Condition-Based Maintenance of CNC Turning Machine

Abstract

In this paper, condition-based Preventive Maintenance (PM) approach is used for a Computer Numerical Control (CNC) turning machine .Wearing of CNC machine is based on the various machining conditions (such as cutting speed, feed and depth of cut) and the time for which it is being used. Larger value of machine power and production rates usually results in more wear and failure of CNC machine. The various machining conditions of CNC machine usually affects the PM requirement. Higher production rate usually results in greater deterioration of the machine, and hence there is a need of frequent preventive maintenance of the machine. In this paper, we adopted the methodology of PM approach to determine the preventive maintenance index function, using the values taken from a CNC machine, to determine the exact time needed between the 2 PM visits, which in turn can help in reducing the frequent breakdowns occurring in the CNC machine.

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Int. J. Mech. Eng. & Rob. Res. 2013 Mayank Srivastava, 2013
CONDITION-BASED MAINTENANCE OF CNC
TURNING MACHINE
Mayank Srivastava1*
*Corresponding Author: Mayank Srivastava, mayank.sri62@yahoo.in
In this paper, condition-based Preventive Maintenance (PM) approach is used for a Computer
Numerical Control (CNC) turning machine .Wearing of CNC machine is based on the various
machining conditions (such as cutting speed, feed and depth of cut) and the time for which it is
being used. Larger value of machine power and production rates usually results in more wear
and failure of CNC machine. The various machining conditions of CNC machine usually affects
the PM requirement. Higher production rate usually results in greater deterioration of the machine,
and hence there is a need of frequent preventive maintenance of the machine. In this paper, we
adopted the methodology of PM approach to determine the preventive maintenance index function,
using the values taken from a CNC machine, to determine the exact time needed between the
2 PM visits, which in turn can help in reducing the frequent breakdowns occurring in the CNC
machine.
Keywords: Preventive Maintenance, CNC machines, Machining Conditions, PM Index,
Tool-Life
INTRODUCTION
Pr even tive Maint enan ce (PM) ca n be
described as maintenance of equipment or
systems before fault occurs. While PM is
generally considered to be worthwhile, there
are risks such as equipment failure or human
error involved when performing preventive
maintenance, just as in any maintenance
operation. In other words, PM is the planned
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Int. J. Mech. Eng. & Rob. Res. 2013
1Department of Mechanical Engineering, MITS Gwalior, MP, India.
maintenance of an equipment with the goal of
improving equipment life by preventing excess
dep reciat i on and d eteri o ratio n . This
maintenance includes, but is not limited to,
adjustments, cleaning, lubrication, repairs,
replacements, and the extension of equipment
life.
We usually observe in our day-to-day life,
that if the machine works continuously under
Research Paper

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Int. J. Mech. Eng. & Rob. Res. 2013 Mayank Srivastava, 2013
certain conditions, then there will be frequent
need for the inspection and repair of the
machine. Thus, in our study, we tries to
determine the exact time needed to do the
preventive maintenance of the CNC machine,
from the data gathered by working on the CNC
turning machine type AUTOCOMP-55. On this
machine, a particular type of design or product
is made and the values concerning to that
product is taken into analysis, to determine the
various parameters like optimum values of
cutting speed and the corresponding feed rate,
processing time of an operation using a
particular tool type under some constraints,
which are due to some technical limitations of
machine and tool and design requirement for
the part.
LITERATURE REVIEW
The literature review has been carried-out in
the areas of maintenance of CNC machines.
Techniques of preventive maintenance index
function and geometric programming model
have been reviewed to assess the various
parameters of CNC machines.
Nolden (1987) used the devi ces like
vibration monitoring, wear particle analyzers,
etc. to pr edi ct fail ures which helps in
determining the predictive maintenance of a
CNC machine.
For optimization of the condition-based
maintenance Banjevic et al. (2001) presented
a control-limit policy and software respectively.
To minimize the total weighted tardiness
(lateness) of jobs in a CNC machine Cassady
and Kutanoglu (2003) considered the PM
planning and Production Scheduling (PS)
decisions simultaneously.
Various Mathematical models and their
solution procedures for various machining
conditions selection problems specially for the
turning operation are done by Hitomi (1996).
Formulating the machining conditions
selection problem as a multi -obje ctive
decision-making problem is done by the
Malakooti and Deviprasad (1989).
According to the Gray et al. (1993), the
decisions like preventive maintenance and
tool replacement are solved at different levels
of the decision-making hierarchy keeping in
view that the preventive maintenance (PM)
decisions are always handled at a higher level
than the tool replacement decisions.
For tool life distributions, some models are
proposed by the Lakovou et al. (1996) to
determine the selection of optimal cutting
speed and policies for tool replacement in
machining problems.
Koomsap et al. (2005 ) st udies that
integrates both the process planning and
Co ndition -Base d Mainte nance (CBM)
scheduling decisions. They proposed a
system that usually collects information on the
current condition of a machine in order to
determine its working lifetime.
Figure 1: AUTOCOMP 55 CNC
Turning Machine

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Int. J. Mech. Eng. & Rob. Res. 2013 Mayank Srivastava, 2013
Cassady and Kutanoglu (2003 and 2005)
also considered the preventive maintenance
planning and production scheduling decisions
simultaneously to minimize the total weighted
tardiness (lateness) and total expected
weighted completion time, respectively.
PRESENT WORK
Condition Based Maintenance (CBM) is a
maintenance which is usually performed
whenever its need arises. This type of
maintenance is performed when we get the
kind of indications showing that the equipment
or machine’s performance is deteriorating or
is going to fail.
In this present work, I usually put emphasis
on finding the time between the two PM visits,
using the data collected from the CNC turning
machine (AUTOCOMP-55, which is making
certain specific types of products).
METHODOLOGY
Proposed PM Index Methodology
The notation used throughout the paper is as
follows.
Parameters
A, B, k, T Coefficients for the proposed PM
index function



,

Speed , feed , dep th of cut
exponents for tool
KjTaylor’s tool life constant for tool j
Cm, b, c, e Speci f i c co effi cients of the
machine power constraint
Cs, g, h, l Specif i c coeff icients and
exponen ts of th e surfa ce
roughness constraint
CoOp eratin g cost of th e CNC
machine (Rs/min)
Ctj Cost of tool j (Rs./tool)
DiDi ame ter of th e genera ted
surface for operation i (in.)
diDepth of cut for operation i (in.)
HMaximum available mac hine
power (hp)
LiLength of the generated surface
for operation i (in.)
SiMaxi mum allowabl e surface
roughness for operation i (min.)
CPM Cost of a PM visit (Rs./visit)
PM Duration of a PM visit (min)
trTool replacement time (min)
Decision Variables
vij Cutting speed for operation i
using tool j (fpm)
fij Feed rate for operation i using tool
j (ipr)
Dependent Variables
tmi j Processing time of operation i
using tool j (min)
riProduction rate of the machine for
operation i (part/min)
Zij Expected tool life of tool j for
operation i (min)
Uij Expected usage rate of tool j for
operation i
Pij PM index for operation i used
Data Collection and Calculations
The values of some of the constraints taken
from the previous research papers are

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Int. J. Mech. Eng. & Rob. Res. 2013 Mayank Srivastava, 2013
The various relations that are using for
finding the PM index are as given below:
•

12
i i
mij
ij ij
D L
t
v f
•













iijijj
ijijii
ij
mij
ij
dfvK
fvLD
Z
t
U
../
12/
•PM Cost = A + B.rk
•n = T/(tm + tr . U)
• PM Cost per Operation
=
T
1( A.tm + A.tr.U +

1
k
m
B
t
+k
m
r
t
UtB ..
)
• PM Index =
PM
CT.
1(A.tm + A.tr.U

1
k
m
B
t
+ k
m
r
t
UtB ..
)
In terms of vij and fij, it can be expressed as
•Pij = P1vij
-1fij
-1 + P2vij

-1fij

-1 + P3vij
k-1fij
k-1
+ P4 vij

+k-1fij

+k-1
where
P1 = ,
12 PM
ii
TC
LDA

P2 = ,
12 PMj
iiir
TCK
dLDAt


P3 =
)1(
12







k
iiPM LDTC
B

,
P4 =
)1(
12 





k
iiPMj
ir
LDTCK
dBt


•Mij(v) = C1(Cs
')(1/h)vij
(g-h)/h +
C2(Cs
')-(

-1)/hvij
[h(

-1)-g(

-1)]/h +
C3(Cs
')-(k-1)/hvij
[(h-g)(k-1)]/h +
C4(Cs
')-(

+k-1)/hvij
[h(

+k-1)-g(

+k-1)]/h
Data collected from CNC turning machine
are as follows:
S.No. Machining Parameters & their values
1. Depth of Cut (di) = 3mm = 0.118 in
2. Max. Allowable Surface Roughness (Si) = 300
in
3. Dia. of gener ated surface (Di) = 543mm =
21.378 in
4. Length of generated surface (Li) = 500mm =
19.685 in
5. Operating Cost of CNC machine (Co) = Rs.350
per 8 h = 0.72916 Rs/min
6. Cost of a PM visit (CPM ) = Rs.526 Rs/visit
7. Tool Replacement time (tr) = 3 min
8. Cost of tool j = Rs.1000 (Assume Avg)
9. Operating time period (T) = 1680 min
Cycle Time = 25 min
Clamping / Declamping Time = 5 min
Tool Used : Single Tool Type
Taylor’s Tool Constant, Cj: 125321000
Tool-Life Constraints

= 4.3

= 1.6

= 1.2
Machine Power Constraints b = 0.96
c = 0.70
e = 0.71
Cm=1.637
Surface Roughness g = -1.60
Constraints h = 1.005
l = 0.30
Cs=259500000
PM cost of m/c when it is idle A = 10 (Assume)
B,k (Dependent on T and B = 30 (Assume)
on cost of PM visit) k = 2.5 (Assume)
Table 1: Values of Constraints Used
Table 2: Data Collected from
CNC Turning Machine

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Int. J. Mech. Eng. & Rob. Res. 2013 Mayank Srivastava, 2013
where
,
12
1
0
1PC
CLD
CPM
ii 

,
12 22 PC
K
CdLD
CPM
j
tjiii 


C3 = CPMP3 , C4 = CPMP4 ,
,
12 j
iii
tK
dLD
C



 ,
H
dC
C
e
im
m

i
l
iS
SS
dC
C

•fij = (1/ (

S
C
vij
g))1/h
The algorithm used for the single machining
operation problem is as follows:
Step 1: Finding (v,f) pair subjected to surface
roughness and machine power constraints.
,
,
1
/1 g
h
BS
BfC
V









   
 


 
 
 
1 /
bh gc
g b
B m s
f C C
Thus, fB= 0.0435 , vB= 480.2
Step 2: Finding (v, f) pair subjected to surface
roughness and tool life constraints.
Figure 2: Possible Intervals or Range to Determine Optimum Value
vopt vmintm vm+t

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Int. J. Mech. Eng. & Rob. Res. 2013 Mayank Srivastava, 2013
,
,
1
/1 g
h
AS
AfC
V









   








11/1
1
 



hg
g
tsA CCf
Thus, fA = 0.0207, vA= 301.184
Step 3: vmintm = min [vA, vB]
i.e., vmintm = vA = 301.184
Step 4: Find the derivative of M(v) w.r.t. v, 2
conditions arises:
(i) M'(v)  0, then vopt = vmintm(Case 1 in Figure
2).
(ii) M' (v) > 0, then vopt = vm + t
Step 5: Find the cutting speed minimizing the
machining plus tooling cost on the surface
roughness constraint.
 
 
   





ghh
h
Stm gh
gh
C
C
C
V











/
/
2
1
11
or, vm+t = 148.543
Step 6: Determine the upper and lower
bounds of the interval.
• If vmintm < vm+t, then v1 = 0 and
v2 = vmintm (Case 2 in Figure 2)
• If M' (vm+t) < 0, then vm+t = v1 and
vmintm = v2 (Case 3 in Figure 2)
• If M' (vm+t) < M' (vmintm), then
v1 = 0 and v2 = vm+t (Case 4 in Figure 2)
Thus,
vm+t < vopt < vmintm
or, 148.543 < vopt < 301.184
Step 7: Optimal value of speed and feed can
be obtained by employing the one-dimensional
optimization method like Newton-Raphson
method, Bisection method, etc.
Thus, after performing various iterations,
the optimal value of cutting speed is
vopt = 148.6175 rpm
Similarly, the optimal value of feed rate is
fopt = 0.0006723 ipr
Step 8: Now, Processing time of an operation,
tm = 110.265 min
Expected Tool Usage, Uij = 0.0495
PM/Operation = 0.65724
PM Index = 1.2495 X 10-3
= 0.0012495
Time b/w 2 PM visits
= tm/PM Index
= 88247.077 min
= 183.85  184 days
 6 months
RESULT/CONCLUSION
In this study concerning to the CNC machine,
we proposed a PM approach, which is used
to find out the time between two PM visits on
the CNC turning machine. The proposed PM
methodology finally results that the preventive
maintenance concerning to the operation of
CNC turning machine should be carried out
every 6 months, thus making the life of the tool
as well as machine to increase without any
failure or breakdown.

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