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ORIGINAL RESEARCH
Process-based tolerance assessment of connecting rod machining
process
G. V. S. S. Sharma
1
•P. Srinivasa Rao
2
•B. Surendra Babu
3
Received: 6 April 2014 / Accepted: 18 December 2015
ÓThe Author(s) 2016. This article is published with open access at Springerlink.com
Abstract Process tolerancing based on the process
capability studies is the optimistic and pragmatic approach
of determining the manufacturing process tolerances. On
adopting the define–measure–analyze–improve–control
approach, the process potential capability index (C
p
) and
the process performance capability index (C
pk
) values of
identified process characteristics of connecting rod
machining process are achieved to be greater than the
industry benchmark of 1.33, i.e., four sigma level. The
tolerance chain diagram methodology is applied to the
connecting rod in order to verify the manufacturing process
tolerances at various operations of the connecting rod
manufacturing process. This paper bridges the gap between
the existing dimensional tolerances obtained via tolerance
charting and process capability studies of the connecting
rod component. Finally, the process tolerancing compar-
ison has been done by adopting a tolerance capability
expert software.
Keywords Process tolerancing Tolerance chart
DMAIC Process capability Dimensioning and
tolerancing Dimensional mapping
Introduction
The vital governing factor influencing the machining
excellence is the geometric and dimensional tolerance
embedded into the product as well as into the process. The
two main facets of tolerancing include the arithmetic and
statistical tolerancing. In arithmetic tolerancing it is
assumed that the detail part dimension can have any value
but within the tolerance range; whereas, in the statistical
tolerancing scheme, it is assumed that detail part dimen-
sions vary randomly according to a normal distribution,
centered at the mid-point of the tolerance range and with its
±3rspread covering the tolerance interval.
The main disadvantage of arithmetic tolerancing or
worst-case tolerancing is that it does not follow any trend
or pattern within the tolerance zone and part dimensions
resulting from the machining process can possess any value
within the tolerance zone. This results in checking up of
each individual dimension for its correctness within the
tolerance zone, which is impractical in mass production.
The statistical tolerance overcomes this drawback of
arithmetic tolerance and facilitates the machining to yield
dimensions according to a normal distribution. Also sta-
tistical tolerancing allows some cancellation of variation
from normal distribution. Hence, this paper in essence
reflects the theme of statistical tolerancing.
The structure of paper is as follows. The first introduc-
tion part of the paper discusses the required introductory
theoretical domain on tolerancing methods. This is fol-
lowed by a literature review on the process tolerancing. A
&G. V. S. S. Sharma
sarma.gvss@gmail.com
P. Srinivasa Rao
psrao89@gmail.com
B. Surendra Babu
sudeepbs@gmail.com
1
Department of Mechanical Engineering, GMR Institute of
Technology, GMR Nagar, Rajam 532127, Andhra Pradesh,
India
2
Department of Mechanical Engineering, Centurion
University, Parlakhemundi 761211, Odisha, India
3
Department of Industrial Engineering, GITAM Institute of
Technology, GITAM University,
Visakhapatnam 530045, Andhra Pradesh, India
123
J Ind Eng Int
DOI 10.1007/s40092-015-0138-2
tolerance stack analysis with tolerance chain of the con-
necting rod machining is then presented. The tolerances
obtained from tolerance stack are put to test for process
capability studies (Sharma and Rao 2013). Then the
improved C
p
and C
pk
values obtained are compared for
optimum tolerance value using a tolerance capability
expert software. Finally, the paper is concluded with a
discussion on the results obtained.
Figures 1and 2show the part model rod and cap por-
tions of the connecting rod, respectively. Figure 3shows
the assembled view of the connecting rod and Fig. 4
depicts the orthographic projection of the connecting rod
product drawing.
Literature review
Primitive studies on process tolerancing were introduced
through graphical representation of machining tolerance
charting (Irani et al. 1989). The graphical approach and
rooted tree diagram were adopted for tolerance charting
(Whybrew et al. 1990). A tree theoretical representation for
a tolerance chart was presented from the part blue print
dimensions, stock removals and working dimensions (Ji
1993). The manufacturing process sequence was deter-
mined by using a profile representation method which
incorporates a two-dimensional matrix containing a num-
ber coding system to represent the part profile (Ngoi and
Ong 1993). A mathematical rooted tree model incorporat-
ing the linkage between the capability of manufacturing
process and tolerance chart balancing was developed (Wei
and Lee 1995). Geometrical control requirements were
expressed as equivalent linear dimensions and then applied
to a tolerance chart (Ngoi and Tan 1995). A backward
derivation approach was traced for determining the
machining tolerances starting from the last operation and
computing machining allowances backwardly till the first
machining operation (Ji 1996). A graphical method for
presenting the process link and for obtaining the necessary
working dimensions and tolerances was introduced (Ngoi
and Tan 1997).
Process capability of machinery was taken into consid-
eration for standardization of tolerances, through a non-
linear programming model (Lee and Wei 1998). This
minimized the total manufacturing loss occurring due to
Fig. 1 Dimensioned part model of rod-end of connecting rod in
CATIA V5 R14 software
Fig. 2 Dimensioned part model of cap-end of connecting rod in
CATIA V5 R14 software
Fig. 3 Dimensioned part model of assembled view with tolerance
annotations of connecting rod in CATIA V5 R14 software
Fig. 4 Product drawing
J Ind Eng Int
123
non-conforming parts. A continuous, multi-level approach
to design tolerancing of electro-mechanical assemblies was
outlined, wherein the assembly models for tolerancing, best
practices for tolerancing, and the design process are inte-
grated (Narahari et al. 1999). Manufacturing tolerances
were allocated from forward dimensional chains, while the
reverse dimensional chains were used to determine the
nominal dimensions directly (Ji 1999).
Xue and Ji (2001) proposed a methodology for dealing
with angular features in tolerance charting. Ji and Xue
(2002) obtained the mean working dimensions from the
reverse chain matrix containing reverse tolerance chains.
Huang et al. (2005) devised a procedure for determining
the process tolerances directly from multiple correlated
critical tolerances in an assembly. Process-oriented toler-
ancing was focused upon, by considering all the variations
arising due to tool wear, measurement device fluctuations,
tolerance stack-up propagation (Ding et al. 2005). A pro-
cess optimization model was introduced which considers
process means and process tolerances simultaneously, with
sequential operation adjustment to reduce process vari-
ability, and with part compensation to offset process
shifting (Jeang et al. 2007). Peng et al. (2008) derived
quality loss function of interrelated critical-to-quality
dimensions. Through this quality loss function, the design-
tolerances of the component are determined for achieving
an improved product as well as process quality. The tol-
erance chart balancing was mathematically modeled for
minimizing the manufacturing cost and quality loss (Jeang
2011). Concurrent tolerancing was identified as an opti-
mization problem and a feasible solution for systematically
distributing the process tolerances within the design
constraints was proposed (Sivakumar et al. 2012). Contr-
eras (2013) proposed simplification of tolerance chains
through a surface position tolerance (SPT) method for
tolerance chart balancing. Chen et al. (2013) optimized the
process parameters for the plastic injection molding. An
improvement in the process potential capability index (C
p
)
and process performance capability index (C
pk
) was reg-
istered through process capability improvement studies on
thrust face thickness characteristic of connecting rod
(Sharma and Rao 2013).
Recent works on tolerancing include tolerance analysis
simulation during the initial design phase by a computer-
aided tolerancing software (Barbero et al. 2015). The
design tolerances estimated through this simulation sub-
sequently determine the manufacturing tolerances. In
another approach, complex workpiece with intricate shapes
are classified based on its overall discrete geometry and
tolerance analysis is performed on this overall part geom-
etry (Schleich and Wartzack 2014). This simplifies the
tolerance analysis for non-ideal complex workpiece shapes.
Louhichi et al. (2015) performed realistic part tolerancing
taking CAD part geometrical discrepancies into consider-
ation. They identified the future research work as tolerance
allocation by taking the manufacturing variations into
consideration, which is also addressed in this paper. Con-
sidering this literature review, it can be summarized that
the works on process tolerancing concentrated on the
aspects of tolerance synthesis through tolerance chain and
tolerance charting. In the pursuit for striking the balance
between the conflicting issues of quality and cost, part
tolerancing is optimized keeping the manufacturing pro-
cess into consideration.
Fig. 5 Process flow chart of connecting rod manufacturing cell. Refer to Table 1for corresponding description of connecting rod machining
operations and their dimensional values
J Ind Eng Int
123
The process capability studies on thrust face thickness,
bolt hole center distance and crank pin bore diameter
critical-to-quality characteristics of connecting rod were
performed. After making the process capable through
DMAIC approach, the end results of these process capa-
bility studies in the form of process capability values and
tolerances obtained from tolerance charting of connecting
rod machining process are compared with a tolerance
capability expert software (Tec-ease.com 2014) and the
results are documented.
Process sequence
The connecting rod manufacturing process sequence is
depicted in the process flow diagram as shown in Fig. 5.
The raw material from the raw material bin is the starting
point of the connecting rod manufacturing process. The
first roughing operation is operation no. 10 followed by a
sequence of operations. The final operation is operation no.
140 where final quality check, set making and dispatch to
engine assembly line are carried out. Table 1gives the
corresponding description of connecting rod machining
operations.
Tolerance stack analysis of the connecting rod
machining
Before proceeding to the process capability-based toler-
ancing study it is necessary to thoroughly examine the
tolerance stack-up of the various machining processes
involved in the manufacture of connecting rod. Figure 6
shows graphical representation of the tolerance chain
associated with the machining of connecting rod.
The tolerance chain in the tolerance chart depicts the
sequel of machining operations and their working dimen-
sions. The tolerance stack-up and selection of reference
surfaces for the subsequent machining operations is infer-
red from the diagram. Subsequently, the tolerances over the
dimensions and the stock removal on the machining
operation are also derived.
Process capability tolerancing of connecting rod
Based on the process capability improvement studies, the
identified critical-to-quality characteristics in the machin-
ing of connecting rod and their initial and improved pro-
cess capability values are tabulated in Table 2.
Comparison using tolerance capability expert
software
The dimensional tolerances and C
pk
values of the quality
characteristic from Table 2are the inputs into the database
of the tolerance capability expert (TCE) software. In the
TCE software, the worst case of manufacturing is consid-
ered, i.e., manufacturing machinery is not modern and not
in good condition.
The following are the assumptions considered while
using tolerance capability expert software.
1. The component or tooling used has repetitive features
over a multiple references.
Table 1 Description of connecting rod machining operations
Machining operation
number
Machining operation description Dimensional value of machining
characteristic (in mm)
10 Thrust face width rough grinding 27.250 ±0.250
20 Gudgeon pin diameter rough boring [25.000 ±0.200
30 Crank pin diameter rough boring [80.000 ±0.200
40 Side face width broaching 128.300 ±0.500
50 Thrust face width finish grinding on separate rod- and cap-end parts 26.800 ±0.200
60 Bolt hole diameter drilling and reaming [6.000 ±0.200
70 Key way slot milling
80 Assembly of rod- and cap-end parts
90 Thrust face width finish grinding of rod and cap connecting rod assembly 26.500 ±0.050
100 Finish boring of gudgeon pin bore diameter [30.000 ±0.200
110 Finish boring of crank pin bore diameter [84.090 ±0.050
120 Crank pin bore diameter honing [85.077 ±0.015
130 Magnetic crack detection
140 Final quality check, set making and dispatch to engine assembly line
J Ind Eng Int
123
2. The characteristic is not along the die/mould parting
line.
3. For producing this tolerance, simultaneous grinding of
two parallel planes are involved.
4. The manufacturing machinery is not modern and in
good condition.
5. The component size, weight, geometry and material
impose additional limitations to the machine
capability.
6. The feature geometry does not enable the process to be
operated under good conditions of practice.
7. The process involves additional setups (for producing
diesel as well as petrol variants of connecting rod).
Fig. 6 Tolerance chain diagram of connecting rod in AutoCAD version 2005 software
Table 2 Process capability values of critical-to-quality characteristics of connecting rod machining process
S. no. Quality characteristic Dimension Initial value Final value
rC
p
C
pk
rC
P
C
pk
1 Thrust face thickness after thrust face width rough grinding 27.250 ±0.250 0.48 0.12 0.12 0.048 1.72 1.37
2 Bolt hole center distance after bolt hole diameter drilling and reaming 106.750 ±0.100 0.017 0.97 0.57 0.009 1.77 1.49
3 Gudgeon pin bore diameter after finish boring operation 30.000 ±0.200 0.004 1.28 0.33 0.002 2.03 1.45
4 Crank pin bore diameter after honing operation 85.077 ±0.015 0.005 0.5 0.34 0.002 1.52 1.45
Table 3 Graphical output from tolerance capability expert software
Characteristic Graphical plot 1 with predicted C
pk
for
predetermined tolerance
Graphical plot 2 with predicted
tolerance for predetermined C
pk
Thrust face thickness after thrust face width rough grinding
with process dimension as 27.250 mm
See Fig. 7See Fig. 8
Gudgeon pin bore diameter after finish boring operation with
process dimension as 30.000 mm
See Fig. 9See Fig. 10
Crank pin bore diameter after honing operation with process
dimension as 85.08 mm
See Fig. 11 See Fig. 12
J Ind Eng Int
123
Fig. 7 Predicted C
pk
for predetermined tolerance of ±0.250 mm for thrust face width rough grinding dimension of 27.250 mm
Fig. 8 Predicted tolerance for predetermined C
pk
of 1.4 ([1.33) for thrust face width rough grinding dimension of 27.250 mm
J Ind Eng Int
123
Fig. 9 Predicted C
pk
for predetermined tolerance of ±0.200 mm for gudgeon pin bore diameter after finish boring dimension of 30.000 mm
Fig. 10 Predicted tolerance for predetermined C
pk
of 1.4 ([1.33) for gudgeon pin bore diameter after finish boring dimension of 30.000 mm
J Ind Eng Int
123
Fig. 11 Predicted C
pk
for predetermined tolerance of ±0.015 mm for crank pin bore diameter after honing operation dimension of 85.08 mm
Fig. 12 Predicted tolerance for predetermined C
pk
of 1.4 ([1.33) for crank pin bore diameter after honing operation dimension of 85.08 mm
J Ind Eng Int
123
Since the TCE software considers worst case of manu-
facturing, hence the results obtained are reliable with a
certain factor of safety tolerance being inherent. The output
obtained from the TCE software is tabulated in graphical
form in Table 3.
Results and discussion
Table 3depicts the various graphical outputs from the
tolerance capability expert software. The dimension of the
thrust face thickness after thrust face width rough grinding
is 27.250 mm. Figure 7shows that for thrust face thickness
with a target tolerance of ±0.250 mm, the predicted C
pk
lies in the region above 4.00. On the other hand, Fig. 8
shows that for a target C
pk
of 1.4 ([1.33), the tolerance
predicted is 0.027 mm, i.e., about ten times less than that in
Fig. 7. The next critical-to-quality characteristic under
consideration is the gudgeon pin bore diameter after finish
boring operation as 30.000 mm. Figure 9gives that for a
target tolerance of ±0.200 mm, the predicted C
pk
is 3.89;
whereas, Fig. 10 gives that for a target C
pk
of 1.45 ([1.33),
the tolerance is ±0.054 mm for gudgeon pin bore diame-
ter, leaving a large scope for improvement in this quality
characteristic. The third critical-to-quality characteristic is
the crank pin bore diameter after honing operation with
process dimension as 85.08 mm concerning Figs. 11 and
12. In Fig. 11 it can be seen that for a target tolerance
of ±0.015 mm, the predicted C
pk
is 1.5 and from Fig. 12 it
can be deciphered that for a target C
pk
of 2.0, the predicted
tolerance is ±0.020 mm. Figures 11 and 12 show close
resemblance to each other and it can be deduced that the
values of tolerances and C
pk
obtained from tolerance sheet
and process capability studies are in-phase with the values
obtained from the tolerance capability expert software for
the crank pin bore diameter after the honing operation.
Conclusion
Optimal values of the dimensional tolerance bandwidth are
determined from the statistical process control charts. The
process is made capable with the capability indices more
than 1.33, i.e., more than a moderate level of 4r, which is
the industrial benchmark. After having made the process
capable, the upper and lower tolerance bounds are shrunk
to the calculated control limits inherent in the process.
With the newly obtained tolerance values, the process is
again calculated for its capability to be more than 4rlevel.
This iterative procedure of process improvement is carried
out till the convergence is reached and no further notice-
able process improvement is seen. Thus, the dimensional
tolerances are optimized in accordance with the statistical
process control improvement studies.
This paper witnesses an application of process toler-
ancing which proves to be a better way of finding the
optimal tolerancing of the part, leading to fewer process
rejections and improved quality levels. The end results of
the process capability values and tolerances obtained from
tolerance charting of connecting rod machining process are
compared with a tolerance capability expert software. The
results showed further scope of improvement for the thrust
face thickness and gudgeon pin bore diameter, whereas the
crank pin bore diameter after honing operation showed
close resemblance between the values obtained through
process capability and tolerance capability expert software.
Open Access This article is distributed under the terms of the
Creative Commons Attribution 4.0 International License (http://crea
tivecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided you give
appropriate credit to the original author(s) and the source, provide a
link to the Creative Commons license, and indicate if changes were
made.
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