Читать книгу Basic Benchwork for Home Machinists - Les Oldridge - Страница 7

Before looking at the various tasks which are performed at the bench, the materials on which we shall be working and their properties must be discussed. It is important that the most suitable material for the job in hand is chosen. Often this will be specified in the drawing from which we are working, but sometimes we have to decide what to use.

The following properties then have to be considered:

STRENGTH. The strength of a material is its ability to withstand stress without breaking. The load, or stress, may tend to stretch, compress, twist, or cut the material. These are termed tensile, compressive, torsional, or shear forces. See Fig. 2.1. The strength of a material varies with the type of stress to which it is subject. For example, cast iron has good compressive strength but relatively poor tensile strength; it is about four times stronger when it is squeezed than when it is stretched.

ELASTICITY is the ability of a stressed material to return to its original shape when the load is removed. Spring steel has a high elasticity factor. Plasticine has practically no elasticity. Most materials are elastic below a certain limit, known as their elastic limit. If the stress applied exceeds this limit, the material is permanently deformed.

PLASTICITY is the reverse of elasticity and is the property of a material to retain any deformation produced by loads after the load has been removed. Steel is plastic at red heat and can be forged to shape.

DUCTILITY is the ability in a material to be drawn out by tensile forces beyond its elastic limit without breaking. This property is important in the production of wire, the wire being produced by drawing metal through dies that get progressively smaller.

MALLEABILITY is a similar property to ductility, except that the material is deformed beyond the elastic limit by compressive forces, such as rolling or hammering, instead of by a tensile force. Lead is a malleable material but lacks ductility because of low tensile strength.

BRITTLENESS. A material is brittle where fractures occur with little or no deformation. Glass is a classic example of a material with this property.

TOUGHNESS is the ability to withstand shock loads.

HARDNESS is the ability of a material to resist penetration, scratching, abrasion, indentation, and wear. In the laboratory, it is measured by applying a load to a small area of material by a hard steel ball or pointed diamond, and measuring the depression made into the material under a given load. Chisels, lathe tools, and center punches, for example, must have this quality to do the job for which they are intended. Unfortunately, the harder carbon steel tools are made the more brittle they become, so some hardness must be sacrificed for toughness in the tempering process. This will be discussed more fully in the chapter on hardening and tempering.

Fig. 2.1 Compressive, tensile, shear, and torsional stresses.

SOFTNESS, obviously, is the opposite property to hardness. Soft materials may be easily shaped by filing, drilling, or machining in a lathe, milling machine, or shaper. In many cases the component is hardened by one means or another, to be discussed later, after the shaping process is completed.