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Stress relaxation

The primary concern in elevated-temperature applications is stress relaxation. Stress relaxation is the loss of load or spring length that occurs when a spring is held at load or cycled under load. Heat affects modulus and tensile strength. In addition to the factors of stress, time, and temperature which affect relaxation, other controllable
factors are
1. Alloy type—the highly alloyed materials are generally more temperatureresistant.
2. Residual stresses—such stresses remaining from forming operations are detrimental to relaxation resistance. Use the highest practical stress-relief temperature.
3. Heat setting—procedures employed to expose springs under some load to stress and heat to prepare them for a subsequent exposure. The effect is to remove the first stage of relaxation.
(Standard Handbook of Machine Design (3rd Edition), Edited by: Shigley, Joseph E.; Mischke, Charles R.; Brown, Thomas H. Jr., p24-5)
Acetal(Delrin, Celcon)

These combine very high strength, good temperature and abrasion resistance, exceptional dimensional stability, and low coefficient of thermal expansion. They compete with nylon (but with many better properties) and with metal die castings (but are lighter). Chemical resistance is good except for strong acids. Typical applications are water-pump parts, pipe fittings, washing machines, car instrument housings, bearings, and gears.
(Mechanical Engineering Handbook; Ed. Frank Kreith,p12-20)

Acrylics (Methylmethacrylate, PMMA)

These are noted for their optical clarity and are available as sheet, rod, tubings, etc., as Perspex (U.K.) and Plexiglas (U.S., Germany, etc.). They are hard and brittle and quite resistant to discoloring and, especially, weathering. Applications include outdoor display signs, optical lenses and prisms, transparent coverings, drafting instruments, reflectors, control knobs, baths, and washbasins. They are available in a wide range of transparent and opaque colors.
(Mechanical Engineering Handbook; Ed. Frank Kreith,p12-20)

Spindle Head

The CNC machine differs from a conventional manual machine in several respects. The principle of operation of a numerical controlled machine can be explained with the help of Fig. 12.1. The figure shows a vertical milling machine. For carrying out an operation like end milling the spindle head is to be positioned in Z- axis and the table in X and Y coordinate axes. The feed movement is to be realized by the individual or simultaneous movement of X and Y axes. Thus the milling machine requires three
slide movements, which are usually referred as axes feed drives. A special feature of a CNC machine is that a separate motor called a servomotor individually drives each axis. AC servomotors are the preferred choice for this purpose today. DC servomotors were widely used earlier. The slides are driven by the servomotors through recirculating ball screw and nut assemblies. The use of re-circulating ball screw reduces friction, backlash and wear. The low friction reduces the torque required at the motor and the lost motion through torsional deflection of the screw. The use of ball screws also improves the dynamic response of the system. In some modern designs, particularly in the case of high-speed machines, linear motors are used in the place of servomotor ball screw combination.

In this type of machining X-axis table or column provides traverse and spindle head provides Y-axis traverse. The saddle or column, or headstock or spindle head provides the Z-axis traverse. These machines are invariably used with a rotary indexing table to facilitate multiphase machining at different angles in a single setup. The axis of rotary table is parallel to Y-axis and is called ‘B’ axis.
( CAD/CAM/CIM, P. Radhakrishnan S. Subramanian V. Raju,p343-356)