KOBA Step Gage, Artifact

Measurement and Standards: KOBA-step Step Gages

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Step GagesStep GagesStep GagesStep Gages A complete system for monitoring coordinate

measuring machines.

Measurement and StandardsMeasurement and StandardsMeasurement and StandardsMeasurement and Standards


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Step Gages, At a GlanceStep Gages, At a GlanceStep Gages, At a GlanceStep Gages, At a Glance

⊕ Available in five standard

lengths: 420, 620, 1020,

1540 and 2020mm

⊕ The gage body is manu-

factured using special

steel, stabilized by a long

aging process which

provides dimensional

stability.

⊕ Material expansion is

U = 11.5 x 10-6 / 6° C

⊕ Distortion-free support of the gaging cylinder faces is accomplished

by a patented, machined form shaped to hold each cylinder firmly in

its installed position.

⊕ The unique form of the KOBA-step body configuration produces

identical principal planes that provide the neutral bend feature at

specified, marked bearing points along the gage body.

⊕ The gage blocks and faces are cylindrical in form with lapped faces

on either end of the cylinder. As the cylinder approaches the gaging

surface it is tapered to allow easy access for measurement and

cleaning of the gage faces.

⊕ Ceramic gage blocks

⊕ Gage faces are spaced in 20mm nominal increments. The diameter

of the gage faces is 5mm. Gage cylinders are available in either steel

or ceramic.

⊕ Gage faces are not set to exact nominal positions for measurement—

this provides higher testing variation against a certified nominal data-

base. KOBA sources for certification are PTB, NIST, DKD and Works

(in-house).

⊕ Access for probing gage faces is available through:

a) holes on the side of the body,

b) the longitudinal groove or

c) at either open end of the gage body.

⊕ KOBA-step is ideal for horizontal and vertical quills operating from

“under-the-table” type machines.

⊕ Gage faces are held along the center line fiber of the gage body which

allows the gage to be held horizontally, vertically or on various inclines

without spacing deformation, this is accomplished through the gages

neutral-bend body design. ⊕ The swivel base option provides proper clamping and support at

bearing points and allows angular positions to be made quickly. ⊕ Leveling feet and handles provide fast and safe set-ups ⊕ Purpose-built transport containers minimize the risk of damage

during transportation. ⊕ Also available in a “mini” version for users of smaller CMMs

-6


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Step Gages for Checking the Accuracy of Step Gages for Checking the Accuracy of Step Gages for Checking the Accuracy of Step Gages for Checking the Accuracy of

Coordinate Measuring MachinesCoordinate Measuring MachinesCoordinate Measuring MachinesCoordinate Measuring Machines

In industrial metrology, actual physical bodies of known length which can

be contacted by mechanical sensors have an important part to play as

reference standards when measuring geometric parameters. They have

become increasingly important for assessing the accuracy of 2- and 3-axis

coordinate measuring machines (CMMs) that employ mechanical sensors.

Checking the length measurement uncertainty has proved to be a highly

informative and economical method for the acceptance testing and the

ongoing monitoring of CMMs. The KOBA-step step-gage can be used for

this in a variety of ways, given the advantages of unidirectional and bidirec-

tional targeting and of measurements from all the gage faces along the line

of measurement in succession while needing only a short time for prepara-

tion and measurement. Local errors can be detected in the CMM and char-

acteristics can be derived for individual coordinate axes of the machine.

With the aid of the length measurement uncertainty, the manufacturer or

user can specify and check the accuracy of a coordinate measuring

machine to establish its suitability for length measurement. This fundamental

task in metrology is of particular importance since, in practice, the majority

of measuring requirements are for the measurement of lengths. “Length

measurement uncertainty” is defined by VDI/VDE guideline 2617 part 2.1

as the uncertainty with which a CMM allows the precisely known distance

between two points on two parallel gage faces situated in succession along

a line of measurement to be re-measured. The illustration below shows a

measurement of this kind being made, taking as an example an individual

parallel gage block with an outside length Le which is arranged obliquely in

three dimensions and whose length is re-measured by successive contacts

with the block of the probe head in positions I and II.

Individual gage blocks arranged obliquely in three dimensions on the

table of the CMM, showing an outside dimension Le measurement.


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On the step-gage, spacing of different kinds for making test measurement

are all available simultaneously as follows:

⊕ Outside dimension Le, e.g. with the probe head in positions

I and II (figure 2)

⊕ Inside dimension Li, e.g. with the probe head in positions

III and IV (figure 3)

⊕ Rear face to rear face dimension Ls, e.g. with the probe head in

positions III and V (figure 3)

⊕ Front face to front face dimension, Lx, e.g. with the probe head

in positions VI and IV (figure 3)

⊕ Positional length Lp of a gage face from the datum gage face,

e.g. with probe head in positions VI and 0 (figure 3)

The illustrations show only one of many options available for each type and

size of spacing. The value of the length measurement uncertainty is gener-

ally given in the form of a length dependant formula: U = A + K x L </= B.

Figure 2 (above), Castellated step gage arranged obliquely in three dimensions

on the table of a CMM, with an outside length Le being measured.

Figure 3 (below), Step gage arranged obliquely in three dimensions on the table of

a CMM, showing measurement of an inside dimension Li, of an outside dimensions Ls

or of the position Lp of a gage face as a distance from the datum face.

A distinction is made between the figure U1, specified for one dimensional

test measurements along a coordinate axis (with terms A1, K1, B1), the figure

U2 for two dimensional test measurements made diagonally in a coordinate

place (with terms, A2, K2, B2) and the figure U3 for three dimensional test

measurements made diagonally in the three dimensional space defined by

the coordinates (with terms A3, K3, B3).


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Length Measurement Length Measurement Length Measurement Length Measurement

Uncertainty PlotUncertainty PlotUncertainty PlotUncertainty Plot

For the purposes of graphic

analysis, the differences

∆ L = La - Lr which are found and

plotted, with the correct signs,

for the individual measured

lengths and runs in a length

measurement uncertainty grid (right). The top and bottom boundary lines

produce a funnel shaped outline with the neck of the funnel measuring

2A (A = the figure specified by the manufacturer for length measurement

uncertainty, irrespective of length). 95% of all the test measurements must

lie within, or on, the boundaries. A quantitative analysis is made simply by

counting the number of measurements which lie outside the boundary lines.

Gage Face Position PlotGage Face Position PlotGage Face Position PlotGage Face Position Plot

It is possible to test the positions Lp of the gage faces as distances from the

datum face using the step-gage. If the relevant length errors ∆ Lp given by

position measurement in line with VDI guideline 2617 part 3 are entered in

a plot, then it is possible to see both the position of the test length and

also the sequence, for example, if the measurement points in a run are

connected by straight lines. With individual gage blocks this is not possible

because they do not have any true common reference point and are not

situated on a measurement line.

A gage face position grid (right),

similar to the length measure-

ment uncertainty grid, is used

for analysis. The outline is sym-

metrical and similar in shape to

a butterfly with a width of 1A

across the waist. The parame-

ters in this case correspond to

the appropriate figures A1, K1, B1

and so on. As this grid is moved along the measured length L, at least 95%

of all the measurements must always lie within or on the boundary lines,

meaning that all the measurement points must do so consistently whatever

the position of the waist. This ensures that any pairing of two gage faces

(even from different runs) in the form of outside, inside or face-to-face dimen-

sions will also lie inside the funnel of the length measurement uncertainty

grid. Thus, the grid forms a combined graphic expression of both the equa-

tions given above for all points of measurement.

Comparisons Between the Test Standards Comparisons Between the Test Standards Comparisons Between the Test Standards Comparisons Between the Test Standards

Gage Blocks and Step GagesGage Blocks and Step GagesGage Blocks and Step GagesGage Blocks and Step Gages

Apart from the step-gage, parallel gage blocks are the reference standards

known for the greatest accuracy of length. However gage blocks are rela-

tively flexible and have to be mounted at the AIRY points (symmetrical spac-

ing a = 0.57735 x L) so that they are free of bending moments if the parallel-

ism of the gage faces is to be maintained. The gage blocks for the individual

test lengths can be individually placed one behind the other for shorter

lengths and next to one another for longer lengths. When this is done, how-

ever, there is no way of obtaining the different gage points along a line of

measurement which are desirable for measurement purposes.


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Special Features and Advantages of the KOBASpecial Features and Advantages of the KOBASpecial Features and Advantages of the KOBASpecial Features and Advantages of the KOBA----stepstepstepstep©©©© GageGageGageGage

The step gage has a castellated body containing a large number of forward

and backward facing gage faces along a single line of measurement. This

line of measurement is the same for measurements between any faces and

the position of the work-piece, that is to say the orientation of the carrying

body only has to be determined once to find this line. There are numerous

possible combinations in various positions along the measurement line; the

actual number of different interface dimensions for a step gage with 26

castellations (i.e., KOBA-step with a nominal size of 1020mm) is 1326.

A special feature of the KOBA-step step gage is the fact that the actual gage

points are situated on the neutral fiber of the carrying body; this means that

there are no first-order changes in length if the state of bending changes.

The configuration of the gage body and the fact that the line of measure-

ment is situated on the neutral fiber prevents any increase in the distance

between the gage faces at the points where the carrying body is supported

and also prevents them from moving closer together at intervening points.

In the KOBA-step step gage, which is neutral in bending, cylindrical gage

blocks are fixed in position individually in an internal longitudinal groove

formed in a steel 55 x 55mm steel carrying body. The axis of the gage

blocks is situated on the fiber of the gage body which is neutral in bending

and they form a series of castellations; this arrangement provides excep-

tional protection for the gage faces. The strength of the carrying body and

the fact that the lengths do not vary if there are changes in bending to which

it is subject, mean that the KOBA-step step gage can be mounted in a wide

variety of fashions, e.g. cantilevered with “zero position” support or with

support at the Bessel points.

AccessoriesAccessoriesAccessoriesAccessories

The range of accessories available for use with the step gage, such as

swivel support and base, allows the gage body to be mounted on the CMM

in such a way as to be free of torsion. A support of this kind produces a

particularly stable connection between the step gage and the table of the

coordinate measure machine (figures 7, 9 and 10)

The combination of the step gage and its accessories produces a complete

system for making an overall check on the CMM. One particularly important

point is that the procedure of checking the coordinate measuring machine

can be carried out fully automatically under computer control.

Figure 7 (far left top) gage aligned

horizontally along the X-Y diagonal

plane , approx. 500mm above the

table, stylus probe vertical.

Figure 9 (far bottom left) gage aligned

horizontally along the X-Y diagonal

plane, turned 90°, stylus probe

horizontal, contact through cross-

drills. Figures 7 and 9 are shown with

base and swivel support accessories.

Figure 10 (left) shows the KOBA-step

aligned vertically along the Z-axis,

standing in the accessory base.


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TraceabilityTraceabilityTraceabilityTraceability

Since the acceptance or refusal of a coordinate measuring machine may

depend on the outcome of the length measurement uncertainty test, it is

advisable to always use officially calibrated testing equipment in order to

avoid unpleasant surprises and incorrect interpretations. The KOBA-step

step gage is available with either a DKD calibration certificate (German

calibration service, DKD) or Works calibration.

The length measurement uncertainties which can currently be achieved

with the step gage are:

⊕ DKD: U = 0.12µm + 0.6 x 10-6 x L (Length)

⊕ Works: U = 0.3µm + 0.8 x 10-6 x L (Length)

RecalibrationRecalibrationRecalibrationRecalibration

As is normal with all measurement standards, the KOBA-step step gage

should be recalibrated after a certain period. We recommend the first

calibration after one to two years and successive recalibration after two

to three years.

Alignment of the KOBAAlignment of the KOBAAlignment of the KOBAAlignment of the KOBA----stepstepstepstep

To define the position and orientation of the KOBA-step respective to the

measuring line within the measuring volume, we recommend proceeding

as follows:

As the gage body with the fixed cylindrical gage blocks has been designed

symmetrically to both sides, one of the external side surfaces and the upper

external surface at the end of the large U-groove can be used for alignment.

Any points which are as large as possible can be sensed at either the side

surface or at the upper external surface. Both places so defined are squared

off mathematically. A parallel line 27.5mm to the side surface and 30mm to

the upper surface is the measurement line situated on the neutral fiber.

It is also possible to use both ends of the 7 x 10mm interior groove for the

alignment and proceed in a similar way as before. The distance from the

wall of the groove and the floor of the groove to the line of measurement is

5mm in both directions.

Figure 11, KOBA-step vertically oriented along the Z-axis, standing in the base. This

alignment is also suitable for checking height gages as shown in Figure 12 (right)


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Practical UsePractical UsePractical UsePractical Use

Practical experience using the

KOBA-step step gage to check

the accuracy of coordinate

measuring machines and

access and interpret the results

has demonstrated the useful-

ness and potentially high

information yield of the “length

measurement uncertainty”

method. The advantage lies in

the fact that the measurement

is carried out at precision gage

faces using the normal gaging

procedure and, in general, the

manufacturer’s standard processing software. It is a good idea to make full

use of the opportunities offered by the step gage since (a) the unidirectional

and bidirectional gaging of the inside and outside dimensions, (b) the di-

mensions between similarly oriented surfaces and successive dimensions

from a given point occur even in the routine measurement of work pieces

and (d) the measurements which have been obtained can all be correlated

with one another as desired even after the event. What is more, a series of

measurements at all the gage faces in succession provides a large amount

of interrelated information and requires only a short time for measurement.

Since the length measurement uncertainty characteristic depends very

much on the geometry of the coordinate measuring machine, the lines

along which measurements are made should be as follows:

⊕ 3 to 4 measurement lines which are diagonal in 3 dimensions (i.e.,

along the diagonals of an inscribed cube—at an angle of approximately

35° - or along the diagonals of the cuboid shape representing the

measured space).

⊕ 2 diagonal lines of measurement in each coordinate plane (i.e.,

along the diagonals of a square—at a gradient of 45° - or the diagonals

of a rectangle).

⊕ At least one line of measurement parallel to each coordinate axis.

At least 18 suitable lines of measurement have to be selected in order to

arrive at a complete definition of the 18 geometrical cuboid characteristics

of the entire measurement volume.

It must be stressed that reliable measurement results for a complex system

cannot be achieved unless stylus probes are included. Checking the CMM

without mechanical contact, i.e., without using a probe, will not result in a

comprehensive statement about the accuracy of the measuring machine.

Figure 13, KOBA-step shown with swivel support and

base, illustrated at various inclines.

Figure 14, KOBA-step, shown with

swivel support and base, size 1,

for accommodating a 1020mm

long step-gage.


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Conducting Formal Acceptance Procedures and Monitoring the Conducting Formal Acceptance Procedures and Monitoring the Conducting Formal Acceptance Procedures and Monitoring the Conducting Formal Acceptance Procedures and Monitoring the

Precision of Machine Tools and Robotic Dimensional Gaging SystemsPrecision of Machine Tools and Robotic Dimensional Gaging SystemsPrecision of Machine Tools and Robotic Dimensional Gaging SystemsPrecision of Machine Tools and Robotic Dimensional Gaging Systems

The KOBA-step gage can be used for checking the positioning accuracies

of machine tools and articulated-arm robotic dimensional gaging systems,

applications where it offers the same elegant approach and speed as when

checking coordinate measuring machines. The swivel support and base

accessories are required for this application.

Regular use of the KOBA-step for monitoring dimensional gaging precision

of machine tools is essential to total quality assurance. Complete quality

assurance demands more than the mere precision dimensional gaging of

finished work and components; regularly monitoring the dimensional gaging

precisions of those machine tools employed in fabrication is indispensible.

Finished products must be machined to within prescribed dimensional

tolerances if they are to pass final acceptance checks. Regularly monitoring

machine tool dimensional gaging precision provides valuable information on

the degrees to which work and component dimensional tolerances are

maintained during machining operations.

Figure 15, KOBA-step installed on a swivel

support and base, shown here spanning

the major diagonal of the workspace of a

vertical milling machine equipped with a

dimensional gaging head.

Figure 16, KOBA-step shown horizontally

aligned along the x-axis of the rotary bed of

a machine tool equipped with a dimensional

gaging head.

Figure 17, KOBA-step shown aligned

vertically for checking the z-axis of a

machine tool equipped with a dimensional

gaging head

Figure 18, KOBA-step shown installed on

a flexible machining center to verify its

positioning accuracy (the center’s dimensional

gaging head is integrated into its tool chuck.)


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KOBAKOBAKOBAKOBA----step Ministep Ministep Ministep Mini

A system for calibrating and monitoring

multi-sensor measuring instruments

and coordinate measuring instruments

of small measuring volume.

The KOBA-step mini is a step gage with

a reduced cross section and gage

block distance resized according to the

measuring length and measuring task.

The advantages of the regular sized

step gage, such as cylindrical gage

block embedded in the neutral fiber of

the carrying body and access on three

sides, have been maintained. The

length and distance of the gage blocks

were reduced to 10mm according to

the total lengths which allows recognition of local errors of the measuring

instrument. The cylindrical gage blocks are made of zirconia ceramic and

are corrosion and wear resistant.

The KOBA-step Mini system is completed by the swivel support, which is

included, allowing the Mini to be mounted horizontally, vertically or in the

volume.

Traceability to the National Length Standard is guaranteed by DKD, PTB

or Works calibration.

Standard Lengths:Standard Lengths:Standard Lengths:Standard Lengths:

Nominal Lengths,

MM

Number of

Castellations /

Probing Faces

Division,

MM

Overall Length,

MM

210 11 / 22 10 220

310 16 / 32 10 330

Includes: KOBA-step Mini with zirconia ceramic probing elements, base

and swivel support for positioning from 0° to 90° (horizontally or vertically)

and an aluminum storage case.


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Standard Nominal LengthsStandard Nominal LengthsStandard Nominal LengthsStandard Nominal Lengths

Nominal Length Nominal Length Nominal Length Nominal Length

MMMMMMMM

Number ofNumber ofNumber ofNumber of

Gage FacesGage FacesGage FacesGage Faces

OverallOverallOverallOverall

LengthLengthLengthLength

WeightWeightWeightWeight

kgkgkgkg

420 22 480 6.5

620 32 680 9.5

1020 52 1080 15.0

1540 78 1600 23.0

2020 102 2080 30.0

The nominal length of a step gage is the distance from the first gage face to the last gage face.

Special sizes on request up to 2500mm.

CrossCrossCrossCross----section of holdersection of holdersection of holdersection of holder: 55x55mm Length of castellationsLength of castellationsLength of castellationsLength of castellations: 20mm

Space between castellationsSpace between castellationsSpace between castellationsSpace between castellations:

20mm (10mm for 210 and 310mm

step gages)

CrossCrossCrossCross----section of cylindrical gage section of cylindrical gage section of cylindrical gage section of cylindrical gage

blocksblocksblocksblocks: 10mm diameter

The following characteristics are identical for all lengths listed above:

210 22 220 —

310 22 330 —

AccessoriesAccessoriesAccessoriesAccessories

Swivel Support Lockable in positions –50° to 50° in 5°

increments

Base, Size 1 For mounting swivel support to step gages

420, 620 and 1020

Base, Size 2 For mounting swivel support to step gage

1540 and 2020

StorageStorageStorageStorage

Step Gages 420, 620

and 1020

Includes one storage box for gage, clamps

and swivel support and one box for the base

CalibrationCalibrationCalibrationCalibration

PTB U = 0.2µm + 0.8 x 10-6 x L

DKD U = 0.2µm + 0.5 x 10-6 x L

Works U = 0.6µm + 1.5 x 10-6 x L

NIST U = 0.22µm + .18 x 10-6 x L

“L” is the distance between any measuring faces in meters.

Step gages 1540 and

2020

Includes one storage box for gage, clamps,

one box for the swivel support and one box

for the base


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Step GageStep GageStep GageStep Gage

⊕ Ceramic Gage Blocks

⊕ Corrosion resistant body

⊕ Includes aluminum

transport / storage case

⊕ 210 and 310mm step gages

include a swivel support and

a base

Order Number Material Nominal

Length

Number of

Blocks

Contact

Points

KOB-420STEPCER Ceramic 420 11 22





KOB-210STEPCERS Ceramic 210 11 22

KOB-310STEPCERS Ceramic 310 11 22

Swivel Arm Support:Swivel Arm Support:Swivel Arm Support:Swivel Arm Support:

⊕ Swivel arm and clamps

⊕ Swivels and locks in position

–50° to 50° in 5° increments

⊕ Includes aluminum transport /

storage case

⊕ Requires Base for use (base is partially visible in photo at left)

Order Number Description

KOB-####SWISUP Locking, swivel support for Step-gages

(replace “####” with the nominal length of the step gage).

Note: 210 and 310 step gages include a swivel support.


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Step Gage Base 1 and 2:Step Gage Base 1 and 2:Step Gage Base 1 and 2:Step Gage Base 1 and 2:

⊕ Mounts swivel arm to

the step gage

⊕ Includes aluminum transport /

storage case

Order Number Description

KOB-####Base-1 Base for step gages 420, 620 and 1020

KOB-####Base-2 Base for step gages 1540 and 2020

(replace “###” with the nominal length of the step gage).

Note: 210 and 310 step gages include a base.

SOLUTIONS© “How it works”

KOBA-step gage

horizontal set-up (above)

and vertical (right)


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⊕ 210 Step Gage Set



⊕ Includes swivel support, base

and storage case

KOB-210STEPSET

Contents Description Qty

KOB-210STEPCER Step gage, 210mm nominal length 1

— Swivel support for 210 step gage 1

— Base for 210 step gage 1

Case Case for step gage, swivel support &

base

1

Note: Set does not include certification, please request PTB, DKD, Works or NIST

certification if required.





and storage case

KOB-310STEPSET



— Swivel support for 310 step gage 1

— Base for 210 step gage 1

Case Case for step gage, swivel support &

base

1




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and storage cases

KOB-420STEPSET



KOB-420SWISUP Swivel Support for 420mm step gage 1

KOB-420BASE1 Base for 420mm step gage, size 1 1

— Fixing clamp 2

Case Storage case for gage 1

Case Storage case for base 1



Case Storage case for swivel and clamps 1





and storage cases.

KOB-620STEPSET





— Fixing clamp 2







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and storage cases

KOB-1020STEPSET





— Fixing clamp 2










and storage cases

KOB-1540STEPSET





— Fixing clamp 2







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and storage cases

KOB-2020STEPSET





— Fixing clamp 2






Body Description

PTB U = 0.2µm + 0.8 x 10-6 x L

DKD U = 0.2µm + 0.8 x 10-6 x L

Works U = 0.6µm + 1.5 x 10-6 x L

NIST U = 0.22µm + 1.8 x 10-6 x L

CertificationsCertificationsCertificationsCertifications

SOLUTIONS© How it works

All KOBA-step© step gages

are available with the following

certifications


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SOLUTIONS© How it works

Business

KOBA Step Gage, Artifact