Encoders
For Machine Tool Inspection
and Acceptance Testing

05/2021

Contents

Machine tool performance from the point ofview of compliance to tolerances, surface
definition, etc., is determined essentially by
the accuracy of machine movement.

Introduction

For precision machining it is therefore important to measure and, if necessary, compensate for deviations in motion. Standards
and directives for inspecting machine tools,
such as ISO 230-2, ISO 230-3, ISO 230-4
and VDI/DGQ 3441, stipulate a number ofmeasuring methods for determining staticand dynamic deviations.

Areas of application

4

Configuration

5

ACCOM evaluation software

6

Measurement methods

In conjunction with the powerful evaluation
software , HEIDENHAIN measuring
devices for the inspection and acceptanceof machine tools permit meaningful machinemeasurements with minimal mounting
and adjustment effort.

Dynamically sampledmeasurements• Circular interpolation test
• Free-form test
• Step response test

6

Static measurement• Positioning accuracy• Guideway error

89Measurement of the thermal drift
Measuring devices for inspection of linear axes
Grid encoders for• Short linear movements• Circular movements• Free-form tests

10

General information

Fundamentals and mounting instructions

12

Angle encoders

ROD 880, RON 886, RPN 886, RON 905 overview

13

EIB 74xExternal interface box

14

KGM 281KGM 282

Measuring devices for inspection of rotary axes

Accessories

15

Adapter cables

This brochure supersedes all previous
editions, which thereby become invalid.
The basis for ordering from HEIDENHAIN
is always the brochure edition valid when
the order is placed.
Standards (ISO, EN, etc.) apply only
where explicitly stated in the brochure.

23Introduction
Areas of application

The acceptance testing and inspection ofmachine tools essentially includes thestatic verification of the geometricalmachine structure in the unloaded state
and—for NC-controlled machines—on theverification of positioning accuracy. The
final results of machining are increasingly
influenced by dynamic deviations from thenominal contour and by high acceleration
rates in the machine tool. Test workpieces
are therefore produced and inspected for
dimensional accuracy in order to draw
conclusions about the dynamic behaviorof the machine.
HEIDENHAIN offers measuring devices
for direct capture of dynamic and static
deviations. The advantage of this direct­inspection method over inspecting only the
results of the machining lies in its separationof technological influences from machine
influences, and in its capability of distinguishing individual factors of influence.

Configuration

Dynamic measurements—particularly
at high traverse speeds—provide infor­
mation on contouring behavior from whichconclusions can be drawn about both thecondition of the machine tool as well as
the parameter settings of the control loop
consisting of the CNC control, drives, andposition feedback systems. This information(e.g., kv factor, reversal spikes) can be used
to optimize the machine's behavior.
Static measurements, such as themeasurement of position error in linear androtary axes using a comparator system,
permit conclusions about the geometricaccuracy and thermal behavior of themachine.

Machine tool builders use the results ofmachine measurements to develop design
measures for improving accuracy. Suchmeasurements also help them to optimize
the commissioning parameters of thecontrol loop wherever they influence theaccuracy of a CNC machine.
Machine-tool users can use the measuring
devices for acceptance testing and regularaccuracy inspection of their machine.

A typical setup for inspecting a machine
tool consists of the following components:• Measuring device for the inspection ofaxis movements (KGM or angle encoder)• EIB 74x external interface box• PC with ACCOM evaluation software
The measuring systems for inspectionof linear axes—KGM 281 or KGM 282
grid encoders—measure the actual pathof traverse without contact but highly
dynamically. Both measuring devices permit
highly accurate, real 2-D measurement.
Angle encoders are used to measurerotary axes. They are attached to the rotarytable or tilting axis, and are connected tothe stationary machine element via ameasuring bridge.

Since the inspection setup operates completely independently, no communication isnecessary between the PC and the CNC.
Machines with any type of control can beinspected. The ACCOM evaluation software
simply needs to be used to program thesame traverse motions on the CNC and on
the PC.
ACCOM offers the possibilities of importing
NC programs as well as exporting the test
NC programs created with ACCOM. This
reduces programming efforts, since, forexample, freeform contours can simply andquickly be loaded from existing NC programs.
HEIDENHAIN Klartext programs can easily beexchanged directly between a TNC control
from HEIDENHAIN and the PC. ISO pro­grams in simplified G-code format can also
be imported by ACCOM.

ACCOM automatically detects the beginningof the inspection procedure—for example
when a certain distance or angle has been
moved from the starting position.
Measuring points are also recorded automatically whenever predefined conditions
have been fulfilled (position window, speed
window).
The measured data is processed by
ACCOM, and then displayed in a clearly
understandable manner. The data can also
be loaded by other programs (e.g. Matlab,
Origin, Excel, etc.), since they are savedin ASCII format.

CNC
NC program
Test program

PC
ACCOM
Test program

Circular interpolation tests with very small radiiand free-form tests provide information on
the dynamic behavior of the control, and circularinterpolation tests with large radii provide
information on the machine geometry.

4

Position accuracy and repeatability, as well as
guideway errors of linear machine axes,
are determined with a comparator system.

The position accuracy and repeatability of rotaryaxes, rotary tables, and tilting tables can also be
determined. A very precise angle encoder serves
as comparator system.

5

Measurement methods
ACCOM evaluation software

The measurement methods for inspection
and acceptance testing of machine tools
are governed by national and international
standards and directives. The ACCOM
evaluation software for PCs from
HEIDENHAIN is an easy‑to-use programfor measured value acquisition andevaluation according to the DIN ISO 230‑2,
ISO 230-3, DIN ISO 230-4, and ISO 10791-6(K2 and K3) standards, as well as the VDI/
DGQ directive 3441. The ACCOM evaluation
software runs on all PCs with Windows
Vista (32-bit), 7, 8, and 10 (32/64-bit).

Dynamic measurements

Free-form interpolation tests are performed
with KGM 281 or KGM 282 282 grid
encoders.
The free form shown features someinteresting contour transitions:• Continuous transition from line to arc• Continuous transition from arc to line
• Abrupt transition from line to arc• Abrupt transition from arc to line
• Abrupt transition from line to line

Circular interpolation test
In the circular interpolation test, the CNC
control performs a circular interpolation in
the working plane.
The ACCOM evaluation software compares
the values measured by the grid encoderwith the ideal (programmed) circular path,and shows the deviations enlarged on the
PC screen. ACCOM also calculates thenumerical values, such as circular error,
circular backlash, and radial error, according
to DIN ISO 230-4.
The data measured with the circularinterpolation tests permit conclusions
about the causes of the errors:• Orthogonality errors of the machine axes
• Reversal spikes during quadranttransitions• Hysteresis, reversal error• Incorrect error compensation values in
the control• Errors resulting from irregular thermal
expansion of machine components• Tilt and sag in the machine axes
• Axis adjustments• Influences of traversing speeds• Influence of acceleration

Free-form test
In the free-form test, the CNC moves themachine axes in a plane on any programmed
path. The KGM is used to measure thepath actually traversed. ACCOM displays
the errors in various views. The dynamic
behavior of the machine can be evaluatedat corners and transitions in the contour.
Free-form paths as per ISO 10791—K2
feed rates and K3 interpolation of twoaxes—can be inspected.

Other typical free-form tests can be
performed with the KGM to detect thefollowing errors or effects of the control
or mechanics, for example:• Orthogonality of two axes (large cross)• Natural vibration (slanted lines atapprox. 45°, corners)• Path interpolation of two axes (slantedlines at small angles)

Standardized representation of a circular interpolation test with a KGM:
The reversal peaks at the quadrant transitions are visible, as is thedifference between clockwise and counterclockwise traverse

Result of a free-form test with excessive errors

Circular interpolation tests performed overlarge radii provide information on the machinegeometry. On the other hand, circularinterpolation tests with small radii provide
information on the accuracy of the control
under high axis acceleration rates. At small
radii, the influence of the machine geometryon the result of measurement is insignificant.
The control and drives, however, have astrong effect.

Step response test
The step response test can be used tomeasure the smallest possible positioning
increment (step-response function) andprovide information on the influence ofstatic friction and the accuracy with which
positions can be held. This test is also
intended for high-precision tasks requiring
increments of as small as 0.1 μm to 0.01 μm.
ACCOM permits graphical representationof distance over time (Xt, Yt) and of speed
over time (vt).

Circular interpolation tests are performed
with KGM 281 or KGM 282 grid encoders.

Step response tests can be performed
with KGM 281 or KGM 282 grid encoders.

The line graph shows a magnified view of the reversal peaks at 90°

6Standardized representation of a free-form test with a KGM and detail zoom:
here a normal view shows an overshoot with the resulting rounding-off error.
(Nominal path in black, actual path in red)

Result of a step response test as an “Xt” graph7

Measurement methods
Static measurements

The positioning accuracy and repeatabilityof a machine tool is measured after themachine axis has been moved to certain
positions.

Determining the thermal behavior offeed axes
The influence of frictional heat in ball screwsof linear axes or worm gears of rotary axeson the position behavior of the feed axis
becomes obvious when positioning tests
are performed as per the ISO 230‑3
standard.

Determining the static positioning
accuracy of linear axes
The KGM can be used to determine thepositioning accuracy of a machine tool whenmoving machine axes to specified positions.
Besides the positioning accuracy, thesedevices can also measure the guideway
error perpendicular to the direction of themachine tool’s slide.

This standard contains recommendations
for uniform measurements of thermal
shifts of lathes and milling machines as aresult of external and internal heat sources.

ACCOM displays the errors clearly according
to the respective standards.
Traverse paths of up to 230 mm can bemeasured with KGM 281 or KGM 282 grid
encoders.

Measurement of the static positioning accuracy as per DIN ISO 230‑2 andthe guideway error in transverse direction

Determining the static positioning
accuracy of rotary axes
By using an angle encoder as reference,
any angular positions can be traversed to,
and a detailed graph of the accuracy canbe recorded.

To test the feed axes, it proposes a
repeated positioning to two points that lie
as near as possible to the ends of thetraverse range at an agreed percentage ofthe rapid traverse velocity. The change ofthe positions with respect to the initialvalue is recorded. The test is to be
conducted until a satiation effect is clearlyobservable.

Determining the thermal behavior of a linear axis as per ISO 230‑3
measurement and feedback via the rotary encoder on the motor(“Semi-Closed Loop”) ...

The example shows the graphs of two
measurements on a linear axis. The uppergraph, with a rotary encoder in the motor
for position capture, shows increasing
position error over time due to heating of
the ball screw. The same measurement of
the linear axis, but with a linear encoder
for position capture, is shown below. The
position error is independent of the heatingof the ball screw, since the linear encoder
always captures the actual position of theaxis slide.

The high-precision ROD, RON, or RPN
angle encoders from HEIDENHAIN areused to determine the positioning accuracyof rotary encoders.
ACCOM evaluates the measurement anddisplays the results clearly.
The example shows the graphs of two
high-resolution measurements of a rotarytable with a worm gear. The figure above
illustrates position feedback via the rotary
encoder in the motor (Semi-Closed Loop).
It shows the errors caused by the worm
shaft (short-wave oscillations) and the worm
wheel (long-wave oscillations) of the rotarytable. The measurement of the same rotarytable but with an angle encoder integrated
for position capture (Closed Loop) shows amuch smaller range of error.

Measurement of the thermal drift

The thermal behavior of linear axes ismeasured with KGM 281 or KGM 282 grid
encoders. The ROD, RON, or RPN angle
encoders are used for rotary axes.
Measurement of the static positioning accuracy of a rotary table withworm gear with the RON 905 and feedback via the rotary encoder in the
motor (Semi-Closed Loop) ...

... and for feedback from an angle encoder (Closed Loop)

Further information:
You will find more information on this
topic in the following Technical Information
documents:• Accuracy of Feed Axes
• Linear Encoders Improve Machining
Accuracy... and for feedback from an angle encoder (Closed Loop)89Measuring devices for inspection of linear axes
KGM 281 and KGM 282 grid encoders

The KGM grid encoders consist of ascanning head and a grid plate with awaffle-type graduation, which is embedded
in a mounting base. The plate and the baseare decoupled mechanically, so thattensions resulting from mounting do not
influence the measurement accuracy.
During measurement, the scanning headmoves over the grid plate without making
contact. The KGM encoders capture any
motions in a plane and separately transmit
the values measured for the two axes.
Area of application
The KGM encoders dynamically test thecontouring accuracy of controlled machine
tools. For example, they make circularinterpolation tests possible with radii
ranging from 115 mm down to 0.1 mm at
feed rates up to 72 m/min. Especially atvery small radii, the errors resulting fromthe machine’s geometry no longer have an
influence on the measurement results.

KGM 281KGM 282

Measuring standard
Coefficient of linear expansion

Two-coordinate TITANID phase grating–6 –1
Þtherm  8 x 10 K

Accuracy grade±1 µm

Measuring range¬ 140 mm

Incremental signals» 1 VPP

Signal period

4 µm in measuring directions I and II

Measuring step 0.001 µm (with EIB 74x)

Supply voltage

5 V ±0.25 V/< 100 mA (per axis)

Mount for scanning head

¬ 20h7

Traversing speed
Mass¬ 230 mm

KGM 281 KGM 282

 72 m/min

Grid plate 2.3 kg
Scanning head  0.6 kg

Accessories:• EIB 74x external interface box• ACCOM evaluation software• Two adapter cables between KGM andEIB 74x• Mounting kit for XZ/YZ plane for
KGM 282

10

304

B

180

280

D1

 228

 328

D2

 170

 262



Measuring setup
For setup, the mounting base is fixed onto
the workpiece-holding element (such as
the machining table) and aligned to theaxes. The scanning head is mounted on the
tool-holding element (for example, thespindle of a machining center) so that it
cannot rotate and is also approximatelyaligned to the axes.

Items supplied:• KGM 281 or KGM 282• Adapter for mounting the scanning headat an angle of 90° (for 20 mm mating ¬)• Mounting kit for XZ/YZ plane (only for
KGM 281)204

 4.9 kg
 0.6 kg

The contact-free scanning also permits
free-form tests over any contours in twoaxes.

An adjustment plate is included in delivery
for simple adjustment of the scanning gap
to 0.5 mm ±0.05 mm. The setting screwsof the scanning head are then used for thefine adjustment. They are used to optimize
the measurement signals displayed in the
ACCOM evaluation software.

L

Scanning
head
I, II = Directions of measurement
F = Machine guideway* = Maximum change during operation
1 = Hose connection nipple for vacuum connection (for fastening on plane surfaces/stone plates)
2 = Adjusted during mounting

Grid

Mounting
adapter

Direction of
measurement

11Measurement of rotary axes
General information

Angle encoders for the measurement of rotary axes

Angle encoders from HEIDENHAIN serve as
high precision reference encoders for themeasurement of rotary axes. They permit
measurements at any positions. Since norestrictions are necessary, such as to only
12 positions per 360°, even short-range
position errors can be measured.

Reference angle encoder

The shaft of the ROD 880 is connected
via a separate shaft coupling to the shaft
to be measured. Suitable shaft couplings,
such as a K01 diaphragm coupling or K16
and K17 flat couplings are described in
the Angle Encoders with Integral Bearing
brochure.

In addition, angle encoders make highly
dynamic motions of the rotary tablebetween the points of measurement
possible (as per ISO 230-3).

Measuring setup
Due to the different possibilities for
mounting on the machine (rotary and tilting
axes, various diameters of rotary tables, etc.),
the customer must install the reference
angle encoder himself.
A stiff connection between the stator androtor of the reference encoder must be
ensured. Since a certain amount of torqueis needed in order for the reference
encoder to move, the measuring accuracy
will be affected if the connection is not
rigid enough.
A connecting element with length L anddiameter D between the shaft of thereference encoder and the stationary partof the measuring setup becomes twisted
as shown in the graph. Whether the shaft
is solid or hollow is of secondary
importance.

RON 886 and RPN 886 encoders have an
integrated stator coupling. The shaft to bemeasured is directly connected with thehollow through shaft.
The RON 905 also has an integrated stator
coupling. The shaft to be measured is
directly connected with the blind hollow
shaft.

ROD 880

Position error in µm 

At this time, rotary and tilting axes are not
involved as simultaneously moving axes inmost cases. The positioning accuracy, as
per ISO 230-2 for example, is definitive forsuch index axes. In addition, the dynamic
and thermal behavior according to ISO 230-3
is important for the increasing number ofsimultaneously moving axes.

Radius in mm 
Influence of the distance R of the machining position from the center ofthe rotary table on the positioning accuracy Δx at various angular errors Δjof the rotary tablea

RON 886

RPN 886

RON 905

System accuracy±1”± 0.4”

Incremental signals» 1 VPP» 11 µAPP

Line count

36 00090 00036 000( 180 000 signal periods)

Measuring step
With EIB 74x

0.000005°

0.0000005°

Shaft

Solid shaft D = 14 mm

Hollow through shaft D = 60 mm

Blind hollow shaft

Starting torque 0.012 Nm at 20 °C

 0.5 Nm at 20 °C

 0.005 Nm at 20 °C

30 Nm
30 Nm at shaft end––

 2.0 kg 2.5 kg

 4.0 kg

Shaft load
			

Axial
RadialMass
Torsion error in ” Areas of application
The position errors of rotary axes (rotary or
tilting tables, swivel heads) are often decisive
factors in a machine’s overall accuracy.

Due to their accuracy and mechanicaldesigns, the angle encoders listed here areespecially suited for the measurement ofrotary axes. They have integral bearings,
but are coupled to the shaft differently:

ROD 880

RON 886/RPN 886

0.000005°

RON 905b
Diameter in mm 
Torsion error of the 100-mm long coupling of an ROD 880 via aa) Solid shaft with various diameters D1b)	Hollow shaft with outside diameter D1 = 25 mm and various inside
diameters D2Further information:
Brochure: Angle Encoders with Integral
Bearing

12

13

Accessories
EIB series of external interface boxes

Adapter cables

The EIB 700 series consists of external interface boxes for precise position measurement. They are suited for inspection stations
and multi-gauging fixtures as well as formobile data acquisition, such as in machineinspection and calibration.

The cables necessary for connecting theencoders to the EIB 74x subsequent
electronics are available as accessories.
The maximum cable length of 10 m should
not be exceeded.

The EIB 700 series is ideal for applications
requiring high-resolution encoder signals
and fast measured-value acquisition. Its
Ethernet transmission also enables the useof switches or hubs for connecting morethan one EIB. Wireless LAN transmission,for example, can be used as well.
A maximum of four HEIDENHAIN encoders
either with sinusoidal incremental signals( 1 VPP,  11 µAPP upon request) orwith EnDat interface (EnDat 2.1 and
EnDat 2.2) can be connected to theEIB 700 series.
The EIB 700 series subdivides the periodsof the incremental signals up to 4096-foldfor measured-value generation. The
deviations within one signal period are
reduced by the automatic adjustment of
the sinusoidal incremental signals.
The integrated measured-value memory
enables the EIB 700 series to save typically
250 000 measured values per axis. Internalor external triggers can be used for axisspecific storage of the measured values.
A standard Ethernet interface using TCP/IPor UDP communication is available fordata output, permitting direct connection
to a PC, laptop, or industrial PC. The typeof measured-value transmission can beselected through the operating mode
(single values, as a block, or upon software
request).
Items supplied:• EIB 74x
• Driver software• Example programs• EIB application software

EIB 741
EIB 742
Encoder inputs

15-pin D-sub connections (female, X11 to X14),
for four encoders

Input signals (switchable) 1 VPP( 11 µAPP
upon request)

EnDat 2.1

EnDat 2.2

Input frequency

 500 kHz––

Subdivision factor

4096-fold––

Cable length 150 m 150 m 100 mData register formeasured values

48 bits (only 44 bits are used)

Interval counter

Derived from axis 1 (only 1 VPP)
Interpolation factor can be set from 1-fold to 100-fold
Can be used as trigger source or additional counting axis

Measured-value memory

Typically 250 000 position values per channel

Software• Software drivers for Windows, Linux, and LabVIEW• Example programs• EIB application software

Data interface

Ethernet as per IEEE 802.3 (max. 1 Gbit)

Dimensions

Approx. 213 mm x 152 mm x 42 mm

Supply voltage

EIB 741: 100 V to 240 V AC
EIB 742: DC 24 V

Remark:
The features can be enhanced by updating the firmware.

1415

Mastering nanometer accuracy

DR. JOHANNES HEIDENHAIN GmbH
Dr.-Johannes-Heidenhain-Straße 583301 Traunreut, Germany +49 8669 31-0
 +49 8669 32-5061
info@heidenhain.dewww.heidenhain.com
208871-2A · 0.5 · 05/2021 · H · Printed in Germany

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