Uses of a Centre Lathe

Liam Pourhana

The lathe is a useful machine tool. As a maintenance machinist you will often use a lathe to carry out repairs or machine new components. To do these tasks you must be able to operate the lathe safely and correctly. This training unit covers the skills and knowledge you will need to operate the center lathe correctly.

A lathe is a machine tool that carries out many machine shop tasks. It can cut and smooth the face of a work piece. It can make holes to an exact size and position. It can also make external or internal threaded parts on a work piece. This ability to do different tasks makes it an important tool. In most tasks a lathe's use is only limited by the size of the work piece. As a maintenance machinist, it is important that you know how a lathe functions and how to operate it correctly.

To identify a lathe, look for its headstock and tailstock. The lathe holds and rotates the work piece about the center line of these two parts. It can also machine a part between its headstock and tailstock centers. This is why we call it a center lathe.

A maintenance machinist is responsible for keeping plant equipment and machinery in good working

order. Most mechanical equipment contains round or cylindrical parts such as shafts, bearings, and

couplings. Lathes can work on both cylindrical and non-cylindrical parts. But, it is important to

remember that a lathe is the only workshop machine that you will use to work on cylindrical parts at this

stage of your professional development.

There will be times when a replacement part is not available. At these times you will have to repair an old

part or machine a new one. At other times, you will need to fit a part to the special requirements of

specific installations. Often new parts supplied by a manufacturer will require machining. Therefore, you must know how to use the lathe correctly to repair,

fabricate or fit cylindrical components. The lathe shapes a metal work piece by rotating it

against a sharp cutting tool edge. The shape and the finish of the part you produce is determined by how

you set up and cut the work piece.

Tasks a Lathe Can Perform

The five most common tasks on a center lathe that you will perform are:o facingo turningo drillingo boringo thread cutting

FacingFacing operations square the end of a work piece to its sides as shown in figure 3. You normally fit the work into the chuck. When you start the lathe, the chuck is rotated by the lathe spindle through gears in the headstock. You then move the cutting tool across the lathe bed and across the face of the work. Facing can also shorten a work piece to a specific length. You may carry out facing operations with the lathe using a manual or automatic feed setting.

TurningTurning, as shown in figure 4, reduces the outside diameter of a work piece. As with facing, you normally fit the work into the chuck. To turn the work you move the cutting tool parallel to the bed of the lathe and along the side of the work piece. You may carry out turning operations with the lathe using a manual or automatic feed setting.

DrillingWhen you drill a hole on the lathe, the work piece rotates and the drill bit stays still. With the tailstock clamped, you feed the drill bit into the work piece by using the tailstock hand-wheel as shown in the image to the right

BoringBoring is the internal machining of a cylindrical hole as shown in the image to the right. It lets you enlarge an existing hole to a very fine tolerance. It is standard practice to have a turned part of a fixed size and the bored part made to size to allow the parts to fit together correctly.

Thread CuttingThread cutting, as shown in the image, is making a screw thread on, or in, a component or part. The thread cutting function of a lathe allows it to cut many different types and sizes of screw threads. The workpiece size and material determine the speed required for the rotating spindle and the carriage. You will not perform thread cutting in this unit.

1. What are the main tasks you perform on a center lathe?a. b. c. d.

2. What two parts let you identify a center lathe easily?a. b.

3. What lathe task reduces the outside diameter of a work piece?

4. What lathe task squares the end of a work piece to its sides?

5. What lathe parts rotate when drilling?

Like any machine, to operate a lathe you must know its major parts and how to control them. There are many different models of center lathes.Each manufacturer makes lathes with slight differences. Some make lathes only to complete tasks on very large or very small work pieces. Even with differences in manufacturer design, size, and names, most lathes look very much alike and carry out similar tasks.

The major parts and controls of two designs of a center lathe as shown in figure 8 are as follows:o headstocko chucko speed selectoro spindle direction controlo feed selectoro feed rodo lead screwo safety guardso carriage and saddleo bedo tailstock

HEADSTOCKThe headstock is the housing for the lathe gears and main drive components. A hollow shaft called a spindle passes through the headstock shell. A number of different attachments, such as a chuck or a faceplate are available to fit on the spindle. These attachments hold the work piece while the spindle provides rotary movement. You should note that 3 and 4 jaw chucks are available but the 4 jaw chuck is not used for this unit of instruction.

A chuck fits onto the spindle passing through the headstock. Its function is to center and hold the workpiece securely. The simplest chuck to use is one that is self-centering. This makes it easier to fit the workpiece. On a self-centering chuck, turning the chuck key tightens or releases all the jaws by the same amount. Because it is so easy to use, most Saudi Aramco machine shops use a self-centering three-jaw universal chuck.

Three-jaw chucks do have limitations. One important limitation is the shape of the work piece it can hold. A three-jaw chuck can only hold a round or hexagonal shaped work piece. Also, you cannot accurately center a worn part in a three-jaw chuck because it centers only to the chuck or spindle. To overcome this, you must use the center of the work piece itself. For this, you need a 4 jaw or another independently adjustable jaw chuck, so called because it allows you to move each of its jaw one at a time. Adjusting the jaws independently helps you to accurately center the work piece. You should also know that there are other ways of holding the work piece, like a faceplate or between centers that are not covered in this unit.

SPEED SELECTORYou use the speed selector to set the spindle RPM. This sets the RPM of the chuck or faceplate holding the work piece. Located on the headstock, the speed selector takes power from the lathe electric motor and transmits it to the spindle. To do this, the speed selector provides drive to the spindle through a gear train or drive belts inside the headstock. Through this drive and gearing the speed selector controls or adjusts the output RPM. The spindle control is either a lever or a dial, depending upon which manufacturer made the lathe. Different manufacturers put the control on the headstock, on the carriage, or on both.

SPINDLE DIRECTION CONTROLThe spindle direction control controls the way the spindle turns. This allows you to set up and use the lathe for different operations. The control normally has forward and reverse settings that allow you to drive the spindle in the selected rotation. For example, you set a counter clockwise (CCW) rotation, for normal turning operations. You may also find a neutral and stop setting. The neutral setting allows you to move the spindle by hand in either direction. The stop setting locks the spindle in place and prevents it from turning. The spindle direction control starts the rotation after the isolation mechanism is canceled.

FEED SELECTORNormally; there are at least four (4) levers used for feed selection. They let you control how much movement and in what direction. However, the number of controls depends upon the lathe manufacturers and lathe type.The standard feed controls are:o feed-on/offo how much feedo direction of feed movement () ←↑→↓o feed for carriage or cross slide () ↔ Between their different levers, feed selectors set the rotational speed of the lead screw and feed rod. They also set the feed direction.

FEED RODThe feed rod transmits drive from the headstock gear train to the saddle or cross-slide. The rod is either splined or keyed along its length, depending upon which manufacturer made the lathe.

LEAD SCREWThe lead screw transmits drive from the headstock gear train to the carriage. Unlike the feed rod, you use it for thread cutting tasks only. You do not use the lead screw for any other lathe operations. You will not perform thread cutting in this unit. Figure 12 shows a typical feed rod and lead screw.

SAFETYGUARDSSafety guards are fitted at a variety of points on the center lathe. The number and positioning of guards depends upon the lathe type and manufacturer. However, all lathes have a cutting area guard. The cutting area guard deflects flying metal cuttings or swarf away from the operator.

CARRIAGEAND SADDLEThe carriage and saddle are an arrangement of parts that support and control the movement of the cutting tool parallel to the centerline of the machine. They fit on the lathe bed (as shown in figure 13) and slide along the center lathe guideways. Because lathe guideways are part of the lathe bed, they are sometimes called bedways

The main parts of the carriage and saddle are:o aprono carriage hand wheelo cross slide and hand wheelo compound slide and hand wheelo tool post

ApronThe apron contains the controls and gears that allow you to move the carriage and cross slide. Lathe operations require you to cut with precision tolerances. Therefore, to stop unwanted movement, bolts secure the apron to the saddle. The apron also contains the half nuts that engage the lead screw for threading operations.

Carriage Hand wheelThe carriage hand wheel allows you to manually control the movement of the carriage. Through this movement the cutting tool moves parallel to the lathe center line. On most lathes it is easy to identify because it is the largest hand wheel on the carriage.

Cross Slide and Hand wheelThe cross slide is mounted on the carriage. It moves the cutting tool 90° to the work piece. You can set the feed of the cutting tool to either manual or automatic depending on the task you are doing. Using the hand wheel permits you to move the cross-slide back and forth across the bed.Compound Slide and Handwheel

The compound slide is mounted on top of the cross-slide. You can make adjustments to position it through 360°. The compound slide handwheel then provides manual movement feed only for angular cuts or tapers. If you position it parallel to the lathe center line, the compound slide controls the depth of cut. In this position you can use it for adding exact cut depths during facing operations.

Tool PostThe tool post supports and secures the cutting tool or tool holder. It is secured on top of the compound slide. It is held firmly in position on the compound slide T-slot. This prevents unwanted movement from spoiling the cut. Note: Lathe operations produce friction. Friction causes heat. Therefore, many lathes have a coolant or cutting fluid supply system. The nozzle for these systems is near the tool post. Most times this fluid nozzle is movable and has a small valve that allows you to regulate the flow as needed.

BEDThe bed forms the base of the lathe with the bedways running along it. Its upper part supports the headstock, carriage and tailstock. Through the bed and bedways, the carriage is maintained in precise alignment. This permits you to machine to precision tolerances along the length of the workpiece.

TAILSTOCKThe tailstock is a heavy casting that moves back and forth along the bed of the lathe. It holds tools such as revolving centers, drill chucks and taper shank drills which fit into the tapered spindle.

SUMMARYYou can now name and identify the main parts of most center lathes. You can also state their functions, find their controls, and understand how they work. In lathe operations, a work piece turns at high speed. You feed cutting tools to, and around, the work piece. You know that fast moving machinery, cutting tools, and flying metal cuttings are dangerous. If not operated correctly; a lathe will cause you painful injury Therefore, before you learn detailed lathe procedures; you must learn the lathe safety rules. Following these simple rules will stop you from hurting yourself and may even save your life.