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SAE J390: Dual Dimensioning on Engineering Drawings โ Standards and Best Practices ๐ ๏ธ
For engineers working with global teams, dual dimensioning – including both inch-pound and SI metric units on the same drawing – is essential. SAE J390 (revised 1999) provides a uniform method for combining these units while maintaining functional interchangeability. This article covers key identification methods, conversion principles, and practical tips for implementing J390 in your workflows.
Methods for Identifying Units on Dual-Dimensioned Drawings
The standard defines four acceptable methods to clearly distinguish inch and millimeter dimensions. Only one method should be used per drawing to avoid confusion.
| Method | Description | Example Arrangement |
|---|---|---|
| A (Position) | Millimeter dimension above or to the left of the inch dimension | [mm above] / [in below] |
| B (Position) | Inch dimension above or to the left of the millimeter dimension | [in above] / [mm below] |
| C (Brackets) | Square brackets [ ] around the millimeter dimension, adjacent to inch | 1.00 [25.4] |
| D (Brackets) | Square brackets [ ] around the inch dimension, adjacent to millimeter | 25.4 [1.00] |
Each drawing must include a note in the title block indicating which method is used, e.g., Dimensions in brackets are millimeters. For existing drawings where space limits positioning, the converted dimension may be placed nearby with the unit symbol (mm or IN).
⚠️ Important: Mixing identification methods on the same drawing can lead to misinterpretation. Choose one method and apply it consistently.
Conversion Principles and Interchangeability
When converting dimensions between systems, proper rounding is critical to maintain part function. The standard references ANSI IEEE/ASTM SI 10 for conversion rules. Key points include:
- Use decimal dimensioning for new drawings – this simplifies dual dimensioning.
- General tolerances must also be dual dimensioned.
- Angular dimensions require no conversion (degrees are common to both systems).
- Nominal designations (thread sizes, tire sizes) stay unchanged.
Engineering Design Insight: For critical features where interchangeability is vital, specify the referee dimensional units on the drawing. This ensures that if a conversion discrepancy occurs, the referee unit determines acceptance. Also, when using dual dimensioning, specify the angle of projection with the ISO symbol; J390 recommends third-angle projection for consistency with typical US practice but notes that first angle is used in many metric countries.
🔍 Design Tip: On new drawings, dimension solely in decimals (inches and millimeters) to avoid fraction conversions. Include a general note for tolerance units. This practice reduces rounding errors.
Frequently Asked Questions
How do I clearly indicate which unit is which?
Use one of the four methods from SAE J390: position (mm above/in below or vice versa) or square brackets around one set of dimensions. Add a note in the title block.
Can I use commas to separate thousands in large numbers?
No. Do not use commas or spaces to denote thousands. Write 32541, not 32,541 or 32 541.
What are the rules for decimal places in metric dimensions?
For millimeter values less than one, place a zero before the decimal point (e.g., 0.13). Nonsignificant zeros are omitted unless needed for uniformity in limit or bilateral tolerances.
How do I handle conversions without compromising part function?
Follow the rounding practices in ANSI IEEE/ASTM SI 10. For critical dimensions, identify the referee (controlling) measurement system on the drawing. Use exact conversion (1 in = 25.4 mm) and ensure that tolerance zones are preserved.
By adhering to SAE J390, engineers can create dual-dimensioned drawings that communicate clearly across international teams and manufacturing sites.
