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Measurement and Standards: KOBA-Ball BarsBall BarsA Monitoring System for Periodic Calibration ofLarge Volume Coordinate Measuring MachinesSphere Plates, At a Glance⊕ Ideal for calibrating large volume measuring devices⊕ Portable, accurate and easy to assemble⊕ Available in standard lengths of 3 or 5 meters —longer available by request⊕ Body consists of three rectangular aluminum extrusions which are foam filled for light weight and extreme rigid- ity.⊕ Ceramic spheres are mounted along the top of the bar at certified intervals⊕ Each sphere is held in place by patented, spring- mounted devices allowing movement along the length of the bar only.Phone (586) 759-2400 Fax (586) 759-2423www.pmargage.com www.shoppwm.com 1
Measurement and Standards:KOBA-Ball Bars KOBA-Significant Features of the KOBA-Ball Bar⊕ Repeatability of measuring lengths by means of a 3-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)Construction and HandlingThe carrying body is a CFC-GFCaluminum box filled with foam, themost significant features of whichare high rigidity, good vibrationdampening and low weight. Thenumber of individual elements de-pends on the total length. Theseelements are securely held togetherby fastening elements.The fixing elements are ceramicballs connected over spring ele-ments which have retainers in their bases. The spring element providesfreedom of movement in axial direction which is essential for perfect bond-ing between the ball and the distance tube. Stress is applied to the rodsand the balls which are lined up by means of a clamping unit with repro-ducible force in the axial direction and which support themselves upon afixed skewback (springer) at the opposite end, thereby avoiding changesin the length occurring due to varied pre-stress forces.Phone (586) 759-2400 Fax (586) 759-24232 www.pmargage..com www.shoppwm.com
Measurement and Standards: KOBA-Ball BarsThe tube elements are designed to enable self-centering of the skewbacksand the clamping units. The distance tubes are mode of stainless steel orCFC and have a carbide 3point contact at the ends for the ceramic balls.This form of contact ensures the best repeatability of measured lengths.Why are distance tubes available in two materials? ⊕ 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. ⊕ Carbon Fiber: 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 or two supports can be used for horizontalmounting.Because of its well-planned design, a completely dismounted test bodycan be set up within 20 minutes, by one person. Changing the position ofthe test body within the measuring volume can be made in minutes.Ball Bar Application and EvaluationThe ball bar is adjusted and measured in various positions within themeasuring volume according to specific requirements. The arrangementcan be carried out in the direction of individual axes, of diagonals in themeasuring planes or of the space diagonals in the measuring volume ofthe CMM to be examined. The recorded measuring values can be evalu-ated in our GUK-KS software, which is independent of the CMM manufac-turer and can administer up to 100 coordinate measuring machines.Technical Data:Attainable Lengths: 2,000mm t0 8,000mmDivisions 200mm, 300mm, 400mm, 500mm,(distance between balls): special distance on requestUseable vertical range 250mm to 3,000mm, dependingof height: on lengthProbing elements: Ceramic balls, 30mm diameterDistance tubes: Stainless steel or CFC tubes with carbide 3-point contactsWeight: Approximately 8kg/mLength of individual elements: Maximum 1,800mmPhone (586) 759-2400 Fax (586) 759-2423www.pmargage.com www.shoppwm.com 3
Measurement and Standards:KOBA-Ball BarsBall Bar CalibrationCalibration of individual measuringlengths can be carried out by the useron an exact coordinate measuringmachine of normal construction size,consequently the cost of calibrationis greatly reduced. For the purposeof calibration, only the first supportingbody element is used. On this ele-ments a small number of measuringlengths, corresponding to the size ofthe measuring machine, are mountedusing the clamping unit; the respectivepairs (i.e., ball 1 → rod 1 → ball 2 / ball2 → rod 2 → ball 3, etc.) are meas-ured and the values of the distancesof center points of balls are noted in acalibration document.1. Calibration Object: The calibration object is an artifact (ball bar) which can be dismantles and is used for the calibration and interim checking of coordinate measuring machines. The artifact consists of 10 distance tubes and 11 ceramic spheres.2. Calibration procedure: The ball bar is calibrated using a “calibration device for linear comparison standards” (DK Nr. 0372). The measur- ing system of the de- vice consists of a plane mirror laser interfer- ometer 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 ac- cordance with the Edlen formula (parameter method). Traceability to the unit length “meter” is estab- lished 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 1,000mm (official label 110 OTB 96). The respective sphere mid-point distances of the different combina- tions , e.g. sphere 1 → bar 1 → sphere2, sphere2 → bar 2 → sphere 3, etc. were measured.3. 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.Phone (586) 759-2400 Fax (586) 759-24234 www.pmargage..com www.shoppwm.com
Measurement and Standards: KOBA-Ball Bars4. Measurement Results: the calibration results indicate the sphere mid-point distances of combinations: The lengths are specified relative to the reference temperature of 20° C and to the metrological condition of the ball during calibration. A linear thermal expansion coefficient of 11.5—10-6 K1 was used to reduce the results of 20° C.5. Measuring Uncertainty: the measuring uncertainty for the sphere mid-point distance is U=0.4µm + 2 x 10-6=L, L=the length of the bar. The specification indicates the upgraded measuring uncertainty Combination Sphere mid-point T1 (°C) distance (mm) Sphere 0 → Bar 1-2 → Sphere 1 500.09970 20.05 Sphere 1 Bar 1-2 Sphere 2 200.20707 20.03 Sphere 2 Bar 2-3 Sphere 3 500.26838 20.04 Sphere 3 Bar 3-4 Sphere 4 500.46535 20.04 Sphere 4 Bar 4-5 Sphere 5 500.30931 20.05 Sphere 5 Bar 5-6 Sphere 6 500.39038 20.04 Sphere 6 Bar 6-7 Sphere 7 500.05702 20.03 Sphere 7 Bar 7-8 Sphere 8 500.44678 20.06 Sphere 8 Bar 8-9 Sphere 9 500.00810 20.09 Sphere 9 Bar 9-10 Sphere 10 500.5244 20.081. Temperature of ball bar during measurement (uncertainty=0.03k) resulting from the multiplication of the standard measuring uncer- tainty by the factor k=2. It was determined in conformity with DKD-3. The values of the measuring parameter lie within the specified range with probability of 95%.Phone (586) 759-2400 Fax (586) 759-2423www.pmargage.com www.shoppwm.com 5