ISO 26303:2012

INTERNATIONAL ISO
STANDARD 26303
First edition
2012-06-01

Machine tools — Short-term capability
evaluation of machining processes on
metal-cutting machine tools
Machines-outils — Évaluation de la capacité des procédés d'usinage
des machines travaillant par enlèvement de métal




Reference number
ISO 26303:2012(E)
©
ISO 2012

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ISO 26303:2012(E)

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ISO 26303:2012(E)
Contents Page
Foreword . iv
Introduction . v
1  Scope . 1
2  Normative references . 1
3  Terms and definitions . 2
4  Symbols . 3
4.1  Upper case letters . 3
4.2  Lower case letters . 4
4.3  Greek letters . 5
5  Preliminary remarks . 5
6  Procedure for short-term capability evaluation . 6
6.1  General . 6
6.2  Agreements . 7
6.3  Warm-up procedure . 9
6.4  Adjustment . 9
6.5  Production . 10
6.6  Measurement . 10
6.7  Computation and analysis . 11
7  Factors influencing short-term capability evaluation . 16
7.1  General . 16
7.2  Thermal influences . 16
7.3  Influences due to measuring uncertainty . 18
7.4  Influences arising from statistical analysis . 18
Annex A (informative) Additional information related to statistical evaluations . 19
Annex B (normative) Agreement form . 26
Annex C (normative) Evaluation — Forms . 30
Annex D (informative) Examples of capability agreements and analysis . 34
Bibliography . 42

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ISO 26303:2012(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 26303 was prepared by Technical Committee ISO/TC 39, Machine tools, Subcommittee SC 2, Test
conditions for metal cutting machine tools.
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ISO 26303:2012(E)
Introduction
The evaluation of the short-term capability of the machining process is a different approach in machine tool
assessment compared with machine tool performance testing methods, which are covered by a number of
International Standards, e.g. ISO 230 (all parts) and other machine tool type specific standards. The main
differences are machining a sample batch of test pieces and definition of the relevant influencing factors as
well as the statistical conditioning and analysis of the workpiece quality related data obtained during such
tests.
This International Standard is the result of a project guided closely by an international working group, and
summarized in order to make the information available to as many interested parties as possible.
Especially for large batch production, short-term process capability estimates, as well as capacity measures,
are very often applied in addition to testing of machine tool performances. In fact, machine tool users
increasingly employ statistical process control (SPC) techniques in their activities and frequently ask the
machine suppliers/manufacturers to become system suppliers as well, giving them responsibilities for the
machining process too.
Statistical methods in process management are covered by ISO 22514 (all parts).
For the purposes of machine tool acceptance based on the test of its capability in machining a specified
workpiece, both requirements and methods stated by individual users differ widely, due to the absence of a
recognised International Standard. Long-winded discussions and adaptation processes during the acceptance
tests are, therefore, often necessary, delaying delivery to the customer and causing great time- and cost-
related expenditure. This International Standard provides a unified procedure for the acceptance test of a
machine tool based on its short-term process capability. It introduces
 the short-term capability of a given process, which employs the machine under test, the machining
process, tooling and clamping applied, as well as the workpiece properties, and
 the estimate of relevant machine capability indexes.
This International Standard adapts to and complies with the specifications established in ISO 22514 (all parts).
However, the term “process performance” specified in ISO 22514-3 corresponds to the term “short-term
capability” in this International Standard. The term “short-term capability” has been widely used in the machine
tool industry for many years; therefore, ISO/TC 39/SC 2 decided to maintain this term.
Combined with the statistical evaluation, many influencing factors significantly restrict the fraction of tolerance
interval covered by machine tool variations. As a consequence, the machine capability indices are specified in
conjunction with the test conditions and the required tolerance limits.
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INTERNATIONAL STANDARD ISO 26303:2012(E)

Machine tools — Short-term capability evaluation of machining
processes on metal-cutting machine tools
1 Scope
This International Standard defines procedures for acceptance of metal-cutting machine tools based on the
tests of their capability in machining a specified workpiece (i.e. indirect testing). It gives recommendations for
test conditions, applicable measurement systems and the requirements for machine tools.
This International Standard is consistent with ISO 22514 (all parts) describing statistical methods for process
management and deals with the specific application of those methods to machine tools and machining of a
sample batch of test pieces. This International Standard covers neither functional tests, which are generally
carried out before testing the accuracy performance, nor the testing of the safety conditions of the machine.
Annex A gives additional information related to statistical evaluation, (normative) Annexes B and C provide
agreement and evaluations forms for short-term capability tests, while Annex D gives an example.
NOTE 1 Direct testing aims to investigate individual machine properties, such as geometric or positioning accuracy.
Short-term capability evaluation is meant to prove that a machine has the capability to fulfil a specific process task. It is,
therefore, important to recognise that the short-term capability test is focused only on the manufactured product. This
means that direct testing methods are more suited for the determination of error sources on the machine tool and for
deriving constructive improvements of a machine tool that is used in a wide production spectrum; a short-term capability
test is less suited for detection of error sources of the machine tool. Therefore, it is expected that short-term capability
evaluation for the acceptance of metal-cutting machine tools in machining processes be primarily carried out on
workpiece-dependent special-purpose machines, e.g. working stations of transfer lines, with a process-determined cycle
time of less than 10 min, so that at least 50 workpieces are manufactured in one shift as the statistical uncertainty
increases strongly for a smaller number. In principle, short-term capability evaluation can also be performed on universal
machines, such as machining centres used for large batch production if they meet the above-mentioned statistical
requirements.
NOTE 2 The term “short-term capability”, which is a widely used term in machine tool industry, corresponds to the term
“process performance” specified in ISO 22514-3.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 4288, Geometrical Product Specifications (GPS) — Surface texture: Profile method — Rules and
procedures for the assessment of surface texture
ISO 22514-3:2008, Statistical methods in process management — Capability and performance — Part 3:
Machine performance studies for measured data on discrete parts
ISO/TR 22514-4:2007, Statistical methods in process management — Capability and performance — Part 4:
Process capability estimates and performance measures
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ISO 26303:2012(E)
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
short-term capability
ability of a manufacturing unit to produce a given part within specified tolerances at a specified confidence
level, a concept mainly applied to batch production
NOTE 1 A manufacturing unit may be a single machine tool, one spindle of a multi-spindle machine tool, one station of
a transfer line, etc.
NOTE 2 Process capability is defined in ISO/TR 22514-4:2007, 2.2.1, as: statistical estimate of the outcome of a
characteristic from a process which has been demonstrated to be in a state of statistical control and which describes that
process’s ability to realize a characteristic that will fulfil the requirements for that characteristic.
NOTE 3 In this International Standard, short-term capability indices, C and C , are estimated under the assumption of
s sk
normal distribution of the characteristic value considered. If this assumption is not fulfilled, short-term range values, R
V,s
and R , are evaluated instead of capability indices.
V,sk
NOTE 4 This International Standard adapts to and complies with the specifications established in ISO 22514 (all parts).
However, the term “process performance” specified in ISO 22514-3 corresponds to the term “short-term capability” in this
International Standard. The term “short-term capability” is widely used in the machine tool industry; therefore,
ISO/TC 39/SC 2 decided to maintain this term.
3.2
short-term capability index
C
s
ratio of the specified tolerance itself to the standard deviation of the measured values quantifying the scatter
NOTE Measured values are also known as characteristic values.
3.3
critical short-term capability index
C
sk
ratio of the specified tolerance itself to the standard deviation of the measured values quantifying the scatter
under consideration of the location of the mean value
NOTE 1 If the mean value of the measured values is in the centre of the tolerance zone, this is called a centred
distribution; if the mean value is not in the centre of the tolerance zone, this is called a shifted distribution. For the
relationship between centred and shifted distributions, see A.1.
NOTE 2 Measured values are also known as characteristic values.
3.4
short-term range value
R
V,s
ratio of the range of the measured values to the specified tolerance itself
3.5
critical short-term range value
R
V,sk
ratio of the range of the measured values to the specified tolerance itself under consideration of the location of
the mean value
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ISO 26303:2012(E)
3.6
control chart
chart, with upper and lower control limits, on which values of some statistical measure for a series of samples,
spiked samples and blanks, are plotted, usually in date or sample number order
[ISO 5667-14:1998, 3.10]
3.7
control chart
control chart for individuals
variables control chart for evaluating the process level in terms of the individual observations in the sample
[ISO 3534-2:2006, 2.3.15]
3.8
control limit
line on a control chart used for judging the stability of a process
[ISO 3534-2:2006, 2.4.2]
3.9
lower specification limit
specification limit that defines the lowest value a quality characteristic may have and still be considered
conforming
[ISO 22514-1:2009, 2.1.13]
3.10
upper specification limit
specification limit that defines the highest value a quality characteristic may have and still be considered
conforming
[ISO 22514-1:2009, 2.1.12]
4 Symbols
4.1 Upper case letters
C Capability index
C Critical capability index
k
C Short-term capability index (corresponds to machine performance index P in ISO 22514-3:2008);
s m
C nominal short-term capability index
s,nom
C Critical short-term capability index
sk
C Nominal critical short-term capability index
sk,nom
C Actual capability index
act
th
K i class (histogram)
i
U Uncertainty (of measurement or capability index)
U Upper control limit for the standard deviation s
CL,si i
U Upper control limit for the average values x
CL,xj j
U Upper specification limit
SL
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ISO 26303:2012(E)
R Range
R Short-term range value
V,s
R Nominal short-term range value
V,s,nom
R Short-term critical range value
V,sk
R Nominal short-term critical range value
V,sk,nom
T Tolerance
T Minimum usable tolerance for capability evaluation
min
L Lower control limit for the standard deviation s
j
CL,sj
L
Lower control limit for the average values x
CL,xj j
L Lower specification limit
SL
4.2 Lower case letters
e Shift of the average value
f Feed speed
i Running index for measurements
j Running index for groups of measurements
k Running index for measurements within one group
m Number of groups of parts for control charts
n Number of evaluated parts
n Number of manufactured parts
mp
n Number of classes (histogram)
K
n Minimum value of necessary parts
min
r Resolution of the measuring device
s Estimator of the standard deviation
s Average standard deviation of the samples (groups)
s ' Sample standard deviation of shifted distribution
s Standard deviation of the measurement (gauging) system
g
s Actual standard deviation of the measurement system
g,act
s Standard deviation of the jth sample (group)
j
t Manufacturing time
m
t Total manufacturing time
tot
x Mean value of population (of 50 measurements)
x ' Mean value of population with shifted distribution
x Mean value of group means x
j
x ith measurement value
i
x ith measurement value (trend corrected)
i,T
x Upper class limit of the kth class (histogram)
u,k
x Mean of the jth sample (group)
j
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ISO 26303:2012(E)
x Maximum value
max
x Minimum value
min
4.3 Greek letters
 X Total trend (in relation to all values)
tot,T
 X Total trend per workpiece
tot,w
 X Trend due to thermal drifting
td
 X Trend due to thermal drifting per workpiece
td,w
 X Permissible trend due to thermal drift per workpiece
td,perm
 X Trend due to tool wear
a
 X Expected trend due to tool wear
a,exp
d Distance between the maximum value and the upper tolerance limit
u
d Distance between the minimum value and the lower tolerance limit
l
d Critical distance between the extreme values and the tolerance limits
c
X Class width (histogram)
k
X Border line of class (histogram)
K,k
X Critical distance of the average value to the tolerance limits
c
X Distance between the average value and the upper tolerance limit
u
X Distance between the average value and the lower tolerance limit
l
 Ambient temperature gradient
amb
 Maximum ambient temperature gradient
amb,max
 Mean value for population
P
 Temperature
 Ambient temperature at beginning of test
amb,0
 Maximum temperature
max
 Minimum temperature
min
 Standard deviation of the population

 Estimation of the standard deviation of the population
 Thermal time constant
 Shift ratio for shifted distribution
5 Preliminary remarks
Short-term capability evaluation belongs to the class of indirect testing methods and, hence, is a different
approach to machine acceptance testing in comparison to the direct testing defined in several series of
International Standards, e.g. ISO 230.
The measured feature shall be machined on one machining unit only. If the same feature is machined on
different, but similar, machining units, the statistical analysis shall be carried out separately for each
machining unit.
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ISO 26303:2012(E)
6 Procedure for short-term capability evaluation
6.1 General
The basic procedure during a short-term capability evaluation is shown in Figure 1. Acceptance according to a
short-term capability evaluation is only recommended for machine tools used in large batch production with a
process cycle time of less than 10 min. In addition, the adequate short-term capability (see 6.6) of the
measuring process is a necessary requirement for measuring the workpieces.
NOTE In some cases, preparatory studies are performed in order to demonstrate that the operator can successfully
interact with the machining process and that the subsequent process capability study will be successful [31].
Before initiating the test and evaluation process, the supplier/manufacturer and the user shall reach necessary
agreements concerning the test plan, including the workpiece features to be measured and analysed, the
procedure, the test conditions and the characteristic values. Hereinafter, all agreements which are referred to
are between the supplier/manufacturer and the user. The evaluations process is started by warming-up the
machine. The subsequent adjustment is for the setting of the manufacturing process to the required
tolerances (e.g. the middle of the tolerance zone in the case of characteristic with two-sided tolerances or zero
for a zero-limited characteristic). The 50 workpieces are then manufactured in series and measured with a
suitable measuring device. The measurements attained are then statistically evaluated in the final step.
If the short-term capability indices or the range values and, if applicable, thermal drift are beyond specified
tolerances, the reasons shall be investigated. These can be, for example, faults which can be recognised as
outlier values in the control chart for individuals (see 6.7.3). If improvements are possible, these shall be
carried out and the tests shall be repeated in part or whole.
Agreements
Warming-up

Adjustment
Test plan
Warmed up machine tool
 
Adjustment of process to
or separate evaluation of
Procedure
targeted value
thermal drift
Test conditions
Characteristic values
Measurement
Analysis
Production
Requirement:
measurement system with

suitable resolution and Usually 50 workpieces are
Statistical evaluation of
 
uncertainty for produced successively
measurement results
measurement of short-term
capability

This is recommended only for large batch production machine with cycle time  10 min.
Figure 1 — Basic procedure for short-term capability evaluation
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ISO 26303:2012(E)
6.2 Agreements
Before the actual acceptance test is carried out, agreements between the manufacturer/supplier and the user
are necessary in order to ensure that:
a) the machine and the applied machining process are evaluated with as few interfering influences as
possible,
b) requirements, which cannot be fulfilled due to the various influencing factors and the narrowing of the
tolerance caused by the statistical analysis, are not set,
c) contractual agreements between the manufacturer/supplier and the user can be formulated, defining the
scope, procedure and evaluation factors for the acceptance, and
d) tolerances that are subject to a short-term capability evaluation are identified considering the associated
costs.
The relevant agreements are listed in the forms provided in Annex B; Annex D provides an example. The test
conditions under which the machine is evaluated shall be negotiated between the manufacturer/supplier and
the user. These include, among others, the ambient temperature and its allowable variation during the test
period. The limits depend on the manufacturing task, as does the location where the machine is installed in
the machine shop or in an air-conditioned room. The following limits shall be used as default values for normal
manufacturing tasks: ambient temperature, i.e. temperature change within 3 °C during time of test;
temperature gradient, i.e. within a maximum of 2 °C/h or 2 °C/h.
Since the aim of the acceptance test is to prove the short-term capability and not the long-term capability,
which is influenced by additional factors, a defined and uniform quality of the oversized blanks shall be
ensured. The composition and characteristics of the material shall not be influenced by a change of batch. An
oversize tolerance shall be agreed upon by the manufacturer/supplier and the user in order to limit the
differences in static deformation due to back forces (component of the total cutting force perpendicular to the
working plane) for varying oversizes.
Machining of blanks can have a direct influence (e.g. differences in machined dimension) and an indirect
influence (e.g. differences in flatness of machined clamping faces) on the scattering of the measured features
resulting from the process. Therefore, tolerances for machining of blanks shall be compatible with the required
process short-term capability. In addition, it can be necessary to further limit the tolerances of the blanks
depending on the machining process and sequence.
Fifty workpieces shall be manufactured in series. The total manufacturing time shall not exceed 8 h, resulting
in a permissible manufacturing time of 10 min per workpiece. In special circumstances of longer
manufacturing times per workpiece, a lower number of workpieces may be agreed upon by the
manufacturer/supplier and the user; but in any case, the number of workpieces shall not be less than 30. If
workpieces with small cycle times are being manufactured, a total manufacturing period of 6 h to 8 h and the
production of more than 50 workpieces with the taking of samples from the larger set, which results in a total
of 50 measurements (sample size multiplied by number of samples) may be negotiated.
Furthermore, the manufacturing technology and an adequate warm-up procedure shall be agreed upon by the
manufacturer/supplier and the user before starting the acceptance test, in order to ensure that the machine is
in thermal equilibrium (see 6.3 and 7.2).
The resolution and measuring uncertainty of the measuring device shall be taken into account. The short-term
capability of the measuring device shall be verified. Generally, one needs a measurement equipment
investigation, including the influence of the operator, at the time of evaluating the short-term capability
(see 6.6).
As an alternative to the short-term capability indices, C or C , the evaluation of the range values, R or
s sk V,s
R , may be agreed upon by the manufacturer/supplier and the user. Additional information on the
V,sk
relationship between standard deviation and range values is given in A.2. The range values only take account
of the greatest and least values and are very susceptible to outliers in the set. Therefore, they do not provide
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ISO 26303:2012(E)
enough information about the process behaviour within the extreme values. Consequently, if range values are
used, the evaluation of the process using the control chart for individuals, the control chart and a histogram is
of special importance (see 6.7).
NOTE The definition of the short-term capability indices or range values is of great economic importance. On the one
hand, the conformance to stringent requirements can guarantee reliable production. On the other hand, this does not
necessarily mean that the manufacturing costs can be reduced. Generally, much higher expenditure is needed for
achieving greater short-term capability indices or lesser range values. Such costs result from supplementing or equipping
the machine with additional components (e.g. direct measuring systems, probing devices) and additional control circuits
(e.g. measurement control, thermal compensation) or changing to a more expensive manufacturing method (e.g. from
turning to grinding).
The required values shall be specified with considerations of the technical possibility and economical
feasibility. In this sense, it is not suitable to set uniform boundaries for all processes. The direct relationship
between the short-term capability indices and the required tolerances shall be taken into special consideration.
As proof of short-term capability naturally guarantees a statistical confidence regarding the manufacturing
process, current tolerances set by the designer for safety reasons should be re-thought. According to current
short-term capability indices, the thresholds given in Table 1 are recommended for e
...