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KOBA Ball Bar

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  • KOBAs new modular ball bar assembly is ideal for calibrating large volume measuring devices. It is portable, highly accurate and easy to assemble. Lengths of 3 or 5 meters are available - longer on request.

  • The body of the ball bar consists of three rectangular aluminum extrusions which are foam-filled for light weight and extreme rigidity. A series of ceramic spheres are mounted along the top of the bar at certified intervals. Stainless steel tubes are fitted between these spheres. Both ends of each steel tube feature a hard metal three-point contact.

  • Each sphere is held in place by a patented, spring-mounted device. The spheres can move in one place only - along the length of the bar. This arrangement provides an extremely stable and accurate method for certification of all machine-probing characteristics.

  • The KOBA Ball Bar was developed in close collaboration with the PTB and the German automotive industry.

  • Find more information about KOBA-Ball Bar calibration below.

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Special connecting devices are used to link the three ball bar segments together. These connectors ensure accurate line-up and help to guard against vibration. An optional, adjustable-height support is available for angular positioning.

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Significant Features of the KOBA-Ball Bar

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  • Repeatability of measuring lengths by means of a three-point contact on the joint of the ball-distance tube.

  • Carbide contact points provide long-term stability

  • Simple calibration

  • Short set-up and take-down times

  • Easy handling due to light weight and maximum element lengths of 1800mm

  • Variable lengths and divisions

  • Constant axial force on the joint of the ball distance tube

  • Ceramic balls and special steel distance tubes are corrosion resistant

  • Carbon fiber distance tubes offer coefficient of thermal expansion close to 0 (zero)

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​Construction and Handling

 

The carrying body is a CFC-GFC aluminum box filled with foam, the most significant features of which are high rigidity, good vibration dampening and low weight. The number of individual elements depends on the total length. These elements are securely held together by fastening elements.

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The fixing elements are ceramic balls connected over a spring elements which have retainers in their bases. The spring element provides freedom of movement in axial direction which is essential for perfect bonding between the ball and the distance tube. Stress is applied to the rods and the balls which are lined up by means of a clamping unit with reproducible force in the axial direction and which support themselves upon a fixed skewback (springer) at the opposite end, thereby avoiding changes in the length occurring due to varied pre-stress forces.

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The tube elements are designed to enable self-centering of the skewbacks and the clamping units. The distance tubes are made of stainless steel or CFC and have a carbide three-point contact at the ends for the ceramic balls. This form of contact ensures the best repeatability of measured lengths.

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Why are distance tubes available in two materials?

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  • Stainless Steel: measuring results from distance tubes of stainless steel, with a coefficient of thermal expansion of 16 x 10^-6 K_-1, reflect environmental influences. This means that fluctuations in temperature are reproduced in length deviations and present the actual uncertainty of measurement.

  • CFC: distance tubes of CFC, with a coefficient of thermal expansion of 0 (zero) do not react to fluctuations in temperature and merely show the uncertainty of measurement of the measuring instrument with no influence from environmental conditions.

 

The KOBA-Ball Bar is mounted by means of a step bearing and a stable, adjustable height support; two supports can by used for horizontal mounting.

 

Because of its well-planned designed, a completely dismounted test body can be set-up within 20 minutes, by one person. Changing the position of the test body within the measuring volume can be made in minutes.

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Ball Bar Calibration

Calibration of individual measuring lengths can be carried out by the user on an exact coordinate measuring machine of normal construction size, consequently the cost of calibration is greatly reduced. For the purpose of calibration, only the first supporting body elements is used. On this element a small number of measuring lengths, corresponding to the size of the measuring machine, are mounted using the clamping unit; the respective pairs (i.e, Ball 1-Rod 1-Ball 2 / Ball 2-Rod 2-Ball 3, etc.) are measured and the values of the distances of center points of balls are noted in a calibration document. See more Ball Bar calibration details below.

Ball Bar Application and Evaluation

 

The ball bar is adjusted and measured in various positions within the measuring volume according to the specific requirements. The arrangement can be carried out in the direction of individual axes, of diagonals in the measuring planes or of the space diagonals in the measuring volume of the CMM to be examined. The recorded measuring values can be evaluated in our software, which is independent of the CMM manufacturer and can administer up to 100 coordinate measuring machines.

KOBA Ball Bar Technical Data

KOBA Ball Bar® Calibration

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  • Calibration Object:
    The calibration object is an artifact (ball bar) which can be dismantled and is used for the calibration and interim checking of coordinate measuring machines. The artifact consists of 10 distance bars and 11 ceramic spheres.

  • Calibration Procedure:
    The ball bar was calibrated using a "calibration device for linear comparison standards" (DK No. 0372). The measuring system of the device consists of a plane mirror laser interferometer. It consists of the interferometer with a mobile probing component (Zeiss probe system) and a permanently mounted plane mirror. A stabilized dual-frequency He-Ne laser is used whose wave length is corrected by the refractive index of air determined in accordance with the Edlen formula (parameter method).
    Traceability to the unit length "meter" is established by calibration of the overall measuring system, using parallel gage blocks of the nominal size 50mm (official label 0274 PTB 98), of the nominal size 500mm (official label 111 PTB 96) and of the nominal size 1000mm (official label 110 PTB 96).
    The respective sphere mid-point distances of the different combinations, e.g. sphere 1/bar 1/sphere 2, sphere 2/bar 2/sphere 3, etc. were measured.

  • Ambient Conditions
    The temperature, air pressure and humidity during measurement have been recorded on a data carrier in the calibration lab and can be communicated if necessary.

  • Measuring Uncertainty
    The measuring uncertainty for the sphere mid-point distances is U=0.4µm + 2 * 10^-6 = L, L = the length of the ball bar.
    The specification indicates the upgraded measuring uncertainty resulting from the multiplication of the standard measuring uncertainty by the factor k = 2. It was determined in conformity with DKD-3. The values of the measurement parameter lie within the specified range with probability of 95%

Measurement Results:

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