Study about the typology of the machines medidoras by coordinates (MMC)

The first MMC appeared to finals of the decade of the years 50 of the 20th century and, at present, have turned into one of the instruments of the metrology whose presence is fundamental in the department of quality of the greater part of the companies [Hansen et al, 1999]. Said equipment are able to offer an effective control of the process of manufacture of any product, characterised by a fast and precise evaluation of the dimensional appearances of this [Carmignato et al, 2010].

The MMC consist of two main parts: the palpador or probe and the system that allows his movement by all the volume of measure of the team. The palpador or probe, that can be of contact or without contact, collects the information of the piece and sends it to a computer where, through a software, can determine so many dimensional measurements (distance between elements, positions, lengths, angles, etc.) like deviations of the geometrical regularity (errors of redondez, of planitud, of rectitud or of parallelism, among others).

The architecture of a machine medidora by coordinates changes depending on several parameters between which find the steerings of trip of his components, how realise these and his volume of measurement. The norm JOINS-IN ISO 10360-1 realises a classification of the distinct types (see figure 1). Each one of them has a series of advantages and inconvenient that will do it more adapted for some determinate applications to the detriment of others for which will not be so effective.

The first of them is the machine of cantilever [JOIN-IN ISO 10360-1, 2001]. It consists of three mobile components along three perpendicular guides between himself. The system of palpado goes solidario with the first component, which moves vertically with regard to the second and both displace with regard to the third horizontally. This, besides, moves horizontally with regard to the base of the machine through some guides and finds supported in an alone extreme. They exist two possibilities of design: the one of fixed table, in which the piece situates on the base of the machine, and the one of mobile table, in which the piece goes directly placed on the third component.

Is a type of architecture that does not use a lot at present and originally applied in machines medidoras by manual coordinates. Like the table of support does not include the guides of movement, can measure pieces relatively weighed without influencing this in the precision of the measure. When being the mobile part quite light, is a quite agile machine in manual way and works to high speeds in automatic way [Hocken et al, 2012].

The volume of measure is long in a steering but limited in the others two axles, by what is particularly adapted to measure long and thin pieces. Besides, when having three open sides, possesses a good accessibility to all the parts of the piece [Dotson et al, 2003].

Like problem, fits to stand out that when taking measures in the extreme of the arm of measure, the inclination of this can cause small variations in the measures, which has to be corrected by means of the software of control of the machine. Besides, the natural frequencies of the system are lower, which limits the size of the machine [Hocken et al, 2012].

A modification of this is the one of type bridge in shape of L [JOIN-IN ISO 10360-1, 2001]. In her, the third component moves horizontally on two guides, one to the level of the base and another above this, that is to say, incorporates a support in the opposite side of the beam in cantilever. As usually they are adaptations of this, are used to to have speeds of slow work and possess low frequencies of resonance, what limits his size [Hocken et al, 2012].

Another type of machine would be the MMC of mobile bridge [JOIN-IN ISO 10360-1, 2001]. Has three components that move along perpendicular guides between himself and the system of palpado goes solidario with the first of them. This first, besides, moves vertically with regard to the second and together move horizontally with regard to the third. This last supports on two columns, which reposan in opposite sides of the base of the machine, and moves horizontally with regard to the base on which situates the piece.

Is the most used in the industry and his volumes of measure vary from the smallest, of 0,1 dm³, until the biggest, of 8 m³, being this the limit in which it is possible to realise measurements of an efficient form. Can measure pieces with a rank of small sizes-half and gives place to values of small uncertainty, being possible therefore achieve a big precision in the measure [Hocken et al, 2012]. Besides, the movement of the axles can be manual or automatic.

The advantage of this type of MMC with regard to the previous is that the imprecisions been due to the existence of an only arm disappear when explaining, in this case, with two [Hocken et al, 2012]. However, when existing a column to each side, could originate problems of accessibility of the probe depending of the type of pieces that wish measure.

In the Figure 2 shows an image of the machine medidora by pertaining coordinates to the Public University of Navarra (UPNA). Said machine, that is of mobile bridge, is a DEA Global Image Climate with cabezal articulated of the type PH10MQ. The program of measurement that employs is PC-DMISTM 3.7, that is one of the most widespread and popular inside the field of the machines medidoras by coordinates. The specific uncertainty (or) of the machine comes given by the following expressesion: or = (1,7 + 3·L/1.000) µm, where L represents the length that wishes evaluate. Likewise, his volume of measure comes given by: 850 mm (X) x 1460 mm (And) x 780 mm (Z).

A variation of this type of machine would be the one of fixed bridge [JOIN-IN ISO 10360-1, 2001], in which the bridge remains rígidamente joined to the rest of the machine and is the table in which they situate the pieces the one who provides a degree of freedom, since it moves horizontally with regard to the bridge.

This rigid structure does that it was the MMC of greater precision and is used to use in the calibración of patterns [Dotson et al, 2003]. However, his performance is not very well since his speed of work sees reduced by the fact that the mobile table has a quite high weight [Hocken et al, 2012].

A fourth type of machine is the one of type portico or gantry. According to the norm JOINS-IN ISO 10360-1, finds composed by three components that move along perpendicular guides between yes, being the system of palpado fixed to the first component, which finds positioned and moves vertically with regard to the second. The group of the first and of the second move horizontally with regard to the third and this moves horizontally on two rails fit in the base of the machine, one to each side of this.

Is the machine of greater volume of measure since it can arrive until 100 m3, or even until volumes quite upper to the previous, and uses to realise precise measurements in pieces very big, such like motors, vehicles or components of aerospace structures [Dotson et al, 2003].

Another advantage is that his architecture allows a free access to all the parts of the volume of measure. Besides, the fact that only the horizontal beam move does that the precision was quite acceptable, even for the biggest. The basic configuration is used to to be relatively economic but if they require special characteristics that increase the precision, the costs can see increased of important way [Hocken et al, 2012].

The following type of machine would be the one of column [JOIN-IN ISO 10360-1, 2001]. The horizontal movement realises by means of the table in which it situates the piece and the system of palpado displaces vertically on a fixed column. This configuration can give place to a big precision but when having a structure with form of C, can see affected by thermal sensors variations. This does that they are used in environingingments especially conditioned, that is to say, without big thermal sensors gradients, such like laboratories of metrology and no so much in productive surroundings [Dotson et al, 2003].

Finally, the last type of MMC that contemplates in this classification would be the one of horizontal arm [JOIN-IN ISO 10360-1, 2001] that, to his time, can have different configurations: with cabezal mobile, with fixed table, with fixed table and dish giratorio or with mobile table, between others.

The first of them, of cabezal mobile, is composed by three components that move along perpendicular guides between himself. The system of palpado goes fixed to the first component and moves horizontally with regard to the second. Both move vertically with regard to the third that moves also horizontally regarding the base of the machine where situates the piece. In the one of fixed table, the difference is that the first component only supports in the second component in an extreme in disposal type cantilever. The one of fixed table and dish giratorio is similar but incorporates a dish giratorio in the table of the machine and, on this, places the piece that wants to measure . Finally, the one of mobile table is similar but the piece situates directly on the third component.

All they have like advantage his excellent accessibility to all the parts of the piece that wishes measure and his high speed of work, what does them adapted for his use in the sector of the automotive sector, for example, to the hour to measure bodyworks or motors. By the contrary, the precision sees a bit affected by the fault of rigidity of his components, what forces many times to compensate the possible variations by means of the software of control of the machine. His mobile arm can arrive to have a field of measure of until 25 m [Hocken et al, 2012].

In the table 1 shows a summary with the configurations of MMC more employees, comparing some of his characteristic metrológicas more important.

The measurements in the machines medidoras by coordinates realise through the probe, that is the mobile part and that displaces in the steerings of the axles by all the volume of measure of the machine. The norm JOINS-IN ISO 10360-1 designates it of a more technical form like system of palpado. His mission is to take the coordinates (X, And, Z) of concrete points of the piece so that, of this form, the software associated to the machine can obtain the necessary measurements.

The probe or system of palpado is the base of the operation of the MMC. At present, they find available in the market a big variety of systems and, although they are used to to be compatible with the greater part of the machines, each user would have to determine which is the one who better adjusts to his concrete needs.

Exist two fundamental types of probes: the ones of contact and the ones of no contact. As its name suggests, the first touch physically the piece and, to his time, can classify in fixed probes, probes of palpado point ready or probes of palpado continuous (‘scanning').

The fixed probes use in MMC manuals since the operario puts in contact, manually, the probe with the piece in the position that more suit, wait to that the MMC stabilise and is he same who orders to this register his position. They exist in big variety of configurations and use at present to measure surfaces curves, distances between elements, angles and diameters of axles or of holes in applications that do not require of big precision. The problem that can have is that, when being handled directly by the operator, his repetibilidad can not ensure if there is a change of operator, appearing possible inconsistencias in the measures.

The probes of palpado point ready (also called probes of shot by contact) obtain different points on the piece of discreet form and are the most used at present. Every time that the tip of the probe goes in in contact with the piece, generates an electronic signal so that the coordinates of the position of this in this moment remain registered. They can be handled manually or automatically and have a big variety of cabezales and accessories, what gives them a big versatilidad. Besides, they delete the influence of the operario in the measurements. The possible errors caused by flexión of the tip of the probe or by different sensitivities of this, according to the angle of contact with the piece, amend by means of the application of piezoelectric sensors or galgas extensométricas, respectively, to achieve precisions underneath of a micrometer.

To way of example, in the gigura 3 shows a probe of contact point ready of the mark Renishaw. The probe of contact, that is one of the available in the MMC of the UPNA, is a probe model TP-200 and employs in his manufacture the technology of galgas extensométricas. The group vástago-tip of the palpador presents a union of magnetic type, what facilitates a fast change of the same. In the case of the figure 3, the length of the vástago of the palpador is of 20 millimetres and his tip is a sphere of rubí with diameter of 3 millimetres.

The last type of systems of palpado of contact are the probes of palpado continuous (also known like probes of inspection), which, although they also can be used like the previous to take points of discreet way, his main function is the continuous register of the coordinates of a series of points of the surface of the piece every second, determining his form and size. To achieve this high speed of register of points, requires of special software since the geometrical elements that use the traditional programs are not sufficient to generate paths with the number of necessary points. This type of probes uses to inspect and measure with a big precision form irregular, such as álabes of turbine or distinct parts of a car. Besides, they can be used in applications of reverse engineering.

The extreme of the probe that goes in in contact with the surface of the piece, that knows with the name of tip of the palpador, is generally a sphere of rubí synthetic in what it knows like palpadores straight. It uses like material of manufacture of the tip the rubí by his high hardness, what minimises the wear, as well as by his low density, that does that the mass of the tip was smaller and, therefore, that reduce the possibility of an erroneous detection of a point because of the vibrations of the machine. Before beginning to realise the measures with the MMC, has to ensure the location and the diameter of the sphere of the most precise form possible and, for this, uses a sphere calibrated of some 30 millimetres of diameter, generally manufactured of ceramic material, with which the machine can determine these previous parameters to any measurement.

In the figure 4 presents a probe of continuous contact also of the mark Renishaw. The one who appears in said figure is one of the model SP-25. It carries mounted a group vástago-tip with a length of 21 millimetres and a spherical tip of rubí of 5 millimetres of diameter.

In addition to palpadores straight, exist other types of palpadores whose use is more specific as they can be: the palpadores in star used to inspect diverse surfaces, palpadores of tip to inspect small holes or threaded forms with precision, palpadores with vástago of ceramics for holes and deep surfaces, palpadores of disk for muescas and groove in inner diameters and palpadores cylindrical to measure holes in plates, the centres of these and threaded surfaces, among others.

The another type of probes that exists are the probes of no contact and that, usually, are used to use in applications where the piece was flexible or of a soft material (for example, of foam) and a contact of the probe can distort the geometry of the own piece. However, in the actuality have lower precision that the ones of contact.

The first type of probe of no contact is the probe laser. It works the same that the probe of shot by contact but, in this case, is a do of laser that works like optical switch and that interactúa with the surface of the piece. The probe projects a do of laser on the surface of the piece and, then, the position of this determines by triangulation in the lens that find in the receptor. With this type of probes can appear some problems as, for example, in the measure of spherical surfaces, if the do reflects in another steering that was not the one of the detector of photodiode.

In the figure 5 shows the probe available laser in the machine medidora by coordinates of the UPNA. In this case, it treats of a model Metris LC-50 with a laser of class 2 that presents an uncertainty of 15 μm and a speed of scanned until 19.200 points/s with a step of 0,1 millimetres.

The another type of probes of no contact are the probes of vision. These do not do a direct measurement but they obtain a species of ‘photography' of the piece, that digitaliza and from which obtain the dimensions of the same. They are used to to be fast and precise and use fundamentally where require a fast inspection of the piece.

Once that the probe llevar the contacts with the surface of the piece, the information collected has to be analysed through a specific software, so that it was translated in a group of geometrical elements known (such as flat, spheres, circles, etc.) by means of models of adjust that they can be more or less complex. Once realised these approximations to geometrical elements, will be able to calculate diverse types of operations between them (as, for example, intersections), positions, distances between elements or geometrical errors of the pieces (such as of redondez, of cilindricidad, of planitud, etc.).

According to the norm JOINS-IN ISO 10360-1:2001, it defines to a software of reference like which calculates the values of parameters of reference as well as the waste of reference of the gaussian element associated to each group of data, being said values of reference the numerical values of the parameters employed in the parametrisation of reference for a particular group of data of reference.

In consequence, a software of measurement comprises the group of programs, procedures and documentation employed to control a machine medidora by coordinates. The utilisation of a good program of measurement of the MMC ensures a good precision in the measures, statistical analyses of high level completed with databases and, in definite, allows a control in real time of the machine. Therefore, the development of software of measurement adapted in the case of the MMCs is one of the most important factors to the hour to explain the importance that have reached these inside the field of the industrial metrology.

As already it mentioned previously, the program of measurement that uses in the concrete case of the machine of the Public University of Navarra is PC-DMISTM. Said software employed is developed by the North American company Hexagon Metrology S.A. and gives access, of a simple form, to functions of programming, evaluation and obtaining of reports about the concrete piece that is analysing .

PC-DMIS is, in general, the software used in the machines medidoras by coordinates manufactured by Hexagon Metrology S.A. but, besides, can be used in the majority of equipment of other manufacturers. PC-DMIS appears in different versions that have optional modules, to end to satisfy the specific requests of each user [PC-DMIS, 2012].

On the other hand, PC-DMIS can be also executed of off-line ‘form', that is to say, without need of connection to the MMC, what allows to generate the programs while the machine follows working on other pieces. This is very useful in surroundings of high productivity, where has to optimise the time of work of the MMCs.

MCOSMOS (Whose acronym means ‘Mitutoyo Controlled Open System for Modulate Operation Support') is another software that uses in the control of MMC. It is developed by the Japanese company Mitutoyo S.A., inside his platform of computer supports MiCAT (‘Mitutoyo Intelligent Computer Aided. Technology') [MiCAT (Mitutoyo S.A.), 2003]. It allows the treatment of a big quantity of information of simple form, transmitting it through all the diverse areas of the surroundings of production. It has different modules instalables to satisfy the specific needs of each customer.

Calypso, developed by the German company Carl Zeiss S.A., account also, to the equal that the two previous, with different modules and versions adapted to the needs of the customer [Calypso (Carl Zeiss S.A.), 2010].

In addition to these three programs of measurement for MMCs, that are the most widespread and representative, exist other a lot of more since the greater part of the manufacturers of machines medidoras by coordinates are used to to have his own software developed. Therefore, the most comfortable for the possible user is to purchase the software that better adapt to his needs at the same time that the machine of measurement of coordinates.

As already it has mentioned , the machines medidoras by coordinates use to measure all type of complex geometries so much in laboratories of metrology as in the own companies. However, his precision in the measures realised can see affected over time because of the progressive apparition of some errors in the MMC.

Exist 21 geometrical errors possible between which find : the errors of linear trip in each one of the axles (3 in total), the errors of rectitud in a steering because of the movement in another (6 in total), the angular errors with regard to each one of the axles (9 in total, where each one of the three possible angular errors receives, in English, the particular name of ‘roll', ‘pitch' or ‘yaw') and, finally, the errors of quadrature between each pair of axles (3 in total) [Hocken et al, 2012].

Besides, the operation of a MMC sees also affected by his own weight, the material of the that is manufactured, his years of service, the accelerations to which sees subjected, the vibrations that can appear, the system of palpado, the controller and the surroundings in which it finds situated [Agapiou et al, 2007].

Fortunately, the software of current control allow to compensate these errors from periodic procedures of calibración, so that the operator of the machine enters the necessary information in the software on each error, that is to say, how much diverts each measurement of his real value, and the software adjusts it properly for future readings.

According to the norm JOINS-IN ISO 10360-2:2010, that collects the essays of acceptance and periodic verification of machines of measurement by coordinates, exist different types of procedures of calibración although all they directed to ensure the maximum precision in the measurements.

In the first place, exists an essay of acceptance that is not more than a group of agreed operations between the manufacturer and the user with the end to verify that the provision of the machine fulfil .

Second, defines an essay of periodic verification so that, as its name suggests, verify periodically (the manufacturers recommend a frequency, at least, annual) that the provision of the machine follow fulfilling the stipulated by the user. This essay llevar according to the same procedures that the ones of the essay of acceptance.

Besides, to ensure that it keeps the level of confidence in the measurements realised with the MMC, exists another essay, designated like periodic control, that is specified by the user and that llevar between the essays of periodic verification, with the frequency that the own user estimate necessary.

The procedure of calibración comes defined in the different parts of the norm JOINS-IN ISO 10360 (1 to 6) and resumido in the Procedure GAVE-027 of the Spanish Centre of Metrology (CEM) although they exist other procedures such as the ASME B89 or the VDI/VDE 2617. All they base in precise measurements of patterns previously calibrated with an upper precision to the one of the machine, to check that the measures go in inside some previously defined tolerances by the manufacturer in the essay of acceptance. However, they differ mainly in the number and the form of the comprobaciones that have to realise .

In addition to these already clear-cut procedures, is recommended the periodic control of the machine, for which, the user realises the pertinent measurements already known of a determinate instrument and checks that the machine follows offering the level of precision required and does not present important errors with a quite upper frequency to the one of the essays of periodic verification (monthly or, even, weekly).

The instruments that use are miscellaneous although all have to have fixed known distances between simple geometrical elements to measure, be mecánicamente robust, have a superficial finishing well so that it do not affect to the measurement and a coefficient of similar thermal sensors dilatation to the of the pieces that usually measure with the machine.

Between the different objects that use stand out the dishes and bars of balls, the dishes and bars of holes, circular artefacts, pieces especially designed (see figure 6), fulfilling the previous conditions and that they are similar to the measures realised usually with the machine, or even, systems laser. Although some of these could be used in the essays of calibración periodic, usually use to ensure a control of the routine and simple machine for the user.

Therefore, is totally recommended to follow an exhaustive control of the measurements of the machine since, although it treats of machines very precise and of extraordinary reliability, a small variation in some part of the system of measurement can drive to errors that would have to be detected to the greater possible shortness.

In the present work has realised a study about the most characteristic typology of the machines medidoras by coordinates (MMC). This type of equipment are used in great quantity of industrial applications as well as, in general, in the field of the metrology, since these are able to obtain measures with big dimensional precision. Likewise, they have described the main types of probes more important that can carry incorporated, in addition to appearances such as the main softwares of measurement used in the same as well as the object of his calibración.

bibliographic References

[1] Agapiou, J.S.; Du, H., Assuring the day-to-day accuracy of coordinate measuring machines - To comparison of tools and procedures, Journal of Manufacturing Processes, Vol. 9 (2), 2007, pp. 109-120.

[2] CALYPSO (Carl Zeiss S.A.). http://www.zeiss.com/4125682000247242/

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[3] Carmignato, S.; Voltan, To.; Savio, And., Metrological performance of optical coordinate measuring machines under industrial conditions, CIRP Annals - Manufacturing Technology, Vol. 59 (1), 2010, pp. 497-500.

[4] Dotson, C.; Harlow, R.; Thompson, R.L., Fundamentals of Dimensional Metrology, Ed. Thomson Delmar Learning, New York, 2003.

[5] Hansen, H.N.; Of Chiffre, L., An industrial comparison of coordinate measuring machines in Scandinavia with focus on uncertainty statements, Precision Engineering, Vol. 23 (3), 1999, pp. 185-195.

[6] Hocken, R.J.; Pereira, P.H. (Editors), Coordinate Measuring Machines and Systems, Ed. CRC Presses, Mouth Raton, 2012.

[7] Leach, R.H., Fundamental Principles of Engineering Nanometrology, Ed. Elsevier, Oxford, 2010.

[8] MiCAT (Mitutoyo S.A.). http://www.mitutoyo.com/pdf/1701.pdf.

[9] PC-DMISTM (Hexagon Metrology S.A.). http://www.pcdmis.com.

[10] Procedure of Calibración GAVE-027: Measurers of Three Coordinates, Ed. Spanish centre of Metrology (CEM), Madrid, 2003.

[11] JOIN-IN ISO 10360-1: Geometrical Specification of Products (GPS). Essays of Acceptance and of Periodic Verification of Machines of Measurement by Coordinates (MMC). It splits 1: Vocabulary (ISO 10360-1:2000), Ed. AENOR, Madrid, 2001.

[12] JOIN-IN ISO 10360-2: Geometrical Specification of Products (GPS). Essays of Acceptance and of Periodic Verification of Machines of Measurement by Coordinates (MMC). It splits 2: MMC Used for the Measurement of Linear Dimensions (ISO 10360-2:2009), Ed. AENOR, Madrid, 2010.