Time study.ppt automated production line

Julius Caesar Puoza (PhD)
MANUFACTURING
TECHNOLOGY
- MEC 228 –
Lecture 1: Time Study

Time study is also called work measurement.
Work measurement is the application of
techniques used to determine the time taken by a
qualified worker to carry out a specified job at a
defined level of performance.
Work measurement means measuring the work in
terms of time content, (i.e., minutes or hours)
required for completion of job.
Work measurement is concerned with
investigating, reducing or eliminating all
ineffective time.
Time Study in Manufacturing

The different techniques used in work
measurement are
1. Stop watch time study
2. Production study
3. Work sampling or Ratio delay study
4. Synthesis from standard data
Time Study in Manufacturing

Steps in taking time study are:
 Selecting the job to be studied
 Recording the specifications
 Breaking operation into elements
 Examining each element
Stopwatch Time Study

The following procedure is followed in conducting
stop watch time study:
 Assessing the rating factor
 Calculating the basic time
 Determining the allowances
 Compiling the standard time
Standard Time: Standard time is the total time in
which a job should be completed at standard level
of performance
Stopwatch Time Study

Allowance
Allowance is an amount of time added to the
normal time to provide personal delays,
unavoidable delays and fatigue of the operator
Types of allowances
1.Relaxation allowances
 Personal need allowances [washing hands,
tea, laboratory] 5-7%
 Fatigue allowances [excessive work, poor
lighting, repeated work machine noise, visual
strain and mental strain]

Allowance
Types of allowances
2. Contingency allowances
 [Unavoidable allowances- tool sharpening,
tool replacing, coolant filling, discussion
with supervisors, cleaning, power failure –
5%]
3. Process allowances
 [Loss of time due to no work, power failure,
faulty material, method, machine and
operation ]

Allowance
Types of allowances
4. Interference allowances
 [when one worker is attending more than one
machine]
5. Special allowances – not a part of operation
cycle but essential for satisfactory performance of
the work
 Start-up allowance
 Shut- up allowance
 Cleaning allowance
 Tool allowance (for machine and tool)
 Changeover allowance

Example 1: Assuming that the total observed time
for an operation of assembling a bench vice is
1.00 min. If the rating is 120%, find normal time. If
an allowance of 10% is allowed for the operation,
determine the standard time.
Time Study

The major aim and objectives in machining
industries generally are
 reduction of total manufacturing time, T
 increase in MRR, i.e., productivity
 reduction in machining cost without
sacrificing product quality
 increase in profit or profit rate, i.e.,
profitability.
All those objectives are commonly and
substantially governed by the total machining
time per piece.
ECONOMICS OF MACHINING

Machining Time: is the time for which the machine
works on the component, i.e. from the time when
the tool touches the work piece to when the tool
leaves the component after completion of
operation.
The machining time depends on the type and
extent of machining required, material being
machined, speed, feed, depth of cut and number
of cuts required.
MACHINING TIME

Major Factors That Govern Machining Time
The major factors considered for selecting VC
are:
• Nature of the cut;
 Continuous cut like turning, boring, drilling
etc. are done at higher VC
 Shock initiated cuts in shaping machine,
planing machine, slotting machine etc. are
conducted at lower VC
 Intermittent cuts, as in milling, hobbing etc.
are done at quite lower speed for dynamic
loading
MACHINING TIME

Work material (type, strength, hardness, heat
resistance, toughness, chemical reactivity etc.)
For instance;
 Harder, stronger, heat resistant and work
hardenable materials are machined at lower
VC
 Soft, non-sticky and thermally conductive
materials can be machined at relatively
higher cutting velocity
MACHINING TIME

Cutting tool material
HSS tools are used at within 40 m/min only in
turning mild steel whereas for the same work
cemented carbide tools can be used at VC, 80 to
300 m/min
High performance ceramic tools and cBN tools
are used at very high speed in machining steels
of different strength and hardness.
Diamond tools can be used in machining
various materials (excepting Fe-base) at VC
beyond 500 m/min
MACHINING TIME

Cutting fluid application:
 Proper selection and application of cutting
fluid may allow increase in VC by 20 to 50%
Purpose of machining:
 Rough machining with large MRR is usually
done at relatively low or moderate velocity
 Finish machining with small feed and depth
of cut is usually done at high VC
Kind of machining operation:
 Unlike turning, boring etc. the operation like
threading, reaming etc. are carried out at
much lower (20 to 50%) cutting velocity for
achieving quality finish.
MACHINING TIME

Capacity of the machine tool
 Powerful, strong, rigid and stable machine
tools allow much higher VC, if required and
permissible
Condition of the machine tool
 Cutting velocity is kept lower than its
normal value stipulated for a given tool –
work material pair , if the machine tool is
pretty old and / or having limitations due to
wear and tear, backlash, misalignment,
unstability etc.
MACHINING TIME

STEPS IN MACHINING TIME CALCULATION

SHAPPING

DRILLING
Cutting speed is the speed at which the cutting
edge of tool passes over the job and it is usually
expressed in meters per minute.
The cutting speed depends on the cutting tool
material, the work piece material and the
operation.

DRILLING
Once the cutting speed has been selected, the
revolutions per minute of job/machine are
calculated as follows:

Planing

The machining time, TC required for a particular
operation can be determined
 roughly by calculation i.e., estimation
 precisely, if required, by measurement.

The factors that govern machining time will be
understood from a simple case of machining. A
steel rod has to be reduced in diameter from D1 to
D2 over a length L by straight turning in a centre
lathe as indicated in Fig. 1.

LC = actual length of cut
= L + A + O
A, O = approach and over run as shown
N = spindle speed, rpm
so = feed (tool), mm/rev
np = number of passes required
Speed, N, is determined from cutting velocity, VC

where, D = diameter of the job before cut
The number of passes, np is mathematically
determined from,
where, t = depth of cut in one pass, mm.

In turning to a given diameter and length, the
cutting time, TC is governed mainly by the
selection of the values of cutting velocity, VC and
feed, so.

The factors that are considered during selecting
the value of feed, So are,
 Work material (type, strength, hardness
etc.)
 Capacity of the machine tool (power,
rigidity etc.)
 Cutting tool; material, geometry and
configuration
 Cutting fluid application
 Surface finish desired

Estimation Of Machining Time By Calculations
In case of turning in lathes
The machining time for facing, grooving, taper
turning, threading, parting etc. in lathes can also
be determined or estimated following the same
principle and method.

Example
How much machining time will be required to
reduce the diameter of a cast iron rod from 120
mm to 116 mm over a length of 100 mm by turning
using a carbide insert. Reasonably select values
of VC and so.

Solution
For turning C.I. by carbide insert, VC is taken as 100
m/min and so = 0.2 mm/rev

In case of drilling and boring

The machining time, TC is estimated from
Speed, N and feed so are selected in the same way
as it is done in case of turning. Therefore, the
drilling time can be determined from

Therefore, the drilling time can be determined from
In the same way TC is determined or estimated in
boring also. Only the portion ‘C’ is not included.
For blind hole, only over run, ‘O’ is excluded.

Example
Determine the time that will be required to drill a
blind hole of diameter 25 mm and depth 40 mm in
a mild steel solid block by a HSS drill of 1180
cone angle. Assume suitable values of VC and so.

Time study.ppt automated production line

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