koba ball bar

Measurement and Standards: KOBA-Ball Bars

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Ball BarsBall BarsBall BarsBall Bars

A Monitoring System for Periodic Calibration of

Large Volume Coordinate Measuring Machines

Sphere Plates, At a GlanceSphere Plates, At a GlanceSphere Plates, At a GlanceSphere 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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Phone (586) 759-2400

Significant Features of the KOBASignificant Features of the KOBASignificant Features of the KOBASignificant Features of the KOBA----Ball BarBall BarBall BarBall 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 HandlingConstruction and HandlingConstruction and HandlingConstruction 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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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 EvaluationBall Bar Application and EvaluationBall Bar Application and EvaluationBall 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:Technical Data:Technical Data:Technical Data:

Attainable Lengths: 2,000mm t0 8,000mm

Divisions

(distance between balls):

200mm, 300mm, 400mm, 500mm,

special distance on request

Useable vertical range

of height:

250mm to 3,000mm, depending

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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Ball Bar CalibrationBall Bar CalibrationBall Bar CalibrationBall 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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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

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%.

Combination Sphere mid-point

distance (mm)

T1 (°C)

Sphere 0 → Bar 1-2 → Sphere 1 500.09970 20.05

Sphere 1 Bar 1-2 Sphere 2 200.20707 20.03









1. Temperature of ball bar during measurement (uncertainty=0.03k)

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