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Metalworking

Machining a bar of metal on a lathe.

Metalworking is the process of working with metals to create individual parts, assemblies, or large scale structures. The term covers a wide range of work from large ships, bridges and oil refineries to delicate jewellery. It therefore includes a correspondingly wide range of skills and the use of many different types of metalworking processes and their related tools.

Metalworking is an art, hobby, industry and trade. It relates to metallurgy, a science, jewellery making, an art-and-craft, and as a trade and industry with ancient roots spanning all cultures and civilizations. Metalworking had its beginnings millennia in the past. At some point in history, modern man's ancestors discovered that certain rocks now called ores could be smelted, producing metal. Further, they discovered that the metal product was malleable and ductile and thus able to be formed into various tools, adornments and put to other practical uses. Humans over the millennia learned to work raw metals into objects of art, adornment, practicality, trade, and engineering.

Contents

Prehistory

Metalworking predates history. No one knows with any certainty where or when metalworking began. The earliest technologies were impermanent to say the least and were unlikely to leave any evidence for long. The advance that brought metal into focus was the connection of fire and metals. Who accomplished this is as unknown as the when and where, but the Egyptians are thought to have been one of the first civilizations to work gold.

Not all metal required fire to obtain it or work it. Isaac Asimov speculated that gold was the "first metal."[1] His reasoning is that gold by its chemistry is found in nature as nuggets of pure gold. In other words, gold, as rare as it is, is always found in nature as the metal that it is. There are a few other metals that sometimes occur natively, and as a result of meteors. Almost all other metals are found in ores, a mineral bearing rock, that require heat or some other process to liberate the metal. Another feature of gold is that it is workable as it is found, meaning that no technology beyond eyes to find a nugget and a hammer and an anvil to work the metal is needed. Stone hammer and stone anvil will suffice for technology. This is the result of gold's properties of malleability and ductility. The earliest tools were stone, bone, wood, and sinew. They sufficed to work gold.

At some unknown point the connection between heat and the liberation of metals from rock became clear, rocks rich in copper, tin, and lead came into demand. These ores were mined wherever they were recognized. Remnants of such ancient mines have been found all over what is today the Middle East.[2] Metalworking was being carried out by the South Asian inhabitants of Mehrgarh between 7000–3300 BCE.[3] The end of the beginning of metalworking occurs sometime around 6000 BCE when copper smelting became common in the Middle East.

The ancients knew of seven metals. Here they are arranged in order of their oxidation potential:

  • Iron +0.44,
  • Tin +0.14
  • Lead +0.13
  • Copper -0.34
  • Mercury -0.79
  • Silver -0.80
  • Gold -1.50

The oxidation potential is important because it is one indicator of how tightly bound to the ore the metal is likely to be. As can be seen, iron is significantly higher than the other six metals while gold is dramatically lower than the six above it. Gold's low oxidation is one of the main reasons that gold is found in nuggets. These nuggets are relatively pure gold and are workable as they are found.

Copper ore, being relatively abundant, and tin ore became the next important players in the story of metalworking. Using heat to smelt copper from ore, a great deal of copper was produced. It was used for both jewelry and simple tools. However, copper by itself was too soft for tools requiring edges and stiffness. At some point tin was added into the molten copper and bronze was born. Bronze is an alloy of copper and tin. Bronze was an important advance because it had the edge-durability and stiffness that pure copper lacked. Until the advent of iron, bronze was the most advanced metal for tools and weapons in common use (see Bronze Age for more detail).

Looking beyond the Middle East, these same advances and materials were being discovered and used the world around. China and Britain jumped into the use of bronze with little time being devoted to copper. Japan began the use of bronze and iron almost simultaneously. In the Americas things were different. Although the peoples of the Americas knew of metals, it wasn't until the arrival of Europeans that metal for tools and weapons took off. Jewelry and art were the principal uses of metals in the Americas prior to European influence.

Around the date 2700 BCE, production of bronze was common in locales where the necessary materials could be assembled for smelting, heating, and working the metal. Iron was beginning to be smelted. Iron began its emergence as an important metal for tools and weapons. The Iron Age was dawning.

History

Turret lathe operator machining parts for transport planes at the Consolidated Aircraft Corporation plant, Fort Worth, Texas, USA in the 1940s.

By the historical periods of the Pharaohs in Egypt, the Vedic Kings in India, the Tribes of Israel, and the Mayan Civilization in North America, among other ancient populations, precious metals began to have value attached to them. In some cases rules for ownership, distribution, and trade were created, enforced, and agreed upon by the respective peoples. By the above periods metalworkers were very skilled at creating objects of adornment, religious artifacts, and trade instruments of precious metals (non-ferrous), as well as weaponry usually of ferrous metals and/or alloys. These skills were finely honed and well executed. The techniques were practiced by artisans, blacksmiths, atharvavedic practitioners, alchemists, and other categories of metalworkers around the globe. For example, the ancient technique of granulation is found around the world in numerous ancient cultures before the historic record shows people traveled seas or overland to far regions of the earth to share this process that still being used by metalsmiths today.

As time progressed metal objects became more common, and ever more complex. The need to further acquire and work metals grew in importance. Skills related to extracting metal ores from the earth began to evolve, and metalsmiths became more knowledgeable. Metalsmiths became important members of society. Fates and economies of entire civilizations were greatly affected by the availability of metals and metalsmiths. Today modern mining practices are more efficient, but more damaging to the earth and to the workers that are engaged in the industry. Those that finance the operations are driven by profits per ounce of extracted precious metals. The metalworker depends on the extraction of precious metals to make jewellery, build more efficient electronics, and for industrial and technological applications from construction to shipping containers to rail, and air transport. Without metals, goods and services would cease to move around the globe on the scale we know today.

More individuals than ever before are learning metalworking as a creative outlet in the forms of jewellery making, hobby restoration of aircraft and cars, blacksmithing, tinsmithing, tinkering, and in other art and craft pursuits. Trade schools continue to teach welding in all of its forms, and there is a proliferation of schools of Lapidary and Jewelers arts and sciences at this- the beginning of the 21st Century AD.

General metalworking processes

A combination square used for transferring designs.
A caliper is used to precisely measure a short length. Here, a digital caliper is measuring a 2 euro coin.

Metalworking generally is divided into the following categories, forming, cutting, and, joining. Each of these categories contain various processes.

Compatibility chart of materials versus processes[4]
Material
Process Iron Steel Aluminium Copper Magnesium Nickel Refractory metals Titanium Zinc
Sand casting X X X X X X 0
Permanent mold casting X 0 X 0 X 0 0
Die casting X 0 X X
Investment casting X X X 0 0
Closed-die forging X 0 0 0 0 0 0
Extrusion 0 X X X 0 0 0
Cold heading X X X 0
Stamping & deep drawing X X X 0 X 0 0
Screw machine 0 X X X 0 X 0 0 0
Powder metallurgy X X 0 X 0 X 0
Key: X = Routinely performed, 0 = Performed with difficulty, caution, or some sacrifice, blank = Not recommended

Prior to most operations, the metal must be marked out and/or measured, depending on the desired finished product.

Marking out (also known as layout) is the process of transferring a design or pattern to a workpiece and is the first step in the handcraft of metalworking. It is performed in many industries or hobbies, although in the repetition industries the need to mark out every individual piece is eliminated. In the metal trades area, marking out consists of transferring the engineer's plan to the workpiece in preparation for the next step, machining or manufacture.

Calipers are handtools designed to precisely measure the distance between two points. Most calipers have two sets of flat, perpendicular edges used for inner or outer diameter. These calipers can be accurate to within one-thousandth of an inch (25.4μm). Different types of calipers have different mechanisms for displaying the distance measured. Where larger objects need to be measured with less precision, a tape measure is often used.

Forming processes

These forming processes modify metal or workpiece by deforming the object, that is, without removing any material. Forming is done with heat and pressure, or with mechanical force, or both.

Casting

A sand casting mold.

Casting achievies a specific form by pouring molten metal into a mold and allowing it to cool, with no mechanical force. Forms of casting include:

  • Investment casting (called lost wax casting in art)
  • Centrifugal casting
  • Heat treatment
  • Die casting
  • Sand casting
  • Shell casting
  • Spin casting

Plastic deforming

A red-hot metal workpiece is inserted into a forging press.

Plastic deformation involves using heat or pressure to make a workpiece more conductive to mechanical force. Historically, this and casting were done by blacksmiths, though today the process has been industrialized.

  • Cold sizing
  • Extrusion
  • Forging
  • Powder metallurgy

Sheet metal forming

A metal spun brass vase.

These types of forming process involve the application of mechanical force at room temperature.

  • Bending
  • Coining
  • Decambering
  • Deep drawing
  • Drawing
  • Spinning
  • Flow turning
  • Raising
  • Roll forming
  • Repoussé and chasing
  • Rolling
  • Rubber pad forming
  • Shearing
  • Stamping
  • Wheeling using an English wheel (wheeling machine)

Cutting processes

Cutting

A CNC plasma cutting machining.

Cutting is a collection of processes wherein material is brought to a specified geometry by removing excess material using various kinds of tooling to leave a finished part that meets specifications. The net result of cutting is two products, the waste or excess material, and the finished part. If this were a discussion of woodworking, the waste would be sawdust and excess wood. In cutting metals the waste is chips or swarf and excess metal. These processes can be divided into chip producing cutting, generally known as machining. Burning or cutting with an oxyfuel torch is a welding process not machining. There are also miscellaneous specialty processes such as chemical milling.

Cutting is nearly fully represented by:

  • Chip producing processes most commonly known as machining
  • Burning, a set of processes which cut by oxidizing a kerf to separate pieces of metal
  • Specialty processes

Drilling a hole in a metal part is the most common example of a chip producing process. Using an oxy-fuel cutting torch to separate a plate of steel into smaller pieces is an example of burning. Chemical milling is an example of a specialty process that removes excess material by the use of etching chemicals and masking chemicals.

There are many technologies available to cut metal, including:

  • Manual technologies: saw, chisel, shear or snips
  • Machine technologies: turning, milling, drilling, grinding, sawing
  • Welding/burning technologies: burning by laser, oxy-fuel burning, and plasma
  • Erosion technologies:by water jet or electric discharge.

Cutting fluid or coolant is used where there is significant friction and heat at the cutting interface between a cutter such as a drill or an end mill and the workpiece. Coolant is generally introduced by a spray across the face of the tool and workpiece to decrease friction and temperature at the cutting tool/workpiece interface to prevent excessive tool wear. In practice there are many methods of delivering coolant.

Milling

A milling machine.

Milling is the complex shaping of metal (or possibly other materials) parts, by removing unneeded material to form the final shape. It is generally done on a milling machine, a power-driven machine that in its basic form is comprised of a milling cutter that rotates about the spindle axis (like a drill), and a worktable that can move in multiple directions (usually two dimensions [x and y axis] relative to the workpiece, whereas a drill can only move in one dimension [z axis] while cutting). The Spindle usually moves in the Z axis. It is possible to raise the table (where the workpiece rests). The motion across the surface of the workpiece is usually accomplished by moving the table on which the workpiece is mounted, in the x and y directions. Milling machines may be operated manually or under computer numerical control (CNC), and can perform a vast number of complex operations, such as slot cutting, planing, drilling and threading, rabbeting, routing, etc. Two common types of millers are the horizontal miller and vertical miller.

The pieces produced are usually complex 3D objects that are converted into x, y, and z coordinates that are then fed into the CNC machine and allow it to complete the tasks required. The milling machine can produce most parts in 3D, but some require the objects to be rotated around the x, y, or z coordinate axis (depending on the need). Tolerances are usually in the thousandths of an inch (Unit known as Thou), depending on the specific machine. In order to keep both the bit and material cool, a high temperature coolant is used. In most cases the coolant is sprayed from a hose directly onto the bit and material. This coolant can either be machine or user controlled, once again depending on the machine. Materials that can be milled range from aluminium to stainless steel and most everything in between. Each material requires a different speed on the milling bit and changes the amount of material that can be milled off at a time. Harder materials are usually slower speeds with small amounts of removed material. Softer materials vary, but usually are milled with a high bit speed.

The use of a milling machine adds costs that are factored into the manufacturing process. Each time the machine is used coolant is also used, which must be periodically added in order to prevent breaking bits. A milling bit must also be changed as needed in order to prevent damage to the material. Time is the biggest factor for costs. Complex parts can require hours to complete, while very simple parts take only minutes. This in turn varies the production time as well, as each part will require different amounts of time.

Safety is key with these machines. The bits are traveling at high speeds and removing pieces of usually scalding hot metal. The advantage of having a CNC milling machine is that it cuts out the machine operator. This in turn prevents many potential accidents.

Turning

A lathe cutting material from a workpiece.

A lathe is a machine tool which spins a block of material so that when abrasive, cutting, or deformation tools are applied to the workpiece, it can be shaped to produce an object which has rotational symmetry about an axis of rotation. Examples of objects that can be produced on a lathe include candlestick holders, table legs, bowls, baseball bats, crankshafts or camshafts.

The material may be secured in the lathe with a chuck and/or one or two "centers", of which at least one can be moved to accommodate varying material lengths. In a metalworking lathe, metal is removed from the workpiece using a hardened cutting tool which is usually fixed to a solid movable mounting called the "toolpost". The toolpost is then moved around the workpiece using handwheels and/or computer controlled motors, while the workpiece is rotated. Modern CNC lathes can do secondary operations, like milling, by using driven tools. When driven tools are used the work piece stops rotating and the driven tool executes the machining operation with a rotating cutting tool. The CNC machines use x, y, and z coordinates in order to control the turning tools and produce the product. Most modern day CNC lathes are able to do most turned objects in 3D.

Objects are usually finished after production using metal dyes, polish, or other metal finishes. Materials used in this process is mostly softer metals. Harder metals can be turned, but with a bit more time and effort. Turning tools are usually steel or harder in order to be able to function. The tool material must be harder than the material being turned in order for the process to work. Production rates for this process depend on the object being turned and the speed at which it can be done. More complex materials, therefore, will take more time.

In order to prevent damage to the material and also the machine, a high-temperature resistant coolant is used to keep the objects cool. The total cost to run a lathe depends on the setup of the lathe itself. Costs would include the materials, turning tools, coolant, and manpower. While turning, the lathe will remove chunks of material that would usually be at a very high temperature. In order to prevent injury, caution must be taken to avoid the hot materials. Also avoid placing fingers or hands in places that might cause harm.

Drilling and tapping

Three different types and sizes of taps.

Drilling is the process of using a drill bit in a drill to produce holes. Normally, swarf is carried up and away from the tip of the drill bit by the fluting. The continued production of chips from the cutting edges pushes the older chips outwards from the hole. This continues until the chips pack too tightly, either because of a deep hole or insufficient "backing off" (removing the drill slightly or completely from the hole). Lubricants (or coolants), such ascutting fluid, are sometimes used to ease this problem and to prolong the tool's life by cooling, lubricating the tip and improving chip flow.

Taps and dies are tools commonly used for the cutting of screw threads in metal parts. A tap is used to cut a female thread on the inside surface of a pre-drilled hole, while a die cuts a male thread on a preformed cylindrical rod.

Grinding

A surface grinder in action.

Grinding uses an abrasive process to remove material from the workpiece. A grinding machine is a machine tool used for producing very fine finishes, making very light cuts, or high precision forms using a abrasive wheel as the cutting device. This wheel can be made up of various sizes and types of stones, diamonds or inorganic materials.

The simplest grinder is a bench grinder or a hand-held angle grinder, for deburring parts or cutting metal with a zip-disc.

Grinders have increased in size and complexity with advances in time and technology. From the old days of a manual toolroom grinder sharpening endmills for a production shop, to today's 30000rpm CNC auto-loading manufacturing cell producing jet turbines, grinding processes vary greatly.

Grinders need to be very rigid machines to produce the required finish. Some grinders are even used to produce glass scales for positioning CNC machine axis. The common rule is the machines used to produce scales be 10 times more accurate than the machines the parts are produced for.

In the past grinders were used for finishing operations only because of limitations of tooling. Modern grinding wheel materials and the use of industrial diamonds or other man-made coatings (cubic boron nitride) on wheel forms have allowed grinders to achieve excellent results in production environments instead of being relegated to the back of the shop.

Modern technology has advanced grinding operations to include CNC controls, high material removal rates with high precision, lending itself well to aerospace applications and high volume production runs of precision components.

Filing

A file is an abrasive surface like this one that allows machinists to remove small, imprecise amounts of metal.

Filing is combination of grinding and saw tooth cutting using a file. Prior to the development of modern machining equipment it provided a relatively accurate means for the production of small parts, especially those with flat surfaces. The skilled use of a file allowed a machinist to work to fine tolerances and was the hallmark of the craft. Today filing is rarely used as a production technique in industry, though it remains as a common method of deburring.

Other

Broaching is a machining operation used to cut keyways into shafts. Electron beam machining (EBM) is a machining process where high-velocity electrons are directed toward a work piece, creating heat and vaporizing the material. Ultrasonic machining uses ultrasonic vibrations to machine very hard or brittle materials.

Joining processes

Mig welding

Welding

Welding is a fabrication process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, but sometimes pressure is used in conjunction with heat, or by itself, to produce the weld.

Many different energy sources can be used for welding, including a gas flame, an electric arc, a laser, an electron beam, friction, and ultrasound. While often an industrial process, welding can be done in many different environments, including open air, underwater and in space. Regardless of location, however, welding remains dangerous, and precautions must be taken to avoid burns, electric shock, poisonous fumes, and overexposure to ultraviolet light.

Brazing

Brazing is a joining process in which a filler metal is melted and drawn into a capillary formed by the assembly of two or more work pieces. The filler metal reacts metallurgically with the workpiece(s) and solidifies in the capillary, forming a strong joint. Unlike welding, the work piece is not melted. Brazing is similar to soldering, but occurs at temperatures in excess of 450 degrees Celsius. Brazing has the advantage of producing less thermal stresses than welding, and brazed assemblies tend to be more ductile than weldments because alloying elements can not segregate and precipitate.

Brazing techniques include, flame brazing, resistance brazing, furnace brazing, diffusion brazing, and inductive brazing.

Soldering

Soldering a printed circuit board.

Soldering is a joining process that occurs at temperatures below 449 Celsius. It is similar to brazing in the fact that a filler is melted and drawn into a capillary to form a join, although at a lower temperature. Because of this lower temperature and different alloys used as fillers, the metallurgical reaction between filler and work piece is minimal, resulting in a weaker joint.


See also

  • Metalworking hand tools
  • National Ornamental & Miscellaneous Metals Association
  • Taxonomy of manufacturing processes
  • Timeline of materials technology

References

  1. ^ Asimov, Isaac: "The Solar System and Back", page 151 ff. Doubleday and Company,Inc. 1969.
  2. ^ Percy Knauth et al.: "The Emergence of Man, The Metalsmiths", page 10-11 ff. Time-Life Books, 1974.
  3. ^ Possehl, Gregory L. (1996)
  4. ^ Degarmo, p. 183.

Bibliography

  • Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2003), Materials and Processes in Manufacturing (9th ed.), Wiley, ISBN 0-471-65653-4 .

Further reading

  • Possehl, Gregory L. (1996). Mehrgarh in Oxford Companion to Archaeology, edited by Brian Fagan. Oxford University Press.

External links