Develop procedures to determine geometric measurement errors after work-piece machining Dirk Beger,...

Develop procedures to determine geometric measurement errors after work-piece machiningDirk Beger, Lisa Groos, Klaus Wendt

TIM Workshop, London

5th November 2014

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Traceable in-process dimensional measurement

Objective: traceability of dimensional 3D

measurements on machine

tools

Requirement: reliable and traceable

measurement uncertainties;target accuracy

5 µm/m

Challenge: harsh environmental conditions e.g. no air-conditioning

Methods: use of • different material standards• different procedures• climate simulation chamber

Source: renishaw.com

Figure 3 Prismatic shapes

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Benefits of on-machine measurement

finishmachining

on-machinemeasurement re-machining ?

dismountworkpiece

remountworkpiece

measuringroom

workpiececoordinate setting

workpiececoordinate setting

Challenge: measurement under shop floor conditions complex determination of measurement

uncertainty

Benefit: shortened measurement duration reduction of energy costs (no expensive air-conditioning)

Target accuracy: 5 µm/m

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Procedures

• volumetric error mappingmapping of geometric errors across the entire working volume (generally requires a mathematical error model)

• verification of MPEvalidation of error compensation and MPE compliance test (ISO 230-6, ISO 230-10)

• task-specific error mapping and correctiondetermination of size, form and position errors of shape measurements / typical geometric features (generally does not require a mathematical error model)

Source: Hüser et al., 1996

x

y

zD3 D4

D1 D2Source: Modified from ISO 230-6

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Procedures

• task-specific measurement uncertaintydetermination of measurement uncertainty for shape and form measurements / typical geometric features

• fitness-of-purposeassessment of the capability of a machine tool to measure geometric features with sufficient small accuracy compared to the required manufacturing tolerance

lower specification limit

upper specification limit

work piece tolerance

non-conformity

uncertaintyregion

uncertaintyregion

non-conformityconformity

measurementuncertainty

measurementuncertainty

measurement uncertainty

operator

machine tool

probing system

measuring method

environment workpiece

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Correlation: procedure, standard and error

procedure materialstandards

error sources

volumetric error mappingverification of MPE

geometric, thermo-mechanical

task-specific error mappingtask-specific measurement uncertainty

+ dynamic forces+ motion control+ control software

fitness-of-purpose++ loads++ heat

Source: eµmetron


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Geometric errors of linear and rotary axes – ISO 230-1

6 error motions

EAX roll

EBX yaw

ECX pitch

EXX positioning

EYX vertical straightness

EZX horizontal straightness

+ 3 location / orientation errors

6 error motions

EAC tilt

EBC tilt

ECC angular positioning

EXC radial

EYC radial

EZC axial

+ 5 location / orientation errors

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Determination of machine tool errors using a hole plate

thermo-invariant Zerodur® hole plate

• hole plate made of Zerodur®

• mapping of error motions and location/orientation errors (ISO 230-1)

• volumetric errors are determined for

temperatures between 15° and 45° (use of mobile climate simulation chamber)

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• systematic evaluation of the geometric errors based on measurements of a hole plate in four positions

• developed for coordinate measurement machines at the PTB

• more information (e.g.):

A reference object based method to determine the parametric error components of coordinate measuring machines and machine tools

E. Trapet and F. WäldeleMeasurement Vol. 9 No. 1, pp. 17-22 (1989)

P osition 1

P osition 2

P osition 3

P osition 4

Linear axes subsystem:

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Rotary axis subsystem:

x

y

Approach:• measure coordinates of at least 3 points

Assumption: • errors of linear axes already corrected

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x

y


• rotation around C-axis and measurement of the chosen points


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• rotation around C-axis and measurement of the chosen points

• description of the rotation with homogeneous transformation matrices yields:



ZCACBC

YCACCC

XCBCCC

EyExE

EzExE

EzEyE

z

y

x

• calculation of errors by a Gaussian fit (method of least-squares)

x

y


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Determination of measurement uncertainty

Approach:

• use of Monte-Carlo-Simulation within a virtual machine tool → expanded measurement uncertainty U

(of type B according to GUM)

Advantage:

• determine smallest measurable tolerance Tmin in advance via

G

UT min

G: limiting value specified by user

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error sources






Source: eµmetron


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Task-specific error compensation – material standard

• close to zero thermal expansion (made from Zerodur®)

• specifications:• form errors< 1 µm• roughness < 0.2 µm• position and size of features will be

calibrated on CMM

• measurement tasks:• hole• flat surface• coaxiality

Traceability for tactile measurements

Traceability for optical measurements

• extremely low coefficient of thermal expansion (made from Invar)

• specifications:• plated surface• form errors < 5 µm• position and size of features will be

calibrated on CMM, accuracy M 2 µm

• measurement tasks:• hemisphere, cylinder, cone• flat and perturbed surfaces


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Task-specific error compensation – concept

Approach:

Calibrated artefact

• acquire local error vectors by measuring a calibrated artefact (e.g. Zerodur® Multi-Feature-Check or a workpiece replica)

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Real workpiece

• measure real workpiece using tactile or optical probe

• use the local error vector to correct the points measured point by point

• evaluation of the corrected points

Approach:

Calibrated artefact


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Real workpiece

• measure real workpiece using tactile or optical probe

• use the local error vector to correct the points measured point by point

• evaluation of the corrected points

Approach:

Calibrated artefact


Advantageno specific geometric error model of

the machine tool is required

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Task-specific error compensation

DesignCAD model

Manufacture5-axis machine tool

Measuretask-specific errors

Software interfaceAdjustment of manufacturing

parameters in numerical control for on-line error correction

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Task-specific error compensation

DesignCAD model

Manufacture5-axis machine tool

Measuretask-specific errors

Software interfaceAdjustment of manufacturing

parameters in numerical control for on-line error correction

Adjust CAD model

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error sources






Source: eµmetron


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Fitness-of-purpose

within tolerance

uncertaintyregion

region of conformity

lower specification limitspecification phase

verification phase

region of non-conformity

region of non-conformity

work piece toleranceout of tolerance out of tolerance

incr

easi

ng m

easu

rem

ent

unce

rtai

nty U

uncertaintyregion

upper specification limit

1.03.0 GT

Ug

Illustration adapted from ISO 14253-1

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Assessing fitness-of-purpose – concept

Experimental determination of measurement uncertainty by

workpiece replica material standard (WR-MS) according to ISO 15530-3:

Manufacturing of WR-MS on machine tool

Repeated measuring of WR-MS on machine tool

Calibration of WR-MS on a CMM

Additional error sources:Loads and internal

heat sources

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heat sourcesMean value + standard

deviation of size, form or position measurements

ypu

Reference result for size, form or position +

calibration uncertainty ucalu

calx

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ISO 15530-3


heat sources

Fitness-of-purpose: U/T ≤ G

Mean value + standard deviation of size, form or

position measurements

ypu

Reference result for size, form or position +

calibration uncertainty ucalu

calx

Additional uncertainties uw

from manufacturing process

Systematic error: calxyb

2222 buuukU wpcal Expanded measurement uncertainty:

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Conclusions

• Geometric errors of machine tool axes can be systematically mapped using a hole plate for instance, which allows to assess the task-specific measurement uncertainty by a virtual CMM / MT for instance

→ determination of smallest measurable tolerance

• Task-specific error mapping allows improving manufacturing accuracy

→ by adjustment of manufacturing parameters

• Measurement uncertainty can be experimentally determined by repeated measurements of calibrated artefacts

→ fitness-of-purpose

Ensure traceability of dimensional 3D measurements on machine tools

for quality assurance.

Physikalisch-Technische BundesanstaltBundesallee 10038116 Braunschweig, Germany

Dirk Beger, Lisa Groos, Klaus WendtWorking Group 5.32Coordinate Measuring Machines

Phone: +49 531 592-5349E-mail: [email protected]

As of: 10/2014

Develop procedures to determine geometric measurement errors after work-piece machining Dirk Beger,...

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