INTRODUCTION:
What Is Lathe?
A lathe is a machine tool which rotates the workpiece on its axis to perform various operations such as cutting cutting,, sanding sanding,, knurling knurling,, drilling drilling,, or deformation with tools that are applied to the workpiece to create an object which has symmetry about an axis of rotation. rotation.
Development of Lathes:
Turret and capstan lathes are the natural development of the engine lathe, where the tailstock is replaced by an indexable multistation tool head, called the capstan or the turret. This head carries a selection of standard tool holders and special attachments. A square turret is mounted on the cross slide in place of the usual compound rest in engine lathe. Sometimes a fixed tool holder is also mounted on the back end of the cross slide. Dimensional control is affected by means of longitudinal (for lengths) and traversal (for diameters) adjustable stops. Therefore, capstan and turret lathes bridge the gap between manual engine lathes and automated lathes and are most practical for batch and short-run production. In comparison with manual lathes, the chief distinguishing feature of capstan and turret lathes is the multiple tool holders that enable the setting up of all the tools
necessary to produce a certain job. Except for sharpening, the tools need no further handling. Considerable skill is required to set and adjust the tools on such machines properly. But once the machines are set, they can be operated by semiskilled operators. Eliminating the setup time between operations reduces the production time considerably. The development of this group of lathes has been enhanced to provide the level of accuracy required for interchangeable production. A capstan lathe or a turret lathe is a production lathe used to manufacture any number of identical pieces in the minimum time. These lathes are development of engine lathes. The capstan lathe was first developed in the United States of America by Pratt and Whitney sometimes in 1860. Special characteristics of a capstan or turret lathe enable it to perform a series of operations such as drilling, turning, boring, thread cutting, reaming, chamfering, cutting- off and many other operations in a regular sequence to produce a large number of identical pieces in a minimum time.
Construction of Turret Lathe:
Turret Lathe A Headstock B Spindle C - Pilot Bar D - Tool post E - Rear Tool post –
–
F - Cross slide G -Intermediate saddle H - Turret I - Turret Clamp Lever J - Saddle Looking Lever
K - Star Handle for Turret Operation L - Turret Apron M - Turret saddle N - Intermediate saddle Apron O - Feed Gearbox
Construction of Capstan Lathe:
A - Turret Locking Handle B Turret –
C - Capstan Slide D - Capstan Rest
E - Capstan Slide Block F - Sliding Bridge
Scope of Work:
The main purpose of these types of lathes is to eliminate the setup time between operations which reduces the production time considerably. As mentioned before, Turret and Capstan Lathes are mainly used in Mass/Batch Production to produce products such as:
Collets. Collets sleeves. Drill collets. Drill collets holder.
And many other products as shown:
Differences between a Capstan Lathe, and a Turret Lathe:
The first picture shows a Capstan lathe turret, and this description is the cause of the confusion. The turret is the toolholding and rotating part which is carried on a fixed but movable saddle. The turret itself moves on the saddle.
The second picture shows a turret off a Turret lathe, and the turret itself moves with the saddle at all times. This moving saddle on the turret lathe is what makes it different in operation from the Capstan machine .
The headstock of a turret lathe is similar to that of an engine lathe in construction but possesses wider range of speeds, and is of heavier in construction.
Similar sizes of capstan and turret lathe and engine lathe, when an engine lathe will require a motor of 3h.p. to drive its spindle and other parts, a capstan and turret lathe will demand power as high as 15h.p. for high rate of production. In a turret lathe, the tailstock of an engine lathe is replaced by a turret. This is a six sided block (or more), each of which may carry one or more tools. These tools
may be indexed one after the other to perform different operations in a regular order. This is a decisive advantage in mass production. In a turret lathe, combination of cuts can be taken. Two or more tools may be mounted on the same face of the turret, making it possible to machine more than one surface at a time. This feature reduces total operational time. A semiskilled operator can operate a capstan or turret lathe after the machine has been set up by a skilled machinist. A skilled machinist may be requisitioned for setting up only for a large number of machines, where as actual production may be given by a semiskilled operator. Capstan and turret lathe is fundamentally a production machine, capable of producing large number of identical pieces in a minimum time. The centre lathe is suitable for odd jobs having different shapes and sizes.
The essential components and operating principles of capstan and turret lathes are illustrated schematically
In the shown figure. Capstan lathes are mainly used for bar work, whereas turret lathes are applicable for large work in the form of castings and forgings. In a capstan or ram-type lathe, the hexagon turret is mounted on a slide that moves longitudinally in a stationary saddle (Shown Figure a). During setup of the machine, the saddle is positioned along the bed to give the shortest possible stroke for the job. The advantage of the capstan lathe is that the operator has less mass to move, resulting in easier and faster handling. The disadvantage is that the hexagonal turret slide is fed forward such that the overhang is increased, resulting in the deflection of the ram slide, especially at the extreme of its position, which produces taper and reduces accuracy. In the turret- or saddle-type lathe, the turret is mounted directly upon a movable saddle, furnished with both hand and power longitudinal feed (Shown Figure b). This machine is designed for machining chuck work, in addition to bar work. Owing to the volume of the swarf produced, the guide ways of the machine bed are flame-hardened and provided with covers that protect the sliding surfaces. The bed must be designed to allow free and rapid escape of swarf and coolant.
FIGURE 6.1 Essential components and operating principles of capstan and turret lathes.
Differences
between
(Capstan
Lathe
and
Turret Lathe) versus Conventional Lathe:
Centre lathe (Conventional)
Capstan/Turret Lathe
1
It is a manually operated lathe
It is a semi automatic lathe
2
It has only one tool post tool changing time is more
Front and rear tool posts are available. Tool changing time is less
3
It has tail stock
It has turret head instead of tail stock
4
Only one tool can be fitted in the tail stock
Six different tools can be fitted in the turret head
5
Number of speeds is less
Number of speeds is more
6
Tool changing time is more
Tool changing time is less
7
The machine should be stopped for changing tool
Tool can be changed without stopping the machine
8
It is not suitable for mass production
The tools are controlled by feed stops
9
The tool is centered manually after changing the tool
The tool is centered automatically
10
Only one operation is done at a time
More than one operation can be done at a time
Tools:
Tools of capstan lathe machine: Tool Geometry: For cutting tools, geometry depends mainly on the properties of the tool material and the work material. The standard terminology is shown in the following figure. For single point tools, the most important angles are the rake angles and the end and side relief angles.
The back rake angle affects the ability of the tool to shear the work material and form the chip. It can be positive or negative. Positive rake angles reduce the cutting forces resulting in smaller deflections of the work piece, tool holder, and machine. If the back rake angle is too large, the strength of the tool is reduced as well as its capacity to conduct heat. In machining hard work materials, the back rake angle must be small, even negative for carbide and diamond tools. The higher the hardness, the smaller the back rake angle. For high-speed steels, back rake angle is normally chosen in the positive range. Most lathe operations are done with relatively simple, single-point cutting tools. On righthand and left-hand turning and facing tools, the cutting takes place on the side of the tool; therefore the side rake angle is of primary importance and deep cuts can be made. On the round-nose turning tools, cutoff tools, finishing tools, and some threading tools, cutting t akes place on or near the end of the tool, and the back rake is therefore of importance. Such tools are used with relatively light depths of cut. Because tool materials are expensive, it is desirable to use as little as possible. It is essential, at the same, that the cutting tool be supported in a strong, rigid manner to minimize deflection and possible vibration. Consequently, lathe tools are supported in various types of heavy, forged steel tool holders, as shown in the figure.
The tool bit should be clamped in the tool holder with minimum overhang. Otherwise, tool chatter and a poor surface finish may result. In the use of carbide, ceramic, or coated carbides for mass production work, throwaway inserts are used; these can be purchased in great variety of shapes, geometrics (nose radius, tool angle, and groove geometry), and sizes.
Tools of turret lathe machine: Boring Tool:
Face Tool:
Tapping Tools:
And many other tools:
Operations: 1. Form tools: For the machining of complex shapes, it is common to use form tools. This contrasts with the cutting that is performed on an engine lathe where the cutting tool is usually a singlepoint tool. A form tool has the form or contour of the final part but in reverse, so it cuts the material leaving the desired component shape. This contrasts to a single-point tool, which cuts on one point at a time and the shape of the component is dictated by the motion of the tool rather than its shape.
2. Rotary broaching: Rotary broaching is another common operation. The broach holder is mounted stationary while its internal live spindle and end cutting broach tool are driven by the workpiece. As the broach is fed into or around the workpiece, the broach's contact points are constantly changing, easily creating the desired form. The most common form made this way is a hexagonal socket in the end of a cap screw.
3. Boring: The process of using a single-point tool to enlarge a preexisting hole.
4. Center drilling: A process in which a rigid drill precisely locates a hole in the center of a work piece. The purpose of center drilling is to establish a true centerline diameter.
5. Drilling: A process in which the work piece is held stationary, while a rotating cutter (the drill) makes a round hole into the work piece to a certain depth.
6. Facing: An operation performed on a lathe that feeds a single-point tool into the end of a cylindrical work piece to create a flat surface.
7. Parting: Also called cutoff, a shearing operation that separates a finished part from bar stock.
8. Reaming: The process of using a cutting tool with straight cutting edges to enlarge or smooth holes that have been previously drilled.
9. Tapping: The process of cutting internal threads in a work piece with a multi-point tool.
10. Threading: The process used to machine a long, spiraling ridge down the outer or inner surface of a work piece.
11. Turning: An operation performed on a lathe that feeds a cutting tool along the length of a cylindrical part to reduce its diameter.
Features:
I. Bar feeding mechanism of capstan lathe: Fig. 4.7.9 typically shows the kinematic arrangement of feeding and clamping of bar stock in capstan lathes. The bar stock is held and tightly clamped in the push type spring collets which is pushed by a push tube with the help of a pair of bell-crank levers actuated by a taper ring as shown in Fig. 4.7.9. Bar feeding is accomplished by four elementary operations; unclamping of the job – by opening the collets bar feed by pushing it forward clamping of the bar by closing the collets free return of the bar-pushing element
After a job is complete and part off, the collets are opened by moving the lever manually rightward to withdraw the push force on the collets. Further moving of the lever in the same direction causes forward push of the bar with the help of the ratchet – paul system shown. After the projection of the bar from the collets face to the desired length controlled by a pre-set stop – stock generally held in one face of the turret or in a separate swing stop, the lever is moved leftward resulting closing of the collets by clamping of the barstock. Just before clamping of the collets, the leftward movement of the lever pushes the bar feeder (ratchet) back freely against the paul.
Fig. 4.7.9 Typical bar feeding mechanism in capstan lathe
II. Turret indexing mechanism in capstan and turret lathes: Turret indexing mechanism of capstan and single spindle turret lathe is typically shown schematically in Fig. 4.7.10. The turret (generally hexagonal) holding the axially moving cutting tools have the following motions to be controlled mechanically and manually; Forward axial traverse comprising; - Quick approach: manually done by rotating the pinion as shown - Slow working feed: automatically by engaging the clutch - Stop at preset position depending upon the desired length of travel of the individual tools Quick return: manually done by disengaging the clutch and moving the turret back
o
Indexing of the turret by 60 (or multiple of it): done manually by further moving the turret slide back.
Just before indexing at the end of the return stroke, the locking pin is withdrawn by the lever which is lifted at its other end by gradually riding against the hinged wedge as indicated in Fig. 4.7.10 (a). Further backward travel of the turret slide causes rotation of the free head by the indexing pin and lever as indicated in Fig. 4.7.10 (b). Rotation of the turret head by exact angle is accomplished by insertion of the locking pin in the next hole of the six equispaced holes. After indexing and locking, the turret head is moved forward with the next cutting tool at its front face when the roller of the lever returns through the wider slot of the wedge without disturbing the locking pin as indicated in the figure. The forward motion of the turret head is automatically stopped when the set-screw corresponding to the working tool is arrested by the mechanical stop. The end position and hence length of travel of the tool is governed by presetting the screw. There are six such screws, each one corresponds with particular face or tool of the turret. The drum holding those equispaced six screw with different projection length is rotated along with the indexing (rotation) of the turret head by a pair of bevel gears (1:1) as indicated in Fig. 4.7.10 (a). The bottom most screw, which corresponds with the tool on the front face of the turret, when hits or touches the stop, the turret movement is stopped either manually by feeling or automatically by disengaging the clutch between the feed rod and the turret slide.
(a) sectional view
(b) top (inner) view Fig.4.7.10 Turret indexing in capstan and turret lathe.
III. WORK HOLDING DEVICES: The standard practice of holding work between two centers in an engine lathe finds no place in a capstan or turret lathe as there is dead centre to support the work at the other end. Work is, therefore supported at the spindle end by the help of chucks and fixtures. The usual methods of holding work in a capstan or turret lathe are: 1. Jaw chucks: • Self centering chuck.
• Independent chuck.
• Combination chuck.
• Air operated chuck.
2. Collet chucks: • Push out type.
• Draw in type.
• Dead length type.
The collet chucks: The collet chucks are used for gripping bars introduced through the head stick spindle of a capstan or turret lathe and is one of the most common method of holding work. Different sizes of spring collets having square, hexagonal, round or any other shaped bore are fitted in the chuck body for holding different sizes of bar having different sections. PUSH OUT TYPE: To grip the work, the tapped portion of the spring collet is pushed in to the mating taper of the chuck. There is a tendency of the bar to be pushed slightly outward when the collet is pushed in to the chuck body for gripping. If the bar is fed against a stop bar fitted on the turret head, this slight outward movement of the bar ensures accurate setting of the length for machining. DRAW IN TYPE: To grip the work, the tapered portion of the spring collet is pulled back in to the mating taper of the chuck which causes the split end of the collet to close in and grip the bar. The machining length of the bar in this type of chuck cannot be accurately set as the collet while closing will draw the bar slightly outward towards the spindle. DEAD LENGTH TYPE: For accurate positioning of the bar, both the push out and draw in type collet present some error due to the movement of the bar along with the collet while gripping. This difficulty is removed by using a stationary collet on the bar. A sliding sleeve closes up on the tapered collet which is prevented from any end movement by the shoulder stop.
Fixtures:
A fixture may be described as a special chuck built for the purpose of holding, locating and machining a large number of identical pieces which cannot be easily held by conventional gripping devices. Fixtures also serve the purpose of accurately locating the machining surface. The main functions of a fixture are as follows: 1. They accurately locate the work. 2. They grip the work properly, preventing it from bending or slipping during machining operations. 3. They permit rapid loading and unloading of work pieces.
Holding Workpiece in a Lathe:
