QMT Features: February 2013
About to buy a CMM?
The way CMMs are specified is about to change. The NPL highlight some of the changes to ISO 10360 to consider when buying a CMM


Are you thinking about buying a new co-ordinate measuring machine (CMM) within the next year? Well, if you are, the way CMMs are specified is about to change. The main parts of the international standard ISO 10360 that relate to traditional tactile-based CMMs changed in 2010 and, over the next year or so, these changes will be seen in specifications for new CMMs. So, what are the main changes you need to look out for when buying a CMM?

What’s new in ISO 10360?
If you are familiar with previous versions of ISO 10360 you will notice that the following changes have been introduced in the latest issues.

• The principle of the assessment method is, as before, to use a calibrated test length, traceable to the metre (Figure 1), to establish whether the CMM is capable of measuring within the stated maximum permissible error of length measurement. However, ISO 10360 now introduces a specified ram axis stylus tip offset (both zero offset and by default 150 mm offset). Previously no offset was specified.
• The calibrated test length may now be a ball bar or laser interferometer system.
• The single stylus probing test, that appeared in ISO 10360-2: 2001, does not appear in the current edition of ISO 10360-2 [1]. It now appears in the new edition of ISO 10360-5 [2].
• Many of the symbols used have changed.

Some of these changes are covered in more detail in the following sections.

Symbols
One of the first things you will notice about the latest revision is that many of the symbols used have changed. For those readers that are familiar with previous methods of specifying CMMs, Table 1 shows the evolution of the symbols over the years.

Table 2 lists the current symbols and some alternatives for use in product documentation. As an example, the length measurement error EL can be written as E0 or E150 depending on the ram axis stylus tip offset. The corresponding maximum permissible error would be E0, MPE and E150, MPE. Note that there is now a requirement to determine the repeatability range of the three repeated length measurements that form part of the test, and this value is given the symbol R0. Although the standard uses the symbols in column two of Table 2 ,it is accepted that these symbols may not be suitable for product documentation, etc. and so the alternatives listed in column two are those that you are most likely to see in product documentation.

Ram axis stylus tip offset
The offset stylus test is a test that has not previously been specified in ISO 10360. The latest version of part 2 of the standard introduces the ram axis stylus tip offset. The ram axis stylus tip offset L (see Figure 2) is the distance (orthogonal to the ram axis) between the stylus tip and a reference point. The reference point is defined by the manufacturer and is usually in or near the probe system. This is a more severe test of the machine and L is usually specified as 150 mm.

Location of single stylus probing test
The single stylus probing test that appeared in ISO 10360-2: 2001 does not appear in the current edition of ISO 10360-2. It now appears in the new edition of ISO 10360-5. Apart from that, the test is essentially unchanged. However, ISO 10360-2: 1995 and ISO 10360-2: 2001 recommended that the orientation of the stylus should not be parallel to any CMM axis.  ISO 10360-5 now states that the stylus orientation should be parallel to the ram axis, unless otherwise specified.

Use of laser interferometers
One major change to ISO 10360 is that a calibrated test length can now be produced using a laser interferometer and a gauge block. The calibrated test length is then the sum of the calibrated length of the gauge block and the displacement recorded by the calibrated laser interferometer system. The use of laser interferometers (Figure 3) is particularly advantageous for larger CMMs which have, in the past, been difficult to verify using material artefacts.

Table 1 How the symbols have changed over the years

 Meaning                          2009  2001   1995
 Length measurement error                     EL   E  delta L
 Repeatability range of the length measurement error      R0  
 Maximum permissible error of length measurement                         EL,  MPE   MPEE
 Single stylus form error                    PFTU   P   rmax - rmin
 Maximum permissible single stylus form error           PFTU  MPEr  ,MPEr

Table 2 Symbols used in ISO 10360-2

 Symbol                       Alternative for product documentation  Meaning
 EL         EL   Length measurement error
 R0        R0   Repeatability range of the length measurement error
 EL, MPE                   MPE(EL)   Maximum permissible error of length measurement
 R0,MPL                     MPL(R0)   Maximum permissible limit of the repeatability range

 

Future work
The technical committee responsible for the CMM standards are keeping abreast of changes in the field of co-ordinate measurement. Many new technologies are being introduced which the technical committees need to keep up with. Expect to see new parts of the standard relating to optical sensors, articulated arm CMMs and laser tracking interferometer systems in the near future. If you want to influence the standardisation process it is worth looking out for the draft standards prior to publication or becoming involved in your national technical committee.



Further reading
This article has given a quick overview of the latest edition of ISO 10360. However, many of the changes to the standard are outside the scope of a short technical article such as this. Therefore, NPL has updated NPL measurement good practice guide no. 42 on CMM verification. As well as highlighting the differences between the various editions of the standard the guide gives a comprehensive description of the CMM verification process. l

The guide is free and is available from NPL’s website:             http://www.npl.co.uk/.

A limited numbers of printed copies are also available on request:
e-mail dimensional_enquiries@npl.co.uk.


References

[1] ISO 10360-2: 2009 Geometrical product specifications (GPS) – Acceptance and reverification tests for coordinate measuring machines (CMM) — Part 2: CMMs used for measuring linear dimensions (International Organization for Standardization)

[2] ISO 10360-5: 2010 Geometrical Product Specifications (GPS) – Acceptance and reverification tests for coordinate measuring machines (CMM) — Part 5: CMMs using multiple-stylus probing systems (International Organization for Standardization)

  
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