08 Jun Using Critically-Dimensioned Drawings
The organic shape of our MEMS-based windmeter
By now, most mechanical design engineers are used to sending off a solid model and having plastic parts show up in less than a week. While this technique is perfectly suitable and cost-effective for rapid prototyping, it doesn’t quite make the grade for production.
What’s in a model?
3D CAD models can provide a manufacturer with every dimension and feature necessary to make a part and importing the model directly eliminates any errors of translation between paper drawings and the computer-controlled tooling or machining software. Additionally, generating fully dimensioned drawings of plastic parts with complex surfaces is impractical and may even prohibit certain design features as a result. However, part models need the context of a 2D drawing package to fully define the part and its interaction with the whole product.
Highlighting The Good Stuff
2D drawings are effectively the contract between the designer and the manufacturer. In addition to part dimensions, 2D drawings can describe variations in tolerances throughout the part, surface textures, flatness, material specifications, and many more characteristics that the manufacturer must meet. Expectations are well understood and parts can be either accepted or rejected based on this document.
Formerly, 2D drawings included at least three views and dimensions for every feature. These are known as fully-dimensioned drawings. These drawings were required prior to the existence of computer-controlled machining capabilities and were the only way to communicate with manufacturers. As machines become more integrated with 3D CAD software packages, designers enjoy more flexibility, including the ability to design part features with organic, complex, three-dimensional surfaces. With this added functionality comes the complication of adequately communicating those features.
With a single drawing note similar to, “All dimensions shall be held to within ±0.010” of the provided 3D part model unless otherwise shown,” a drawing can be limited to just highlighting the most critical features. This is known as a critically-dimensioned drawing, and it is generally easier to produce and far easier to read. The drawing can include dimensions to features that require a tighter tolerance than the standard, surface callouts, and manufacturing instructions such as limitations for regrind and minimum cure times. A standard dimension can still be included if it is particularly important just to call it to the manufacturer’s attention or to show that it should be inspected for acceptance or statistically monitored as part of the quality assurance program.
Put it all together
In addition to the 2D part drawings, 2D assembly drawings show how all of the parts fit together and list the part numbers and quantities required. This drawing brings the pile of unrelated parts together into a cohesive product. The assembly drawing may also include instructions regarding screw torques, adhesive cure times, or any number of other product-level requirements.
For effective and flexible communication with the manufacturer, the best solution is a combination of solid models and drawings. The goal is to limit the possibility of misunderstandings or miscommunication while recognizing that complicated, fully-dimensioned drawings just are not necessary in most instances. Used well, critically dimensioned drawings combined with 3D models can convey the information required to make a high-quality part as well as or better than our predecessors could.
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