|FOR IMMEDIATE RELEASE
How to Select the Optimum Preferred Metric Sizes and Help Cut Manufacturing Costs
Choose the best metric standard sizes and compete on the global market.
Last modified: 2012-01-27
How do you select the optimum metric sizes when you design a new metric product? Will material stock, cutting tools, fasteners, etc. be readily available where parts are to be produced? If not, this may cause a cost penalty and added manufacturing time. If you select from the First, Second or Third choice sizes shown in Figure 1 Preferred Metric Sizes, you will ensure that design and manufacturing practices follow the established global metric standards.
The selection of a preferred size in the customary inch system has been helped by the fractional system to some extent. In the inch design you would select the whole inch sizes first, and then in increments of 1/2, 1/4, 1/8, etc. The metric system does not use fractions, and there are 25.4 millimeters sizes to choose from in each inch. This is why the preferred numbering system or the preferred metric sizes (ASME B4.2) is so important to use in all metric product designs.
Material presented in this news release will help you in your product designs. If you need more details, use to the electronic or paper editions of the METRIC STANDARDS for Worldwide Manufacturing book.
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 varieties which benefit consumers and producers alike.
To make 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, coating and strength class. Steel plates are interchangeable when the thickness, size, tolerance, and the steel quality are sufficiently close to swapping one manufacturer to another. More importantly, purchasing interchangeable parts and components around the world provides an opportunity to reduce manufacturing costs.
Preferred Metric Sizes
The preferred numbering system has played a major role in the development of metric standards. This is a geometrical series of numbers adopted worldwide. Its first known application was in the 1870s 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 refers to the Renard 5 (first choice sizes 60 % increments), Renard 10 (second choice sizes 25 % increments) and Renard 20 (third choice sizes 12 % increments) series of preferred numbers standardized in ASME Z17.1 and ISO 3.
Nominal metric sizes are identical where the metric system has been in use for several years. Here is how the preferred metric nominal sizes were developed and how these chosen sizes reflect preferred metric standard sizes for threaded fasteners, steel plates, sheets, bars, etc already in use throughout the world.
How do the preferred metric sizes relate to the customary inch sizes and the preferred numbers are shown in Figure 1.
The metric cold finished round bar sizes specified in the new ASME B32.100-2005 standard are according to the sizes shown in Figure 1 below.
The ASME preferred metric sizes are identical to those in the ISO 497 R20' rounded series selected years ago. The intent of the number series shown is to help reduce the number of standard sizes for screw threads, steel plates, steel sheets, round steel bars, lifting capacities, hydraulic cylinder diameters, etc.
The preferred size range from 4 through 40 millimeters may be extended to cover smaller or larger sizes by just multiplying or dividing sizes shown by 10. For instance, 60-mm sizes would be a preferred choice as would 2.5-mm devices.For example, the six first choice thread sizes shown in Figure 2 are recommended to replace the 61 other thread sizes listed. The first choice sizes are according to the R5' series of preferred sizes shown in Figure 1. Less savings will be achieved if you select the twelve ISO (green) first and second choice sizes shown or the 14 ASME (bold) first choice sizes. The cost reduction becomes substantial when you figure thousands of dollars savings for each unique fastener size that can be eliminated from the product design.
The metric coarse thread pitch is slightly smaller (finer) compared to the customary unified coarse thread pitch. For example; the metric coarse thread M8 has the thread pitch 1.25 mm and the inch thread 5/16 18 (7.94 mm) has the thread pitch 1.41 mm.
Preferred metric, standard metric and customary inch steel plate sizes are shown in Figure 3. It is recommended selections be made from the preferred metric column marked first and second choice. Selection of first choice sizes will further reduce the number of plate sizes from eight to four. This is a huge cost reduction compared with the 19 inch plate sizes that used to be stocked by a large USA company. Steel plate sizes shown are those specified in the new ASME B32.100-2005 standard. Preferred metric sizes for thin flat products (sheets) in the range from 0.06 through 4 mm follow the sizes shown in Figure 1 in the above standard.
You probably do not have to reduce the numbers of ropes used to hold balloons, but the Renard series of sizes gives you a great tool to help cut costs in manufacturing.
ISO tolerance standards offer industry a savings opportunity. New software programs (KOK ISOTOLTM Computer Tolerance Software) make those standards more easily available and can maximize those opportunities and save time as well. Rating basic sizes and tolerances helps reduce the number of hole and shaft sizes specified.
Hole basis fits with hole tolerances identified by ASME and ISO as H11, H9, H8 and H7 help rationalize on standard cutting tools and gages, whereas shaft basis fits with shaft tolerances h11, h9, h7 and h6 help rationalize on standard round steel bars available in all major industrial countries. The recently released ASME B32.100-2005 standard specify the above shaft tolerances. See Figure 4.
The ASME standard and software have ten preferred hole and shaft basis fits ranging from LOOSE RUNNING to FORCE fits shown in Figure 5. and in KOK ISOTOLTM and KOK ISOGAGETM software. It is recommended you use hole basis fits in most applications since it helps you reduce the cost of cutting tools and gages. However, it may be to your advantage to use shaft basis fits where you have a standard shaft size in a machine with bearings, couplings, sprockets, gears, and other components attached to it. Examples include knitting, printing, and farm machines.
Each preferred fit has the same clearance or interference for hole or shaft basis fit listed on the same line.
The system needs no particular CAD
software. Users still must use standard hole or shaft sizes in order to
save in manufacturing. Engineers in the EU countries have taken advantage
of the system for over 70 years, and many users there still use published
tables or recall the allowances from memory.
ISO screw thread tolerances are the medium tolerance class 6H for nuts and 6g for bolts as shown in Figure 6. The identical nut tolerance 6H is used for both close- and medium-fit nuts. The close bolt tolerance 4g6g is recommended for set screws and socket head screws and the fastener user taps identically threaded holes for all threaded fasteners.
Tolerances for other steel products are typically greater in many ISO product standards, and may sometimes be twice as large as those used for customary inch products. Refer to specific standard in order to find the tolerance applicable. A large number of ISO steel product standard tolerances are shown in the referenced metric standards book, and a major steel producer use it to prepare bids for sales outside USA.
Due to extensive work and negotiations by the fastener industry, we have worldwide agreements on threaded fastener standards. The ISO 898/I on strength classes for fasteners was published years ago, and it covers the most frequently used classes ISO 5.8, 8.8, and 10.9, which correspond to our customary grades SAE 2, 5, and 8 as shown in Figure 7.
Other important ISO strength classes; ISO 4.6, ISO 4.8, ISO 9.8 and ISO 12.9
Unifying steel material quality standards worldwide is a tremendous task. ISO has done an outstanding job through its many technical committees. Over 60 standards on the subject have been published and more are being developed. The EU has eliminated many differences between German, French, British, Italian and other European steel designations by issuing EN (European Norm) standards designated DIN EN, NF EN, BS EN, UNI EN, etc. and published in each countrys language. For specific details about the ISO fastener strength classes or material quality standards see the referenced metric standards book.
The 2007 Electronic 7th Edition of METRIC STANDARDS for Worldwide Manufacturing. or hard copy (2007 or 1996 second editions) ASME Press, 806 pages, 500 tables, ISBN 978-0-7918-0261-8. Do you need help with bringing your business in tune with Global Manufacturing and Engineering? Contact Knut , and I shall be happy to help you. Global METRIC Training Program. The new KOK ISOTOLTM Computer Tolerance Software provides the powerful tool you need to help cut costs in manufacturing and engineering. The new KOK ISOGAGETM Computer Software for Gages provides the powerful tool you need to reduce cost of gages. This software provides limit dimensions for plug and ring GO-NOGO gages for all tolerances in the ASME and ISO standards.
The ebook and software are hosted and distributed by the ANSI Digital Standards Store. and by the American Institute of Physics <http://asmedl.aip.org/ebooks/asme/asme press/802612>
E-mail kok at gometricusa.org
Phone 704 990 8449