Going metric--inch by inch.

Author:Rose, Steve
Position:Shop talk

We are slowly going metric, inch by inch. I heard this old joke at a machine tool trade show. The discussion was regarding the slow adoption of the metric system by U.S. companies. Like it or not, we are living in a metric world. There are many metric prints being issued from the various U.S. design offices. Consider this, your car is probably half built with metric dimensions.

One day we will be lucky enough to use the metric system in all aspects of design and manufacture. Until then, there are three conversions helpful for working with both systems.

  1. Converting inches to metric dimensions (recommended formula): millimeters to inches: divide by 25.4 inches to millimeter: multiply by 25.4

  2. Cutting speeds: converting mpm to sfpm: A local machine shop finish turns hardened gear blanks (50-58 Rockwell C). This operation finish turns the outside diameter of gear teeth. This is a tough application for carbide inserts, so a special cutting material called Amborite is used. Amborite, a trade name of this insert style, is produced by DeBeers, the world's largest diamond company. This insert is a solid piece of CBN (cubic boron nitride). DeBeers, a South African company, provides all recommended cutting speeds and feeds in metric units. Cutting speeds are recommended in meters per minute (mpm).

    How can we convert metric units, mpm into sfpm?

    Turn case hardened steel: 58Rc

    Recommended speed is 80mpm to 120mpm.

    Convert meters per minute to feet per minute (fpm). (conversion factor = 3.28)

    1 meter = 1,000mm

    1,000mm = 39.3701"

    39.3701" / 12 = 3.28 feet

    80 to 120 mpm = 262.4 - 393.6fpm

  3. Surface finish: Using values based on Ra, roughness average.


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