There are hundreds of methods to make metal parts. The list of technologies and techniques can be overwhelming. To make matters worse, each approach technology comes with a variety of strengths and weaknesses. The purpose of this article is to give a quick overview of the predominant metal part fabrication techniques and discuss their strengths and weaknesses.
Stamping (also known as pressing) is the process of placing flat sheet metal, either in blank or coil form, into a stamping press where a tool and die surface forms the metal into a net shape. Stamping includes a variety of sheet-metal forming manufacturing processes including punching using a machine press or stamping press, blanking, embossing, bending, flanging, and coining. This could be a single-stage operation where every stroke of the press produces the desired form on the sheet metal part or could occur through a series of stages. The process is usually carried out on sheet metal, but can also be used on other materials such as polystyrene. Stamping is usually done on cold metal sheet. See Forging for hot metal forming operations.
Forging is a manufacturing process involving the shaping of metal using localized compression forces. The blows are delivered with a hammer (often a power hammer) or a die. Forging is often classified according to the temperature at which it is performed: cold forging (a type of cold working), warm forging, or hot forging (a type of hot working). For the latter two, the metal is heated, usually in a forge. Forged parts can range in weight from less than a kilogram to hundreds of metric tons. Forging has been done by smiths for millennia; the traditional products were kitchenware, hardware, hand tools, edged weapons, cymbals, and jewelry. Since the Industrial Revolution, forged parts are widely used in mechanisms and machines wherever a component requires high strength; such forgings usually require further processing (such as machining) to become a finished part. Today, forging is a major worldwide industry.
Drop forging is a forging process where a hammer is raised and then “dropped” onto the work piece to deform it according to the shape of the die. There are two types of drop forging: open-die drop forging and closed-die drop forging. As the names imply, the difference is in the shape of the die, with the former not fully enclosing the work piece, while the latter does.
Press forging works by slowly applying a continuous pressure or force, which differs from the near-instantaneous impact of drop-hammer forging. The amount of time the dies are in contact with the work piece is measured in seconds (as compared to the milliseconds of drop-hammer forges). The press forging operation can be done either cold or hot.
Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process. Casting materials are usually metals or various cold setting materials that cure after mixing two or more components together; examples are epoxy, concrete, plaster, and clay. Casting is most often used for making complex shapes that would be otherwise difficult or uneconomical to make by other methods.
Casting is a 6000-year-old process. The oldest surviving casting is a copper frog from 3200 BC.
Metal injection molding (MIM) is a metalworking process by which finely-powdered metal is mixed with a measured amount of binder material to comprise a “feedstock” capable of being handled by plastic processing equipment through a process known as injection molding. The molding process allows dilated (oversized due to binder) complex parts to be shaped in a single step and in high volume. After molding, the powder-binder mixture is subjected to steps that remove the binder (debinding) and sintering to densify the powders. End products are small components used in various industries and applications. MIM is typically cost-effective for small, intricate, high-volume products which would otherwise be quite expensive to produce by alternate or classic methods. The variety of metals capable of implementation within MIM feedstock is broad.
Machining is any process in which a piece of raw material is cut into a desired final shape and size by a controlled material-removal process. These controlled material removal processes are today collectively known as subtractive manufacturing, which differs from processes of controlled material addition, which are known as additive manufacturing. Exactly what the “controlled” part of the definition implies can vary, but it almost always implies the use of machine tools (in addition to just power tools and hand tools).
CNC Machining is a process used in the manufacturing sector that involves the use of computers to control machine tools. Tools that can be controlled in this manner include lathes, mills, routers, and grinders. The CNC in CNC Machining stands for Computer Numerical Control.
There are many kinds of machining operations, each of which is capable of generating a certain part geometry and surface texture.
In turning, a cutting tool with a single cutting edge is used to remove material from a rotating workpiece to generate a cylindrical shape. The primary motion is provided by rotating the workpiece, and the feed motion is achieved by moving the cutting tool slowly in a direction parallel to the axis of rotation of the workpiece.
Drilling is used to create a round hole. It is accomplished by a rotating tool that typically has two or four helical cutting edges. The tool is fed in a direction parallel to its axis of rotation into the workpiece to form the round hole.
In boring, a tool with a single bent pointed tip is advanced into a roughly made hole in a spinning workpiece to slightly enlarge the hole and improve its accuracy. It is a fine finishing operation used in the final stages of product manufacture.
Reaming is one of the sizing operations that removes a small amount of metal from a hole already drilled.
In milling, a rotating tool with multiple cutting edges is moved slowly relative to the material to generate a plane or straight surface. The direction of the feed motion is perpendicular to the tool’s axis of rotation. The speed of motion is provided by the rotating milling cutter. The two basic forms of milling are:
Other conventional machining operations include shaping, planing, broaching and sawing. Also, grinding and similar abrasive operations are often included within the category of machining.
3D printing, also known as additive manufacturing (AM), refers to processes used to create a three-dimensional object in which layers of material are formed under computer control to create an object. Objects can be of almost any shape or geometry and are produced using digital model data from a 3D model or another electronic data source. Thus, unlike material removed from a stock in the conventional CNC machining process, 3D printing or AM builds a three-dimensional object from computer-aided design (CAD) model (STL, STP, STEP, SLDPRT) by successively adding material layer-by-layer.
The term “3D printing” originally referred to a process that deposits a binder material layer-by-layer onto a powder bed with inkjet printer heads. More recently, the term is being used in popular vernacular to encompass a wider variety of additive manufacturing techniques. United States and global technical standards use the official term additive manufacturing for this broader sense. ISO/ASTM52900-15 defines seven categories of AM processes within its meaning: binder jetting, directed energy deposition, material extrusion, material jetting, powder bed fusion, sheet lamination and vat photopolymerization.
There are many metal additive manufacturing methods and will be the subject of a separate post.
Take a walk to the nearest park or ball field and look down; that’s not just a bunch of rocks and dirt down there. Every metal known to humankind comes from the ores and minerals found underground. To a manufacturing person, all that stuff under our feet is what rocks our world. Pardon the pun.
Let’s take another walk, this one through the Periodic Table of the Elements. Yes, most of us learned about the elements in high school chemistry class, but it’s probably been a while, so a refresher might be in order.
We’ll zoom through most of it. Hydrogen, oxygen, and argon—gases are exciting to welders and neon sign makers, but unless we’re short of breath after a brisk walk, most of us take them for granted. Without silicon and germanium, computer chip manufacturers would need new jobs. Plutonium is important to bomb makers, as are lead and krypton to those in lighting. To everyone else, the elements are a pretty boring subject.
In between all those rare earths and noble gases, however, sit metals. Aluminum, titanium, iron, and nickel—these are the building blocks of modern society. Without the raw materials trapped in the earth’s crust, and the technology to extract and process those minerals into various alloys, humans would still be living in grass huts and chasing their food with wooden clubs. Protolabs uses a range of metals for its manufacturing services. These can be classified as either hard or soft, with metals like steel and stainless steel on one side of the fence, and brass, copper, magnesium, and aluminum on the other. The last on this list—aluminum—is the most abundant metal in the earth’s crust, and the third most common element after oxygen and silicon. Despite making up 8 percent of the earth’s crust by weight, aluminum is rarely found in its pure metallic form, however, since most of it is locked up in bauxite and other ores.
Elemental aluminum is soft and highly malleable, making it a poor candidate for mechanical purposes. Instead, aluminum is usually blended with a mix of other elements, including silicon, copper, magnesium, and zinc, then heat-treated to make the strong, lightweight alloys used today in airframes, automobiles, and various consumer products.
Protolabs’ machining service makes parts from two types of aluminum: 6061-T651 and 7075-T651. The T-suffix signifies how the material was processed, in this case mechanically stretched by 1 to 3 percent after heat treatment to eliminate residual stress, thus making it more stable when machined. 6061 aluminum is alloyed with magnesium and silicon, and in its wrought form offers yield strength of 40,000 psi or more. It is very corrosion resistant and weldable given the proper equipment, making it an ideal choice for low-fatigue applications such as structural components in machinery, hydraulic valve bodies, marine, and automotive parts, and most any application requiring robust, lightweight material.
The other horse in Protolabs’ aluminum stable is 7075 aluminum. Harder and stronger than 6061, it offers yield strength nearly twice that of its less robust cousin, but at nearly three times the cost. Its primary alloying elements are zinc, magnesium, and copper. The American military uses 7075 in many of its firearms, connecting rods made of forged 7075 aluminum are used in top fuel dragsters, and the wing spars in Boeing aircraft are made of 7075. It’s tough stuff. In fact, the only place where 6061 wins out is in corrosion resistance, and in parts that need a little more “give” than those made of 7075. Both materials offer easy machining, although 7075 is a bit abrasive.
Another popular lightweight material is magnesium, which is the fourth most abundant element in the earth’s crust. Two-thirds the weight of 6061 and nearly as strong, it is the lightest of all structural metals. Camera and cell phone bodies, frames for power tools, laptop computers chassis—magnesium is a preferred material wherever good strength and low weight is important. In an effort to improve fuel efficiency, automobile manufacturers make extensive use of magnesium in transmission cases, seat frames, and intake manifolds.
Magnesium is most commonly alloyed with aluminum and zinc. It has excellent dampening characteristics, is very machinable, and readily molded or die-cast.
In addition to magnesium’s susceptibility to corrosion, another drawback is its reduced strength at high temperatures, although Volkswagen used magnesium successfully in the crankcase of its air-cooled Beetle engine for more than 50 years. Price-wise, it’s more expensive than aluminum, but this is largely mitigated by the relative ease with which magnesium components are manufactured. Note that Protolabs no longer manufacturers magnesium parts.
Rounding out the soft metal lineup are brass and copper, the kissing cousins of the metal family. Of the two, brass is by far the most versatile. With the exception of environments high in ammonia and some acids, it is extremely weather and corrosion resistant. If you’ve ever replaced a car radiator, soldered a kitchen faucet, or played the French horn, you’ve handled parts made of brass.
Protolabs offers parts machined from C260 cartridge brass, long a favorite for ammunition casings. It contains 70 percent copper and 30 percent zinc, and is considered the most general purpose of all brass alloys. There are literally dozens of brass grades though, all with subtle differences and distinct uses. Cutting back on the copper percentage by a few points produces a brass suitable for rivets and screws. Cut back a bit more, add a little iron, and you’ve created Muntz metal, good for architectural trim and lining the bottoms of boats. Increase the copper content, toss in some manganese and a pinch of nickel, and you have the makings of Sacagawea one dollar coins. Brass is the ultimate switch hitter.
To a machinist, brass is as easy as it comes: coolant is optional, tool life exceptional, and feedrates quite high. Don’t let its easygoing nature fool you, however—brass is sturdy stuff, offering tensile strength rivaling that of mild steel. Ironically, copper is a far different story. Even though it’s the primary ingredient in brass, unalloyed copper’s machinability is roughly five times worse, and even the most patient of machinists avoid it due to copper’s tough, stringy nature. Chips are virtually impossible to break and, due to its high thermal conductivity, the material heats up very quickly during cutting.
Copper is only second to silver in electrical conductivity, a factor that makes it one of the most important metals in use today. Copper (and aluminum) wiring basically make electricity possible. Without it, lights would remain unlit, cars wouldn’t run and it would be impossible to blend a frozen margarita.
Copper is easy to braze but difficult to weld. Its extreme ductility makes it both strong and flexible, a rare occurrence among metals. Yet copper does far more than conducting the power needed to heat our grills. It’s used in semiconductor manufacturing as an element of high-temperature superconducting, in glass-to-metal seals such as those needed for vacuum tubes, and has even been approved by the United States EPA for use in hospitals and public places as an antimicrobial surface.
Because elemental copper exists in nature, people first started pounding it into coins and cutlery millennia ago. Today, it’s an ingredient in more than 570 different metallic alloys, of which cartridge brass is one. Tellurium copper, nickel copper, bronze, gunmetal, aluminum, and steel alloys—the list goes on. Copper can also be used for electrodes in electrical discharge machining (EDM), a technology often seen in injection molding and metal stamping. In the modern world, copper is indeed king.
The world needs hard metals as well. Steel is used in everything from cars to cruise ships, cables to crescent wrenches. Regardless of alloy type, steel is mostly composed of iron. Iron smelting and limited steel manufacturing has been in use for thousands of years, but it wasn’t until the Bessemer steel process, invented in the mid-1800s, that mass production of high-quality steel was made possible. Thus began the industrial revolution.
As with the soft metals, a small quantity of alloying elements can have a dramatic effect on steel’s properties—the addition of less than 1 percent carbon and manganese, along with a little metallurgical legerdemain, is what makes brittle iron into tough 1018 steel. And 4140 alloy steel, suitable for aircraft use, is made by combining an equally small amount of chromium along with a dusting of molybdenum.
Carbon steels such as these can be hardened to one extent or another, and are easily welded. There’s just one problem: They rust, making plating or painting a requirement for most any application involving carbon steel.
The 300-series stainless steels offered by Protolabs carry at least 20 percent chromium along with a fair amount of nickel, making them more difficult to machine. Still, these popular materials are commonly used for medical instruments, vacuum and pressure vessels, and for food and beverage equipment. 300-series stainless is quite tough, but cannot be hardened like carbon steel. If hardness is a requirement for your application, consider kicking it up a notch with 17-4 PH.
This versatile but very tough material contains nickel, chromium, and copper. Although considered part of the stainless steel family, its machinability in the annealed state approaches superalloy status. When heat treated, it easily achieves hardness of 45 Rc and tensile strength of 150,000 psi or higher, three times that of carbon steel. It’s most commonly used in the medical, aerospace, and nuclear industries, or anywhere a combination of high strength and good corrosion resistance is needed.
Since rust never sleeps, metallurgists developed stainless steel. By increasing the amount of chromium to at least 10.5 percent, corrosion resistance is greatly enhanced. Stainless steel is widely used in the chemical industry, textile processing, and for marine applications. Many stainless steels are temperature resistant as well, and are able to withstand temperatures upwards of 2,700 degrees F, hot enough to turn aluminum, brass and copper into molten puddles. 316 stainless, for example, is excellent for heat exchangers, and sees regular use in steam turbines and exhaust manifolds.
If you’re looking for some truly robust alloys, look no further than cobalt chrome and Inconel. Protolabs doesn’t machine these materials, but its 3D printing service is happy to sinter them for you through a direct metal laser sintering (DMLS) process. Each material has unique, high-performance properties.
Inconel contains 50 percent or more of nickel, giving it excellent strength at a range of temperatures. It’s used for extreme demands such as gas turbine blades, jet engine compressor discs, and even nuclear reactors and jet engine combustion chambers. The high nickel content makes Inconel one of the most difficult materials to a machine, requiring wear resistant coated carbide and a rigid machine tool. Sitting right next to nickel on the periodic table is cobalt, the main ingredient in cobalt chrome alloy. This material is known for superb wear resistance and human biocompatibility, making it ideal for dental implants, hip and knee replacements, and arterial stents.
Finally, there’s titanium. This lightweight element is alloyed with aluminum and vanadium, providing a strong, corrosion-resistant material. Like cobalt chrome, titanium is biocompatible and is used extensively for bone screws, pins, and plates. Its tensile strength is roughly twice that of mild steel but weighs just half as much. This makes titanium appealing to the aerospace industry and high-performance vehicle manufacturers.
Metallurgy—it’s a pretty cool subject, right? As we’ve seen, a dozen or so raw elements provide for hundreds of important, life-altering materials. None of these metals would be worth a wooden nickel without the means to shape them, however. Principal among these is machining, which evolved in lockstep with steel processing. Over the past 150 years, machine tools have grown from crude pulley and steam driven devices to the high-tech, ultra-precise computer numerical control (CNC) equipment of today.
Protolabs employs a veritable army of these machine tools, one that’s several hundred strong, standing ready to machine custom parts from most of the materials just discussed. Chief among these are machining centers, which work by rotating a cutting tool such as an end mill or drill to remove material. The workpiece is gripped in a vise or similar clamping device and moved in one or more axes against the cutter, thus creating complex geometries. Five-axis machining centers may use all axes simultaneously to generate the free-form shapes common in artificial knees and propellers, or indexed to machine multiple sides of the workpiece in one clamping.
CNC lathes use a chuck or collet to grip the workpiece and rotate it against a fixed cutting tool. Need to cut a set of candlestick holders or a fitting for a garden hose? Lathes make short work of these parts and more. Mill-turn machines, like Protolabs uses, take lathes one step further with the addition of rotating tools and secondary spindles, eliminating what were once secondary machining operations.
For large-volume production, machined parts are often transitioned to casting or molding processes. Metal injection molding, or MIM, is the process whereby metal powders such as nickel steel, 316 stainless, 17-4 PH or chrome-moly are mixed with a binder composed of wax and thermoplastic.