ENG 2159 TUTORIAL II
WHAT IS MACHINING?
Machining is a subtractive manufacturing process that involves
material removal, usually in the form of chips, from a workpiece.
Machining may be considered as a system consisting of the
workpiece, the cutting tool, and the equipment (machine tool). In
machining, there exists a relative motion between the tool and the
work; the primary motion is called cutting speed, whereas the
secondary motion is called feed.
In general, there are three fundamental cutting conditions:
(a) cutting speed,
(b) feed, and
(c) depth of cut.
Cutting speed is the greatest of the relative velocities of cutting tool or
workpiece. For example, in turning machining operation, the surface speed
of the workpiece is the cutting speed (v), usually expressed in m/min (see
Figure 1.1). Feed (f) is the distance moved by the tool (or by the work) per
revolution, usually expressed as mm/rev. Depth of cut (d) is the distance the
cutting tool penetrates into the work.
ROUGHING AND FINISHING IN MACHINING
Machining is usually carried out in two steps with varying cutting
conditions (cutting speed, feed, and depth of cut).
The two steps in machining are (a) roughing pass and (b) finishing
pass.
In the roughing pass, a bulk amount of material is quickly removed
from the workpiece as per required feature. In this step, higher feed
rate and depth of cut are employed so as to achieve a high material
removal rate from the work. The roughing pass creates a shape
close to desired geometry but leaves some machining allowance
(material unremoved) for finish cutting. The roughing pass cannot
provide good surface finish and close tolerance. This is why a
finishing pass is carried out to improve surface finish, dimensional
accuracy, and tolerance level; here, the feed rate and depth of cut
are low. Thus, the material removal rate (MRR) is reduced in the
finishing pass, but the surface quality is improved.
TABLE 1.1 Difference between Roughing and Finishing
Roughing in Machining
It is performed prior to finishing
pass
Its objective is to remove bulk
excess material from workpiece
in every pass
Finishing in Machining
It is performed only after
roughing pass
Its objective is to improve surface
finish, dimensional accuracy,
and tolerance
It involves higher feed rate and
depth of cut
It results in higher MRR
It involves higher cutting speed
The surface finish is poor
The surface finish is good
The MRR is low
It cannot provide high
It can provide high dimensional
dimensional accuracy and close accuracy and close tolerance
tolerance
It permits the use of an old cutter It requires the use of a sharp
cutting tool
Machinability
Machinability refers to the ease of machining a material to obtain
desired results at low cost. There are a number of quantitative
measures of machinability; these measures include
(a) tool life,
(b) surface finish, and
(c) other measures, such as cutting force, power, temperature, and
chip formation.
The tool life refers to the service time in minutes or seconds to a total
failure of the cutting tool at certain cutting speed. The surface finish
refers to the acceptable surface finish produced at standardized
cutting speeds and feeds.
A good machinability may mean one or more of the following: (a)
minimum cutting forces, power, and temperature; (b) longer tool life
(minimum tool wear); and (c) a good surface finish.
Material Removal Rate (MRR)
The material removal rate is the volume of material removed per
unit time. Volume of material removed is a function of speed, feed
and depth of cut.
The Higher the values of these, the higher the material removal rate.
If D represents the original diameter of the workpiece in mm, d
represents the depth of cut in mm, f represents the feed in mm/rev,
then material removed per revolution is the volume of chip whose
length is πD and whose cross section area is d × f.
That is, volume of material removed/revolution = π × D × d × f
Since job/workpiece is making N revolutions per minute, the MRR
in mm3/min is given by
MRR = π × D × d × f × N
In terms of the cutting speed v in m/min,
(4)
MRR = 1000 × v × d × f
(5)
You can check the dimensional accuracy of these equations by
substituting the units.
For the Equation (4), (mm)(mm)(mm/rev)(rev/min) =>
mm3/min and for Equation (5),
(mm/m)(m/min)(mm)(mm) => mm3/min.
Machining Time
The time required to machine a component is called machining time.
Machining time depends on size of the workpiece, amount of
material to be removed and the operating conditions employed such
as speed, feed and depth of cut.
Consider the speed of the job as N rpm, length of the job as Lj mm
and f as feed in mm/rev. The product of feed and speed f × N is the
feed rate in mm/min.
It gives the distance the tool moves (f × N) in mm in one minute.
Hence, for a distance Lj, the time
required for one complete cut, t in minutes is given by
Note that the above equation of time doesn’t include setup time and
the time required for tool approach and retraction.
Operating conditions in drilling
The cutting speed v in drilling is the peripheral speed of the drill.
The cutting speed that should be used
in drilling depends on number of factors properties of material being
drilled, drill material, drill diameter, rate of feed, coolant use.
The feed f is the movement of the drill along its axis. It is usually
specified in millimeters per revolution of the drill
The depth of cut (d) in drilling is not a significant parameter. It may
be defined as the distance from the machined surface to the drill
axis.
That is,
The choice of operating conditions in drilling operations becomes
more critical with an increase in the hole depth.
As the depth of hole increases (i) the chip ejection becomes more
difficult, and (ii) fresh cutting fluid is not able to reach to the
cutting zone.
These factors lead to overheating of the drill and shorten its life.
Hence, for machining lengthy holes reduced feed rates are used.
For machining holes of very large length a special type of drilling
process known as gun drilling is used. By using this process, it
is possible to machine the holes having length greater than 300
times the diameter.
Material Removal Rate
The cutting speed v decides the speed of rotation of the drill N based
on the diameter of the drill D. The cutting speed is given by:
Where,
D is in mm,
N is in rev/min or rpm, and
v is in m/min.
The material removal rate (MRR) in drilling is the volume of
material removed by drill per unit time. For a drill with a diameter
D, the cross-sectional area of the drilled hole is D^2/4.
The material removal rate is the product of cross-sectional area of
the drilled hole, feed rate f (in mm/rev) and the rotational speed N
of the drill. That is,
Machining time in drilling
The machining time for drilling a hole depends upon the length of
the hole to be drilled L, rpm of the drill N and feed f. If t is time
required to drill a hole, then
where ,
L is in mm,
f is in mm/rev and N is in rpm.
In order to drill a through hole, the drill must pass through the
thickness of workpiece. This means the travel of the drill is more
than the length of hole. Thus, in actual practice, for the hole length
Lj we add length of approach (or approach) L1 and length of recess
(or over travel) L2 for the calculation of time t.
Power required = Unit power * MRR
Example 1
The part shown below will be turned in two machining steps. In the
first step a length of (50 + 50) = 100 mm will be
reduced from Ø100 mm to Ø80 mm and in the second step a length
of 50 mm will be reduced from Ø80 mm to Ø60 mm.
Calculate the required total machining time T with the following
cutting conditions:
Cutting speed V = 80 m/min,
Feed is f = 0.8 mm/rev,
Depth of cut = 3 mm per pass.
SOLUTION:
V = 80 m/min
F = 0.8 mm/rev
The turning will be done in 2 steps. In first step a length of (50 + 50) =
100 mm will be reduced from Ø100 mm to Ø80 mm and in second
step a length of 50 mm will be reduced from Ø80 mm to Ø60 mm.
Example 2:
A 150 mm long 12 mm diameter stainless steel rod is to be reduced
in diameter to 10 mm by turning on a lathe in one pass. The spindle
rotates at 500 rpm, and the tool is traveling at an axial speed of 200
mm/min.
Calculate,
(a) the cutting speed,
(b) material removal rate and
(c) the time required for machining the steel rod.
Example 3
Calculate the time required to drill a 25 mm diameter hole in a
workpiece having thickness of 60 mm to the complete depth.
The cutting speed is 14 m/min. and feed is 0.3 mm/rev. Assume
length of approach and over travel as 5 mm.
Solution
Given data: D = 25 mm, Lj = 60 mm, v = 14 m/min, f = 0.3 mm/
rev. with usual notations.
Substituting the data in the Equation (1), we calculate the drill
rotational speed, that is
or
N = 178 rpm.
Length of tool travel = Lj + Length of approach and over travel.
= 60 + 5 = 65 mm
Substituting the values of f, L and N in Equation
we get
t = 65 / (0.3×178) = 1.21 minute.
End of MEC 2159 MACHINING TUTORIAL
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