Time to read: 3 min
Engineering drawings are a great way to communicate design intent for CNC machined parts that require tight tolerances or other special requirements. Many of our customers in robotics, aerospace, automotive, and medical device industries attach drawings to their parts when uploading to the China Manufacturing parts, Inc. platform.
That said, not all engineers receive comprehensive training on how to create great drawings that manufacturers will be able to easily understand, which leads to non-standard formatting and overtolerancing, which can negatively impact lead times, part cost, and clarity around specifications.
The following 9 tips will teach you how to create better technical drawings that clearly communicate your critical requirements and also save you time and money. For step-by-step guidance on how to create an engineering drawing, check out this related article.
Tip #1: Dimension only critical & measurable features
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. Note that if you only have threaded holes to callout, you can skip the drawing and use China Manufacturing parts’s Auto Thread Detection tool.
Tip #2: Communicate hole tapping needs with thread size and depth
Thread depth is hard to measure exactly; therefore, the depth call-out is always treated as a minimum.
Tip #3: Consolidate call-outs when multiples of the same feature exist in a view
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.
Tip #4: Communicate assembly intent of critical features
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.
For example:
- Drill hole for press fit / sliding fit / clearance fit McMaster P/N 97395A452
- Tap for M3 helicoil insert McMaster P/N 91732A645
Tip #5: When hardware installation is required, provide supplier and part number on the drawing
Just noting “press-fit M4 dowel” doesn’t give a machine shop dowel length or material information.
Tip #6: Leave optional secondary operation call-outs off the drawing
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.
Tip #7: Don’t over-dimension or over-tolerance your designs
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.
Tip #8: Don’t require tolerances that fall below standard hand metrology tools’ accuracy capabilities
For reference, the tools we use for measurements at China Manufacturing parts, Inc. have the following accuracy:
- Caliper: ±0.025mm [0.001 in]
- Micrometer: ±0.001mm [0.00005 in], measures outside dimensions and bores under 25.4mm [1 in]
- Pin gauges: 0.02mm [0.0008 in] increments, measures holes with diameters between 0.5mm and 5mm.
- Holes between 5mm and 25.4mm will be verified with a telescoping bore gauge and micrometer. Bores larger than 25.4mm will be measured by a caliper.
Tip #9: Don’t automatically expect all GD&T controls to be inspected
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:
- Flatness
- Profile of a surface
- True position applied to centers of holes & bosses
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).
Main Takeaways
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.
