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河南理工大學萬方科技學院畢業(yè)論文
Fundamentals of Machine Tools
In many cases products from the primanry forming processes must undergo further refinements in size and surface finish to meet their design specifications. To meet such precise tolerances the removal of small amounts of material is needed. Usually machine tools are used for such operation.
In the United States material removal is a big business-in excess of $36×10 per year, including material, labor, overhead, and machine-tool shipments. Since 60 percent of themechanical and industrial engineering and technology graduates have something connected with the machining industry either through sale, design, or operation of machine shops,or working in related industry, it is wise for an engineering studet to devote some time in his curriculum to studying material removal and machine tools.
A machine tool provides the means for cutting tools to shape a workpiece to required dimensions; the machine supports the tool and the workpiece in a controlled relationship through the functioning of its basic member,which are as follows:
(1) Bed, Structure or Fram. This is the main member which provides a basis for, and a connection between, the spindles and slides; the distortion and vibration under load must be kept to a minimum.
(2) Slides and Slideways. The translation of a machine element(e.g. the slide) is normally achieved by straight-line motion under the constraint of accurate guiding surfaces(the slideway).
(3) Spindles and Bearings. Angular displacements take place an axis of rotation; the position of this axis must be constant within extremely fine limits in machine tools, and is ensured by the provision of precision spindles and bearings.
(4) Power Unit. The electric motor is the universally adopted power unit for machine tools. By suitably positioning individual motors, belt and gear transmissions are reduced to minimum.
(5) Transmission Linkage. Linkage is the general term used to denote the mechanical, hydraulic, pneumatic or electric mechanisms which connect angular and linear displacements in defined relationship.
There are two broad divisions of machining operations:
(1) Roughing, for which the mental removal rate, and consequently the cutting force, is high, but the required dimensional accuracy relatively low.
(2) Finishing, for which the metal removal rate, and consequently the cutting force, is low, but the required dimensional accuracy and surface finish relatively high.
It follows that static loads and dynamic loads, such as result from an unbalanced grindingwheel, are more significant in finishing operations than in roughing operations. The degree of precision achieved in any machining process will usually be influenced by the magnitude of the deflections, which occur as a result of the force acting.
Machine tool frames are generally made in cast iron, although some may be steel casting or mild-steel fabrications. Cast iron is chosen because of its cheapness, rigidity, compressive strength and capacity for damping the vibrations set-up in machine operations. To avoid massive resistance to bending and torsional stresses. Tow basic types of ribbing are box and diagonal. The box formation is convenient to produce, apertures in walls permitting the positioning and extraction of cores. Diagonal ribbing provides greater torsional stiffness and yet permits swarf to fall between the sections; it is frequently used for lathe beds.
The slides and slideways of a machine tool locate and guide members which move relative to each other, usually changing the position of the tool relative to the workpiece. The movement generally takes the from of translation in a straight line, but is sometime angular rotation, e.g. tilting the wheel-head of a universal thread-grinding machine to an angle corresponding with the helix angle of the work piece threat. The basic geometric elements of slides are flat, vee, dovetail and cylinder. These elements may used separately or combined in various ways according to the applications. Features of slideways are as follows:
(1) Accuracy of Movement. Where a slide is to be displaced in a straight line, this line must lie in two mutually perpendicular planes and there must be no slide rotation. The general tolerance for straightness of machine tool slideways is 0~0.02 mm per 1000mm; on horizontal surfaces this tolerance may be disposed so that a convex surface results, thus countering the effect of “sag” of the slideways.
(2) Means of Adjustment. To facilitate assembly, maintain accuracy and eliminate “play” between sliding members after wear has taken place, a strip is sometimes inserted in the slides. This is called a gib-strip. Usually, the gib is retained by socket-head screws passing through elongated slots; and is adjusted by grub-screws secured by lock nuts.
(3) Lubrication. Slideways may be lubricated by either of the following systems:
(i) Intermittently, through grease or oil nipples, a method suitable where movements are infrequent and speed low.
(ii) Continuously, e.g. by pumping through a metering valve and pipe-work to the point of application; the film of oil introduce between surface by these means must be extremely thin to avoid the slide “floating”. If sliding surfaces were optically flat oil would be squeezed out, resulting in the surface sticking. Hence in practice slide surface are either ground using the edge of a cup wheel, or scraped. Both processes produce depressions which retain “pocket” of oil, and complete separation of the parts may not occur at all points; positive location of the slides is thus retained.
(4) Protection. To maintain slideways in good order, the following conditions must be met:
(i) Ingress of foreign matter, e.g. swarf, must be prevented. Where this is no possible, it is desirable to have a form of slideway, which does not retain swarf, e.g. the inverted vee.
(ii) Lubricating oil must be retained. The adhesive property of oil for use on vertical or inclined slide surface is important; oil are available which have been specially developed for this purpose. The adhesiveness of oil also prevents it being washed away by cutting fluids.
(iii) Accidental damage must be prevented by protective guards.
Lathes
A machine tool performs three major functions: (i) it rigidly supports the work piece or its holder and the cutting tool; (ii) it provide relative motion between the work piece and the cutting tool; (iii) it provides a range of feeds and speeds.
Machines used to remove metal in the form of chips are classified as follows:
Machines using basically the single-point cutting tools include: engine lathes, turret lathes, tracing and duplicating lathes, single-spindle automatic lathes, multi-spindle automatic lathes, shapers and planers, boring machines.
Machines using multipoint cutting tools include: drilling machines, milling machines, broaching machines, sawing machine, gear-cutting machines.
Machines using random-point cutting tools (abrasive) include: cylindrical grinder, centreless grinders, surface grinders.
Special metal removal methods include: chemical milling, electrical discharge machining, ultrasonic machining.
The lathe removes material by rotating the work piece against a cutter to produce external or internal cylindrical or conical surfaces. It is commonly used for the production of surfaces by facing, in which the work piece is rotated while the cutting tool is moved perpendicularly to the axis of rotation.
The engine lathe, shown in Fig.1, is the basic turning machine from which other turning machines have been developed. The drive motor is located in the base and drives the spindle through a combination of belts and gears. The spindle is a sturdy hollow shaft, mounted between heavy-duty bearings, with the forward end used for mounting a drive plate to impart positive motion to the work piece. The drive plate may be fastened to the spindle by threads, by a cam lock mechanism, or by a threaded collar and key.
The lathe bed is cast iron and provides accurately ground sliding surfaces (way) on which the carriage rides. The lathe carriage is an H-shaped casting on which the cutting tool is mounted in a tool holder. The apron hangs from the front of the carriage and contains the driving gears that move the tool and carriage along or across the way to provide the desired tool motion.
A compound rest, located above the carriage, provides for rotation of the tool holder through any desired angle. A hand wheel and feed screw are provided on the compound rest for linear motions of the tool. The cross feed is provided with another hand wheel and feed screw for moving the compound rest perpendicular to the lathe way. A gear train in the apron provides power feed for the carriage both along and across the way. The feed box contains gears to impart motion to the carriage and control the rate at which the tool moves relative to the work piece. Since the transmission in the feed box is driven from the spindle gears, the feeds are directly related to the spindle speed. The feed box gearing is also used in thread cutting and provides from 4 to 224 threads per in.
The connecting shaft between the feed box and the lathe apron are the feed rod and the lead screw. Many lathe manufacturers combine these two rods in one, a practice that reduces the cost of the machine at the expense of accuracy. The feed rod is used to provide the accurate lead necessary for the thread cutting. The feed rod is driven through a friction clutch that allows slippage in case the tool is overloaded. This safety device is not provided in the lead screw, since thread cutting cannot tolerate slippage. Since the full depth of the thread is seldom cut in one pass, a chasing dial is provided to realign the tool for subsequent passes.
The lathe tailstock is fitted with an accurate spindle that has a tapered hole for mounting drill, drill chucks, reamers, and lathe centers. The tailstock can be moved along the lathe ways to accommodate various lengths of work pieces as well as to advance a tool into contact with the work piece. The tailstock can be offset relative to the lathe ways to cut tapers or conical surfaces.
The turret lathe is basically an engine lathe with certain additional features to provide for semiautomatic operation and to reduce the opportunity for human error. The carriage of the turret lathe is provided with T-slots for mounting a tool-holding device on both sides of the lathe ways with tools properly set for cutting when rotated into position. The carriage is also equipped with automatic stops that control the tool travel and provide good reproduction of cuts. The tailstock of the turret lathe is of hexagonal design, in which six tools can be mounted. Although a large amount of time is consumed in setting up the tools and stops for operation, the turret lathe ,once set, can continue to duplicate operations with a minimum of operator skill until the tools become dulled and need replacing. Thus , the turret is economically feasible only for production work, where the amount of time necessary to prepare the machine for operation is justifiable in terms of the number of part to be made.
The multi-spindle automatic lathe is provided with four, five, six, or eight spindles, with one workpiece mounted in each spindles. The spindles index around a central shaft, with the main tool slide accessible to all spindles. Each spindle position is provided with a side tool-slide operated independently. Since all of the slides are operated by cams, the preparation of this machine may take several days, and a production run of at least 5000 parts is needed to justify its use. The principal advantage of this machine is that all tools work simultaneously, and one operator can handle several machines. For relatively simple parts, multi-spindle automatic lathes can turn out finished products at the rate of 1 every 5 sec.
Shapers, Planers, Drilling and Milling Machines
A shaper utilizes a single-point tool in a tool holder mounted on the end of the ram. Cutting is generally done on the forward stroke. The tool is lifted slightly by the clapper box to prevent excessive drag across the work, which is fed under the tool during the return stroke in preparation for the next cut. The column houses the operating mechanisms of the shaper and also serves as a mounting unit for the work-supporting table. The table can be moved in two directions mutually perpendicular to the ram. The tool slide is used to control the depth of cut and is manually fed. It can be rotated through 90 deg. on either side of its normal vertical position, which allows feeding the tool at an angle to the surface of the table.
Two types of driving mechanisms for shapers are a modified whitworth quick-return mechanism and a hydraulic drive. For the whitworth mechanism, the motor drives the bull gear, which drives a crank arm with an adjustable crank pin to control the length of stroke. As the bull gear rotates, the rocker arm is forced to reciprocate, imparting this motion to the shaper ram.
The motor on a hydraulic shaper is used only to drive the hydraulic pump. The remainder of the shaper motions are controlled by the direction of the flow of the hydraulic oil. The cutting stroke of the mechanically driven shaper uses 220 deg. Of rotation of the bull gear, while the return stroke uses 140 deg. This gives a cutting stroke to return stroke ratio of 1.6 to 1. The hydraulic shaper has an advantage of infinitely variable cutting speeds. The principal disadvantage of this type of machine is the lack of a definite limit at the end of the ram stroke, While may allow a few thousandths of an inch variation in stroke length.
Planers are similar to shapers because both machines are primarily used to produce flat and angular surfaces. However ,planers are capable of accommodating much larger workpieces than horizontal plane providing a straight-line cutting and feed action. Single-point cutting tools are mounted on an overhead cross rail and along the vertically supported columns. The cutting tools are fed into or away from the workplace on either the horizontal or vertical plane, thus being capable of four straight-line feed motions.
Cutting speeds are slow on the planer because of the workplace size and type of cutting tool being used. In order to increase the production of the planer, multiple tooling stations are employed. Another method of increasing production is to mount a number of workpieces on the table at the same time. The planer size is designated by the maximum workplace capacity of the machine. The height, width, and length of the workplace that can be accommodated on the planer’s worktable varies with the type of planer.
Upright drilling machines or drill presses are available in a variety of sizes and types, and are equipped with a sufficient range of spindle speeds and automatic feeds to fit the needs of most industries. Radial drilling machines are used to drill workpieces that are too large or cumbersome to conveniently move. The spindle with the speed and feed changing mechanism is mounted on the radial arm; by combining the movement of the radial arm around column and the movement of the spindle assembly along the arm, it is possible to align the spindle and the drill to any position within reach of the machine.
Plain radial drilling machines provide only for vertical movement of the spindle; universal machines allow the spindle to swivel about an axis normal to the radial arm to rotate about a horizontal axis, thus permitting drilling at any angle.
A multispindle drilling machine has one or more heads that drive the spindles through universal joints and telescoping splined shafts. All spindles are usually driven by the same motor and fed simultaneously to drill the desired number of holes.
The milling operation involves metal removal with a rotating cutter. It includes removal of metal from the surface of a workpiece, enlarging holes, and form cutting, such as threads and gear teeth.
Within a knee and column type of milling machine the column is the main supporting member for the other components, and includes the base containing the drive motor, the spindle, extremity by a bearing in the overarm. The knee is held on the column in dovetail slots, the saddle is fastened to the knee in dovetail slots, and the table is attached to the saddle. Thus, the build-up of the knee and column machine provides three motions relative to the cutter. The fourth motion may be provided by swiveling the table around a vertical axis provided on the saddle.
Fixed-bed milling machines are designed to provide more rigidity than the knee and column type. The table is mounted directly on the machine base, which provides the rigidity necessary for absorbing heavy cutting load, and allows only longitudinal motion to the table. Vertical motion is obtained by moving the entire cutting head.
Tracer milling is characterized by coordinated or synchronized movements of either the paths of the cutter and tracing elements, or the paths of the workpiece and model. The tracing finger follow the shape of the master pattern, and the cutter heads duplicate the tracer motion.
The following are general design considerations for milling:
(1) Wherever possible, the part should be designed so that a maximum number of surfaces can be milled from one setting.
(2) Design for the use of multiple cutters to mill several surfaces simultaneously.
(3) The largest flat surface will be milled first, so that all dimensions are best referred to such surface.
(4) Square inside corners are not possible, since the cutter rotates.
Grinding Machines
Grinding machines utilize abrasive grains, bonded into various shapes and sizes of wheels and belts to be used as the cutting agent. Grinding operations are used to impart a high-quality surface finish on the workpiece is improved since tolerances of 0.00025 mm are possible in grinding operations. Grinding machines are classified according to the type of surface produced. Common surfaces and classifications of grinding machines are surface, cylindrical, and special machines.
The grinding process is of extreme importance in production work for several reasons.
(1) It is the most common method for cutting hardened tool steel of other heat-treated steel. Parts are first machined in the un-heat-treated condition, and then ground to the desired dimensions and surface finish.
(2) It can provide surface finish to 0.5 um without extreme cost.
(3) The grinding operation can assure dimensions in a relatively short time, since machines are built to provide motions in incremente of ten-thousandths of an inch, instead of thousandths as is common in s can be other machines.
(4) Extremely small and thin parts can be finished by this method, since light pressure is used and the tendency for the part to deflect away from the cutter is miniminzed.
On a cylindrical grinding machine the depth of cut is controlled by moving the wheel head, which includes both the wheel and its drive motor. Coolants are provided to reduce heat distortion and to remove chips and abrasive dust.