1. Introduction to the Lathe Machine
Module 3
Introduction to the Lathe Machine

2. Module Objectives
Upon 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

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

4. Major parts of a lathe

5. Major parts of a lathe

6. Major parts of a lathe

7. 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.

8. 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.

9. Major parts of a lathe Carriage
Controls and supports the cutting tool, and
composed of a number of parts:

10. 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).

11. Major parts of a lathe Lead screw:
Lead screw transmits power to the carriage through a gearing and clutch arrangement in the carriage apron

12. The power feed lever is located on the carriage apron
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

13. Split-nut (Half nut) lever
Major parts of a lathe
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.

14. Major parts of a lathe Tailstock
The function of the tailstock is to support long work-pieces during machining, and to hold some tool.

15. Lathe attachments
An attachment is a device mounted on the lathe so that a wider range of operations could be performed

16. Lathe attachments 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.

17. 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

18. Collet chucks
Collet chucks are used to grip small diameter work pieces.
Collets are made in sets, to fit different sizes.

19. 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.

20. Centers
Support the work piece between the headstock and the tailstock.
Follower rest
Is 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.

21. Steady rest
Is used to support long cylindrical work pieces. It is attached directly to the machine bed.

22. Lathe attachments 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.

23. Lathe attachments Grinder
Grinding can be done in the lathe if the machine is equipped with an electric grinding attachment

24. 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.

25. 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

26. 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

27. 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

28. Lathe machine cutting tools

29. 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

30. Standard terminology of cutting tool’s angles

31. 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

32. 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.

33. 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.

34. 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 mm); this information will help you to select the correct cutting operation that you should use to have the correct value of roughness

35. 7. Lathe Safety
The 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

36. 7. Lathe Safety
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.

37. 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.

38. 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.

39. 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.

40. 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.

41. 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

42. 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.

43. 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

44. 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).

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

46. 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:
Where
n: Spindle speed in rev/min (RPM)
cs: Cutting speed in m/min
d: Diameter of the workpiece in (m)
Π: Constant = 3.14

47. Example:
Calculate the rev/min required to rough turn a 40 mm diameter piece of machine steel?
Solution:
Given:
diameter (d) = 40 mm
d = 40/1000 = 0.04 m
CS 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

48. 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.

49. 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 diameter
to size and produce a good surface finish, so a fine feed should be used

50. Recommended feeds for cutting various materials
For general-purpose machining, a 0.25 to 0.4 mm feed for roughing and a to 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.

51. 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.

52. 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.

53. 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.