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Model
5528A
Appendix
B:
M easurement
Consideratio
ns
YOll
Cil
n think
o
f
repea
t.lbilit
y
in
a
similar
way
wh en makin
g
measurement
s w
ith
the
laser
.
Operator skill
,
machine
stabilit
y,
machin
e geo metry,
and th
e
environm ent affect repeatabilit
y.
ACCURACY
is how closely the measurements agree to
a given standa
rd for the measu remenl.
Repea tability. by
comparison,
is how
d
ose the measurements agree to
each
o
ther.
In our
exa mple o
f th
e marks man,
Ih
e close ness
of
his sho
ts
to
thc
bull's eye
is
th
e meas ure o
f
his
accuracy.
It
is possible
for
all
his shots to have bee
n
close
together
(repea
tabl
e),
but to
have
been
fa
r
from
the
bull's
eye. Repeatability
and
accuracy are no
t
thc
same.
Accuracy
in
linea
r
posi tioning is the difference betwee
n
the average
position
o
f a group
of
measu rements
and
the loca
tion of
the
target
position
for
those measu rements. Accurate paris are
exactl
y li ke the drawing
for those
parts.
Repeatabl
e
parts
are
exactl
y like
each
o
ther.
A
batch
o
f
pariS,
all rejected
fo
r th
e same
flaw
,
are repea
tabl
e
but
no
t
accurate.
RESOLUTION
is the
smallest increment
o
f
scale. In the ta rget
exampl
e,
resolut ion
is t he
distance
between the rings in the
targel.
FACTORS AFFECTING ACCURACY AND PRECISION
Th
e accuracy
and
precis
ion
o
f a measurement ar
e
influ enced
by many factors, w
hi
ch
can
be
separated int
o two groups -
•
M achin
e-related,
and -
•
Set-up- related.
Machine-
relat ed
facto rs
in
cl ude
-
•
M
achine geomet ry,
•
M
ach
ine
sca
le accuracy
and
precisio
n,
•
M achin
e environment,
and -
•
M
achine res po
nse to
the
changing environm ent
.
O ne
set-up-related
error Is
A
bbe Erro
r.
Machine-Related Factors
A machine
calibratio
n
is
mor
e
compl
ex
than checkin
g
positi onin
g accu racy
and preci
sion.
Th
e
accu
racy
and
preCisio
n
o
f th
e wo
rk a
machine
can
be ex
pected
to
tu
rn o ut depends o
n
-
•
The
mach
ine's
environment
(incl
udin
g temperature
and
vibration
),
-
•
The
machine's res po nse to
chan
gi ng o
perat
ing
and environment al
conditio ns.
(Ma
ch
in
e
stabilit
y) -
•
The accura
cy
and precision
of
Ihe
machin
e's geometry,
and
-
•
The accuracy and
precision
of the
machin
e's sca
le(s).
Each
of these factors is eq ually
importa
nt.
Measuremen ts of accuracy and
pr ecision
have
meanin
g
onl
y
when th
e
environment and
geo metry are know
n.
B-3
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