4.4 Measurement strategy for a production-related multi-scale
inspection of formed work pieces
A. Loderer, B. Galovskyi, W. Hartmann, T. Hausotte1
1 Institute of Manufacturing Metrology, Friedrich-Alexander-Universität Erlangen-Nürnberg, Germany
Abstract
The technology of sheet-bulk metal forming provides numerous advantages in the field of manufacturing.
Work pieces with filigree and complex structures can be formed by only a few forming steps. To ensure a
sustainable and effective production, the forming process has to be controlled by a production-related
measurement system. A measurement system, which meets the high requirements of a forming process like
a short measuring time, a high measuring point density and the ability to measure different features at the
same time, is a multi-scale fringe projection system with multiple sensors of different resolutions. However, an
adapted definition of a measurement strategy is necessary in order to enable a rapid conformity decision of
the manufactured work piece based on the evaluated measurement data and thus to be able to inspect as
many work pieces as possible. It allows to correct the manufacturing during a primary forming process and to
assure a sustainable forming process.
Keywords:
Fringe projection; multi-scale measurement; sheet-bulk metal forming
1 INTRODUCTION
In times of increasing raw material costs, saving resources
provides not only economical and sustainable advantages but
also monetary benefits. Especially by improving production
technologies, the reduction of the consumption of raw
materials is possible. The aim is on the one hand, to
generate the product with as less material and energy as
possible. On the other hand, the product itself should be as
light but also stabile as possible. In many cases, these
demands could only be partially met by the current production
processes. Indeed, they are able to deliver lightweight
structures and products, but only at high cost. Therefore, this
state of the art motivated the search for a new high
performance forming method. The development of the sheet-
bulk metal forming enables the production of high quality
sheet metal components with highly loaded functional
elements [1].
With the sheet-bulk metal forming technology the production
of complex work pieces in only a few forming steps is
possible with a minimum of required material. The increasing
number of integrated function elements and the reduction of
required material and process steps, which is attended by the
reduction of energy consumption, is possible by an three
dimensional material flow. Due to this new method filigree
structures as well as geometric unequal features can be
produced in only one step [1]. In order to research the new
technology and their possibilities, the transregional
collaborative research center (Transregio) 73 as a joint
research initiative was found [2].
But a sustainable production is not only characterized by a
reduction of material requisition and energy consumption [3].
At the same time, the reduction of wastage is important task.
This requires a control of the produced work pieces [4]. By
evaluating the work piece’s quality, for example the
geometrical dimensions, control variables can be derived and
thereby the process could be regulated. The high output of
work pieces and their complex structure generate high
requirements on the measuring technology. Because of the
fact that all surface features are formed in only one step, all
of the features have to be controlled nearly at the same time.
That means a holistic inspection has to be arranged in a way
that features of all sizes are measured by measuring systems
which accuracies are precise enough and measurement
areas are big enough [5]. These high demands could be meet
best with the optical measurement technology. This
measurement method is contactless and enables complete
detection of the work piece’s surface in a very short time [6].
Besides laser scanning systems, fringe projection systems
are already in use for production-related or even in-line
measurements [7]. Due to the different scale of the work
piece feature of sheet-bulk metal forming parts, a
combination of systems with different resolutions is needed.
To guarantee the appropriate accuracy for each work piece
feature multiple fringe projection sensors with different
measurement areas and accuracies has to be arranged.
Thus a multi-scale measurement is possible [5].
In order to guarantee a high level of sustainability, the
wastage has to be as low as possible. Therefore the rate of
measured and controlled work pieces has to be as high as
possible. The more work pieces are controlled, the faster
deviations from the ideal are detected, and the faster
variables for the process regulation could be derived. For
such measurement not only the measuring systems and
sensors have to be optimized and adapted to the sheet-bulk
metal forming parts, also the measurement strategy has to be
specified. Because of the complex surface and the high
output-rate, measurement strategies, which are used in
current in-line measuring systems, are not appropriate. The
surface structures are in most cases not as complex as on
G. Seliger (Ed.), Proceedings of the 11th Global Conference on Sustainable Manufacturing - Innovative Solutions
ISBN 978-3-7983-2609-5 © Universitätsverlag der TU Berlin 2013
143
A. Loderer, B. Galovskyi, W. Hartmann, T. Hausotte
sheet-bulk metal forming parts. Also the output-rate is
incommensurable.
In this article an approach for the definition of a measurement
strategy for a production-related multi-scale inspection of
sheet-bulk metal forming parts is given.
2 CHARACTERISTICS OF SHEET-BULK METAL
FORMING
2.1 Demonstrator work piece
In order to demonstrate the performance of the sheet-bulk
metal forming a demonstrator work piece was worked out,
which is shown in figure 1. For the development of the
demonstrator work piece the hoped benefits of the new
technology were analyzed. Leading work piece functions are
integrated in a smaller number of single components due to
the requirement on light weight construction. But also the
requirements on the geometry and its accuracy are
increasing. Thus technologies like bending or cutting cannot
be used for the manufacturing of this new generation of work
pieces. Therefore, function features like local tooth systems,
carriers and butt straps have to be considered [1].
Figure 1: Demonstrator work piece
The tooth system symbolizes a very filigree feature. With a tip
circle diameter of merely 0.4 mm, it is the smallest feature of
the demonstrator work piece.
Material accumulation should be represented by a closed
carrier. Thus the challenge is to transport the required
material for a complete molding in the forming tool.
A second carrier in an open design simulates a depth-jump.
Here the challenge in the production is to create a constant
wall thickness and in addition the small internal diameter,
which are in the size of 1 mm.
With a butt strap a possibility is given to check the capability
of forming a comparatively long and thin feature. Challenging
in this case is keeping the condition of the flatness.
Furthermore the diameters of 2 mm at the end of the butt
strap have to be molded correct. To guarantee this, a
targeted material flow has to set up.
Each of the feature elements can be found three times in the
demonstrator work piece.
2.2 Frequently manufacturing defects
Main characteristics of the sheet-bulk metal forming are
three-dimensional material flow and material hardening. If
these processes are not working proper, it can lead to
characteristic defects. The complex geometry of the part
features of the demonstrator work piece are designed in order
to discover characteristic defects in case of an incorrect
working forming process. Figure 2 shows some of the most
frequently manufacturing defects.
Figure 2: Manufacturing defects
Essential for a correct contour molding is a correct mold filling
of the mold cavity. The material flow has to be set up in such
a way, that material is dispersed ideal in the mold cavity and
filled this completely [5]. Due to the complex geometry, the
material flow has to be adapted to the particular volume of
the work piece feature. To give an example, the closed carrier
needs more material for a correct mold filling than the tooth
system [8].
In contrast, there can be also significant surface deviations.
As a consequence of too much available material, there
could be variability of the sheet thickness. Equally, material
hardening can differ locally. Both cases lead to a deviation of
the flatness due to inserted stresses.
The frequently deviations of the ideal surface are essential for
the development of a measurement strategy, adapted to the
properties of the sheet-bulk metal forming. These deviations
have to be detected and evaluated. In order to demonstrate
the development of such a measurement strategy, an
example of a diameter of 2 mm of a butt strap is used.
Besides a simple geometry, a likewise simple defect
characterizes this work piece feature as a proper example. In
case of an incorrect working forming process, the diameter
gets smaller or the roundness deviates clearly.
imperfect mold filling
deviation from the flatness
wall thickness-jump
grooves
R=2mm
tooth system
butt strap
carrier (open)
carrier (closed)
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Measurement strategy for a production-related multi-scale inspection of formed work pieces
3 SELECTION OF AN APPROPRIATE MEASURING
SYSTEM
3.1 Economic and sustainable importance of
measuring systems
The selection of an appropriate measuring system is not only
a question of technical requirements. Also economical and
sustainable aspects have to be considered. Measurement
results are information that is used as a basis for decisions.
In case of the sheet-bulk metal forming, the result of the work
piece inspection leads to a decision, if a work piece meets
the specifications or not and if the variables of the forming
process have to be changed. The less reliable the
measurement results are, the more wastage is produced.
Wastage of course is one kind of dissipation, which should be
avoided absolutely. Because of this, when selecting a
measurement system as defined by sustainability, the system
should be as accurate and as fast as possible [3]. These
properties lead to a high inspection rate with accurate results,
which lead to a better control of the forming process. And
thereby wastage can be avoided. But these properties require
a higher monetary effort, which in turn reduces the economic
benefit of the sheet-bulk metal forming technology. Therefore,
the best agreement between requirements on sustainably
and economic benefits have to be made [4].
To avoid a complex analyze, which systems could be the
best agreement, the “golden rule of measuring metrology”
can help. Georg Berndt developed in 1968 a rule for the
selection of measurement systems. Therefore, the
measurement uncertainty of the measurement system has to
be known. The “golden rule” says, that the measurement
uncertainty should be less than a fifth, better less than a
tenth, of the tolerance width. If this minimum requirement
could be met, it is assured that the measurement results are
able to detect work pieces accurate enough. All systems that
are conform with the “golden rule” can be consulted for the
further selection of an appropriate measurement system [9].
3.2 Technical approach for selecting a measuring
system
After preselecting the measurement systems, a clear
selection based on a technical approach has to be made. The
best measurement system is characterized by the detection
of the work piece feature as fast as possible and as accurate
as necessary. To test this capability of a measurement
system, whether expert knowledge or a test procedure could
be used. For the development of a production-related
measurement strategy for sheet-bulk metal formed parts only
little knowledge exists [5]. The capability of optical
measurement procedures, especially fringe projection, was
already proofed and is used for work piece inspection [10].
But there are different types of fringe projection systems with
different resolutions and measurement areas. Both
parameters have to be adequate large respectively accurate
to detect a work piece feature holistically. To test this on
measurement systems, the work piece feature is compared
to an appropriate reference, from which the dimensions are
known. If the capability of a measurement system to detect
the reference correctly is proofed by repeat measurements, it
can be concluded, that also the work piece feature is
measured correctly.
This approach should explained by using the complex
example of the tooth system. At first a reference for the
contour of the tooth system has to be found. The micro-
contour standard from the PTB [11] seems to be appropriate
for this. It has very filigree surface structures similar to the
tooth system and also the diameter on the standard is almost
equal to the tip circle diameter. The contour-profile can be
seen in figure 3.
Figure 3: PTB micro-contour standard
The advantage of the micro-contour standard is that the
dimensions are known very well and the contour is a bit more
complex than the original contour. If a measurement system
detects the micro-contour standard adequate accurate and
fast, it can be concluded that also the tooth system is
measured adequately. In the present case of the less
complex diameter of the butt strap, the search for a reference
is comparatively easy. As reference a segment of 90° of a
cylinder a known diameter of 2.005 mm is chosen.
After selecting the reference to test the measurement
systems, a selection of appropriate systems has to be done.
For a fast and accurate inspection of sheet-bulk metal
forming parts, fringe projection systems are already in use.
After comparing parameters like resolution and measurement
area, two different fringe projection systems are selected for
the further investigations. System 1 has a resolution of 17 µm
(lateral) and 1.0 µm (vertical) with a measurement volume of
13 x 8 x 3 mm³. System 2 has a resolution of 5 µm (lateral)
and 0.3 µm (vertical) with a measurement volume of
4.4 x 2.8 x 1 mm³. For the detection of the reference as well
as of the work piece feature, that has a dimension of 2 mm
and, related to the tolerance DIN 2768m, a tolerance width of
plus/minus 0.2 mm, both systems are appropriate. Also
considered for the selection of measurement systems should
be the “golden rule of measuring metrology”. In our case the
tolerance width is 0.4 mm. That leads us to a minimum
measurement uncertainty of 80 µm. Better would be a
measurement uncertainty of 40 µm.
With both measurement systems 40 repeat measurements
on the reference are done. Contrary to a measuring system
analysis, the measurements are not done by different
operators. Instead of this, the measurements are done under
almost production-related conditions. Thereby one operator
sets up the measurement system and the reference once and
starts the measuring process again and again. This
procedure is similar to an automatically work piece
inspection.
For the evaluation of the results, normally in addition to the
measuring values also the measuring cycle time is recorded.
It reaches form the beginning of the measurement until the
end of the evaluation. But both considered fringe projection
systems do not have an automatically evaluation
implemented yet. Because of this, the measuring cycle time
is not recorded and considered for the further investigations.
For a fast and reliable selection of an appropriate
measurement system, simple parameters can be used. The
measurement results of repeat measurements of an ideal
measurement system are not normally distributed, but are
always the same value. Because of this, additional
mathematical limitations have to be observed. It is not
possible to calculate parameters like the standard deviation
42 mm
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A. Loderer, B. Galovskyi, W. Hartmann, T. Hausotte
or the measurement capability index on a simple way. Hence,
for the comparison of the measurement systems only simple
parameters are considered. Firstly, the range of the
measurement results is calculated. This parameter is the
result of the difference between the highest and the lowest
value. The parameter shows how good a measurement
system is able to reproduce a measurement result under
prevailing conditions. Next, the differences between each
measurement result and the true value for the reference are
calculated. From this follows how far a measurement result is
away from the true value. The mean deviation is the result of
the average of all differences. As a further parameter, the
median is calculated also. This shows which result had been
measured most. With these three parameters the more
appropriate measurement system for the measurement task
can be selected. The here described parameters are shown
in figure 4.
4 DEVELOPMENT OF A MEASURMENT STRATEGY
If the parameters range, mean deviation, median and
measuring cycle time are known for each work piece feature
and measurement system, the measurement strategy can be
defined. At first it has to be controlled for each available
measurement system, if the minimum requirements of a work
piece feature are reached. These minimum requirements
have to be defined previously. The range should be at least a
fifth of the tolerance width, less than a fifth would be even
better. A minimum value for the measuring time depends on
the production cycle time and the projected inspection rate.
For a production-related inspection, the measuring time
should be as close to the cycle time as possible. The higher
the inspection-rate, the more work pieces can be controlled
and the faster variables for the regulation of the production
process can be generated. All measurement systems which
reach the minimum requirements are compared directly.
Though the intention is to find an appropriate measurement
system for each work piece feature and thereby avoid using a
system more often than it is available. If a measurement
system has to be used more often than it is available, it would
mean, that the work piece or the system itself has to be
moved or positioned new during the inspection. Thus the
measuring cycle time would increase. If the measuring time
of a measurement system is significantly shorter than the
process cycle time, a new positioning of a measurement
system is possible without coming along with any
disadvantages.
After selecting a measurement system for each work piece
feature, the order of the measurements has to be defined. If
possible, the measurements should be done parallel. But if
more than one system is connected to the same computer,
parallel measurements are not possible. Hence the
measurement with the shortest measuring time should begin.
If during this measurement a defective work piece feature is
detected, the work piece inspection can be stopped and the
calculation of variables for the process regulation can be
started. Especially for inspections, which takes more time
than the production cycle time, it is important, to detect
defects as soon as possible. The more time elapses until a
defect leads to the calculation of new process regulation
variables, the more work pieces with the defect are produced.
To increase the efficiency even more, the first measurement
should not have the shortest measuring time, but also should
inspect the most critical work piece feature. This would
assure that the feature with the highest chance for a defect is
controlled at first. That way saves important time, which can
be used for a precise regulation of the sheet-bulk metal
forming process, in order to reduce wastage. And this leads
to a better sustainability again.
For a simple control of the selection of the measurement
systems, an application based on an Excel-sheet was
designed. The application tests, if a measurement system
reaches the minimum requirements of a work piece feature
and how good the system is in comparison to other
measurement systems. With this support, the measurement
strategy can be controlled.
In case of the example work piece, the selection of the
measurement system is done as follows: The minimum
Figure 4: Measurement results and parameters
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Measurement strategy for a production-related multi-scale inspection of formed work pieces
requirement on the ranges, which should be smaller than a
fifth of the tolerance width, is reached by both measurement
systems. The measuring time is not considered. Due to the
small range, the small mean deviation from the true value
and the median, which is almost the true value, measurement
system 1 is more appropriate for the inspection of the
diameter of the butt strap. For the inspection, there are eight
sensors of the same type available. Because there are only
six diameters on the three butt straps of the demonstrator
work piece, each work piece feature can be measured by one
system. A movement or repositioning of the work piece or the
measurement system is not necessary in this case. A
schematic set-up of the measurement systems can be seen
in figure 5.
Figure 5: Set-up of the measurement systems
5 SUMMARY AND OUTLOOK
In this article the development of a production-related
measurement strategy for the inspection sheet-bulk metal
forming parts, in order of a fast and efficient process
regulation, was described. For this, the characteristics of the
sheet-bulk metal forming were described and their challenges
for a production related inspection were explained. An
approach for comparing and selecting measurement systems
was shown as well as helpful parameters for a fast and
simple decision making. The systematic way for selecting a
measurement system and strategies for a work piece
inspection is expended by the shown approach. Figure 6
shows the approach summarized in a flow chart.
To make the inspection even more efficient and thus
increasing the sustainability of the sheet-bulk metal forming,
in a next step the selection of an appropriate measurement
system will be approximated to a mathematical optimization
problem. This can be described and solved by algorithms.
Thereby, the selection of a measurement system can not
only be checked if the selected system reaches the minimum
requirements, but also select the most appropriate system by
solving the mathematical equations.
Figure 6: Flow chart
And as a further improvement, the defects during the
production could be sorted by their number of appearance.
The more often a defect appears, the better would be to start
with the measurement of the related work piece feature. Thus
it would be possible to detect the most frequently defects at
the beginning of an inspection, which leads to a faster
process control.
In Order to verify the advantages of the new measurement
strategies several case studies have to be worked out and
performed. Therefore sheet-bulk metal formed parts with
different known defects will be inspected by several
measurement system and using different strategies. With the
proof of the effectiveness of the measurement strategies, the
development of the sheet-bulk metal forming towards an
industrial forming technology will be made a sustainably step
forward.
6 ACKNOWLEDGMENTS
The underlying research is gratefully funded within the DFG
SFB/TR73.
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