In precision machining, datums make everyone’s lives easier. Datums are lines, planes, or points that serve as a reference for consistently measuring and locating part features. These perpendicular reference points are often marked on a schematic with a boxed letter and an arrow. When used correctly, they ensure a part functions as intended.
Precision machining companies like Reata use datums to manufacture parts accurately. Datums help us create and orient a part in CAD, measure from a consistent point, and mate parts perfectly. The proper use of datums increases production efficiency and part quality.
When creating a part schematic, take extra care when choosing datums. A few minutes of forethought at the beginning of your project can save time, money, and headaches later on.
The Role of Datums in Precision Machining
Datums come into play in assembly and inspection processes. When we assemble parts, datums serve as consistent points of reference, from facilitating proper alignments to GD&T profiling.
Datums are equally important during inspection. Using the same datums to measure each part ensures final parts are accurate and consistent. Datums are also useful for meeting tolerance specifications.
In short, datums impact a part’s fit, form, and function. These reference points play a key role in the early stages of part development, and incorrect usage can create major issues down the line.
3 Key Qualities of Effective Datums
With so many options in more complex parts, it’s hard to know which plane, straight line, or point to choose as your datum.
Here are three key factors to keep in mind when marking datums:
- Ease of measurement. An ideal datum is easy to measure from because it’s the most relatable point. Proximity to other components is essential.
- Feature simplicity. Datums should not be complex features that are difficult to locate or accurately measure.
- Size in relation to part. Datums should be larger than the surfaces being measured to avoid projection errors. In some cases, if you are using a plane as your datum the size would matter for flatness, which would control your projection issues. However, if you’re talking about using a hole as your Datum, the distance from the feature that is important is much more related to the projection issues. (For example if you have a hole as your Datum, and it tilts by 1 degree the more you project that over a distance the greater the deviation becomes.)
For example, examine this schematic. The primary datum (marked by a boxed A) is a good choice because the outside location and large size make it easy to measure. The vertical plane is a simple feature that’s easy to locate, and it’s large enough to avoid projection errors.
Common Problems When Using Datums
As a precision machining company, we see many issues regarding datums. Here are a few to avoid:
- Tolerance stacking. Incorrectly calling out a datum or neglecting to provide one can lead to tolerance stack-up issues. A poorly chosen datum can cause difficulty measuring part features. For parts with tight tolerances, incorrect datum usage can create significant misalignments.
- Size discrepancies. Using a datum smaller than the surface being measured can lead to projection errors, and parts could be incorrectly categorized as bad. If datums are improperly defined, manufacturers can encounter problems fixturing parts.
- Strained relationships. Improper datum usage can strain your relationship with manufacturers. Mistakes at the beginning of a project can lead to longer quality control times, increased costs, and even significant back-and-forth between you and your precision machining company.
These challenges are frustrating for customers and manufacturers. Thankfully, it’s possible to avoid these issues by choosing the right datums from the get-go.
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