Application Examples

WORKPIECE MEASUREMENTS ON NC(CNC)-MACHINE TOOLS: MACHINE CONFIGURATIONS/ APPLICATION EXAMPLES T. Pfeifer and A. Fiirst Department of Metrology, Laboratory of Machin e Tool, R WTH Aachen, F. R. G.

Abstract. The adaption of a 3-D probe system to numeri c ally controlled machine tools enables a measurement of workpiece in machining positions. By measurements before, during and after machining, errors can either be eliminated or recognized well in advance. The selection and adaption of a suitable 3-D probe will be described. The necessary extension of the machine control unit as well as application examples and limitations will also be explained. Keywords. Metropogy, process control, NC(CNC)-machine tools, 3-D probe. 1. INTRODUCTION Due to their flexibility, numerically

immediate vicinity of the machining

controlled machine tools are being

process itself. Then, only such a

increasingly applied in small and

quality control can help to avoid dead

medium scale series trolling the

times between o ccurance and recognition of machining errors and consequently

relative movement be-

tween workpiece dire~t

production. Con-

and tool either by

reduce costs due to waste

pieces or

remachining.A direct check of geometri e s

or indirect measuring systems

leaves little room for geometric

on the machine tool itself guarantee s

deviations. However, other production

short dead times. One possibility for

process parameters non-specific to

realizing direct quality control is to

the machine - for ego fixtures for

use an electronic 3-D probe in

workpiece and tool - lead to geometric

connection with the measuring system

deviations that could far exceed the

of the machine tool. This system

positioning uncertainty of the ma-

substitutes the tool and probes the

chine in an unloaded state. Since the

workpiece just as in case of coordinate

influence of such

measuring machines /1/ (Fig.1). The

disturbing

quantities cannot be predetermined,

modification of the Ne-unit necessary

it is essential to integrate a con-

for this purpose, the choice and

stant or intermittent control of

adapt ion of a suitable 3-D probe as

machined geometries within the pro-

well as the application possibilities

duction process. This control is more

and limitations will be discussed

efficient if conducted in the

below.

279

T. Pfeifer and A. Ftirst

280

also done in the extended control unit.

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3. SELECTION AND ADAPTION OF A

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3-D probes are already known from their

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from probes with integrated measruing systems (measuring 3-D probes) and

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switchable 3-D probes with integrated micro-switches (touch trigger 3-D probes)

(Fig. 2) /3/. The choice of

Fig. 1. Intermittend Workpiece Measurement on CNC-Machine Tool

2. SYSTEM CONFIGURATION The function of an NC-machine is to be extended with respect to measuring a workpiece clamped on the machine by replacing the tool by an electronic 3-D probe. After connecting the probe signals with the machine's position feedback unit the machine can be used as a coordinate measuring device. The coordinate system of the machine tool together with the position feedback

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Parallelogram of Plate Springs

0lnducti~e

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a..ring Points' Micro · Switches

Displacement Pick.Ups

Zeiss

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unit and 3-D probe form the basis of a spatial reference system for a 3-D determination of discrete probing pOints on the workpiece. In order to

Fig. 2. Principles of Measuring and Touch Trigger 3-D Probes

acquire the necessary accuracy of the reference system a correction of the

one of these systems must be made with

systematic positioning error in space

special respect to an application on

of the unloaded machine is essential.

machine tools. Since the probe is to

~his

1s done by registering the in-

be changed like a tool, the following

dividual position errors with com-

case-specific selection criteria are

parison measuring systems - ego Laser

first to be consiJered:

Interferometer /2/ - and storing them

- place of use

in form of a correction matrix in an

- maximum stylus overtravel

extended

- signal decoupling possibilities

CNC unit. Processing the

correction matrix as well as the conversion of the probing coordinates to geometric elements of the workpiece are

- vibratory sensitivity Due to their mechanical structure

281

Workpiece measurements on NC(CNC)-machine tools

- mostly orthogonal oriented parallelo-

ainty of touch trigger systems is larger

grams of plate springs or one spring

than that of measuring systems, it is

membrane - measuring probe systems

yet sufficient for our purpose /3/.

have certain limitations as compared

The cost price is less due to the simple

with touch trigger probe systems with

structure. On the whole, touch trigger

regard to the place of use an<1. maximum

3-D probes are to be preferred for

stylus overtravel. When adapting the

applications on machine tools.

probe to machine tools with horizontal

shows such a

working spindles, the weight of the

tool holder of a NC-boring/milling

Figure 3

3-D probe adapted to the

stylus causes various stylus pretavel according to the stylus geumetry. The

Wireless Irff1smissiGn

01 Probe Sign!1

necessary range of stylus overtravel

~~~~r.- RfCtiving Coil

of 10 mm (brake path of machine tool in rapid traverse) for protecting the

Transmitting Coil

probe against desstruction cannot be easily observed due to the restricted deforming capability of the spring

Triggering J - 0 Probe

elements. From the view-point of automatic probe changing from a tool magazine, a faultless decoupling of the probe signal - by wireless transmission, for example - is essential. As compared with the transmission of the impulse of a touch trigger probe,

Fig. 3. Adaption of a 3-D Probe to the

the transmission of analogous values

Tool holder of a NC-Machine

of a measuring probe requires more

Tool

elaborate electronic circuitry. machine.

As mentioned previously, the

Due to the relatively small pretravel

faultless decoupling of the probe sig-

until the impulse triggerring (see

nal is an important aspect when adapt-

chap. 5), touch trigger probes can be adjusted with relatively high pre-

ing a 3-D probe system to an automatic tool (probe) changer. Basically, sig-

tensions or spring constants which

nal transmission can be done via con-

improves their stability against

tacts or by wireless coupling.

vibrations. The same measures of

mission via contacts is subject to'

c~using

an improvement of the

vibratory sensitivity are not possible using a measuring probe since higher

Trans-

disturbances because of wear and pollution so that wireless transmission wins preference.

Using touch trigger

spring constants easily lead to

Frobes involves affordable electronic

measuring errors resulting from various bending deviations of the

efforts for wireless transmission. One alternative for signal transmis-

stylus /3/.

sion will be illustrated by examples

~part

from the selection criteria

already

described, the measuring un-

shown in Fig. 3 and 4.

Inductive decoupling is done via two coils.

The transmitting coil is ad-

certainty and buying costs are also to

justed onto the changeable probe and

be considered. Investigations show

the receiving coil is mounted on the

that although the measuring uncert-

s~indle

I.C.- L

box.

Figure 4 shows the

T. Pfeifer and A. Furst

282

Control - Control Addition " is discribed. This configuration is necessary for actual process intermittent workpiece measurements with a feedback of measuring results to avoid errors. However, less extended system stages are possible; the coupling of the 3-D probe system enables certain measuring tasks to be integrated within the machining process. In the lowest version the probe signal is used to reset the machine's position feedback unit at Fig. 4. Wireless Transmission of

the probed point (Fig. 5) or to inter-

Probe Signal by Detuning of

rupt machining by an alarm. This zero

Resonant Circuit

pOint can, for example, be used as a reference point to compensate deviation

transmitting and receiving circuit.

between workpiece and machine

The function is based on detunins a

coordinate system due to thermal effects

resonant circuit whose inductance is

or to detect clamping errors of work-

determined by the loosely coupled

piece. The possibility to interrupt the

receiver coil.

By opening one of

process is an interesting application

the switches or deflecting the probe

case for fully-automated processes as,

stylus a resonance shift is caused,

for example, intended for future DNC-

which in turn leads to a voltage drop

system/4,5/. The probe acts as an aid

at the output end of the transformer.

to detect clamping errors or to check

This voltage drop is detected by a

production errors caused by tool

highly selective synchronous demodu-

damage or wear. A simplified quality control by checking complete ma-

lation circuitry.

After rectification,

aaaption ana debouncing, a trigger

chining is ?ossible. If the 3-D probe

impulse is generated to read and

is adjusted on the machine table, it

store data of the machine's position

is possible to conduct an automatic

feedback unit.

langitudinal or radius

Besides the rela-

compensation

tively simple and therefore fair

of tool. A pre-adjustment of the tool

priced set-uf, the following features

is then superfluous. In the next

characterize the transmission system: - signal transmission without auxiliary energy on transmitting end - insensitivity to stray fields - low time constants (50 JUS) - insensitivity to distance alternations (Fig.4, bottom) 4. NECESSARY USE-SPECIFIC

version the probe system is directly connected to the position feedback unit. On probing, the measuring values are read-off and displayed. Thus the machine operator is able to re-adjust the workpiece or make alterations in machining program, when testing the NC-progam at the first part of a batch. Furthermore,

EXTENSIONS OF THE MACHINE

with the aid of a step-gauge, one can

CONTROL

conduct a periodic check of

Within the introduction the system

positioning accuracy of machine tool

configuration "Machine Tool - 3-D Probe

(ref. chap. 5) with relative ease.

Workpiece measurements on NC(CNC)-machine tools

283

Measuring procedures on the machine

Other possibilities for processing

tool described till now can be

measuring

considerably improved with regard to

control unit by additional measuring

reliability and speed, if, in an-

data exist in extending the control

other stage, measuring values are

unit by additional measuring modules.

processed with the aid of an

In future this can be realized in a

data exist in extending the

additional data processing unit . Only

multiprocessor control structure

then can a true 3-D workpiece

whose concept enhances modularity

measurement

realized. The data processing unit is

by virtue of uniform interfaces and a c ommon data way / 6 / . A combination

to compute geometric elements out of

of the last two alternatives yields

on the machine be

probed points and determine

a third for elaborate measuring tasks,

differences between nominal and

The extended control could be useo

actual values. This task can be fulfilled on a separat distant small-

for storing individual probed points

scale computer - ego desk calculator

instructions for such operations

or mini-computer - or on a central

have been masked and interpreted.

process computer in case of inter-

Further processing is done on a

in a storage module after special NC

linked NC-machines. Besides quality

seperate computer after transmitting

control protocols, it may be

the data via a suitable communication

necessary to have a direct feedback

medium.

of data to the machine to correct the

is advantageous since the instruction

NC-program. This can, for example, be

set of the control unit does not

done using a BTR - interface between machine tool control and compute.r.

Such a system configuration

require to be considerably extended It suffices

for measuring tasks.

to have additional instructions for

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to the distant computer that connects measuring data with geometric ele-

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5. APPLICATION EXAMPLES AND LIMITATIONS At our laboratory detailed investi-

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qat ions are beinq conducted reqarding the applications and limitations of process intermittent workpiece measurements on a development system "3-D Probe - NC-boring/milling machine and Process Computer". The most im-

Fig. 5. Possibilities of Using a 3-D Probe on NC-Machine Tools

portant aspect of the investigation are the influence of machine errors and special measuring conditions

T. Pfeifer and A. FUrst

284

during machining. On the one hand it

also varies with the direction of

is important to determine the random errors that cannot be compensated.

approach of the probe /3/. The most

On the other the systematic errors

stylus is deflected over one contact

and their compensation must also be

or bearing pOint(Fig. 2) and the

investigated.

disconnecting sequence of the other

The random errors consist of

measurements (Fig. 6 B) .

inconvenient values arise when the

two contacts changes during repeated positioning dispersion of the un-

- Both, the repeatability and

loaded machine and the measuring

difference between pretravels in-

uncertainty of the probe system to-

crease with larger lengths of the

gether with its fixture errors. Both

stylus.

error components will be explained by

In the example illustrated,both

analysing experimental results. Con-

components when probing accross the

sidering the probe system, the

probe axis

following errors of a triggering

worst case, superimposite to a total

system (Fig. 2) are possible:

error of

- According to the direction of

±

er~or

(xy -plane) can, in the 4.5;um. A convenient

adjustment - eg., the line =45 0 ,225 0

approach of the probe different

corresponds to the x-axis of

pretravel or switching distances are

machining/measuring center - and a

necessary to issue a touch trigger

calibration of probe ball along

impulse by opening the switching

several positions in the xy plane

contacts (Fig. 6 A) This gives rise

leads to a considerable reduction of

to a measuring error principally

this value /7/. Furthermor e , if the

systematic in nature, but difficult

probing is done along axes (x,y,z),

to compensate under operating

the measuring error can be reduced

conditions /7/. This error is there-

to approx.

fore listed as a random one that

investigations, the errors are

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independant of the probing speed which, in fact is particularly advantageous for curtailing measuring ToUC.hltlgVtlOCtty : v - lOO mnv'mln Slyru~ lrnglh : TS "20mm

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mid-point of the probe ball lies without the axis of the tool/probe


clamping system. If, during changing,

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changing systems in machining centers

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- and the probe system does not undergo a rotary shift relative to this

Fig. 6. Measuring uncertainty of a Touch Trigger 3-D Probe cannot be compensated. - Due to varying contact

spindle position, then the maladjustment of the probe ball can be determined by an initial calibration cycle.

conditions,

the repeatability of measurements

In this case the adjustment error i s random in nature and is about as

285

Workpiece measurements on NC(CNC)-machine tools

large as the tool clamping error. The possibilities of avoidung the

as checked by probing a step-gauge with a touch trigger probe (dynamic

systematic clamping error are either

registration of measuring values) .

the establishment of a fixed relation

Although the results of checking the

between the radial spindle and probe

positioning error with a step-gauge

ball position or precise mechanical

contain random errors of both

construction of the probe that

machine tool and probe, the

guarantees a central position of the probe ball even when using various styli. Otherwise, when a fixed

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position between tool axis and center of probe ball is required, an initial calibration cycle at a fixed normal

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must be conducted after each change in order to compensate the error. In view of curtailing measuring times, this alternative is to be given last

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The random part of the positioning error is considerably dependant on the back-lash between machine tables and drives as well as the stability

Fig. 7. Inspection of Position Accuracy of a NC-Machine Tool with Step-Gauge and 3-D Pobe

of the position control loop. Since,

in Comparison to Inspection

in the case of using direct position

with a Laser-Inferferometer

feedback units like scales, the drive elements have littel influence on the

positioning dispersion is

lost motion, the lost motion and

comparatively lower. Besides it must

positioning dispersion are not large.

be mentioned that the positioning

Using a touch trigger 3-D probe

dispersion in the example of this

measurements are done by dynamic

machine tool is relatively low and

probing. In this case the measuring

hence convenient for improvement of

results are not influenced by in-

the measuring accuracy by compensating

stabilities of the position control

the systematic positioning error. This

loop. The functions within a measuring

is shown in the upper part of Fig. 8,

cycle of a position control loop

where the mean positioning error of

consist of positioning the probe ball

both measurement procedures and the

between probed points, stopping within

mean remaining error after online

the probe's overtravel limits and

error compensation is drawn. The

reversing direction of approach for

remaining positioning error amounts

the next point to be probed. This is

about + 1 um and lies within the limits

vivid from the error curve of an NC-

of the positioning dispersion.

boring/milling machine with a medium operating scale (Fig. 7). The lower

Among possible systematic errors of

part of the figure depcits the random

direct workpiece measurements on the

positioning error as measured with a

machine tool the following may be

laser interferometer (registration of

considered: displacements of the

a measuring values in still state) or

reference coordinate system due to

T. Pfeifer and A. FUrst

286

thermal effects, alternations of the

-The displacements (Llx, Ll y, Ll z) occur

reference length of the scales due to thermal effects and the systematic

almost instantaneously with tem-

positioning deviation. As compared

increases in length up to 60;um. A

with measuring machines, large

vertical temperature gradient (y-axis)

thermal errors of machine tools

along the cross section causes

can result from varying

bending, that leads to translational displacements along the y-axis of

perature changes. The spindle box

temperatures of the operating environment, heating of machine tool

upto 35 ?m. Only slight displcace-

caused by friction in the drive elements

ments in the x-axis are observed

and gears as well as heating from the

since the temperature gradi e nt ill the

cutting process itself. Figure a shows

horizontal direction is quickly stabilized. -On cooling (spindle revolutions n=o) the displacements reduce almost instantaneously with the temperature -Even after long warming-up times, load changes cause displacements upto

10 fm.

On the one hand this example shows that intermittent probing of reference edges on a workpiece or on the machine table during machining can compensate thermal displacements by resetting the Fig. a.Spatial Displacement of Cutter Spindle of a Boring / Milling Machine caused by Heating of Spindlebox

machine's position feedback unit. On the other hand it also shows the limitations of measuring on the machine due to long measuring cycles. For example, when measuring during the

the thermal displacements of the

cooling period of machine tool for a

spindle box of a milling machine by

time of 20 min displacements up to

varying the spindle revolutions in the

10

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(Fig. 7,left) are possible. Even

still state. The temperatures of the

in this case a compensation during

drive chassis at a characteristic

measuring is possible by reference

point are also to be seen in the

pOint probing. This is demonstrated

figure.

The displacements are measured on the example where a workpiece is

continually at the tool clamping position with a measuring 3-D probe.

machined and then measured on the came

This

machine. The workpiece is of aluminium

enables the direct measurement of dis-

with equidistant stagad holes that are bored in sequence. Figure 9A shows the

placements in space relative to the machine's coordinate system. The results indicate the following: -After a cold start, the temperature of the spindle box rises up to 20 0 K and does not reach a stable value even after 2 hours.

machining sequence. After machining each stage, a reference point on the workpiece is probed, and the displacement due to thermal influence registered is compensated by shifting the reference pOint in the Ne-program. Inspite of long warming-up times, a

Workpiece measurements on NC(CNC)-ma chine tools

287

thermal displacement of approx. 20jUm

results with respect to the dispersion of

was measured between machining of the first and last boring stages. Results

the hole mid-pOints. Thus it is concluded that roundness-errors of the tool ~pindle

of measurements on a high preciSion

causing form errors of the holes in-

coordinate measuring machine confirmed

fluence the measuring results to a

that these displacements could be

greater extent.

Furthermore, it is

largely eliminated by integrating the

shown tha t thermal influences are

measuring cycle within the machining

largel y compensated so in the cooling

sequence (Fig.9B). The standard

rhase.

The compensation is done by

probing reference pOints after measurAI

ing a complete bored stage.

For probing

of 20 points a measuring time of ap[,rox. 81

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During this 2 , 5 min was re~uired. rel ative l y short int erval there was no

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considerable tem perature changes and conse~uently

only slight thermal dis-

placements arose.

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Fig. 9.Measurement of Bored Holes on

by intermittent probing of reference points during machining and also by

the NC-Machining/Measuring

measuring a finished workpiece.

Center in Comparison to Mea-

Practically the question arises, as to

surement on a Coordinate Mea-

how often probing is to be done or

suring Machine

what rest error is tolerable without increasing measuring times and con-

deviation of mid-pOint coordinates

sequently production times too. A

between successive boring stages(same

decision is to be taken by considering

z-nominal position) amounted to a

the error and the ensuing costs due to it.

;urn'

maximal of 3 Without compensation, the displacements would have caused

of various thermal elongations of

even greater standard deviations. The

workpiece and scale of position feed-

rest dispersions of mid-pOint coordinates are mainly due to roundness

able geometric deviations during ma-

errors of the holes (max. 4

chining or cause measuring errors when

~m)

that

Compensation is more difficult in case

back unit. This can lead to consider-

strongly influence the results of de-

measuring the finished workpiece. An

terminating the geometry of holes by

illustration is given in Fig.9C. The

.measurement of discrete pOints /8/.

curve shows the mean position deviation

A comparison between results of mea-

of the hole mid-pOint with reference to

surements on a coordinate measuring

the axis of the first concentric hole.

machine and on a tool machining/mea-

Considering the results obtained on a

suring center confirms this. Inspite

coordinate measuring machine shows a

of larger random errors of the

rise in negative deviations in pro-

machining/measuring center, there is

portion with the distance of concen-

no significant difference of measuring

tric holes from the reference hole.

288

T. Pfeifer and A. FUrst

This error-characteristic indicates a difference between workpiece and scale temperatures durinq machininq. This is also clear from results of measurements with the machining/measuring center. A difference between both

- The thermal elongations must first be compensated since they are often larger or in fact contrary to the positioning error. - The amount of the positioning error must be estimated for each machine

measurements exists only between the

tool. Should it be less than the

first two hole mid-points. It amounts

error to be expected from other in-

to approx. 4

fID

which lies within

the range of the mean positioning error of this machine (Fig. 7). The relatively good agreement

of the

measuring results was achieved after

fluencing quantities, then no compensation is necessary. - The measuring devices for determining the positioning error must be available; mathematical methods

an online compensation of temperature

for

differences between measuring scale

developed.

and workpiece by means of additional

processing errors are to be

- Since the reference coordinate

temperature measurements. However,

system is subject to wear and there-

compensation of temperature differ-

fore not permanently stable,

ences during machining and during

correction values must be constantly

measuring is e ve n possible without

updated by periodic machine checks

temperature measurements when a known

(eg. checking with a step-gauge).

reference scale is probed in certain intervals. For example, a reference length made of

invar and mounted on

the machine table can be periodically probed. Such a reference length is considered to be thermically stable because of its low extension coefficient. In an integrated machining or measuring cycle, first the reference length and then a particular machined length of the workpiece will be probed at regular intervals. The longitudinal change of the measuring scale results from a difference of the measured value with respect to a value at a reference temperature (T=293k). Thus it is possible to measure changes in length relative to the workpiece. Finally, the following aspects are to be considered when examining the systematix positioning error in space: - Basically, the systematic positioning

The first three points must be anaylsed with regard to a particular application case and will not be discussed here. One dimensional positioning errors such as lost motion pitch error of lead screw or indexing errors on measuring scales can, today, be corrected by controls available to the user. However, the determination, description and compensation of errors with a more complex characertistic or such that superimpose each other is more difficult. In this connection one dimensional errors with short periods, straightness and squareness are to be mentioned.

deviations

Figure 8 (upper

part) shows the curves of a positioning error resulting from a long period due to deviation from straightness and a short period error of the measuring system with a periodic length

oUt, =635;urn and an amplitude of 2/'UIT\. An accurate analysis of this error is

error is to be compensated if it is

only possible by continuous trans-

at least twice as large as the random

mission error measurement/11/. The

error.

mathematical description can be done

Workpiece measurements on NC(CNC)-machine tools

with the aid of a polynome and periodic function. A similar description was used for online error compensation, when measuring a step-gauge on machine

289

description and consequently that of the compensation is dependant on the density of measuring lines which in turn must be adjusted to a praticular machine tool.

tool (Fig 7).

6. CONCLUSION The fundament for connecting measurable error components to the spatial

Adapting a 3-D probe system to a machine

positioning error is the measurement

tool enables a reduction of production

on a network within the machine's

uncertainties. Depending on the com-

operating area /7/. Each measuring line

plexity of the measuring tasks, the

is related to the reference pOint of

machine's control system is to be

the machine's position feedback units.

extended. Controls with modular

The actual coordinate system of the

structures and unified interfaces are

machine is determined from measurements

particularly suitable for the integration of additional functions. In

of the deviation in straightness and squareness in the direction of traverse. order to achieve measuring accuracy, the random error of the unloaded maResults of one-dimensional error meachine and the 3-D probe must be small

surements are connected to this coordinate system. The position

as compared to the machining error.

deviations in any point are calculated

Systematic machining errors such as

from a linear interpolation between

thermal displacements of the coordinate

the deviations along the measuring

system and geometric errors of machine

lines/7,10/. The quality of error

slideways can be compensated.

REFERENCES /1/ Autorenkollektiv, Einsatzschwer-

/5/ N.N.,Probing for that touch of

punkte der Drei-Koordinaten MeB

quality, Machinery and production

technik, Industrie-Anzeiger,Nr.

engineering, 16. August 1978

70, 1978, S. 36-42 /2/ Einsatz des Laser-Interferometers zur Abnahme und Uberwachung der Positionsgenauigkeit von koordi natengesteuerten Fertigungsein

/ 6/ Autorenkollektiv, Neuere Steuerungskonzepte zur Produktivitatssteigerung, Teil 1, Industrie-Anzeiger, Nr. 78, 1978,S.86-98

richtungen, QZ,BD.18(1973) Nr. 9, S.220-229

/ 7/ Pfeifer,T.,Bambach,M. Definition und Prlifung von Kriterien zur Bestimmung

/3/ Pfeifer,T., Bambach,M., Flirst,A.,

systematischer und zufalliger Fehler

Ermittlung der MeBunsicherheit

von Drei-Koordinaten-MeBgeraten,Be-

von 3 D-Tastsystemen, Teil 1 und

richt zum Forschungsvorhaben 11 B

Teil 2, Technisches Messen tm,

8-FA 7057, Westdeutscher Verlag,

H2,1979,S. 47-52 und H4, 1979,

1979

S. 161 - 169 /4/ N.N.,Swedes show the way with unmaned line,Machinery and

I.C.-

l·

/8/ Wirtz,A., Berlicksichtigung von Formabweichungen bei der MaBbestimmung

production engineering, 25 May

auf 3-Koordinaten-MeBmaschinen

1978

Fertigung, Heft 2, 1976, S.71-74

290

T. Pfeifer and A. Flirst

/9/ Pfeifer,T.,Wiechern,R., Ubertra-

/10/Schneider,C.A., Entwicklung eines

gungsfehlermessung-Methodik und

Laser-GeradheitsmeBsystems zur

Einsatzmoglichk c iten bei kinema-

Durchftihrung geometrischer PrUf-

tischen untersuchungen an Werkzeug-

ungen im Maschinenbau, Dissertation

maschinen, VDI-Z, Heft 14, 1979

RWTH-Aachen, 1979