1. The document describes a KOBA ball bar, which is a portable calibration tool for large coordinate measuring machines (CMMs). It consists of ceramic spheres mounted on carbon fiber or stainless steel distance tubes at regular intervals. 2. To calibrate the ball bar, individual sphere-tube-sphere lengths are measured on a high-precision CMM. This establishes traceability of the ball bar's lengths to national standards. 3. A ball bar calibration certificate provides the measured lengths of combinations of spheres and tubes, ambient measurement conditions, stated measurement uncertainty, and other details ensuring the ball bar's lengths are accurately certified.

1. Measurement and Standards:
KOBA-Ball Bars
Ball Bars
A Monitoring System for Periodic Calibration of
Large Volume Coordinate Measuring Machines
Sphere 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.
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2. 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 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 de-
pends on the total length. These
elements are securely held together
by fastening elements.
The fixing elements are ceramic
balls connected over spring ele-
ments which have retainers in their bases. The spring element provides
freedom of movement in axial direction which is essential for perfect bond-
ing 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 repro-
ducible 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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3. Measurement and Standards:
KOBA-Ball Bars
The tube elements are designed to enable self-centering of the skewbacks
and the clamping units. The distance tubes are mode of stainless steel or
CFC 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 horizontal
mounting.
Because of its well-planned design, 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.
Ball Bar Application and Evaluation
The ball bar is adjusted and measured in various positions within the
measuring volume according to 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 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 8,000mm
Divisions 200mm, 300mm, 400mm, 500mm,
(distance between balls): special distance on request
Useable vertical range 250mm to 3,000mm, depending
of height: on length
Probing elements: Ceramic balls, 30mm diameter
Distance tubes: Stainless steel or CFC tubes with carbide
3-point contacts
Weight: Approximately 8kg/m
Length of individual elements: Maximum 1,800mm
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4. Measurement and Standards:
KOBA-Ball Bars
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 element is used. On this ele-
ments 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 meas-
ured and the values of the distances
of center points of balls are noted in a
calibration 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.
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5. Measurement and Standards:
KOBA-Ball Bars
4. 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 20.08
1. 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%.
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