Work holding and supporting is an important aspect of an engineer. All the work has to be held properly for machining. A good method helps in achieving economy and efficient manufacturing. Standard work holding devices used in lathe operation are: 1. Chuck 2. Lathe Centres 3. Collets 4. Carriers and Catch Plates 5. Face-Plate 6. Mandrel 7. Steady Rest 8. Follower Rest.
Type # 1. Chuck:
It is the most important device for holding the workpiece, particularly of short length and large diameter or of irregular shape which can’t be conveniently mounted between centres. It can be attached to the lathe by screwing on the spindle nose.
(а) Independent or Four Jaw Chuck:
It has four jaws and each jaw is independently actuated and adjusted a key for holding the job. This type of chuck is used for irregular shapes, rough castings of square or octagonal in such jobs, where a hole is to be positioned off the centre.
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(b) Three Jaw or Universal Chuck:
In this case, all the three jaws move simultaneously by turning a key and thus the workpiece may be automatically held in the centre of chuck-opening. It is used for holding round, hexagonal bar or other symmetrical work.
(c) Collet Chuck:
It is mostly sued for holding bars of small sizes (below 63 mm) and is normally used where production work is required such as in capstan lathe or automats.
(d) Magnetic Chuck:
They are either electrically operated or are of permanent magnet type. In lathe it does not find widespread use.
Other types of chucks are air or hydraulically operated chucks, drill chucks, etc.
Type # 2. Lathe Centres:
Table below shows various types of lathe centres and their applications:
Centres:
Next to chucks, lathe centres are used for work holding during turning operation. A centre hole of particular depth and shape is made at each end of the workpiece. The lathe centres act as supports for the workpiece and take up the thrust due to metal cutting.
These are made of very hard materials to withstand wear and resist deflection. The included angle of centre is 60° for general purpose work and 75° for heavy work. The shanks of all the centres are machined to the Morse (0 to 6) or Metric (4 to 6) standard tapers.
The various types of centres are:
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(i) Ordinary centre, which is used for most general work;
(ii) Tipped centre, which contains a hard alloy tip brazed into a steel shank;
(iii) Ball centre, which has a ball shape at the end of centre instead of a sharp point, and is used to minimise the wear and strain on the ordinary centre while taper turning by set over method;
(iv) Half centre in which case less than half of the centre is ground away, thus facilitating facing of the bar ends without removal of the centre;
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(v) Rotating dead centre is used in tailstock for supporting heavy work revolving at a high speed.
Type # 3. Collets:
A collet is used for holding small semi-finished or finished parts so that additional operations may be performed. It is a practical device for quickly and accurately chucking symmetrical workpiece. Collets are available in several shapes, i.e., round, square and hexagonal holes to accommodate corresponding shapes of workpiece.
The front portion is made conical and a transverse slope is made in three-fourth of length. The other end is threaded and has a key way to prevent the collet from turning in the collet sleeve.
A tapered collet sleeve is inserted into the spindle nose. The collet fits and is drawn into the sleeve by a hollow draw bar that fits through the spindle. Collet action is controlled by either a lever (pull type) or a hand wheel (screw-type). These extend through the spindle from the back end.
The outside taper of the hardened jaws of the collet fits accurately against the taper of the sleeve. The body is spring tempered. This permits the jaw sections to be drawn together against the work surface, to hold the workpiece securely and concentrically. Collets are furnished in sets.
Type # 4. Carriers and Catch Plates:
Carriers are also known as driving dogs and used to drive the workpiece when it is held between two centres. These are attached to the end of the workpiece by a set screw. Catch plates are either screwed or bolted to the nose of the threaded, stock spindle.
A projecting pin from the dog fits into the slot provided in catch plate. This imparts a positive drive between the lathe spindle and workpiece. Fig. 12.19 shows three types of lathe dogs commonly used.
Type # 5. Face-Plate:
Mounting work on a face plate provides an ideal way of supporting certain types of works. Flat plates may be screwed to the face-plate for operations such as facing, spot fating, drilling and boring. If the work has several holes, it must be re-centred about each successive hole.
It is the lathe operation of finishing the ends of the work, to make the ends flat and smooth and to make the piece of the required length. For this purpose, usually the work is held in a chuck. If considerable material is to be removed later in the turning operation, it is better not to face the exact length until all the rough turning cuts are taken.
A side tool is used for facing operation. To obtain a smooth finish the point of the side tool is slightly rounded with an oilstone, or so ground as to present a short flat surface to the work. The length of this flat surface should be greater than the amount of the cross-feed per revolution of job. The work may be roughed by feeding from the centre hole outward or from the circumference toward the centre. The finishing cut is made from the centred hole outward.
For facing operation, centre holes are drilled deeper for better support during the roughing operation. Usually a right-cut facing tool with 58° point angle is used which gives a slight clearance between the centre point and the work face. (Refer Fig. 12.35).
It is important to ensure that during facing operation the cutting tool point is not damaged by running it into the centre point. If the work is to be faced on the centres and several pieces are to be faced, it is advisable to use a half-centre for facing. (Refer Fig. 12.36).
Workpiece which cannot be turned between centres or chucked because of their unusual shapes, may be clamped to a faceplate, or mounted on an angle plate bolted to a faceplate, so that the surfaces to be worked are concentric with the lathe centres.
A faceplate is a plate which is so machined that it will be square with the lathe centres when mounted on the spindle. It generally has four T slots as well as four elongated holes to accommodate clamp bolts.
Thus, regardless of the shape of the workpiece, the section upon which an operation is to be performed can be held in an on-centre position. Workpiece mounted in this manner should be checked for mounting accuracy.
The various accessories used in connection with faceplate work to facilitate the accurate fastening or clamping of the work in position on the faceplate are:
i. Square head bolt,
ii. Shouldered stud,
iii. U-Clamp,
iv. End measuring rod,
v. Parallel strip,
vi. Angle plate,
vii. Stop block,
viii. Indicator,
ix. Weight for counter balance.
Type # 6. Mandrel:
It is a hardened piece of round bar with centres and flats at each end. It is used for holding and obtaining a hollow piece of work that has been previously drilled or bored. It is held between two centres and should be true with accurate centre holes for machining outer surface of the workpiece. It is made of high carbon steel to avoid distortion and wear.
Its ends are made slightly smaller in diameter and flattened to provide effective gripping surface of the lathe dog set screw. It is tapered about 0.5 mm per metre so that work can be forced on it with a press fit and then removed after working.
It is used for holding the bored jobs (gear blanks, pulleys or tubes) on a lathe for the purpose of turning outside surface of the job. Several types of mandrels are in common use. Some common types are shown in Fig. 12.19.
Types of Mandrels:
i. Plain (suitable for one size of bore).
ii. Stepped (having steps of different diameters).
iii. Collar (provided with solid collars. Helps in reducing weight of mandrel).
iv. Screwed (used for mounting workpiece with internal threads).
v. Cone (used for workpiece of different hole diameters).
vi. Gang (with one fixed collar at one end and movable collar at other end).
vii. Expansion (provided with three longitudinal slots, two of which are cut nearly through and third splits it completely).
viii. Taper shank.
Type # 7. Steady Rest:
The steady rest is used when a long piece is machined or drilled at its end by holding the job in a chuck. It avoids the undue deflection of the job at the other end.
Type # 8. Follower Rest:
A follower rest performs the same function as a steady rest, but it is attached to the saddle and moves along with the tool. It thus supports the work opposite to the tool and counteracts the cutting forces.