A general-purpose machine tool is basically comprised of power drive and kinematic system for the essential formative and auxiliary tool – work motions and a rigid body or structure to accommodate all the above. But several additional elements or devices called accessories are also essentially required for that machines’ general functioning, mainly for properly holding and supporting the workpiece and the cutting tool depending upon the type and size of the tool – work and the machining requirements. Before reading this, you should Know basic Information about lathe and Lathe specifications.
Workpieces are held to the headstock spindle of the lathe with chucks, faceplates, or lathe centers. A lathe chuck is a device that exerts pressure on the workpiece to hold it secure to the headstock spindle or tailstock spindle. Commonly used with the lathe are the independent chuck, the universal scroll chuck, the combination chuck, the hollow headstock spindle chuck, the lathe tailstock chuck, the collet chuck, and the step chuck.
Independent Chuck – The independent chuck generally has four jaws which are adjusted individually on the chuck face by means of adjusting screws. The chuck face is scribed with concentric circles which are used for rough alignment of the jaws when chucking round workpieces. The final adjustment is made by turning the workpiece slowly and using gages to determine its concentricity. The jaws are then readjusted as necessary to align the workpiece to desired tolerances.
The jaws of the independent chuck may be used or may be reversed so that the steps face in the opposite direction; thus, workpieces can be gripped either externally or internally. The independent chuck can he used to hold square, round, octagonal, or irregular shaped workpieces in either a concentric or eccentric position due to the independent operation of each jaw.
Because of its versatility and capacity for fine adjustment, the independent chuck is commonly used for mounting workpieces that require extreme accuracy.
Universal Scroll Chuck – The universal scroll chuck usually has three jaws which move in unison as an adjusting pinion is rotated. The advantage of the universal scroll chuck is its ease of operation in centering the work for concentric turning. This chuck is not as accurate as the independent chuck but, when in good condition, it will center the work automatically within 0.003 of an inch of complete accuracy.
The jaws are moved simultaneously within the chuck by means of a scroll or spiral threaded plate. The jaws are threaded to the plate and move an equal distance inward or outward as the scroll is rotated by means of the adjusting pinion. Since the jaws are individually aligned on the scroll, the jaws cannot be reversed. However, the chuck is usually supplied with two sets of jaws which can be interchanged.
The universal scroll chuck can be used to hold and automatically center round or hexagonal workpieces. Having only three jaws, the chuck cannot be used effectively to hold square, octagonal, or irregular shapes.
Combination Chuck – A combination chuck combines the features of the independent chuck and the universal scroll chuck and can have either three or four jaws. The jaws can be moved in unison on a scroll for automatic centering or can be moved individually if desired by separate adjusting screws.
Drill Chuck – The drill chuck is a small universal-type chuck which can be used in either the headstock spindle or in the tailstock for holding straight-shank drills, reamers, taps, or small-diameter workpieces. The drill chuck has three or four hardened steel jaws which are moved together or apart by adjusting a tapered sleeve within which they are contained. The drill chuck is capable of centering tools and small-diameter workpieces to within 0.002 or 0.003 of an inch when firmly tightened.
Hollow Headstock Spindle Chuck – The hollow headstock spindle chuck is similar to a drill chuck but is hollow. It is provided with threads to screw it onto the headstock spindle nose. This chuck can be used to hold rods, tubes, or bars which are passed through the headstock spindle. It is generally capable of centering workpieces to an accuracy of 0.002 of an inch.
Collet Chuck – The collet chuck is the most accurate means of holding small workpieces in the lathe. The collet chuck consists of a spring machine collet and a collet attachment which secures and regulates the collet on the headstock spindle of the lathe.
The spring machine collet is a thin metal bushing with an accurately machined bore and a tapered exterior. The collet has three lengthwise slots to permit its sides to be sprung slightly inward to grip the workpiece. To grip the workpiece accurately, the collet must be no more than 0.001 inch larger or smaller than the diameter of the piece to be chucked. For this reason, spring machine collets are generally supplied in sets with various capabilities in 1/16, 1/32, or 1/64 inch increments. For general purposes, the spring machine collets are limited in capacity to 1 inch in diameter.
The collet attachment which, with the spring machine collet, forms the collet chuck and consists of a collet sleeve, a draw bar, and a handwheel or hand lever to move the draw bar. The collet is fitted to the right end of the headstock spindle. The draw bar passes through the headstock spindle and is threaded to the spring machine collet. When the draw bar is rotated by means of the handwheel, the collet is pulled inward and the collet walls are cammed together by contact with the collet sleeve, tightening the chuck to the workpiece.
Collet chucks are usually standard equipment on toolroom-type engine lathes and on horizontal turret lathes. Spring machine collets are available in different shapes in order to chuck square and hexagonal workpieces of small dimensions as well as round workpieces.
Jacobs Rubber-Flex Collets – The rubber flex collet is comprised of hardened alloy steel jaws bonded together with oil-resistant Hycar synthetic rubber. The collets have a range of 1/8 inch per collet and the gripping power and accuracy remain constant throughout the entire collet capacity. These collets are designed for heavy-duty turning and possess 2 to 4 times the grip of conventional split steel type collets.
Step Chuck – The step chuck is a variation of the collet chuck, but is intended for accurate holding of workpieces larger than 1 inch in diameter. The step chuck consists of the handwheel or hand lever collet attachment and a step chuck machine collet in place of the regular spring machine collet. The step chuck machine collet, which is split into three sections like the spring machine collet, is threaded to the draw bar of the collet attachment. As the step chuck machine collet is drawn into the collet sleeve, the three sections of the collet are cammed against the workpiece by an inside taper in the collet sleeve. The step chuck is supplied in 2, 3, 4, and 5 inch sizes, the size indicating the maximum diameter of the workpieces that can be supported. The step chuck machine collets are furnished blank and machined on the lathe to the desired step diameter.
Lathe Tailstock Chuck – The lathe tailstock chuck is a device designed to support the ends of the workpieces in the tailstock when a lathe center cannot be conveniently used. The chuck has a taper arbor that fits the lathe tailstock spindle. The three bronze self-centering jaws of the chuck will accurately close upon the workpieces that are between 1/4 and 1 inch in diameter. The bronze jaws provide a good bearing surface for the workpiece. The jaws are adjusted to the diameter of the workpiece and then locked in place.
2. Lathe Faceplates
A lathe faceplate is a flat, round plate that threads to the headstock spindle of the lathe. The faceplate is used for clamping and machining irregularly-shaped workpieces that cannot be successfully held by chucks or mounted between centers. The workpiece is either attached to the faceplate using angle plates or brackets, or is bolted directly to the plate. Radial T-slots in the faceplate surface facilitate mounting workpieces. The faceplate is valuable for mounting workpieces in which an eccentric hole or projection is to be machined. The number of applications of the faceplate depends upon the ingenuity of the machinist.
A small faceplate, known as a driving faceplate, is used to drive the lathe dog for workpieces mounted between centers. The driving faceplate usually has fewer T-slots than the larger faceplates. When the workpiece is supported between centers, a lathe dog is fastened to the workpiece and engaged in a slot of the driving faceplate.
3. Lathe Centers
General. Lathe centers are the most common devices for supporting workpieces in a lathe. Most lathe centers have a tapered point with a 600 included angle to fit the workpiece holes with the same angle. The workpiece is supported between two centers, one in the headstock spindle and one in the tailstock spindle. Centers for lathe work have standard tapered shanks that fit into the tailstock directly and into the headstock spindle, using a center sleeve to convert the larger bore of the spindle to the smaller taper size of the lathe center. The centers are referred to as live centers or dead centers, depending upon whether they move with the workpiece or remain stationary. The most common types of centers are described below.
Male Center. The male center or plain center is the type used in pairs for most general lathe turning operations. The point is ground to a 60° cone angle. When used in the headstock spindle, where it revolves with the workpiece, it is commonly called a live center. When used in the tailstock spindle, where it remains stationary when the workpiece is turning, it is called a dead center. Dead centers are always hard and must be lubricated very often to prevent overheating.
Pipe Center – The pipe center is very similar to the male center, but its cone is ground to a greater angle and is larger in size. It is used for holding pipe and tubing in the lathe while they are being machined.
Female Center – The female center is conically bored at the tip and is used to support workpieces that are pointed at the end.
Half-Male Center – The half-male center is a male center that has a portion of the 60° cone cut away. The half-male center is used as a deadcenter in the tailstock where complete facing is to be performed. The cutaway portion of the center faces the cutting tool and provides the necessary clearance for the tool when facing the surface immediately around the drilled center in the workpiece.
V-Center – The V-center is used to support round workpieces at right angles to the lathe axis for special operations such as drilling or reaming.
4. Lathe Dogs
Lathe dogs are cast metal devices used in conjunction with a driving plate or a faceplate to provide a firm connection between the headstock spindle and the workpiece that is mounted between centers. This firm connection permits the workpiece to be driven at the same speed as the spindle under the strain of cutting. Frictional contact alone, between the live center and the workpiece, is not sufficient to drive the workpiece. Three common types of lathe dogs. Lathe dogs may have bent tails or straight tails. When the bent tail dogs are used, the tail fits into a slot of the driving face plate. When straight tail dogs are used, the tail bears against a stud projecting from the faceplate.
The bent tail lathe dog with a headless setscrew is considered safer than the dog with the square head screw because the headless setscrew reduces the danger of the dog catching in the operator’s clothing and causing an accident.
The bent tail clamp lathe dog is used primarily for holding rectangular workpieces.
General – A workpiece that cannot be held between centers because its axis has been drilled or bored, and which is not suitable for holding in a chuck or against a faceplate, is usually machined on a mandrel. A mandrel is a tapered axle pressed into the bore of the workpiece to support it between centers. A mandrel should not be confused with an arbor, which is a similar device used for holding tools rather than workpieces. To prevent damage to the work, the mandrel should always be oiled before being forced into the hole. When turning work on a mandrel, feed the cutting tool toward the large end of the mandrel, which should be nearest the headstock of the lathe.
Solid Machine Mandrel – A solid machine mandrel is generally made from hardened steel and is ground to a slight taper ranging from 0.0005 to 0.0006 inch per inch. It has a very accurately countersunk center at each end for mounting the workpiece between centers. The ends of the mandrel are smaller than the body and have machined flats for the lathe dog to grip. The size of the solid mandrel is always stamped on the large end of the taper.
Expansion Mandrel – An expansion mandrel is used to hold work that is reamed or bored to a nonstandard size. The expansion mandrel is composed of two parts: a tapered pin that has a taper of approximately 1/16 of an inch for each inch of length and an outer split shell that is tapered to fit the pin. The split shell is placed in the workpiece and the tapered pin is forced into the shell, causing it to expand until it properly holds the workpiece. Since solid machine mandrels have a very slight taper, they are limited to a workpiece with specific inside diameters. An expansion mandrel will not accept workpieces having a greater range of sizes. The expansion mandrel is, in effect, a chuck arranged so that the grips can be forced outward against the interior of the hole in the workpiece.
General – Workpieces often need extra support, especially long, thin workpieces that tend to spring away from the cutter bit. Two common supports or rests are the steady rest and the follower rest.
Steady Rest – The steady rest or center rest, as it is also called, is used to support long workpieces or shafts being machined between centers or for boring operations. It is also used for internal threading operations where the workpiece projects a considerable distance from the chuck or faceplate. The steady rest is clamped to the lathe bed at the desired location and supports the workpiece within three adjustable jaws. The rest prevents the workpiece from springing under cut, or sagging as a result of the otherwise unsupported weight.
The workpiece must be machined with a concentric bearing surface at the point where the steady rest is to be applied. The jaws must be carefully adjusted for proper alignment and locked in position. The area of contact must be lubricated frequently. The top section of the steady rest swings away from the bottom section to permit removal of the workpiece without disturbing the jaw setting.
Follower Rest – The follower rest is used to back up a workpiece of small diameter to keep it from springing under the stress of the cutting operation. The follower rest gets its name because it follows the cutting tool along the workpiece. The follower rest has one or two jaws that bear directly on the finished diameter of the workpiece opposite and above the cutting tool. The rest is bolted to the lathe carriage so that it will follow the cutter bit and bear upon that portion of the workpiece that has just been turned. The cut must be started and continued for a short longitudinal distance before the follower rest is applied. The rest is generally used only for straight turning or threading long, thin workpieces.
7. Taper Attachment
The taper attachment is used for turning and boring tapers. It is bolted to the back of the carriage saddle. In operation, it is connected to the cross-slide so that it moves the cross-slide laterally as the carriage moves longitudinally. This action causes the cutting tool to move at an angle to the axis of the workpiece to produce a taper.
The angle of the desired taper is set on the guide bar of the attachment, and the guide bar support is clamped to the lathe bed. Since the cross-slide is connected to a shoe that slides on this guide bar, the tool follows along a line that is parallel to the guide bar and hence at an angle to the workpiece axis corresponding to the desired taper.
Toolpost Grinding Machine – The toolpost grinding machine is a machine tool attachment specially designed for cylindrical grinding operations on the lathe. It consists primarily of a 1/4 or 1/3 horsepower electric motor and a wheel spindle connected by means of pulleys and a belt. The machine fastens to the compound rest of the lathe with a T-slot bolt which fits into the T-slot of the compound rest in the same manner as the lathe toolpost. The toolpost grinding machine mounts grinding abrasive wheels ranging from 1/4 to 3 or 4 inches in diameter for internal and external grinding operations.
Milling and Grinding Attachment (Versa Mill). The milling and grinding attachment is a versatile, self-powered attachment that fits to the carriage of the lathe. The attachment is used for milling slots, flats, or keyways on lathe mounted workpieces, for performing internal and external grinding operations, for drilling holes in the periphery of lathe-mounted workpieces, for reaming and boring operations, and for milling gear teeth and square tooth threads. The spindle of the milling and grinding lathe attachment is vertically adjustable, riding on four shafts and controlled by a screw and a hand wheel.