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Section carried out with the technical advice of Dea Hexagon Metrology

Verification of the performance of a coordinate measurement machine

15/06/2004

With the rules ISO 10360

Certification benefits metrological of coordinate measurement machines is regulated by international standards that guarantee users the conformity of the product acquired with the standards defined by the commissions of regulations involving manufacturersusers and certification authorities.

In recent years the ISO 10360 has been the most important standard, with wider dissemination. The International Organization for Standardization (ISO) is a worldwide Federation of national standards bodies. International standards drafted the technical committees.

The ISO 10360 regulates the acceptance testing and verification of the performance of the machines of measurement for coordinates, known as MMC.

The body of standards ISO 10360 defines the criteria for acceptance and re-verification of measuring machines and adopt European national standardization authorities.

It consists of several parts, each one takes care of specific tests and testing:

Part 1: Vocabulary
Part 2: MMC to measure linear dimensions
Part 3: MMC with four axles and turntable
Part 4: MMC with measurement by scanning functions
Part 5: MMC with systems of data with multiple tips

Certification and acceptance according to ISO 10360-2 tests

The provision of the machine of measurement verify according to two main parameters:

MPE_And and MPE_P

The Maximum Error Allowed (MPE) has to be specified by the manufacturer and defines the maximum errors And and P of the system of measurement.

And = error of indication of the longitudinal measurement
P = error of taking of point

These two parameters allow to establish the provision of the machine of relative measurement to the length measured (MPE_And) and to the taking of a point (MPE_P).

The two parameters can be declared by the manufacturer for the volume of complete measurement or for a volume reduced of the cual have specified the limits.

Indication of the measured length MPE_anderror

The formula that defines the inaccuracy of the measurement of the length of the volume is:

_And MPE = +/-(A+L/K)

Where:

To = constant inaccuracy of the machine or system expressed in microns
L = length expressed in mm
K = constant positive

The test must be performed with 5 certified components (block patterns, or feet of King) of various lengths, 3 times to guide them in 7 different directions in the measurement volume, each measuring a total of 105 measurements (Fig.1).

Figure 1

All the results of measurements 105 (100%) must be within the limits of the MPEE specified by the manufacturer. Measurements out of tolerance are not supported. He is allowed to repeat the test if within a maximum of 5 measurements of length, the 35 carried out, there is one of the three values repeated outside the zone in accordance.

Error point_PMPE decision-making

The error point P takes verifies measuring 25 points in a ball of calibration of a diameter between 10 and 50 mm with a transducer which is not parallel to an axis of the MMC. This parameter is verified in a single position in the Centre of the measurement volume. The mistake of taking of point P is band dispersion of the 25 radial distances between measured points and the center of the ball which is calculated using the 25 point, i.e. Rmax-Rmin shots. The resulting P error has to be within MPE_P specified by the manufacturer (Fig.2-3).

Figure 2

Figure 3

Verification of benefits of digitization

Verification of the benefits of digitization are described by the standard ISO 19360-4 and it is applicable for the probe point to point.

The benefits of digitizing is determined by the parameters T_{i, j} and t:

T = scanning error
(I) = intensity of points, high (H), low (L)
j = predefined route (P), not predefined (N)
t= time spent for the test in seconds

The manufacturer of MMC can specify the maximum allowed Error for the benefits of digitization according to the following combinations:

MPE,_THP, MPE_BPD,_THNMPE, MPE_TLN

Density of points	Default route	Not predefined route
High	THP	THN
Baja	BPD	TLN

Scanning with high density of points is particularly significant in the scans to define the error of the form, for example in the case of the redondeces.

The error of scanning is verified digitizing a ball of calibration by four sections defined in the time established by the manufacturer. The results of scanning define the Center and radius of the ball. The error T_{i, j} is calculated as the band of dispersion of the radius of the ball, defined by all the points of digitization.

a is the angle that defines the inclination of the transducer on the Z axis of the measuring machine. Please take an angle of 45 degrees.

Benefits of digitization of the MMC are accepted if:

the error T_{i, j} is not superior to the MPET_{(I), (j)} specified by the manufacturer
the difference between the calculated radio and half of the registered diameter of the ball is not greater than the MPET_{i, j}
the total time spent for the test is not superior to t as specified (Fig.4).

Figure 4

Verification of the performance of a MMC according to ISO 10360-5

Part 5 of the 10360 norm regulating verification of the performance of a machine for measuring with fixed head and multiple tips (star point) or a MMC with head of articulated measurement (e.g. Renishaw PH 10 M/MQ).

In the case of a fixed with multiple tips head parameters are:

MPE_MF,_MSMPE, MPE_ML

Where:

MF = shape (sphericity) error
MS = error of dimension (diameter)
ML = error of position (X and Z Center)

The verification procedure is to measure the ball of calibration with the 5 tips that form the head (which must have the same length) taking 5 points each. The calculation of the three errors MF, MS and ML has to use the 125 measured points (Fig. 5).

Figure 5

In the case of articulated head parameters are:

_AF,_ASMPE, MPE ASM_{to the}

Where:

AF = shape (sphericity) error
AS = error of dimension (diameter)
To the = error of position (X and Z Center)

The verification procedure is to measure the ball of calibration in 5 positions of the head (perpendicular to each of them) taking 5 points each. Calculation of three AF, AS errors and the need to use the 125 measured points (Fig. 6-7-8).