Knut O. Kverneland
KOK Precision Tooling Co.
It’s curious that in an international engineering meeting, English is the accepted language. What’s not accepted, however, are the units of measure on the equipment under discussion. Engineering units still cause difficulties in the global exchange of parts and data.
All countries are now in various degrees of transition to the metric system. Many European countries are nearly completely metric whereas the UK, Canada, India, and Australia are about half way. In comparison, the U.S. may be about 20% metric. Many industries have been metric for a long time while others are taking a long time to get there. The American auto industry is an example of one in transition for more than 20 years.
Changing to the metric system presents an opportunity for companies to unify metric standards worldwide and encourage the use of more interchangeable parts. These can be mass produced in fewer variety which benefit consumers and producers alike.
To make more parts interchangeable, other factors must also be interchangeable, such as the nominal size of a part, its tolerances, and material quality. A bolt, for example, must have the same physical size, tolerance, and strength class. Steel plates are interchangeable when the thickness, size, tolerance, and steel quality are sufficiently close to allow swapping one manufacturer for another. More importantly, purchasing interchangeable parts and components around the world provides an opportunity to reduce manufacturing costs.
The idea for developing metric standards worldwide comes from a preferred numbering system. Its first known application was in the 1870’s by Charles Renard, a French army captain who reduced the different diameters of rope for military balloons from 425 to 17. The R5, R10, and R20 series refer to the Renard 5 (first size choices in 60% increments) Renard 10 (second size choices, 25% increments) and Renard 20 (third size choices, 12% increments). This series of preferred numbers has been standardized in ANSI Z 17.1 and ISO 3.
Nominal metric sizes are identical where the metric systems have been in use for several years. These reflect preferred sizes for components such as threaded fasteners, steel plates, sheets, and bars used throughout the world. The accompanying table, Selecting a preferred size shows how the general system works.
For example, if a designer was choosing a hydraulic cylinder, bolt, or plate thickness, the sizes in the First-choice column would be preferred. Second and Third choice columns are self explanatory. The table extends to smaller and larger sizes. For instance, 60-mm sizes would be a preferred choice as would 2.5-mm devices.
The three columns to the far right are the originating Renard numbers. In the First-choice column, each succeeding number is 1.6 times the previous, with some rounding. These three columns provide the basis for the values on the left side of the table. The inch values show close corresponding English units.
The form of the first table carries through to other tables in the standard. The number series shown are recommended to reduce the number of standard sizes for items such as screw threads, steel plates, steel sheets, round steel bars, lifting capacities, and hydraulic cylinder diameters.
The table of Preferred screw threads shows similar first-choice values to the previous table. The eleven first-choice fastener sizes are all metric coarse thread and recommended to replace the other 53 thread sizes listed. Going one step further, the four sizes in bold are based on the R5 series in the table Selecting a preferred size. Cost reductions become substantial considering that thousands of dollars can be saved for each unique fastener size eliminated from a product or inventory.
ISO tolerance standards offer industry an additional savings opportunity. A few software programs make the information in the standard tables more easily available and can maximize savings and cut design time as well.
Rating basic sizes and tolerances helps reduce the number of hole and shaft sizes specified. Hole basis fits (terminology used in the standards) and tolerances are identified by ANSI and ISO as H11, H9, H8, and H7. These help rationalize or zero-in on standard cutting tools and gages. Shaft basis fits have tolerances called h11, h9, h7, and h6, and help rationalize on standard steel bars available in all major industrial countries. The tolerances depend on the diameter and the standard spells out each. For instance, the h11 tolerance on a 25-mm bright-finish round steel bar is +0.000 and -0.13 mm, and h9 on the same bar is +0.000 and -0.052. The h7 and h6 tolerances are tighter.
The ANSI standard includes ten preferred hole and shaft basis fits ranging from Loose Running to Force fits. It is recommended to use hole basis fits in most applications because it helps reduce the cost of cutting tools such as reamers, and gages. It’s also an advantage to use shaft basis fits when a standard shaft size in a machine carries bearings, couplings, sprockets, gears, and other components. Each preferred fit has the same clearance or interference for holes or shafts on the same line.
The ANSI B4.2 standard provides a table of allowances for the ISO tolerance zones and limit dimensions for preferred holes, shaft tolerances, and first-choice sizes. In the U.S., these tables and others are published in the Machinery’s Handbook, and in Metric Standards for Worldwide Manufacturing, published by the ASME Press, New York. The later text holds many additional tolerance tables for a wide range of components.