KOBA Step Gage, Artifact

Measurement and Standards:
Measurement and Standards
KOBA-step Step Gages

Step Gages
A complete system for monitoring coordinate
measuring machines.

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Step 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
-6

⊕ 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

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Step Gages for Checking the Accuracy of
Coordinate 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 L measurement.
e

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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 L s

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
Uncertainty 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 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
Gage 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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KOBA-
Special Features and Advantages of the KOBA-step© Gage
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.
Accessories
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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Traceability
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)
Recalibration
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.
KOBA-
Alignment of the KOBA-step
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 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 Figure 13, KOBA-step shown with swivel support and
faces using the normal gaging base, illustrated at various inclines.
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 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
Precision 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 Figure 16, KOBA-step shown horizontally
support and base, shown here spanning aligned along the x-axis of the rotary bed of
the major diagonal of the workspace of a a machine tool equipped with a dimensional
vertical milling machine equipped with a gaging head.
dimensional gaging head.

Figure 17, KOBA-step shown aligned Figure 18, KOBA-step shown installed on
vertically for checking the z-axis of a a flexible machining center to verify its
machine tool equipped with a dimensional positioning accuracy (the center’s dimensional
gaging head gaging head is integrated into its tool chuck.)

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KOBA-
KOBA-step 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:

Nominal Lengths, Number of Division, Overall Length,
MM Castellations / MM MM
Probing Faces
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 Lengths

Nominal Length Number of Overall Weight
MM Gage Faces Length kg
210 22 220 —

310 22 330 —

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.

The following characteristics are identical for all lengths listed above:
Cross-
Cross-section of holder 55x55mm
holder: Length of castellations 20mm
castellations:
Cross-
Cross-section of cylindrical gage castellations:
Space between castellations
blocks:
blocks 10mm diameter 20mm (10mm for 210 and 310mm
step gages)
Accessories

Lockable in positions –50° to 50° in 5°
Swivel Support
increments
For mounting swivel support to step gages
Base, Size 1
420, 620 and 1020
For mounting swivel support to step gage
Base, Size 2
1540 and 2020

Storage

Step Gages 420, 620 Includes one storage box for gage, clamps
and 1020 and swivel support and one box for the base
Includes one storage box for gage, clamps,
Step gages 1540 and
one box for the swivel support and one box
2020
for the base

Calibration

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.

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Step Gage
⊕ Ceramic Gage Blocks
⊕ Corrosion resistant body
⊕ Includes aluminum
transport / storage case
⊕ 210 and 310mm step gages
include a swivel support and
a base

Nominal Number of Contact
Order Number Material
Length Blocks Points

KOB-210STEPCERS Ceramic 210 11 22

KOB-310STEPCERS Ceramic 310 11 22

KOB-420STEPCER Ceramic 420 11 22





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:
⊕ 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 & 1
base
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 & 1
base

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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 swivel and clamps 1
Case Storage case for base 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

SOLUTIONS©
How it works

All KOBA-step© step gages
are available with the following
certifications

Certifications

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

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KOBA Step Gage, Artifact

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