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GD&T 101: An Introduction to Geometric Dimensioning and Tolerancing
How to Accelerate Your Engineering Builds (and Ensure You Get Parts That Fit)
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While engineering drawings are a great way to communicate design intent for CNC machining, providing a drawing with your prototype RFQ (request-for-quote) may actually make your quote more expensive and incur longer lead times.
That’s because an engineering drawing is a legally binding document. Cautious machine shops carefully review every call-out on a drawing to make sure each one is feasible and it takes longer to quote a fully dimensioned drawing compared to a drawing with only a few inspection call-outs.
Additionally, machine shops will bump up the quoted price when they see drawings full of dimensions and restrictive tolerances because they know they’ll have to spend more time inspecting the part and throw out finished parts that don’t meet spec. For example, if you order quantity one, the shop might have to make three copies to yield one that meets the +/-0.0005in tolerance. Since potential low yield must be accounted for, you’ll pay for the extra effort and wait longer for your prototypes.
In some cases, certain requirements simply must be communicated via an engineering drawing, but it’s imperative for engineers to convey only critical requirements. This saves not only drawing time; it also saves production time, which ultimately translates into lower prototype fabrication costs.
The following 9 tips will teach you how to create better engineering drawings, that clearly communicate your critical requirements and also save you time & money.
In CNC machining, all dimensions can be derived from the 3D model. Therefore, only critical inspection dimensions and threading information are needed on a 2D drawing.
At Fictiv, we assume all dimensional call-outs require inspection. However, by default we can only guarantee a dimension that’s measurable by hand metrology tools, such as calipers, micrometers, and pin gauges.
Not measurable directly with hand tools:
Thread depth is hard to measure exactly; therefore, the depth call-out is always treated as a minimum.
Dimension only one of the features and label the dimension as “#X DIM”, meaning that the feature exists in that view “Number” times. For example, “4X 10-32 TAP” implies that in the view, there are 4 10-32 threaded holes.
If an entire assembly is being machined, provide an assembly drawing or instruction. Alternatively, if you’ll be installing McMaster off-the-shelf hardware by yourself, provide the part number so the machinist can look it up.
Just noting “press-fit M4 dowel” doesn’t give a machine shop dowel length or material information. If your machine shop can’t identify a part, they can’t purchase it.
If secondary operations, such as polishing and anodizing, are optional, not critical, it’s best to request quote and lead time for those add-ons separately, so you know the additional time & cost. In my experience, many people don’t find secondary operations worth the additional lead time and cost until late-stage prototyping.
Similarly, if you are unsure of what material to use or are trying a few different materials, leave material off the drawing so it doesn’t cause confusion in production.
Typically, only a few features on a part are critical to its function, so you want the machinist to pay extra attention to these features. When you over-dimension, the critical requirements are lost in the noise, so assign only tolerances to mission-critical features. Over-dimensioning will also drive up cost of the prototype.
For reference, the tools we use for measurements at Fictiv have the following accuracy:
At Fictiv, we don’t discourage using GD&T controls to communicate design requirements—we appreciate the system’s efficiency compared to traditional linear dimensioning. However, some GD&T’s controls can’t be easily verified with hand metrology tools, which is the standard in quick-turn prototyping.
We commonly see the following GD&T control symbols on prototype drawings:
Standard practice is to skip inspection of these call-outs, unless a machine shop is specifically instructed to provide a CMM inspection report (which may add several hundred dollars and extra days of lead time).
Over-dimensioning (dimensioning too many non-critical features) and fear-tolerancing (unnecessarily requiring less than +/-0.05mm or +/-0.002in on features) are the leading causes of money hemorrhaging in prototyping.
These tips should save you time during design and money during prototyping. When you get the urge to dimension a feature, ask yourself: is this critical for assembly, and is this requirement measurable? Call out that dimension only if you answer “yes” to both.