1. Turning operation on lathe and study of effect of speed, feed and depth of cut on surface roughness

2. Machining Turning
Definition: A machining operation in which a single point cutting tool removes material from a rotating workpiece to form a cylindrical shape. 2

3. Basic Mechanics of Metal Cutting
Metal ahead of the cutting tool is compressed.
This results in the deformation or elongation of the crystal structure -resulting in a shearing of the metal.
As the process continues, the metal above the cutting edge is forced along the “chip-tool” interference zone and is moved away form the work.

4. Direction of feed motion decides type of turning –
Feed motion parallel to axis – longitudinal turning
Feed motion perpendicular to axis – transverse turning
Source: NPTEL

5. Machining Orthogonal cutting model
Cutting edge is perpendicular to the cutting speed 5

6. Machining Terminology
Speed – surface cutting speed (v, m/min)
Feed – advancement of tool through the workpiece in one rotation of spindle (f, mm/rev)
Depth of cut – distance by which tool penetrates in the workpiece (d, mm)

7. Process Parameters
Any machining operation involves three basic parameters
Primary motion (cutting motion or Cutting Speed) v
During the rotation of jobpiece, primary motion makes workpiece to approach cutting edge hence spindle rotates anticlockwise
Secondary motion (feed) f
Distance travel along the axis
Depth of cut d
Distance tool penetrates in the jobpiece. It is along raidial directoion
Relative tool travel in 1 rotation = pD
Cutting Speed = v = pDN/1000 m/min

8. Process Parameters
Three process parameters are perpendicular to each other

9. Output Parameter: Surface Roughness
Roughness: It is defined as closely spaced, irregular deviations on a scale smaller than that of waviness. Roughness is expressed in terms of its height, its width, and its distance on the surface along which it is measured.
Waviness: It is a recurrent deviation from a flat surface, much like waves on the surface of water. It is measured and described in terms of the space between adjacent crests of the waves (waviness width) and height between the crests and valleys of the waves (waviness height).

10. Sample Plot: Surface Roughness v/s Cutting Speed

11. Cutting Speed
Depends on number of rotations (rpm) of the chuck of a lathe
Always expressed in meters per minute (m/min)
Important to use correct speed for a given material to get desired surface finish
Too high: cutting-tool breaks down rapidly
Too low: time lost, low production rates

12. Cutting Speeds Using High-Speed Steel Toolbit Turning Rough Cut
Finish Cut
Material
m/min
Machine Steel 27 30
Tool Steel 21
Cast Iron 18 24
Bronze
Aluminum 61 93

13. Calculating Spindle Speed
Given in revolutions per minute
Cutting speed of metal and diameter of work must be known
Proper spindle speed set by dividing CS (m/min) by circumference of work (mm)

14. Example:
Calculate RPM required to rough-turn 50 mm diameter piece of machine steel (CS 27 MPM):

15. Effect of Cutting Speed
Rough
Finish v
(m/min)
18.5
37.1
51.8
69.1
86.4 f
(mm/rev)
0.3 d
(mm)
As speed increases surface finish increases

16. Feed
Distance cutting tool advances along length of workpiece for every revolution of the spindle
Feed of lathe dependent on speed of lead screw or feed rod
Speed of feed rod controlled by change gears in quick-change gearbox

17. Feed
Lead Screw
Feed Rod

18. Feed
f – the distance the tool advances for every rotation of workpiece (mm/rev)

19. Feed Analysis of Turning
Feed & Feed rate: Both are used to specify the same quantity which is the amount of travel in secondary motion direction in a specified amount of time. Time unit is revolution of workpiece for feed while feed rate is defined in terms of second. 19

20. Effect of Feed on Roughness

21. Rough and Finish cut Generally two cuts are employed - Roughing cut
Purpose to remove excess material quickly
Coarse feed: surface finish not too important
0.25- to 0.4-mm
Finishing cut
Used to bring diameter to size
Fine feed: Produce good finish
0.07- to mm

22. Effect of Feed v (m/min) 25.9 f (mm/rev) 0.5 0.3 0.1 d (mm) Finish
Rough
v (m/min)
25.9
f (mm/rev)
0.5
0.3
0.1
d (mm)
At low feed surface finish is good.

23. Feed High-speed steel cutting tool feed per revolution of spindle
Rough Cuts
Finish Cuts
Material
mm/rev
Machine Steel
Tool Steel
Cast Iron
Bronze
Aluminum

24. Depth of Cut Distance tool penetrates in the workpiece
It is equal to half the difference between the initial diameter and the final diameter.
Generally only one roughing and one finishing cut is used
Roughing cut should be as deep as possible to reduce diameter to within 0.75 to 1 mm of size required
Finishing cut should be less than mm

25. Depth of cut on a lathe

26. Depth of Cut
perpendicular distance between machined surface and uncut surface of the Workpiece d = (D1 – D2)/2 (mm)

27. Effect of Depth of Cut v (m/min) 25.9 f (mm/rev) 0.3 d (mm) 0.5 0.1
At low depth of cut surface finish is good.

28. Material Removal Rate MRR
Volume of material removed in one revolution MRR =  D d f mm3
Job makes N revolutions/min
MRR =  D d f N (mm3/min)
In terms of v MRR is given by
MRR = 1000 v d f (mm3/min)

29. MRR
Note that undeformed chip thickness is equal to feed! 29

30. Calculating Machining Time
Factors such as spindle speed, feed and depth of cut must be considered

31. Example:
Calculate the time required to machine a 2-in. diameter machine-steel shaft 16 inch to diameter finish size
Roughing cut
Roughing feed feed = .020
Total Time
29.1 min
Finishing cut:
Finishing feed = .003

33. For animations on effect of Depth of Cut, click the following
Low Depth of Cut
Medium depth of cut
High depth of cut

34. Effect of Depth of Cut
Cutting Force vs. Depth of cut for a mild steel shaft at 118 RPM and feed rate 0.3 mm/rev

35. Effect of Feed Rate
Cutting Force Vs. Feed Rate for a mild steel shaft at 118 RPM with a depth of cut of 0.5 mm

36. Effect of Cutting Speed
Cutting force vs. Cutting speed at different feed rates for a mild steel shaft cut with depth of cut of 0.3 mm