This standard is confined to milling machine arbors. The reason for confining this standard to this specified mailing machine accessory is that through many years of development and general usage, there already exists good agreement on the structure and dimensions of milling machine arbors between competent manufacturers of such equipment here in the United States and abroad. This agreement is much better than for many other milling machine accessories and equipment. Already considerable interchangeability exists between the products of various suppliers of milling machine arbors.