Durability. The first cost of a milling machine, like any other modern machine tool, is comparatively great, and to make its employment economical, this cost must be spread over a long period of service—in other words, the machine must be durable. Strong design and the use of high quality materials throughout the machine are most essential to durability.
Thorough workmanship is also an important factor. Seemingly small details in construction should receive careful attention, for it is these that many times give rise to serious trouble. The fitting of different parts, and making of all alignments should be carefully done, and means should be provided for taking up wear at any points where it is apt to occur. In connection with the wearing qualities of different parts, the selection of materials is an important factor; parts that are subject to continuous usage, such as the change gears in constant speed drive machines, should be made of a hard material having good wearing qualities. In Brown & Sharpe machines, these gears are made of steel and are hardened.
Where change gears are being thrown into and out of mesh frequently by a tumbler arrangement, it is well to have the tops of the teeth pointed, and the ends of teeth in sliding gears chamfered. These features not only facilitate throwing the gears into mesh, but also reduce the danger of teeth becoming bruised or broken, which is apt to happen when gears with teeth of the ordinary shape are thrown into mesh.
Rigidity is as essential to durability as to accuracy, since the existence of vibrations causes very rapid wearing of parts. Hence, every part should be of stable enough construction to resist vibrations under all practical working conditions.
Beyond these points, and that of provision for lubricating all bearing, the matter of durability is more especially a question of the care devoted to the machine while in use. Its failure to be durable because of lack of proper care cannot be attributed to any faults in design or construction. The information given in the next chapter on the care of milling machines is very important to those who have charge of these machines.
Convenience. Much time is lost in operating a milling machine that is inconvenient in any way for the workman to handle: therefore, from the standpoints of economy and efficiency, convenience is a most desirable quality. To be convenient, a machine must be so designed
