Basic machine tools
Hundreds of varieties of metal machine tools, ranging in size from small machines mounted on workbenches to huge production machines weighing several hundred tons, are used in modern industry. They retain the basic characteristics of their 19th- and early 20th-century ancestors and are still classed as one of the following: (1) turning machines (lathes and boring mills), (2) shapers and planers, (3) drilling machines, (4) milling machines, (5) grinding machines, (6) power saws, and (7) presses.
Turning machines
The engine lathe, as the horizontal metal-turning machine is commonly called, is the most important of all the machine tools. It is usually considered the father of all other machine tools because many of its fundamental mechanical elements are incorporated into the design of other machine tools.
The engine lathe is a basic machine tool that can be used for a variety of turning, facing, and drilling operations. It uses a single-point cutting tool for turning and boring. Turning operations involve cutting excess metal, in the form of chips, from the external diameter of a workpiece and include turning straight or tapered cylindrical shapes, grooves, shoulders, and screw threads and facing flat surfaces on the ends of cylindrical parts. Internal cylindrical operations include most of the common hole-machining operations, such as drilling, boring, reaming, counterboring, countersinking, and threading with a single-point tool or tap.
Boring involves enlarging and finishing a hole that has been cored or drilled. Bored holes are more accurate in roundness, concentricity, and parallelism than drilled holes. A hole is bored with a single-point cutting tool that feeds along the inside of the workpiece. Boring mills have circular horizontal tables that rotate about a vertical axis, and they are designed for boring and turning operations on parts that are too large to be mounted on a lathe.
Shapers and planers
Shaping and planing operations involve the machining of flat surfaces, grooves, shoulders, T-slots, and angular surfaces with single-point tools. The largest shapers have a 36-inch cutting stroke and can machine parts up to 36 inches long. The cutting tool on the shaper oscillates, cutting on the forward stroke, with the workpiece feeding automatically toward the tool during each return stroke.
Planing machines perform the same operations as shapers but can machine longer workpieces. Some planers can machine parts up to 50 feet long. The workpiece is mounted on a reciprocating table that moves the workpiece beneath a cutting tool. This tool, which remains stationary during the cutting stroke, automatically feeds into the workpiece after each cutting stroke.
Drilling machines
Drilling machines, also called drill presses, cut holes in metal with a twist drill. They also use a variety of other cutting tools to perform the following basic hole-machining operations: (1) reaming, (2) boring, (3) counterboring, (4) countersinking, and (5) tapping internal threads with the use of a tapping attachment.
Milling machines
A milling machine cuts metal as the workpiece is fed against a rotating cutting tool called a milling cutter. Cutters of many shapes and sizes are available for a wide variety of milling operations. Milling machines cut flat surfaces, grooves, shoulders, inclined surfaces, dovetails, and T-slots. Various form-tooth cutters are used for cutting concave forms and convex grooves, for rounding corners, and for cutting gear teeth.
Milling machines are available in a variety of designs that can be classified as the following: (1) standard knee-and-column machines, including the horizontal and the vertical types; (2) bed-type or manufacturing machines; and (3) machines designed for special milling jobs.
Grinding machines
Grinding machines remove small chips from metal parts that are brought into contact with a rotating abrasive wheel called a grinding wheel or an abrasive belt. Grinding is the most accurate of all of the basic machining processes. Modern grinding machines grind hard or soft parts to tolerances of plus or minus 0.0001 inch (0.0025 millimetre).
The common types of grinding machines include the following: (1) plain cylindrical, (2) internal cylindrical, (3) centreless, (4) surface, (5) off-hand, (6) special, and (7) abrasive-belt.
Power saws
Metal-cutting power saws are of three basic types: (1) power hacksaws, (2) band saws, and (3) circular disk saws. Vertical band saws are used for cutting shapes in metal plate, for internal and external contours, and for angular cuts.
Presses
This large class of machines includes equipment used for forming metal parts by applying the following processes: shearing, blanking, forming, drawing, bending, forging, coining, upsetting, flanging, squeezing, and hammering. All of these processes require presses with a movable ram that can be pressed against an anvil or base. The movable ram may be powered by gravity, mechanical linkages, or hydraulic or pneumatic systems.
Appropriate die sets, with one part mounted on the movable ram and the matching part mounted on the fixed bed or platen, are an integral part of the machine. Punch presses stamp out metal parts from sheet metal and form the parts to the desired shape. Dies with cavities having a variety of shapes are used on forging presses that form white-hot metal blanks to the desired shapes. Power presses are also used for shearing, bending, flanging, and shaping sheet metal parts of all sizes. Power presses are made in various sizes, ranging from small presses that can be mounted on a workbench to machines weighing more than 1,000,000 pounds (450,000 kilograms).
Modifications of basic machines
Certain machine tools have been designed to speed up production. Although these tools include features of the basic machine tools and perform the same operations, they incorporate design modifications that permit them to perform complex or repetitive operational sequences more rapidly. Furthermore, after the production machine has been set up by a skilled worker or machinist, a less skilled operator also can produce parts accurately and rapidly. The following are examples of production machine tools that are modifications of basic machine tools: (1) turret lathes, including screw machines; (2) multiple-station machines; (3) gang drills; and (4) production milling machines.
Turret lathes
Horizontal turret lathes have two features that distinguish them from engine lathes. The first is a multiple-sided main turret, which takes the place of the tailstock on the engine lathe. A variety of turning, drilling, boring, reaming, and thread-cutting tools can be fastened to the main turret, which can be rotated intermittently about its vertical axis with a hand wheel. Either a hand wheel or a power feed can be used to move the turret longitudinally against the workpiece mounted on the machine spindle.
The second distinguishing feature of the turret lathe is the square turret mounted on the cross slide. This turret also can be rotated about its vertical axis and permits the use of a variety of turning tools. A tool post, or tool block, can be clamped to the rear of the cross slide for mounting additional tools. The cross slide can be actuated either by hand or by power.
Turret lathes may be classified as either bar machines or chucking machines. Bar machines formerly were called screw machines, and they may be either hand controlled or automatic. A bar machine is designed for machining small threaded parts, bushings, and other small parts that can be created from bar stock fed through the machine spindle. Automatic bar machines produce parts continuously by automatically replacing of bar stock into the machine spindle. A chucking machine is designed primarily for machining larger parts, such as castings, forgings, or blanks of stock that usually must be mounted in the chuck manually.
Multiple-station machines
Several types of multiple-station vertical lathes have been developed. These machines are essentially chucking-type turret lathes for machining threaded cylindrical parts. The machine has 12 spindles, each equipped with a chuck. Directly above each spindle, except one, tooling is mounted on a ram. Parts are mounted in each chuck and indexed for up to 11 machining operations. The finished part is removed at the 12th station.
Gang drills
A gang-drilling machine consists of several individual columns, drilling heads, and spindles mounted on a single base and utilizing a common table. Various numbers of spindles may be used, but four or six are common. These machines are designed for machining parts requiring several hole-machining operations, such as drilling, countersinking, counterboring, or tapping. The workpiece is moved from one drilling spindle to the next, where sequential machining operations are performed by one or more operators.
Production millers
Milling machines used for repet-itive-production milling operations generally are classified as bed-type milling machines because of their design. The sliding table is mounted directly onto the massive bed of the machine and cannot be raised or moved transversely; table movement is longitudinal only. The spindle head may be adjusted vertically to establish the depth of cut. Some machines are equipped with automatic controls that require only a semiskilled operator to load parts in fixtures at each end of the table and start the machine. One part can be unloaded and replaced while the other is being machined.
Special-purpose machines
Special-purpose machine tools are designed to perform special machining operations, usually for production purposes. Examples include gear-cutting and gear-grinding machines, broaching machines, lapping and honing machines, and boring machines.
Gear-cutting machines
Three basic cutting methods are used for machining gears: (1) form cutting, (2) template cutting, and (3) generating. The form-cutting method uses a cutting tool that has the same form as the space between two adjacent teeth on a gear. This method is used for cutting gear teeth on a milling machine. The template-cutting method uses a template to guide a single-point cutter on large bevel-gear cutting machines.
Most cut gears produced in large lots are made on machines that utilize the gear-generating method. This method is based on the principle that two involute gears, or a gear and rack, with the same diametral pitch will mesh together properly. Therefore, a cutting tool with the shape of a gear or rack may be used to cut gear teeth in a gear or rack blank. This principle is applied in the design of a number of widely used gear-cutting machines of the generating type. Gear-generating machines that cut with reciprocating strokes are called gear shapers.
Gear-hobbing machines use a rotating, multiple-tooth cutting tool called a hob for generating teeth on spur gears, worm gears, helical gears, splines, and sprockets. More gears are cut by hobbing than by other methods because the hobbing cutter cuts continuously and produces accurate gears at high production rates. In gear-making machines gears can be produced by cutting, grinding, or a combination of cutting and grinding operations.
Broaching machines
In general, broaching is classified as a planing or shaping art because the action of a broaching tool resembles the action of planer and shaper tools. Broaching tools of various designs are available. The teeth on broaching tools are equally spaced, with each successive tooth designed to feed deeper into the workpiece, thus completing the broaching operation in a single stroke. Examples of internal broaching applications include cutting keyways in the hubs of gears or pulleys, cutting square or hexagonal holes, and cutting gear teeth. External grooves can be cut in a shaft with an external broaching tool. Some broaching machines pull or push broaching tools through or over the workpiece.
Lapping and honing machines
Lapping and honing operations are classified under the basic art of grinding. Lapping is a process in which a soft cloth impregnated with abrasive pastes or compounds is rubbed against the surface of a workpiece. Lapping is used to produce a high-quality surface finish or to finish a workpiece within close size limits. Dimensional tolerances of two millionths of an inch (0.00005 millimetre) can be achieved in the hand or machine lapping of precision parts such as gauges or gauge blocks.
Honing is a low-speed surface finishing process used for removing scratches, machine marks, or small amounts of metal, usually less than 0.0005 inch (0.0125 millimetre), from ground or machined surfaces. Honing is done with bonded abrasive sticks or stones that are mounted in a honing head. In a typical honing operation, such as honing automotive engine cylinder walls, a honing machine with one or more spindles is used. The honing head rotates slowly with an oscillating motion, holding the abrasive sticks against the work surface under controlled light pressure.