Machining Types, Tools, and Uses
What is machining?
Machining is a manufacturing process used to produce products, parts, and designs by removing layers from a workpiece. There are several types of machining that include the use of a power driven set of machining tools to chip, cut, and grind to alter a workpiece to meet specific requirements. Metal fasteners, costume jewelry, toys, and hand tools are all formed using the machining process. There are times when a finished part needs a touch up to meet quality standards or manufacturing requirements. In those instances, it may need to be machined to give it the proper appearance.
Though machining is used to shape metals, it can be used for other materials. Molding is a common way of producing plastic and rubber products. In some cases, after being molded, other features need to be added. This is normally completed by machining, which can add holes, remove burrs, or finish shaping. As soft and pliable as paper may be, it can be machined to match a special design or form.
Tools used in machining
A variety of machining tools are used to shape, deform, and mold metal to produce a specific geometric shape. Part of the machining process is to secure the workpiece using a gripping device to hold it in place while the tool runs across it. Prior to the industrial revolution, machining functions were performed by hand. Modern technology has taken those handcrafting skills, using computerized numerical control (CNC), and programmed them in machining equipment to be repeated multiple times accurately and precisely.
Several different tools are required to perform the multiple operations necessary to shape a workpiece. The types of tools chosen are carefully selected to ensure the quality of the final product.
Listed and described below are a few of the kinds of machining tools:
Drilling removes material using a drill bit to cut a hole of circular cross section in the workpiece. It is the most common machining process, which is used about 75% of the time. A drill jig is placed into a chuck connected to a spindle, which is driven by a drill head powered by a pulley and electric motor. Either electronically or by hand, the drilling tool is lowered onto the surface of the workpiece.
Milling produces three dimensional shapes using a rotating multi-edge cutting tool. In CNC manufacturing, the milling tool can be programmed to move in several directions on a fixed workpiece. The process can create parts in a wide range of shapes with features such as slots, pockets, and grooves. There are several kinds of milling tools depending on the type of cuts required.
Boring is the process of enlarging or expanding an existing hole by using a single point cutting tool. It may be used for workpieces that are too large to fit on a lathe or drill press. Standard boring equipment can bore holes up to 12 feet (3.6 m) in diameter.
Grinding or abrasive machining is a cutting tool designed to perform several different operations. Different types of grinding tools include surface, cylinder, centerless, internal, and specially designed. Surface grinders create flat, angular, and irregular surfaces while a cylinder grinder finishes the outside surface of a cylinder. Centerless grinding involves feeding the workpiece into a blade. Internal grinding finishes tapered or straight holes. Over the last decade, grinding has seen increased use in manufacturing and become a manufacturing standard.
In a turning tool operation, the workpiece rotates as the tool removes layers from it. The process is similar to boring but reconfigures the external surface of the workpiece instead of the center. A typical form of turning tool is a lathe, an ancient process dating back to the time of the Egyptians.
A cutting tool is a wedge shaped sharp edged tool used to remove layers from a workpiece by shearing. It can be done using a single or multiple point cutter, which must be harder than the workpiece. These precision tools are designed to withstand the heat generated during the cutting process.
The types of machining listed here are a few of the methods used in modern production. Specialty and custom tools are continually being perfected to meet the needs of innovative designs.
Forms of Burning Machining Technologies
Mechanical methods of machining have been used for many years. Through innovation and technological advances, other processes have developed to remove layers without the need for grinding, boring, or mechanical tools. Some of these techniques are referred to as burning where the workpiece is heated and melted to achieve a shape or design. The most common types are laser, oxy-fuel, and plasma.
A laser beam, of high energy, contacts the workpiece creating thermal energy. The heat created melts, burns, and vaporizes the surface of the workpiece to shape it into a design. There are two types of lasers – gas and solid state. With a gas laser, gases are used to generate heat, which are He-Ne, argon, and Co2. Solid state lasers have different forms, which include YAG (yttrium aluminum garnet), Nd:YAG ( neodymium-doped yttrium aluminum garnet), and ruby. The laser cutting process can shape steel or etch patterns. Its benefits include high-quality surface finishes and cutting precision. Laser machining produces accurately placed cuts of high precision and has the ability to cut or shape any type of material.
Oxy-fuel cutting, also known as gas cutting, is mostly used to cut thick steel plates. The heat source for oxy-fuel cutting is produced by combining oxygen with some type of fuel such as acetylene, gasoline, hydrogen, or propane. The oxy-fuel torch heats the workpiece to kindling temperature, around 960o C. Once the proper temperature is achieved, pure oxygen is directed through a nozzle onto the heated cut. The oxygen changes the heated and unprotected steel into an oxidized liquid by an exothermic reaction. The created slag is blown out of the heated cavity. The process can cut deeper angles up to 70o and is more economical than the other burning methods.
Plasma cutting is a popular economical method for cutting steel. The process of plasma cutting involves using a plasma torch to generate a plasma arc.
The torch fires an electrical arc that transforms an inert, ionized gas, or plasma, which reaches an extremely elevated temperature. The heat from the torch is applied to the workpiece at high speed to melt away unwanted material. Metals machined in this way are electrically charged since an electrical current flows between the electrode of the torch and the workpiece. Plasma cutting can cut thin or thick materials. Handheld torches can cut materials of up to 38 mm thick while CNC devices can cut steel sheets of up to 150 mm thick.
Technologies of Erosion Machining
While burning tools apply heat to melt excess stock, non-traditional methods use a form of erosion. Waterjet cutting and electric discharge machining are non-traditional methods that do not require tools to remove excess material from a workpiece. They use the force of abrasive filled water and electrical discharge.
Waterjet cutting is a versatile fabrication process that uses water under high pressure, mixed with an abrasive, to cut materials into custom shapes and designs. Water is pressurized using an intensifier or direct drive pump, which is capable of producing significant fluid pressure. As the water enters the cutting head, the water goes into an opening containing a hard jewel such as a diamond, sapphire, or ruby. The velocity of the water increases at the opening, which can reach 2500 mph. Abrasive powder is added to the water stream to cause erosion. Waterjet cutting is typically used on materials that can suffer damage or deformation from heat processes.
Electric discharge machining (EDM):
EDM is known as spark machining, spark eroding, die sinking, wire burning or wire erosion. It is a non-traditional method where material is removed from a workpiece through the use of thermal energy. Much like plasma and laser cutting, EDM does not use pressure or force to remove material from a workpiece. Instead, it removes it from conductive materials using an electrode. In a gap between the workpiece and the electrode, a discharge occurs, which removes excess material by melting and vaporizing it. The complete process involves submerging the workpiece in a dielectric fluid.
The CNC Machining Process
Computer numerical control machining is a technological process developed in the 1950’s for the manufacturing of helicopter rotors. In the late 50’s, a project devoted to producing computer aided design software was financed from which came AutoCAD. AutoCAD combined with CNC has become an advanced technological method for producing parts by machining at a lower cost.
CNC has been applied to a broad range of manufacturing, production, and processing equipment. Software and programming, using the G-code computer language, develop commands and instructions to guide a machine through the shaping of a workpiece. The implementation of CNC has led to a decrease in losses due to human error and a significant drop in the amount of waste. Once a CNC machine is coded, it needs minimal maintenance or downtime and completes production at a faster rate.
Speed and lowered labor costs have made CNC a highly cost efficient method for producing high volume production runs. Down times for mishandled materials, insufficient supplies, or other production errors are eliminated. Every product and part is precision produced in accordance with exacting design specifications. Producers using CNC have the added benefit of more control of the total production process.
Precision machining produces parts with very few errors or imperfections and close tolerance finishes. There are several forms of precision machines that include milling, turning, and electrical discharge. The process demands focused attention to the specific requirements of the design.
The process of precision machining is chosen because of its adherence to the exact dimensions of the design. Every part includes a set of tolerances that allows for deviations caused by machining. Parts that require precision machining have very low tolerances for machining errors and become useless. Tolerances can vary between 0.0005 and 0.2 for precision machined parts depending on the parts initial dimensions and the type of metal.
The introduction of CNC machining significantly improved precision machining providing for higher tolerances and the production of high quality finishes. Designs can be downloaded directly from the design computer into the CNC machine, which is programmed to every detail of the design. The variance in quality depends on the type of CNC machine being used.
A key factor in precision machining is the finish of the final part. Surface finishing is measured by its texture characterized by the lay, roughness, and waviness. Each of these factors has a mathematical formula to determine the quality of the finish. Manufacturers of machining equipment provide the specifications regarding the quality of the finishes their equipment produce.
Precision machining is critical for the manufacture of parts for spacecraft since they require parts that exactly meet their specifications and requirements. Also, the aerospace industry and certain medical manufacturers have the same requisites.