5 Main Applications of Lathe | Machine Tools | Industrial Engineering

The following points highlight the five main applications of lathe used in the industries. The applications are: 1. Forming 2. Contour Turning 3. Machining of Cams 4. Ultra Precision Machining 5. Hard Turning.

Application # 1. Forming:

Forming is the operation of producing profile on a lathe using a special form tool which bears the reverse of the required profile of component upon its cutting edge. Form tools are used for turning intricate profile by plunge feeding. Forming tools could be either flat type or of circular form type depending on their shape.

Depending on direction of feed these could be classified as radial or tangential type or disc-shaped. These have greater life as many regrinds are possible before their failure and these are easier to manufacture also.

As the profile produced on workpiece is replica of form of tool, the accuracy of work profile is entirely dependent upon the accuracy of the form of tool profile. It is important that the tool is set exactly at the work centre, otherwise the profile will not be correct.

Though rake angle need not be provided for small form tools, it is essential in case of bigger and complex shapes for efficiency of cutting. Clearance in any case, has to be provided to permit cutting action. Provision of rake angle modifies the profile on workpiece and as such some correction has to be provided on tool profile to get desired profile on workpiece.

Long profile on form tool causes chattering and poor finish. Cuts have to be very light. It is generally difficult to produce complex form tools and these are usually expensive. Complex profiles can best be generated by a single point than by forming. The flat form tools are ground to the required form over the full depth of the shank, the profile remaining constant from the top face to the bottom.

In the case of zero top rake, the tool will have the exact inverse profile of the work to be produced. Such tools are easy to design and manufacture. Tools with positive rake need a correction on the profile to achieve the correct contour on the workpiece.

In the case of circular form tools, the cutting edge is obtained by cutting away a portion of the disc. Tool has tendency to rotate which is prevented by providing a pin hole in the tool in which a locking pin on the holder engages.

Application # 2. Contour Turning:

Sometimes complicated shapes (like turbine blades) are required to be machined for which contour turning method is the best one. In it, first a template of the desired shape is made on plaster, plastic or some other soft material manually. The contour turning then reproduces the shape of the template in all the three dimensions by controlling the cutting tool through the action of the follower which moves over the surface of the template.

Purely mechanical devices consisting of mechanical linkage, spring loaded cam followers, double track cams and lever arms though theoretically can produce the desired shape, but are practically not efficient for high production. In this method, the operator manually controls tool slide to hold the follower against the template at the rear of the machine.

In tracing system, the follower controls the source of power (which may be pneumatic, hydraulic or electric) for the tool slide thus controlling the movement of the tool across the work. Whenever the stylus position is changed, it actuates the power system to hold the tracing head continually against the template and thus reproduce the shape of the template on the work.

Contour tracing equipment can be of two types:

(i) In which the contours can be defined in a single plane and the template or model remains stationary,

(ii) In which the contouring is required both around the periphery and along the template and it has to rotate also.

Hydraulic systems utilising hydraulic cylinders connected to tool slides are more popular. Stylus movement across the template face controls the cylinder either by a directly controlled hydraulic valve or by a pneumatic control to the hydraulic valve. In these systems the tracing pressure is very light and hydraulic pressure varying between 13 and 25 kg/cm² provides sufficient force to hold the tool slide rigid against the heaviest cut.

Usually variable angle tool slide is employed which permits setting the tool slide at the optimum angle for the most difficult cutting conditions in the contour. For step shafts the tool slide is normally set at 45°.

In the two-dimensional controlled tracer system, the carriage feed is also controlled, and the feed rates for both cross and carriage feed slides are arranged in the control valve to give a resultant feed rate tangential to the point of contact between the stylus and the template.

Very rugged and new machines are required for contour turning applications. For higher accuracy, the tracing system characteristics such as time lag between stylus deflection and tool movement, friction of tool slide, and inertia must be fully considered and controlled. Stylus radius should be smaller than the smallest radius to be produced on the work. It should correspond with tool shape for best results.

Application # 3. Machining of Cams:

Cams are usually manufactured on 2½ axis NC machining centre. The tool path is generated either from motion equation of the follower or by the mathematical curves of the cam profile. The number of points to guide the cutter should be taken large enough to get the profile within the required tolerance, taking the geometry of the profile into consideration. Computer aided part program becomes necessary.

The 2½ axis NC machining centres are suitable to do paraxial, linear and circular profile machining. Profiles other than these can be obtained by approximation with linear or circular interpolation. APT language is used. The 2C, L program is used for cam manufacturing.

Operator can produce NC tape for machining cam on NC machining centre. The data related to each cam segment is fed by operator in interactive mode. Appropriate feed and speeds are selected. The program takes into account the cutter diameter in the computation of the cutter location data. The number of milling passes are decided depending on the thickness of work material, roughing cuts and finishing cut. The NC blocks are produced for each cut.

Application # 4. Ultra Precision Machining:

Applications of ultra-precision machining are ‘encountered in precision manufacturing of components for computers, electronics, nuclear and defence applications. For example surface finish of 10^-9 m or 0.001 µm and accuracy of the order pin and sub lim range may be required for components like optical mirrors, computer memory disks etc.

Cutting tool for such ultra-precision applications is exclusively a single-crystal diamond with a polished cutting-edge with a radius of the order of tens of nanometer. To reduce wear of diamond, cryogenic diamond turning (in which tool system is cooled by liquid nitrogen to a temperature of – 120°C) is adopted.

The materials for ultra-precision machining are copper alloys, aluminium alloys, silver, gold, electroless nickel, infrared materials, and plastics (acrylics). For depths of cut in nanometer range, hard and brittle materials produce continuous chips (ductile-regime cutting), whereas deeper cuts produce discontinuous chips.

The machine tools for these applications are built with very high precision and high spindle speed, and work-holding device stiffness. Such machines are made from structural materials with low thermal expansion and good dimensional stability. These are located in lust free environment in a controlled atmosphere where temperature is maintained within a fraction of one degree.

Vibrations from external and internal sources are eliminated. Feed and position controls are attained by laser metrology. Machines are equipped with highly advanced computer control system and with thermal and geometric error compensating features.

Application # 5. Hard Turning:

It is extremely difficult to machine hard materials because of poor machinability and high tool wear and fracture. Hard turning machines are made with high stiffness. These use polycrystalline cubic boron nitride (PCBN) cutting tool. These machines produce machined parts with good dimensional accuracy, surface finish, and surface integrity.

Hard machining involves machining of steels having hardness of 45 HRC and above. The heat generated in hard machining is used to plasticize or anneal the material at the cutting point. The high speed makes the heat flow away from the cutting point and thus chips carry away the heat. In hard machining, the tool just skids over the work place.

The cutting speeds used in hard turning with ceramic and CBN tools are 120—180 m/min. Hard turning is a finishing process. Hard turning allowance is around double that of grinding allowance. Rough cut may be around 0.4 mm and finish cut nearly one-third of it. Feed rate may be around 0.1 mm/rev.

Hard turning requires tool materials like ceramic, silicon nitride (SN), CBN, Poly Crystalline Boron Nitride (PCBN) etc. which are inert and extremely hard at elevated temperatures. Cutting materials being brittle can’t withstand cutting forces generated and as such interrupted cuts are not recommended.

Hard machining calls for an extremely rigid machining set up. The machine requires play free slides, very rigid tool clamping set up, spindle bearing with minimum run out, play free tailstock, minimum overhang of tool, no vibrations etc. Hard milling has found its major application in die and mould manufacturing.