• Contact Us
• Quick Quote
• Projects
• Services
• Home

• Resources

Guide to Metric Dimensioning for Domestic Engineering Design and Craftsmanship

J.P. Leonard, Varient Precision LLC, Madison WI 53704 v2.0 - September 2010

Today many international engineering designs are developed using metric dimensions, yet much work in the United States still involves the U.S. customary (American) system of units. Modern engineering software can readily convert between metric and American lengths, and can provide dual dimensioning in design drawings.

Nonetheless, designers know that there is a significant difference between simply converting the dimensions to metric (or American) units, and designing directly in those units. Much like different languages, a literal word-by-word translation is inferior to one that preserves the meaning and spirit of the original language. In engineering design, all objects or assemblies will inevitably have subtle differences in size and proportions depending on whether they were designed in metric or American units.

In practice, metric designs present some difficulties when parts or assemblies are fabricated. This is in part due to the prevalence of standard mechanical hardware as well as material stock in standard American sizes in the U.S. market. But it can also be due to unfamiliarity or reluctance to work with metric units. Lets consider several different fields in the U.S. that involve design and fabrication:

• Most metalworking today is via CNC automation, which can easily operate in either system of units. One practical problem with metric design is the relative rarity of metric tooling in many shops, as well as difficulty sourcing fastener hardware in metric sizes. Metal stock manufactured in the U.S. usually has nominal dimensions aligned to fractional inch values. Designers, machinists, and craftsmen often are most experienced with inches, and therefore most comfortable using American dimensions.
• In the architecture and building trades, virtually all domestic stock materials and components are based on American units. Although CAD designs can be built using native metric units, the joints, panels, and structural features must match to existing American unit sizes. As well, a translation from metric to American units may still be necessary for fabrication and code compliance, given the unfamiliarity of the metric system in the construction trades.
• Precision woodworking in the U.S., such as millwork or cabinetry, remains less automated than metalworking. Mass market furniture and toys are often built internationally, and therefore likely to be designed in the metric system. Niche areas such as custom residential woodwork, musical instruments, boat- and coach-building in the U.S. often involve small shops with much manual setup and fabrication. Wood is a natural product, of variable size and quality, and most stock must be straightened by cutting and planing to the desired size. Craftsmen involved in precision woodwork recognize that final dimensions are somewhat independent of standard stock sizes, and therefore could be amenable to metric design. What is much more problematic, however, is the dominance of power equipment and cutting tools graduated in American units.

Despite these limitations, it is worth considering the advantages of using the metric system in domestic designs. The practical advantages of using the metric system include simpler decimal-based units and more opportunities for international collaboration, sourcing of components, and fabrication. For the designer there is an additional advantage-- learning a new way of looking at the world is often a channel for new insight and creativity.

Engineers with design experience primarily in the American system must relearn some of the intuitive aspects of dimension relationships before they can design effectively in the metric system. To be effective, one must be able to think directly in metric units, be familiar with standard metric tolerances and clearances. Even more importantly, one must successfully remap the suitable length intervals for the design onto the metric ruler. By following the simple approach and tables shown here, and with practice, one can become fluent in the metric length system and able to produce native metric or American designs.

Measurement

Obtain rulers, calipers, micrometers and indicators that are graduated exclusively in millimeters (mm), or if electronic, set to display only metric units. This will force you to measure directly in the new system, and over time gain an intuitive appreciation for the physical size of objects in metric units.

Translation American-to-Metric

An intermediate step to learning native metric design can involve mapping American intervals and fractional length relationships onto the metric ruler. This is also useful when modifications or retrofits to American-based designs are needed, yet must be executed in the metric units. This remapping is shown in Table 1.



Mapping to the Metric Ruler

Native designs in metric units should generally consist of low-order decimal intervals on the metric ruler. As this system is primarily base-10, the simplest sub-intervals are 1/100, 1/10, 1/5 and their multiples. Other sub-intervals are available using fractional powers of 2 (e.g. 1/2, 1/4, 1/8) , although 1/16 and 1/32 are difficult to achieve. In this case, 1/20 and 1/40 are often the best equivalents. Recommended metric unit lengths are given in Table 2.



Tolerances and Standard Sizes

Tolerances are determined by machining and fabrication techniques and represent true physical lengths independent of the measurement system. Native metric designs should also use appropriate low-order decimal schemes for tolerancing and clearances. Some recommendations are given in Table 3.



Machine Screws

Machine screws and associated tooling are most broadly available here in American sizes, while metric sizes are often still considered special order items. The use of American screws is therefore a possible compromise for metric designs implemented in the U.S.. Table 4 provides common screw and hole sizes mapped to the metric ruler.



In summary, translating designs from American to metric units is possible, but should generally use a remapping to whole or even-valued metric numbers. In this way, the original intent and fractional relationships in the design can be preserved, while producing a design that is compatible with metric tools and tolerancing. A hybrid design that uses metric sizes but American hardware and stock sizes is also possible using this mapping method.

Native metric designs, however, will often have new proportionality relationships such as 1/10, 1/5, 1/2. Proportions such as 1/16, 1/8 and even 1/4 are generally less desirable in metric designs. The result will be parts and assemblies that are subtly different, but just as functional and easy to build as those in the American system. This is particularly true when designs are generated in the U.S. but will be fabricated internationally.