Читать книгу Basic Benchwork for Home Machinists - Les Oldridge - Страница 8

Materials can be divided into a number of groups, such as:

1. Metals, which can be subdivided into ferrous and non-ferrous metals. This is the group with which we are most concerned but the others will be met from time to time.

2. Plastics, are now widely used in industry and which the model engineer will occasionally use them.

3. Timber.

4. Ceramics—the name originally given to materials made from clay but now used to cover a wide range of materials.

FERROUS METALS

These are the metals containing iron. Metals are rarely used in their pure state but are combined with other metals to form an ALLOY. In the case of iron, carbon is the most important addition. Although it is only present in small amounts, it causes big changes in the property of the metal.

CAST IRON this form the iron has been melted and poured into a mold, usually made of sand, in which it is allowed to solidify. This is a simple, convenient, and relatively cheap process to manufacture components of a complicated shape. Cast iron is an alloy of iron and carbon with small amounts of manganese, silicon, sulfur, and phosphorus. It contains about 3% of carbon.

There are two types, gray and white. Both get their names from the appearance of the metal when fractured. In white cast iron, all the carbon present is cementite; in gray cast iron, most of the carbon is present as flakes of graphite, and there is usually a remainder which is in the form of pearlite. Because cementite is intensely hard, white cast iron is hard and durable, though very brittle. Graphite is soft and is a good lubricant, so gray cast iron is readily machinable, less brittle, and suitable for sliding surfaces. Being hard and brittle white cast iron is rarely used alone but it is the material used for the production of malleable iron.

GRAY CAST IRON, then, is the type in common use; it is cheap and easy to cast and machine. As a typical example, a motor car cylinder block contains 93.32% iron, 3.3% carbon, 1.9% silicon, 0.8% manganese, 0.14% sulfur, and 0.18% each of phosphorus, molybdenum, and chromium. The carbon content of approximately 3.3% consists of about 0.7% of combined carbon and about 2.6% of free carbon.

Because of the free carbon content, cast iron is easy to machine and file; the carbon flakes act as a lubricant, enabling the cast iron to be machined dry. Drilling or tapping of cast iron components is fairly easy, no lubricant being required. There is, however, a hard skin in which some of the molding sand may still be present. This is particularly hard on lathe tools, and when it has to be filed, an old file should be used; a new one would probably be ruined.

Cast iron is used for model engine flywheels, internal combustion engine cylinders, model locomotive wheels, and a host of other parts. Because of its self-lubricating properties, it is an ideal material for plummer block bearings. The spindle of the Model Engineer sensitive drilling machine runs directly in cast iron bearings and shows little signs of wear after years of use.

Cast iron has low tensile strength and poor shock resistance.

THE STEELS

There are standard specifications for steels contained in BS 970, which dates back to 1942, but since that date, there have been several revisions. In 1970, the specifications underwent a radical change and in 1983 the Standards were again restructured. Originally an EN code was used but this is now replaced by a six-digit system. It will be some time before the EN numbers disappear altogether and, in fact, some manufacturers show both the EN numbers and the current specifications where the two are closely aligned with only a point or two variation in analysis. For example, the free-cutting steel 212M36 corresponds to the old EN8M.

The North American naming convention uses a four digit number that helps to identify the steel type and its carbon content. Common steel types used by hobbyists include 1018, a common, low carbon steel, and 12L14, a low carbon steel with lead added to improve its machinability.

PLAIN CARBON STEELS. The main difference between cast iron and steel is the carbon content. Plain carbon steel has never more than 1.5% carbon, whereas cast iron, as has been stated above, has about 3%.

MILD STEEL, containing about 0.15% to 0.3% carbon combined with the iron, is ductile and malleable. It is easy to weld, machine, forge, or press into a new shape. It may be worked hot or cold. Because of its low carbon content, it cannot be hardened by heating and quenching, but can be case-hardened, a process which will be described later. It is supplied in bar form with hexagon, round, square, or flat sections in a “black” or “bright” form, and in sheets of varying thicknesses.

MEDIUM CARBON STEEL, with a carbon content of 0.35% to 0.5%, is much stronger than mild steel. Its hardness and strength can be increased by quenching the metal from a red heat. It can be tempered, rendering it suitable for many general engineering purposes where the stresses imposed are greater than could be withstood by mild steel.

HIGH CARBON STEEL, with a carbon content of 0.55% to 1.5%, is used for most tools after being hardened and tempered. Chisels, files, drills, and reamers are made from this steel.

ALLOY STEELS. In order to improve the properties of steel and to suit the metal to special applications, other substances beside carbon are added to the steel. NICKEL improves the ductility and toughness of the metal. CHROMIUM and MOLYBDENUM increase its hardness, while VANADIUM improves the elasticity, strength, and fatigue resistance of the steel. All steel contains MANGANESE but sometimes more is added to improve the steel’s mechanical properties.

STAINLESS STEEL is principally an alloy of iron, nickel, and chromium. It has a high resistance to corrosion, but in some forms, it is difficult to machine. However, by introducing a free machining agent into the alloy, this drawback can be overcome. DRILL ROD, a common tool steel used by model engineers, is a carbon steel with 1.1% to 1.2% carbon, 0.35% manganese, 0.45% chromium, and 0.1% to 0.25% of silicon.

TINPLATE. Sheets of mild steel are coated with tin to provide the metal used for the familiar food containers and for many other purposes. It is a useful material for the model engineer, being easily worked and soldered and can be obtained without cost from discarded cookie tins, etc.

NON FERROUS METALS

ALUMINUM is the lightest of the commonly used metals. It is too soft to use in its pure state, but when alloyed with copper, magnesium, and manganese, it is widely used for many components. It is a good conductor of electricity but is impossible to solder by the usual methods.

COPPER is soft, ductile, and of low tensile strength. It is an excellent conductor of electricity and is easy to solder or braze. It is the base of the brass and bronze alloys. Copper hardens with age and also work-hardens, that is, it becomes hard when it is bent or stretched. It can easily be returned to its soft, ductile state by annealing. This is done by heating to a red color and then allowing it to cool.

LEAD is soft, ductile and of very low tensile strength. It is often added to other metals to make them free cutting. It is typically used for lead acid battery plates and in soft solder.

TIN is corrosion resistant and is used to coat mild steel plate to make “tin plate.” It is used in soft solder and is an alloying agent in bronze, and is the basis of “white metal” bearings.

BRASS AND BRONZE. When copper is alloyed with zinc, brass is formed. Bronze is an alloy of copper and tin, and usually about 10% tin is used. Sometimes about 0.5% of phosphorus is added, the alloy then being called phosphorous bronze. There are various classes of bronze made especially for particular applications. It is, for example, an excellent bearing material.

IDENTIFICATION OF FERROUS METALS

Several metals have a similar appearance and new bar materials are often color coded by painting the end with a distinctive color paint. Often, off-cuts are used up and it is essential that these are identified. Most model engineers have a scrap box where all sorts of odds and ends are stored. Trouble will be experienced if, for example, a piece of high carbon steel is selected when a free cutting mild steel is what is required. There are several ways in which metals can be identified and some tests appear below.

APPEARANCE. Cast iron has a dark rough finish; the mold joint line is probably visible. A section of iron away from the skin has a gray appearance and a fracture appears crystallized.

Mild steel comes in two forms, black and bright. The former, “hot-rolled steel,” has a smooth scale with a blue/ black sheen. Cold-rolled steel (CRS) has a bright, silver-gray surface. Medium carbon steel has a smooth scale and a black sheen, while high carbon steel has a rougher scale.

GRINDING. A popular test is to grind the metal and note the color, quantity, and type of sparks given off. This is a difficult procedure to describe: a video film is really necessary, and it is equally difficult for the beginner to recognize the different types of sparks. It is suggested that an experiment is carried out using steels of known types and comparing the differences.

Cast iron gives off a short stream of red sparks, which at some distance from the grinding wheel, burst into a yellow spark formation. Plain carbon steel produces a lighter and brighter spark in a greater profusion than cast iron. As the carbon content increases, the sparks become lighter, are in greater quantities, and occur nearer the wheel. The high carbon steels produce secondary bursts, bunching out from the primary sparks.

If materials are drilled, it is very noticeable that the cuttings from cast iron are granular in form, while those from steel come off in long spirals. The cuttings (swarf) from medium carbon steel may turn brown or blue, but still be in spiral form. Swarf is very sharp and can cause nasty cuts if handled, so special care is needed when clearing the cuttings away from a drill.

PLASTICS

These materials become plastic above certain temperatures and, while plastic, they can be squeezed into dies or molds to give them the required shape that they retain on cooling. There are two main types, THERMOSETTING and THERMOPLASTIC. The former group do not become plastic on re-heating. They are hard, rigid, and rather brittle. They are used particularly for electrical equipment as they are good insulators. Bakelite comes within this category.

Thermoplastics may be softened by heat so they cannot be used at temperatures much above 212°F (100°C). Some of them, celluloid and plexiglass (Perspex) for example, are transparent and most can be colored by adding a suitable pigment.

POLYVINYLCHLORIDE (PVC) comes in this class, and is the flexible and rubberlike substance commonly used for insulating electric cables.

POLYTETRAFLUORETHYLENE (PTFE) is similar to PVC but has a very low co-efficient of friction, which makes it particularly suitable for making bushes which need not be lubricated. NYLON, one of the earliest plastics, is used for a variety of purposes, including small gearwheels.

REINFORCED PLASTIC. Laminated plastic such as TUFNOL consists of a fibrous material like paper or woven cloth impregnated with phenolic resin. The sheets of fabric are then laid up in a hydraulic press and squeezed and heated so that they become solid sheets, rods, or tubes.

GLASS FIBERS can be bonded together by polyester or epoxy resins to form large and complex moldings. Crash helmets and boat hulls are examples of things made in this way. The customary term is “glass-reinforced plastic” or GRP.