Module ObjectivesUpon the completion of this module, the student will

be able to:• Describe how a lathe operates.• Identify the various parts and attachments of a

lathe and their function.• Describe how the cutting tools cut materials.• Implement and describe the safety rules for

machining using a lathe.• Select the correct cutting speed, feed, and depth

of cut during machining

Introduction

• The centre lathe is a power-driven, general purpose machine tool used for producing cylindrical work-pieces

Major parts of a lathe

Major parts of a lathe

Major parts of a lathe

Headstock• Head stock provides the driving power to control the

rotation of the part (work piece) being machined. The headstock contains the spindle to which the various work holding attachments are fitted.

Major parts of a lathe

Machine Bed• Its function is to provide main frame for the support of

the work-piece and tool during machining.

Major parts of a lathe

CarriageControls and supports the cutting tool, andcomposed of a number of parts:

Major parts of a lathe

• The saddle: is fitted to the ways of the bed and slides along them (Z axis for CNC machines).

• The apron: contains a drive mechanism to move the carriage along the ways, using hand or power feed.

• The cross-slide: allows the tool to move in and out (toward or away from the operator), (X axis for CNC machines).

• The compound rest: allows the tool to move at an angle. (move in X and Z axes at the same time)

• The tool rest: is used to mount the cutting tool.

• Power is transmitted to the carriage through the feed mechanism. This regulates the amount of tool travel per revolution of the spindle (Feed).

Lead screw:• Lead screw transmits power to the carriage

through a gearing and clutch arrangement in the carriage apron

Major parts of a lathe

The power feed lever is located on the carriage apron. The power feed lever activates either the longitudinal power feed of the carriage or the cross feed of the cross slide

Split-nut (Half nut) lever• Half-nut lever is placed on the apron and

engaged for thread cutting • Thread cutting is the process of cutting screws

and helical shapes.

Major parts of a lathe

Tailstock• The function of the tailstock is to support long

work-pieces during machining, and to hold some tool.

Major parts of a lathe

Lathe attachments• An attachment is a device mounted on the lathe so

that a wider range of operations could be performed

Work piece holding devices:Three-jaw universal chuck• The three-jaw universal chuck holds cylindrical or

hexagonal work. All three jaws move together to bring the work on center.

Lathe attachments

The Four-jaw independent chuck

• The four jaws are reversible and can hold work of different sizes and shapes. Each jaw may be moved independently

Collet chucks• Collet chucks are used to grip small diameter work

pieces. • Collets are made in sets, to fit different sizes.

Faceplate

• A faceplate is one of the work piece holding devices. It contains more open slots or T slots so that bolts or T bolts may be used to clamp the work piece to the face of the plate.

Centers• Support the work piece

between the headstock and the tailstock.

Follower restIs used to support long cylindrical work pieces. It is attached to the carriage when in use, and is moved as the tool and carriage move.

Steady rest• Is used to support long cylindrical work pieces. It is

attached directly to the machine bed.

Drill Chuck• The tailstock of a lathe can be used for drilling, with

the aid of a drill chuck attachment. The drill chuck is used to hold the drill bit for drilling.

Lathe attachments

Grinder• Grinding can be done in the lathe if the machine

is equipped with an electric grinding attachment

Lathe attachments

Chip Formation

• In order for the machine tool to cut metal, a sharp cutting tool made of special hard metals must be used

In order for a chip to be formed, a cutting edge must penetrate the material, cutting off a chip.

Basic form of a cutting tool

• The basic form of a cutting edge is a wedge.• Generally there are three angles that permit

the cutting action

• α Clearance angle• ɣ Rake angel• β Wedge angle

• α The clearance angle: is the angle required to make cutting easier and minimize the friction and heat generation

• ɣ Rake angel: is the angle responsible for chip removal

• β Wedge angle: a cutting with too small wedge angle results in an easy penetration of the cutting tool but would lead to early tool failure, while a cutting tool with large wedge angle would need more power

Lathe machine cutting tools

• To machine metal in a lathe, a cutting tool called tool bit is used.

• Tool bits used in training are either High Speed Steel (HSS) or carbide-tipped tools.

• HSS is the most popular type of tool steel; it is tough enough to withstand most cutting shocks and retains its hardness at higher speeds

Lathe machine cutting tools

• Left-hand tools have their cutting edge on the right-hand side. The tool moves toward the tailstock while cutting.

• Right-hand tools have the cutting edge on the left-hand side and move toward the headstock during the turning operation

Lathe machine cutting tools

Standard terminology of cutting tool’s angles

The Tool holder• Tool bits are held on a variety of lathe holders and tool posts• The tool bit should be clamped in the tool holder with

minimum overhang, otherwise, tool chatter and a poor surface finish may result

5. Cutting fluids• Cutting fluids are very important to minimize or

reduce the effects of friction and heat in machining operations.

• They affect the performance of the cutting tool and improve surface quality.

5. Cutting fluids• The cutting fluid must provide lubrication and

cooling for the cutting tool, the chip and the work piece.

• Generally soluble oils are used when cutting steels.

• Soluble oils are mineral oils that contain a soap-like material that makes them mix in water into a milky white solution.

6. Surface finish• Surface finish is the degree of roughness of the

machined surface. It depends on many factors such as, speed, feed, depth of cut, and the use of cutting fluids.

• The roughness symbol in the Fig. means that the roughness required for this surface must be 3.2 μm (3.2 micrometer i.e 0.0032 mm); this information will help you to select the correct cutting operation that you should use to have the correct value of roughness

7. Lathe SafetyThe following points should be considered

during the lathe operation• Do not attempt to operate a lathe until

you know the proper procedures and have been checked out on its safe operation by your instructor.

• Never attempt to operate a lathe while your senses are impaired by medication.

• Dress appropriately; Remove any necklaces, other jewelry, wristwatch, or rings. Secure any loose-fitting clothing and roll up long sleeves. Wear an apron or a properly fitted shop coat, safety glasses are a must

• Clamp all work solidly. Use the correct size tool and work-holding device for the job. Get help when handling large sections of metal and heavy chucks and attachments.

• Remove sharp edges and burrs from the workpiece before dismounting it from the machine. Burrs and sharp edges can cause painful cuts.

7. Lathe Safety

• Be sure that all guards are in place before attempting to operate the machine. Never attempt to bypass a safety switch.

• Turn the face plate or chuck by hand to be sure there is no binding or danger of the work striking any part of the lathe.

7. Lathe Safety

• Keep the machine clear of tools, and always stop the machine before making measurements and adjustments.

• Metal chips are sharp and can cause severe cuts. Do not try to remove them with your hands; Stop the machine and remove them with pliers.

• Do not permit small-diameter work to project too far from the chuck without support from the tailstock.

7. Lathe Safety

• Do not run the cutting tool into the chuck. Check any readjustment of the work or tool to make sure there is ample clearance when the cutter has been moved leftward to the farthest point that will be machined.

• Stop the machine before attempting to wipe down its surface, so the cloth doesn’t become caught on the rotating parts.

7. Lathe Safety

• Before repositioning or removing work from the lathe, move the cutting tool clear of the work area. This will prevent accidental cuts on your hands and arms from the cutter bit.

7. Lathe Safety

• Avoid talking to any one while running a lathe! Do not permit any one to fool around with the machine while you are operating it. You are the only one who should turn the machine on or off, or make any adjustments.

• Before engaging the half-nuts or automatic feed, you always be aware of the direction of travel and speed of the carriage

7. Lathe Safety

• Always remove the key from the chuck. Make it a habit to never let go of the key until it is out of the chuck and clear of the work area.

• When doing filing on the lathe, make sure the file has a securely fitting handle.

• Tools must not be placed on the lathe ways. Use a tool board or place them on the lathe tray.

7. Lathe Safety

• If any odd sounding noise or vibration develops during lathe operation, stop the machine immediately. Get help from your instructor.

• Use care when cleaning the lathe. Chips sometimes stick in recesses. Remove them with a paintbrush or wooden stick, not a dust brush. Never clean a machine tool with compressed air

7. Lathe Safety

8. Cutting Speed, Feed and depth of cut• In order to cut any material the machine must be

adjusted to the correct rotational speed of the part, the correct rate of tool travel and the cutting depth.

• 8.1 Cutting Speed and Spindle Speed• Lathe work cutting speed (CS) may be defined as the rate at

which a point on the work circumference travels past the cutting tool.

• For instance, if a metal has a CS of 30 m/min, the spindle speed must be set so that 30 meters of the work circumference will pass the cutting tool in 1 min i.e. the length of chip produced in one minute = 30 m.

• Cutting speed is always expressed in feet per minute (ft/min) or in meters per minute (m/min).

Lathe cutting speeds in meters per minute using a high-speed steel cutting tools

Spindle speed calculation • After the correct cutting speed (CS) is selected from the table,

the spindle speed (n) of the lathe machine in revolutions per minute (RPM) must be calculated and then set on the machine.

• The formula used to calculate the lathe spindle speed is shown below:

Wheren: Spindle speed in rev/min (RPM)cs: Cutting speed in m/mind: Diameter of the workpiece in (m)Π: Constant = 3.14

Example:• Calculate the rev/min required to rough turn a 40

mm diameter piece of machine steel?

• Solution:Given: diameter (d) = 40 mmd = 40/1000 = 0.04 mCS from table no. 3.1, For machine steel, under rough cutting =

27 m/min

Note:The calculated spindle speed is not necessary to be found on your machine headstock selection plate, so you should use the nearest lower speed available

8.2 Feed

• The feed of a lathe is the distance the cutting tool advances along the length of the work for every revolution of the spindle.

• For example, if the lathe is set for a 0.4 mm feed, the cutting tool will

travel along the length of the work 0.4 mm for every complete turn that the work makes.

The speed is controlled by the change gears in the quick change gearbox.

Roughing cut and Finishing cut• the purpose of a roughing cut is to remove

excess material quickly, so a coarse feed should be used.

• The finishing cut is used to bring the diameterto size and produce a good surface finish, so a

fine feed should be used

Recommended feeds for cutting various materials

• For general-purpose machining, a 0.25 to 0.4 mm feed for roughing and a 0.07- to 0.012-mm feed for finishing is recommended

The following table lists the recommended feeds for cutting various materials when a high-speed steel cutting tool is used.

8.3 Depth of Cut, Rough and Finishing cuts:• The depth of cut may be defined as the depth of the chip

taken by the cutting tool and is one-half the total amount removed from the workpiece in one cut.

8.3 Depth of Cut, Rough and Finishing cuts:• When machining a part, you need to do roughing and finishing cuts. • The roughing cuts are the cuts that are taken to reduce the diameter to

approximate size; the work is left around 0.5 mm oversize for finishing (final) cut.

The depth of a rough-turning cut will depend on the following factors:• The condition of the machine• The type and shape of the cutting tool used• The rigidity of the workpiece, the machine, and the cutting tool• The rate of feed• The material being cut

The depth of a finish-turning cut will depend on the type of work and the finish required. In any case, it should not be less than 0.13 mm. The micrometer graduated collars are used to set the required depth of cut accurately.

References

1. Technology of Machine Tools. Seventh Edition, McGraw-Hill Companies,

2. Machine shop operations and setups, 4th edition, Lascoe nelson Porter.

3. Machine tool and Manufacturing technology, Steve F. Krar, Mario Rapisarda, Albert F. Check., Delmar Publishers.

4. www.en.wikipedia.org/wiki/Machining5. http://www.mini-lathe.com6. http://mdmetric.com/tech/surfruff.htm