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19.3 MetrIc Tooth Standmrds*

The metric module was developed in a number of versions that differ in minor ways. The German module, defined by the DIN standard, is widely used throughout Europe. However, the Japanese have their own version, defined in JIS standards. The deviations among these and other national metric standards are fortunately minor: the various metric standards, differ only with regard to dedendum size and root radii.
Even these minor deviations are resolved by a new unified module standard sponsored and promoted by the International Standards Organization (ISO). This unified version, shown in fIgure 1.52, conforms to the new SI system in all respects. Currently, Germany, Japan, Great Britain and other major industrial countries on the metric system, are shifting to this ISO standard, which has been advocated as the basis for American metric gearing.

                   
ISO standard metric gear tooth is defined by a rack of module m = 1. ISO gears share many features with inch-size American gears: 200 pressure angle, plus similar addendum and dedendum ratios.

Tooth proportions for the standard, which applies to cylindrical gears of the spur and helical varieties, are given in terms of the basic rack, as shown in the illustration. Dimensions, in millimeters, are normalized for module m = 1. Corresponding values for other modules are obtained by multiplying each dimension by the value of the specific module, m. Major tooth parameters are described by the standard:

• Tooth form is straight-sided and full-depth, forming the basis of a family of full-depth interchangeable gears.
• Pressure angle is 200, conforming to world-wide acceptance of 200 as the most versatile pressure angle.
• Addendum is equal to the module, m, which conforms to the American practice of addendum equaling 1/P.
• Dedendum is equal to 1.250 m, which corresponds to American practice for coarse pitch gears (see Table 1.1).
• Root radius is slightly greater than current American standards specify.
• Tip radius has a maximum tip-rounding specified. This rounding is a deviation from American standards, which do not specify rounding. However, as a maximum or limit value, American gear makers are not prevented from specifying a tip radius as near zero as possible.

Note that the basic racks for metric gears and for American inch gears are essentially identical. For metric gears, specific size dimensions are obtained from multiplying by m (the module). Gears conforming to diametral pitch American standards are sized by dividing the basic rack dimensions by the specific diametral pitch (P).
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Apart from minor changes in wording. this paragraph, including figure 1.52, is quoted or reproduced wilh the permissior of Machine Design magazine from the following article: "Shifting to Metric", by G.W. Mchalec and F. Buchsbaum Machire Design, Vol. 45, August 9,1973, pp. 94-97.

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