cmm inspection fundamentals.ppt

7/23/2019 CMM inspection fundamentals.ppt

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2

Which inspection solution will suit your application?

Dynamic effects on scanning

performance

Articulation or fixed sensors?

Stylus changing or sensor

changing?

Touch-trigger or scanning?

ro!ing applications

Active or passive scanning?



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3

ro!ing applications - factors

Manufacturers need a range ofmeasurement solutions.

Why?

• Machining processes have different

levels of stability:− Sta!le form "

− therefore control size and position

⇒ Discrete point measurement

− #orm variation significant "

− therefore form must be measured and

controlled

⇒ Scanning



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4

ro!ing applications - factors

Manufacturers need a range ofmeasurement solutions.

Why?

• eatures have different functions:

− for clearance or location

−form is not important

⇒ Discrete point measurement

− for functional fits

−form is critical and must be controlled⇒ Scanning

Measured values

$est fit circle

Maximum inscri!ed

%functional fit& circle



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5

Discrete point measurement

Ideal for controlling the position or si'e ofclearance and location features

! "ata capture rates of ( or ) points per

second

! #voids stylus $ear

! Touch-trigger pro!es are ideal

% lo$er cost& small size and great versatility

! Scanning pro!es can also be used % passive pro!es can probe 'uic(ly

% active pro!es are slo$er because the probe

must settle at a target force to ta(e the reading



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6

Discrete point measurement

Speed comparison

Touch-trigger pro!es are ideal

for high speed discrete point

measurement

Scanning pro!es can also

measure discrete points

*uic+ly, and provide higher

data capture rates when

scanning



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7

Scanning

Ideal for controlling the form or profile of (no$n features that form functional

fits $ith other parts

! "ata capture speeds of up to ..

points per second

! Incurs $ear on the stylus

Scanning allo$s you to:

! "etermine the feature position

! #ccurately measure the feature si'e

! Identify errors in the form or shape of

the feature



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8

Scanning

Scanning a cylinder !loc+

Scanning provides much more information

a!out the form of a feature than discrete

point measurement

! Typical scanning

routine& measuring

precision features

$here form is critical toperformance



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9

Digitising

Ideal for capturing large amounts of dataa!out an un+nown surface

! )ses many of the same techni'ues as

scanning

! "eflection vector of the probe is used todetermine the motion vector in $hich the

machine moves ne*t

"igitised surface data can be:

! +*ported to CAD for reverse engineering

! )sed to generate a machining program

for re,manufacture



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10

Digitising

/e-manufacture and reverse engineering

Digitising provides large amounts of data

to define un+nown contoured surfaces

! "igitising a master part

to ac'uire an accurate

description of the

surface! Scanning cycle and

data analysis handled

by Tracecut soft$are

! "igitising can be

performed on CMMs&machine tools or

dedicated platforms li(e

Cyclone



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11



performance



changing?


ro!ing applications




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12

0deal applications

! Measurement of size& position

and form of precision

geometric features! Measurement of profiles of

comple* surfaces

! Inspection of -" prismatic

parts and (no$n surfaces

! Size and position processcontrol applications $here

form variation is not

significant

Scanning Touch-trigger



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13

Speed and accuracy

! igh speed data capture , up

to /00 points per second

! arge volume of data gives anunderstanding of form

! igh point density gives

greater datum stability

! "ynamic effects due toaccelerations during

measurement must be

compensated if high speed

scans are to produce accurate

measurement results

! Slo$er data capture rate

! ess information about the

surface

! Simple calibration of probe

and machine yields accurate

point data

! "ynamic performance of themachine has little impact on

measurement accuracy since

probing is performed at

constant velocity




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14

Complexity and cost

! More comple* sensors& data

analysis and motion control

! igher costs than basic touch,trigger systems

% Conventional systems have

higher purchase and

maintenance costs

% enisha$ scanning systems

are more cost,effective and

robust

! Simple sensors $ith a $ide

range of application soft$are

! o$er costs than scanningsystems

% obust sensors

% +asy programming

% Simple to maintain % Cost,effective replacement for

lo$er lifetime costs




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16

The ideal scanning system

Characteristics of the ideal scanningsystem"

− 1igh speed, accurate scanning of the

form of (no$n and un(no$n parts

− /apid discrete point measurement $hen

measuring feature position

− #lexi!le access to the component to allo$

rapid measurement of all critical features on

the part

− 2asy interchange $ith other types of

sensor& including touch,trigger probes and

non,contact sensors.− #llo$s the sensor choice for each

measurement to be optimised

− Minimum stylus wear



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17



performance



changing?


ro!ing applications




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18

Dynamic effects on scanning performance

The scanning paradox3

! Modern CMMs can move 'uic(ly&

yet conventional scanning is

typically performed at lo$ speeds

% less than 1/ mm sec 30.4 in5sec6

Why?



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19

Dynamic effects on scanning performance

Scanning induces dynamic forces in the structure of the CMM and

the probe itself& $hich can affect

measurement accuracy

Dynamic errors are related to

acceleration of the machine andprobe as the stylus is moved over

the surface of the component



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20

1ow do machine dynamic errors arise?

Discrete point measurement isdone at constant velocity ,

acceleration is zero at the point of

contact

% $ith critical damping

Conse'uently there are no inertial

forces on the machine or probe



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21

1ow do machine dynamic errors arise?

Scanning re'uires continuouslychanging velocity vectors as the

stylus moves across a curved

surface

4arying inertial forces are induced&

$hich cause the machine todeflect

4i!ration is also a factor $hen

scanning



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22

What a!out scanning sensor dynamics?

"uring scanning& the deflection of the probe variesdue to the difference bet$een the programmed

path and the actual surface contour

! The probe must accommodate rapid changes in

deflection& $ithout loss of accuracy or leaving the

surface

! The ideal scanning sensor can accommodate

rapidly changing profile due to:

% a high natural fre'uency

% lo$ suspended mass % lo$ overall $eight

7hilst important& pro!e dynamics have a very small

effect compared to machine dynamics



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23

Dynamic errors in practice

2xample" measure a 8/0 mm 32 in6 ring gauge at (. mm5sec %.67in5sec& using a CMM $ith performance of )6 8 95).

orm error

)µ

m

Static errors

dominate atlo$ speed



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25

The dynamic performance !arrier

Dynamic errors increase as speeds rise

+rror

Speed

At higher scanning speeds, machine dynamics

!ecomes the dominant source of measurement

error



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26


+ma*

S1

+rror

Speed

Scanning speeds have to !e +ept low if tight tolerancefeatures are to !e inspected

9eft uncorrected, machine dynamics present a

dynamic performance !arrier to accurate high

speed scanning



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27


We need a way to !rea+ through the dynamic performance!arrier, ma+ing high speed scanning more accurate

+rror

Speed

+ma*

S1 S2



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28



performance



changing?


ro!ing applications




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29


Articulating heads are a standard feature on the ma9ority of computer,controlled CMMs

% eads are the most cost,effective $ay to measure comple* parts

#ixed pro!es are best suited to applications $here simple parts are to be measured

% Ideal for flat parts $here a single stylus can access all features



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30

Articulating heads - !enefits

! #lexi!ility , a single& simple styluscan access features in many

orientations

% Inde*ing and continuously variable

solutions



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31


/epeata!le indexing using +inematic principles"! Method"

% /0 measurements of calibration sphere at #;/&<;/=& then /0 $ith an inde* of the >10M head to #0&<0= bet$een each reading

! T>200 trigger probe $ith 10mm stylus

! /esults"

! Comment"

% Inde*ing head repeatability has a similar effect on measurement accuracy to stylus changing repeatability

esult Span fi*ed Span inde* ∆ ?Span@ ;/epeata!ility<

A 0.0004- 0.0011B 0.000/4 = .6...>7

0.000-B 0.00141 0.00122 = .6...>

D 0.000;/ 0.000E1 0.000-4 = .6...(7

l i ti



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32

Articulating heads - repeata!ility

0ndexing repeata!ility affects the measured position of features

% Size and form are unaffected

Most features relationships are measured @in a plane

% eature positions are defined relative to datum features in the same plane 3i.e. the same inde* position6

!"atum featureused to establish a part co,ordinate system

% Therefore inde*ing repeatability typically has no negative impact on measurement results& but many benefits

appl inno ation



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33


! Speed , inde*ing is faster thanstylus changing 3done during CMM

moves6

! Dynamic response , simple& light

styli ma(e for a lo$er suspendedmass

! Costs

% simple styli $ith lo$ replacementcosts

% small& lo$ cost stylus change rac(s

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34

1(.M indexing head - design characteristics

#lexi!le part access

/apid indexing during CMM positioning moves give

flexi!le access with no impact on cycle times

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35


! Automation , programmable probechanging $ith no manual intervention

re'uired

% touch,trigger& scanning and optical

probing on the same machine

! Stylus changing , even greater

fle*ibility and automation

% optimise stylus choice for eachmeasurement tas(

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36

Articulating heads - disadvantages

! Space , a head reduces available Dtravel by a small amount , can be an

issue on very small CMMs

1(.MB in,'uill

version of >10

inde*ing head

reduces D travel

re'uirements

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37

#ixed sensors - !enefits

! Compact , reduced D dimensionma(es minimal intrusion into the

measuring volume , ideal for small

CMMs

! Simplicity , fi*ed passive sensorsare less comple* for lo$er system

costs

% ote" an active sensor is more

comple* and often more e*pensive

than a passive sensor and an

articulating head combined

! Stylus length , fi*ed sensors can be larger than those fitted

on articulating heads& ma(ing it possible to carry longer styli

Articulating

head

#ixed sensor

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pp y

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38

#ixed sensors - disadvantages

! #eature access , large and comple*stylus arrangements are needed to

access some features

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y

Slide

39


! rogrammingcomplexity , comple*

stylus clusters mean

more attention must be

paid to collision

avoidance

DA2/E

>ossible collisions $ith:

! component! fi*turing

! stylus change rac(

! other styli in rac(

! machine structure

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40


! Machine si'e % large stylus clusters consume

measuring volume

% much larger stylus change

rac(s consume more space

% you may need a largermachine to measure your

parts

Star styli andlarge changer

consume more

wor+ing volume

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41


! Speed , stylus changing ta(es longerthan inde*ing

% up to 10 times slo$er than inde*ing

% inde*ing can be done during

positioning moves

! Dynamic response , heavy styli

increase suspended mass and limit

scanning speed

! Accuracy , comple* styli compromise

metrology performance

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42



performance



changing?


ro!ing applications


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Slide

43

Why change styli?

Fptimise your measurement repeata!ility for each feature by selecting a stylus $ith:

% Minimum length

! onger styli degrade repeatability

% Maximum stiffness

% Minimum Goints % Maximum !all si'e

! Ma*imise the effective $or(ing length 3+76

Test results"

T>200 repeatability $ith stylus lengthStylus length (.mm .mm

)ni,directional repeatability 0.-0 Fm 0.;0 Fm

2" form deviation G0.;0 Fm G0.E0 Fm

-" form deviation G0.4/ Fm G1.00 Fm



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Slide

46

Why change sensors?

Hot all parts can be measured $ith onesensor:

! Scanning pro!e

% ideal for features $ith functional fits

$here form is important

% digitising contoured surfaces

! Fptical pro!es

% ideal for pliable surfaces

% inspection of printed circuit boards

! Touch-trigger pro!e

% ideal for discrete point inspection& forsize and position control

% compact for easy access to deepfeatures

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Slide

47

The solution3

! Automatic, no re*ualification,

easy programming

! automatic s$itching

! automatic sensor recognition

! automatic electrical connections

! automatic alignment of sensor

ro!e sensor changing

The re*uirement666

If the range of features and parts that you must

measure demands a range of sensors& then a

sensor changing system is essential

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Slide

48

ew AC/> pro!e changer for use with 1(.M

ro!e changing

Buic+ and repeata!le sensor changing for

maximum flexi!ility

4ideo commentary

! He$ #C- sensor

changer

! Ho motors or

separate control

! Change is controlledby motion of the

CMM

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49


Dynamic effects on scanningperformance



changing?


ro!ing applications


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Slide

50

assive sensors

Simple, compact mechanism % no motor drives

% no loc(ing mechanism

% no tare system

% no electromagnets

% no electronic damping! springs generate contact force

% force varies $ith deflection

Deflection

Typical scanning

deflection#orce



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Slide

52

Scanning a @defined surface

Most scanning is performed on @+nown or @defined features % feature size& position and form vary only $ithin manufacturing and

fi*turing tolerances

Active force control does not significantly reduce force

variation in most scanning applications

! /enishaw passive scanning"

% CMM moves around feature

! adaptive scanning (eeps

deflection variation to a minimum

% small form errors accommodated

by sensor mechanism

% small force variation due to

deflection range

! Active scanning"

% CMM moves around a pre,

defined path

% form errors accommodated

in the sensor

% force variation is controlled

by probe motors



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54

Scanning pro!e cali!ration

assive sensors! probe characteristics&

including stylus bending&

are calibrated

! simple calibration cycle

! sophisticated non,linear

compensation

Active sensors! smaller variation in contact

force& but styli are less stiff

! calibration of probe

mechanism characteristics

and stylus bending effectsat fi*ed force still re'uired

Compact

passive

sensor

Complex

active

sensor



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S i ! li! ti



Slide

56

Scanning pro!e cali!ration

constant force does not result in !etter accuracy

! ho$ the probe is calibrated is $hat counts

assive sensors

! passive probes have contact

forces that are predictable ateach *&y&z= position

! scanning probe a*is deflections

are driven by the contact vector

! sensor mechanism and stylus

bending calibrated together

Active sensors

! contact force is controlled& and

therefore not related to *&y&z=

position

! no relationship bet$een contact

vector and probe deflections

! separate calibration of sensor

mechanism and stylus bending

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A ti i i l i



Slide

57

Active or passive scanning - conclusion

! <oth active and passive systems

achieve the basics , accurate scanning

$ithin their calibrated operating range

! Their performance and costs differ

! 9oo+ at the specification of the

system !efore ma+ing your choice

?

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B ti t + t l t li



Slide

58

Buestions to as+ your metrology system supplier

! Do my measurement applications re*uire a scanning

solution?

% o$ many need to be scanned

% o$ many need discrete point measurement

! 0f 0 need to scan, what is the performance of the system?

% Scanning accuracy at high speeds

% Total measurement cycle time& including stylus changes

! 0f 0 also need to measure discrete points, how fast can 0 do

this?

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B estions to as+ o r metrolog s stem s pplier



Slide

59

Buestions to as+ your metrology system supplier

! Will 0 !enefit from the flexi!ility of an articulating head

% #ccess to the component

% Sensor and stylus changing

! What are the lifetime costs?

% >urchase price

% 7hat are the li(ely failure modes and $hat protection is provided

% epair 5 replacement costs and speed of service

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Buestions?



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60

Buestions?

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cmm inspection fundamentals.ppt

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