Various Types of Operations Performed on Lathe Machine

As we know that the lathe machine is used for producing basically cylindrical and conical shape jobs with the help of various tools. Different shapes are produced by different operations. Some of operations are as follows:

Turning

Turning is the operation when the metal removal takes place from the surface of the cylindrical work piece. In this process the tool is fed along the axis of the spindle. Turning is the removal of metal from the outer diameter of a rotating cylindrical work piece. Turning is used to reduce the diameter of the work piece, usually to a specified dimension, and to produce a smooth finish on the metal. Often the work piece will be  turned  so  that  adjacent  sections  have  different diameters.

Shoulder Turning

A shoulder is a point at which the diameter of the work piece changes with no taper from one diameter to the other. In other words, there is a 90 degree face moving from one diameter to the other. To get a nice square edge it must be machined with a tool having sharp point. It should be ground to an angle of less than 90 degrees so that it can work right down into the corner of the shoulder. To get a nice square face on the shoulder it will be needed to make a facing cut. While doing so the carriage should be locked. This gives the best result. Face of the shoulder should be cleaned up by locking the carriage until it is square. While using sharp pointed tool it will needed to use fairly high RPM, say 1500, and advancing the tool slowly otherwise it will get little grooves from the pointed tip instead of a nice smooth finish. Finally, sharp corners are to be removed by using a file to make a nice beveled edge on outside edge of the shoulder and on the end of the work piece.

Facing

Facing is the process of making flat surfaces on a lathe. The job is held on a faceplate or chuck and the tool is fed at right angles to the bed to obtain flat surfaces. Most often, the work piece is cylindrical, but using a 4- jaw chuck you can face rectangular or odd-shaped work to form cubes and other non-cylindrical shapes. To safely perform a facing operation the end of the work piece must be as close as possible to the jaws of the chuck. The work piece should not extend more than 2-3 times its diameter from the chuck jaws unless a steady rest is used to support the free end.

Boring

The process of removal of stock from a hole in the workpiece is called boring. Holes are bored by single point cutting tools. The cutting tool shaves off a thin layer of material to an accurate size. Tapered holes are bored in the same manner as in the case of taper turning. The boring processing is said to be difficult for some of the following reasons:

(a) It is hard to see the processing surface.

(b) The scraps are kept in the hole. When the scraps are kept, the surface cannot be finished to high roughness.

(c) Because we cannot see the bottom of a deep hole, it is hard to stop the boring bar at the bottom location. We must depend on the scale of the lathe and the sound.

(d) Especially the case of a small hole, the backside of the boring bar touches the material. We must set the height of the edge suitably.

(e) Because a boring bar is long, the tool has vibrations easily. It is dependent on the rigidity of the boring bar.

Drilling

This is the process of making holes in the workpiece with the help of drills. The drill is held in the tailstock and the drilling operation is carried out by advancing the drill in the workpiece by rotating the handle of the tail stock. On a lathe, drilling is generally done in the centre of the workpiece. Before drilling into the end of a workpiece, face the end. The next step is to start the drill hole using a center drill. If you try to drill a hole without first center drilling, the drill will almost certainly wander off center, producing a hole that is oversized and misaligned.

Reaming

It is the process of enlarging holes to accurate sizes. Reaming is always carried out after drilling. It is similar to the drilling process - the reamer is held in the tailstock to carry out the reaming operation. Two broad categories of commercial reamers are generally available; these are hand reamers and machine reamers. As the name suggests, for use in machine tools the latter are more suitable. Machine reamers, especially the larger diameters, tend to have taper shanks for mounting in the mandrel of machine tools. Machine reamers will remove a greater amount of metal and the lack of taper results in cutting occurring nearer the front edge of the tool, shavings are usually pushed forwards and do not pack the flutes to the same extent. If the shavings are not removed from the flutes regularly and they become packed the reamer will likely seize in the bore and irreparable damage will be done to the work. Even if this doesn't happen, packed chips in the flutes will cause the reamer to cut oversize, off-line, or both.

Milling

Milling is an operation of removing material from a work piece with multi point rotating cutter. The lathe is a practicable method of performing milling operations in the absence of a true milling machine. On a lathe, the milling cutter is held in the headstock and the work piece is clamped in movable vice. The milling operation is carried out by a cutter revolving against the work piece. This process is used for milling small work pieces only, where a milling machine cannot be used. The most common way of milling in the lathe is to use a vertical slide with a small machine vice attached. The only drawbacks are that the lathe needs to be converted for milling operation each time. Also, unless the lathe is particularly large the rigidity is going to be less than that of a milling machine designed for the job. Most slot and channel milling will be done with small end mills or slot drills (up to 1/2" diameter or so). Although between-center bars are available for mounting side and face cutters, these are not really convenient to work with as the work needs to be clamped to the cross-slide and requires shimming up to the correct height for machining. It is difficult to do with the accuracy. End milling of work held in a small machine vice bolted to the vertical slide is a much more practicable solution.

Grinding

This is a process of removing material by means of rotating abrasive wheel for finishing operations. On a lathe, the work piece is held between the centres and the grinding operation is carried out by mounting the tool post grinder on the compound slide. The grinding operation is carried out after rough turning, to provide an accurate finish to the work piece by removing a small amount of material.

Counter boring

The process of boring a hole to more than one diameter on the same axis is known as counter boring. Counter boring is needed for receiving the head of a socket head cap screw. This operation is also carried out with a boring tool.

Knurling

The process of rendering rough the surface of a work piece by making a series of indentations or depressions on it is known as knurling. The knurling tool which is held in the tool post is pressed against the job to carry out the operation. The indentations are generally of a criss– cross pattern and can be classified into three categories -coarse, medium and fine.  Another form of indentation is known as straight knurling and is not used extensively.

Eccentric Turning

The process of performing turning operations at various axis in a single setting job is known as eccentric turning. Many different methods can be employed for such work. The most vital factor is the number of jobs to be made. Such jobs can be best machined with the help of well designed fixtures and proper tools, but their use requires a lot of economic considerations. Where a large number of similar jobs are to be machined, such that a quicker and larger production will compensate for the cost of production of the said fixtures and tools, it is always advantageous to use fixtures. However, if only a few pieces are to be machined it would certainly be uneconomical to have the use of fixtures. In such cases, other methods of eccentric turning are used. A very common method of eccentric turning, using a mandrel having two sets of centres. For such machining, special fixtures are designed and are mounted on face plate for supporting the work during the operation.

(a) Crank Shaft

(b)  Eccentric Bush

Thread Cutting

Threading is the process of creating a screw thread. There are many methods of generating threads. Thread cutting on lathe is an operation that uses a single-point tool to produce a thread form on a cylinder or cone. The tool moves linearly while the precise rotation of the work piece determines the lead of the thread. The process can be done to create external or internal threads (male or female).  In external thread cutting, the piece can either be held in a chuck or mounted between two centers. With internal thread cutting, the piece is held in a chuck. The tool moves across the piece linearly, taking chips off the work piece with each pass. Usually 5 to 7 light cuts create the correct depth of the thread.

Taper Turning

An operation performed on a lathe that feeds a tool at an angle to the length of the work piece in order to create a conical shape. Taper turning falls  into three categories, short tapers of relatively obtuse angles generally turned with the top-slide, longer tapers of a more acute angle produced either by setting the tailstock over or by use of a taper turning attachment, and internal taper. This operation will be taught in detail in further classes.

Recessing

Recessing is a process of producing narrow slot on a cylindrical job. It is also called as grooving or necking. So the recessing tools are sometimes called necking tools. Recessing tools may be either straight or bent shank types. As the recess is usually narrow, the cutting edge is kept narrow. It is relieved by 1^O to 2^O on each side towards the shank. The sides are relieved to make the tool free cutting. The rake angle should be decreased or the face should be made hollow to the radius. The tool should be set exactly in centre. If tool is set below or above the centre, it will break.

Chamfering

Chamfering is the operation of beveling the extreme end of a work piece. This is done to remove the burrs, to protect the end of the work piece from being damaged and to have a better look. The operation may be performed after knurling, rough turning, boring, drilling. Chamfering is an essential operation before thread cutting so that the nut may pass freely on the threaded work piece.

Parting Off

Parting-off is the operation of cutting a work piece after it has been machined to the desired size and shape. The process involves rotating the work piece on a chuck or faceplate at half the speed to that of turning and feeding by a narrow parting off tool perpendicular to the lathe axis by rotating the cross slide screw by hand. Before the operation starts, the carriage is locked in position on the lathe bed and the cutting tool is held rigidly on the tool post with the compound slide set parallel to the lathe axis. The tool should be fed very slowly to prevent chatter. The feed varies from 1.7 to 0.15 mm per revolution and the depth of cut is equal to the width of the tool. In parting off, a work of very large diameter, cuts are made in stages. The parting off tool is first fed through a certain depth, and then withdrawn and two more cuts are made at the two sides of the central groove. The tool is next fed into the central groove until the work is cut off in two parts.