary annotations, including GD&T symbols.
Preview of the Next Lesson:
We have now completed our tour of the theory and practice of detail design and tolerancing. In the next lesson, we will begin Module 6, Project 1: Finalization and Risk Assessment. You will take the components you designed for the linear motion system and apply everything you've learned in this module to apply GD&T and refined dimensions to the Project 1 detailed drawings. It's time to create the first major deliverable for your portfolio.