Saraswanto Abduljabbar

Metrology expert - Consultant

4/4/2015

LETS ACCURACY SPEAKS

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1. PREFACE

A lot of question have been delivered by end-user for the Universal Length

Measurement (ULM), but they do not get the right answer, because the hidden

errors are difficult to identify. Some how the end-user only believe what the sales

engineer said. Some time the sales engineer just only understand for selling. They

do not know how to help you. Don’t worry, you can read this white paper, hopefully

it can assist you to make decision what is the ULM suitable to your gage calibration

in your production.

All of these measures help ensure that calibration will be accurate, but this must

not lead to a false sense of security:

Gage calibration will not eliminate all measuring errors. As we have seen before,

gaging is not simply hardware: it is a process. Calibration lends control over the

instrument and the standard or master, but gage users must continue to seek

control over the environment, the workpiece, and the gage operator.

I have been in quality control as an Inspector in aircarft manufacturing, and for 31

years in metrology equipments, calibration, coach for metrology, including

Coordinate Measuring Machine (CMM). Especially for the Universal Length

Measurement (ULM), I have experience for 22 years. A lot of operator the ULM still

struggle to do measurements because most the reading was not readability. This

white paper is a map to guide you which is the ULM right for you.

Best Regards,

Saraswanto Abduljabbar

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2. WHAT KIND OF ULM DO YOU NEED?

This white paper is intended to help decision maker to choose the right ULM, to

avoid mistake and erase the doubt. Every people understand that quality is

expensive because all the thing have cost for inspecting the quality product. We get

more confusing when we see all of the ULM’s manufacturer trying to provide what

is end-user need, but most of the end-user get disappointed with the machine after

they use it.

2.1 QUALITY CONCEPT

We forget that the methodology of measurement is as important as the gauge itself.

As a mchine operator, you must assume much of the responsibility for gauging

accuracy. Before you make decision to choose the correct GAGES, make sure you

follow these basic steps:

Fig. 1 All must be realistic

Sometimes we think that all of the ULMs (gauges) are similar, and then we are

looking for the “cheapest” and lost control with SOURCE ERROR OF THE

MEASUREMENTS. In the methodologiy of measurement, some engineer make

design how to manufactur the Universal Length Measurement (ULM) with the

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submicron technology by criteria as below: Precision1, Readibility,

Stiffness, and Longterm Stability2. These reasons, we call QUALITY

CONCEPT.

Unfortunatelly, rarely people understand about the methodology of measurement,

so that they think all of the ULM are similar, and they always believe what they see

without trying to get more detail information why the ULM more expensive than

others. Actualy you must not buying the hardware, but you must buy the QUALITY

PROCESS. Yes... The important is not the ULM itself, it is the PROCESS. If you

just think buying the ULM, you will got mistake.

Mistakes can cause disasters...!!!

2.2 Which best suits the purpose?

1. Most end-user almost have WRONG thinking. They think that the

RESOLUTION is important, so there is a manufacture show up the machine

with six digits after comma (0,000 000). In fact in the metrology world,

calibration certificate only shown 4 to 5 digits. The manufacturer knows that

most end-users know the accuracy of the gauges depends on the

RESOLUTION, and then manufacturer makes TRICK by make the DRO

(Digital Read Out) can be read 6 digit after comma. 0.000 000. In fact the

correct Resolution must be the resolution of the SCALE, not the DRO

resolution.

1 Precision: This describes the closeness of results of measurements of the same measured quantity under thesame conditions, i.e. the same thing is measured several times. It is often quantified as the standard deviation ofthe values around the average. It reflects the fact that all measurements include a random error, which can bereduced, but not completely eliminated.

2 The long-term stability of an ULM is the degree of uniformity of reading over time, when the reading data ismeasured under identical environmental conditions, such as gage block, force, and temperature. Long-term datareading changes are caused by changes in the measurement, such as drift.

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Quoting for ACCURACY, REPEATABILITY and REPRODUCIBILITY are not

useful for the instruments. When choosing an instrument, DRIFT is what

really counts, which is often overlooked completely,

or in some cases precision

Be careful it is not the RESOLUTION OF THE DISPLAY thatyou are buying...!

2. Second issue is the accuracy, often calls MPE (Maximum Permissible

Error)3. Most people only see the accuracy stated in the catalog and never

think that the process is proofing the accuracy is correct or wrong to support

the accuracy of the machine. So that is why the manufacture always show

up the best figure for the MPE, because they know the end-user will never

objection. In this case you must understand that the ULM measurement

method is using mechanical contact, so the MPE (Maximum Permissible

Error) must be excecuted by by Gage Blocks. Many ULMs stated the MPE

according to the scale accuracy4 not the Machine accuracy. You can

understand by see the calculation of the Uncertainty Measurement.

We are talking about the ULM manufactured by Mahr GmbH. I has already

use it for 22 years. All of the ULMs have been excecuted by calibration the

scale using laser interferrometer and calibrated the linear accuracy by Gage

Blocks 25, 50, 75, and 100 mm. It was really good concept.

Standardized acceptance testing process for horizontal length measuring

units acc. to VDI/VDE2617 Sheet 2.2, Mahr GmbH:

Consisting of:

1. Testing of the scale (reference element of measuring device)

3 MPE = Maximum Permissible Error As defined in the EN ISO 10360

4 Accuracy: This means closeness of agreement between a measured value and the true value of a quantity. If ameasurement is accurate, the average of the measurement results is close to the “true” value (which may be e.g.the nominal value of a certified standard material). If a measurement is not accurate, this can sometimesbe due to a systematic error. Often this can be overcome by calibrating and adjustment of instruments.

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2. Testing of the complete unit determined with PTB reference gage

blocks

Fig 2. Testing of the scale (reference element of measuring device) by Laser Ferrometric System

Fig 3. Testing of the complete unit determined with PTB reference gage blocks

PROCEDURES:

1. Laser measurement min. 3x mean calculation, (linearity deviation)

2. Storage of correction values in the machine control (RAM)

3. Test measurement with the determined correction values (if necessary)

4. Gage block probing, 3x GB 25mm, 50mm, 75mm, 100mm, pitch determination,

linear proportion

5. If necessary, adaption of the correction values.

Laser (90°umgelenkt)

FM

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6. Gage block measurement with new correction, 10x 25mm, 10x 50mm, 10x 75mm

10x 100mm, at least 5 probings each / gage block center point is stored in the

calibration certificate. Testing of repeatability range ≤ 0.02µm, first commissioning

by manufacturer, max. 0.03µm for ULM 600E and Testing of repeatability range ≤

0.05µm, first commissioning by manufacturer, max. 0.09µm.

2.3 Factors affecting the accuracy of UniversalMeasuring Measurement (ULM).

Fig. 4 Scheme of the uncertainty measurement of the ULM

2.3.1 Factors affecting the standard of measurement:

co-efficient of thermal expansion

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elastic properties

stability with time

geometric compatibility

2.3.2 Factors affecting the work piece to be measured:

co-efficient of thermal expansion

elastic properties

arrangement of supporting work piece

hidden geometry

surface defects such as scratches, waviness, etc.

2.3.3 Factors affecting the inherent characteristics of instrument:

repeatability & readability

calibration errors

effect of friction, backlash, etc

inadequate amplification for accuracy objective

deformation in handling or use

2.3.4 Factors affecting person:

improper training / skill

inability to select proper standards / instruments

less attitude towards personal accuracy measurements

2.3.5 Factors affecting environment:

temperature, humidity, atmospheric pressure, etc.

cleanliness

adequate illumination

heat radiation from lights / heating elements

3. ERROR CAN BE AVOIDED

3.1 Fixtures or accessories are a common source ofgauging error.

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The fixtures establishes the basic relationship between the ULM and

the workpiece, so any error in the fixture inevitably shows up in the

measurements. Make sure you see the fixtures, that fixture is a solid,

one-piece unit will greatly reduce the problem.

Fig. 5 ULM 300 Mfg Mahr GmbH - Germany

3.2 Dirt or stain is second source of errors.

So the machine provide the correct contact point with tungsten carbide tip with the

right purpose.

1. Flat testpiece faces are contacted with a spherical face.

2. Cylindrical testpiece faces are contacted using a hatchet.

3. Spherical testpieces faces are contacted with a flat face.

Fixture

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The smallest area contact more accurate measurement.

In designing a structure or device, how is the engineer to choose from this vast

menu the material.

3.3 HIGHLY HOMOGENEOUS CAST FRAME STEEL

Fig. 6 ULM 100 – Comparator with horizontal base (highly homogeneous castframe)

Direct measuring range 100 mm (4 in)

Length measuring deviation MPE =(0.1 + L/2000) µm5

[L in mm]

Application range outside 0 - 670 mm

inside 1 - 400 mm

Dimensions L x W x H (mm 960 x 390 x 450

Instrument weight 150 kg (330.8 lbs)

Universal working table:

Area of installation 160 mm x 160 mm

Range of height adjustment: 0 - 100 mm

5 Calibrated in Germany at 20° ± 0,1K; 0.1K/hour

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Transverse travel 25 mm

Y-Gradient 3°

The ULM 100, already have beed drawed from the line of precimar group because

found a lot of problems. The result data of measurement is very difficult to accepted

in the cuatomer site. Because of the themperature problem in the customer site,

drift, very poor longterm stability, and difficut to get readilibity data result.

Customers are unhappy with this machine.

After investigation about the problem, Mahr makes decision to do no longer selling

this ULM 100. The ULM 100 with highly homogeneous cast frame, actually was

not correct using MPE = (0.1 + L/2000). A lot of variable error, makes the data

unstable. The Steel Base frame has been choosen because its cheaper and easier

manufactured and it will no reject. It is a good reason for the manufacturing but no

benefit for the customer.

If you read in chapter 2.3 Factors affecting the accuracy of Universal Measuring

Measurement (ULM). The modulus of steel is lower than granite. The modulus

measures the resistance of the material to elastic deflection or bending. The

selection of a material for this application, but it is not the only one. The Machine

Base must have a high yield strength. If it does not, it will bend or twist if you put in

the changed temperature frequently to do so. It was natural characteristic of cast

iron or steel easily bend and twist.

3.4 NATURAL CHARACTERISTIC OF THE STEEL CAST

1) Elasticity (natural)

2) Bending moment and fatique (mechanical)

3) High thermal expansion (environment)

4) Expansion joint failure (assembly)

5) Fatigue (material)

6) Stress (clamping)

7) Hysteresis (electrical)

8) Corrosion (rusty)

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Fig. 7 The modulus measures the resistance of the material to elastic deflection orbending. (Elasticity and Fatique)

3.4.1 ELASTICITY EFFECT

First Problem, before you make measurement, you must make alignment to get both

spindle of head stock and tail stock in one AXIS. It’s call REVERSAL POINT (Pic. 1).

Elasticity effect can make the reversal point MOVED OUT when the head stock SHIFT

AWAY starting from the reversal POINT (see picture 2).

REVERSAL POINT

AXIS

Fig. 8 Reversal Point

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LOAD

Second Problem:

For longterm purpose the condition of machine bed became worst due to fatique and

elasticity factor, make the machine bending. Load on the work table will make bend the

bed of the ULM, caused of:

Bending due to the own weight

Bending due to weight of workpiece and accessories of the ULM TABLE.

Bending due to characteristic material of steel cast or IRON CAST.

Caused of bed made of the steel casting material, the result of the measurement will be

lost of the accuracy and the reading doot get repeatable data. This is the reason why

Mahr GmbH replaced the steel casting material to material of Natural Granite Bed.

Finding Bending Stress and Modulus of Elasticity E (Pic. 9)

The bending of beams is one of the most important types of stress in engineering.

Bending is more likely to be a critical stress than other types of stress - like tension,

compression etc. In this laboratory, we will be determining the Modulus of Elasticity E

(also called Young's Modulus) of the various materials and using Solid Edge to

determine the Second Moment of Area for the different cross-sections.

Fig. 8 Bending due to load and own weight

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Fig.9 Bending Test

Bending Equations

Use units:

Force (N), Length (mm), Stress (MPa)

E = Young's Modulus or Mod of Elasticity (MPa)

I = 2୬ୢ Moment of Area or Area Moment (mmସ).

In our case, we must first convert the massto Newtons (N). W = kg * 9.81(gravity)L is the span length in (mm).I is the Second Moment of Area in (mmସ).We can calculate this for a rectangle using asimple formula;

A Ba b

c

W

STEEL BASE

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L = Overall lengthW = Point loadM = Moment

End Slope Max. Deflection Max. BendingMoment

≤

= ඨ

+) (

=

=+

+

(at position c)

(under Load)

LengthBetweenSupport1150 mm

"۷" Beam Mild Steel(Square)

NaturalGranite

Remarks

Determine the maximum stress

for each mass (load) added to

the beams.

Discuss any sources of error

in the experiment - especially

measurements - and how they

might affect the results. Specify

an overall error for your

calculation of E.

Mass Deflection Deflection Deflection

Kg mm mm mm

0.5 0.35 0.82 0.003

1.0 0.70 1.70 0.006

1.5 1.09 2.62 0.008

2.0 1.465 3.56 0.010

The third problem:

Error due to contact pressure: The variations in the contact pressure between the anvils

of the instrument and the work piece being measured produce considerable difference in

reading. The deformation of the work piece and the anvils of instrument depend upon

the contact pressure and the shape of the contact surfaces.

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A steel bar with a diameter D = 10 mm, a length L = 100 mm and an elasticity module E

= 2.1* 10ହN/mm measured with a force F = 5 N. It is shortened by ΔL = 0.03 µm.

S = (3.14 * 100)/4

= 78.57 mm

=∆ ∗

ૠૡ.ૠ ∗ = .μ

A bar with the same section S, but with a length L = 300 mm, is shortened by 3 x 0.03

µm = 0.09 µm.

E = 2.1 * 10ହN/mm

ΔL = ∗ி

ௌ∗ா

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Fourth Problem:

When the ULM is not in the CONTROL ROOM Condition, and usually when in the

morning, the temperature setting for 20˚C (in the working hour) and in the evening the

measuring room will be shut off the air condition, then the temperature increase arround

27˚C to 30˚C. If the condition happen in every day, so the material will be bending due to

the factor of elasticity dan fatique. The other case, in the evening will be get high

humidity. It will caused stain and rusty for the iron casting bed and the electronic

component may get circuit shock. This problem many happened for the ULM using

CAST IRON BED. So for longterm purpose the ULM with Casting Iron will get worst and

worst. It is not repairable.

Fifth Problem:

Make variation reading unstable caused of the coeffisian material thermal expansion is

not homogenous. For instance, the material of bed steel 10,7 x 10ି, and the material

contact point 11,5 x 10ି and material workpiece 10,5 x 10ି. They will make unstable

result.

Sixth Problem:

Errors due to misalignment: Abbe’s principle of alignment should be followed in

measurements to avoid cosine errors, sine errors, etc. According to Abbe’s principle,

“the axis or line of measurement of the measured part should coincide with the line of

measuring scale or the axis of measurement of the measuring instrument”.

The combined Cosine and Sine error occurs if the micrometer axis is not truly

perpendicular to the axis of the work piece as shown below.

Similarly, the same error occurs when measuring an end gauge in the horizontal

comparator. If the gauge is not supported so that its axis is parallel to the axis of

measuring anvils, or if its ends, though parallel to each other, are not square with the

axis. This is shown below.

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The above combined Cosine & Sine errors can be avoided by using gauges with

spherical ends. If the axis of the two spherical end gauges are not aligned, the error in

length will occur this is equal to (a+b) as shown below.

From the above figure, it is known that:

a = R1 ( 1 - Cos ߙ )

= R1 ( 1 - 2sin1 )

=

221

2

1)RR(

h11R

=

221

2

1)RR(2

h11R

= 221

21

)RR(2

hR

very nearly

Similarly, b = 221

22

)RR(2

hR

Therefore, (a+b) = 221

221

)RR(2

h)RR(

= )RR(2

h

21

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4. GRANITE

It would be a good choice for this application because:

1. Better Thermal Conductivity.

2. Good Fatique Resistance.

3. Good Modulus.

4. Good Yield strength.

5. Good Damping.

6. Low Thermal Expansion.

7. Repairable.

8. Electrical Insulating Properties.

9. Resistance to Corrosion and Oxidation.

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The measurement uncertainty of measuring instruments is known to vary with the

temperature behaviour of the materials involved.

Fig. 10 Granite Bed

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4.1 Rigid body kinematic Teknik

(The error terms inherent to the mechanical design (yaw, pitch, straightness, etc)

This study deals with improvement of the mechanism accuracy of parallel kinematic

machine consisting of closed loop link mechanism. In the same manner as the

orthogonal mechanism, the parallel kinematic machine has actually the following

factors that cause the positioning error:

(1) joint run-outs caused by the motion of the mechanism,

(2) elastic deformations of the links and the joints, generated by transfer of the center

of gravity,

(3) elastic deformation of the machine frame supporting the mechanism, which is

caused by internal and external forces,

(4) thermal expansion of the links and frame, which is caused by the temperature

fluctuation.

For those reason, GRANITE is the best material for the submicron technology.

4.2 WHAT IS THE BENEFIT USING GRANITE...?

4.2.1 The use of air cushioning allows the Abbe measuring headstock and the

tailstock quill to be shifted with much greater speed and ease, which saves

time. Air cushioning is a great deal more economical to implement in

combination with hard rock than with steel or cast iron.

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4.2.2 The use of hard rock allows the instrument maker to quickly respond in an

economically viable way to special custom requirements regarding the lengths

of beds or additional slide-ways.

4.2.3 Hard rock has a more favourable damping behaviour, so that external

vibrations transmitted to the measuring instrument have a substantially lower

influence on the measurement result.

4.2.4 Hard rock beds need considerably less maintenance, as there is no corrosion

- hard rock beds are rust- and acid-proof.

4.2.5 Hard rock does not conduct electricity, and it is nonmagnetic. Its greater

hardness makes it less subject to wear than steel. Small damages to the

guiding surfaces of hard rock beds have no influence on the accuracy of

guiding, as due to the rock structure, only small grains break out which leave

no burrs. Defective metal beds need to be repaired by specialists at high cost.

in this case.

4.2.6 Granite is chosen because of its electrical insulating properties, it also has

good thermal fatigue resistance, resistance to corrosion and oxidation, No

Fatigue, and no elasticity. Contrary steel material.

4.2.7 Hard rock beds do not have material-inherent stresses, so that the bed

surface retains its form for a long time. By contrast, cast iron, even if

appropriately treated before and after machining, is subject to ageing and

therefore does not keep its form.

Summing up, it is obvious that the physical properties of hard rock provide many

good reasons for using hard rock beds in universal length testers.

5. FINITE ELEMENT ANALISYS (FEA)

The Finite Element method originated from the need to solve complex elasticity and

structural analysis problems:

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1) Accurate representation of complex geometry.

2) Inclusion of dissimilar material properties.

3) Easy representation of the total solution.

4) Capture of local effects.

The solution concept ULM made of GRANITE Hard Rock Bed to easier finite element

analisys to make high accuracy measurement and repeatable data.

Quality products usually come with quality concept that they are about the materials,

softwares, and accessories with considering the uncertainty of measurement. For

instance; we produce the machine with accuracy 0.07 µm, it does not make sense if the

material of bed of the machine made of IRON (STEEL) CAST. It can be found bending.

Most of the ULM using Cast Iron, on slide of the head stock use cylindrical bearing

which it can produce friction. For the longterm purpose, the bearing can worn-out and it

will be made the result unstable. But it can not be found at ULM by using Natural

Granite with air bearing for the slide movement.

5.1 First of the problems for the accuracy is F R I C T I O N

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For the longterm usage, ULM with air bearing technology is a good concept. Because no

friction for sliding the head stock and tail stock. In this purpose performance of the ULM

become more efficient, it can be shorter time for set-up and measurement. Of course

this product will get more cost, but it will make benefit to customer.

5.2 Second problem of the ACCURACY is W E A R

Thus the air bearing is the best concept. ULMs of Mahr GmbH are using AEROSTATIC

air Bearing. Consequence using this concept, it will high manufactured cost, BUT

satisfaction for the CUSTOMERS. Because the ULM can be use for longer time. So far

the ULM have been occupied for more than 20 years without any problem.

6. THE CONCLUSION

The concept using GRANITE BED is the best concept due to the other ULMs also still

use CAST IRON BED. The weakness of the cast iron bed is too much, especially for the

longterm stability. Compare ULM Granite Bed more expensive than ULM Cast Iron, but

ULM Granite Bed is WORTH IT.

Fig. 11 ULM 600 E Mfg. Mahr GmbH

Uncertainty budget and CMC estimate, according to ISO GUM (Guide Uncertainty of Measurement).

Procedure:

Artefact: ULM E

Method: Direct comparison

Ref. Standard: Gauge Block

Uncert. Equation uc2(E) = u2(L) + u2(Ls) + Ls

2.qs2.u2(da) + Ls

2.as2.u2(dq)+ u2(Ldrift) + d.u2(Lw) + u2(F)

Constants L 2000 mm 2.000.000 μm

as 3,20E-07 /°C

resol 0.1 µm

qs 0,1 °C

Uncert source/

Component

Unit/

Satuan

Distribusion Symbol Expanded

uncert/

U

Cov. Factor/

Pembagi

Deg. of

freedom/

vi

Std. Uncert/

ui

Sens. Coeff/

ci

ci.ui (ci.ui)2 (ci.ui)4/vi

1 Repeatability μm Normal 0,02 1,00 1,00E+10 0,0200 1,00 0,02 4,00E-04 1,60E-17

2 Instrument Accuracy μm Rectangular 0,06 1,73 1,00E+10 0,0346 1,00 0,03 1,20E-03 1,44E-16

3 Misalignment Error μm Normal 0,02 1,96 1,00E+10 0,0102 1,00 0,01 1,04E-04 1,08E-18

4 CTE °C -1 Rectangular 0,000000032 1,73 50 0,0000 200000,00 0,00 1,37E-05 3,73E-12

5 Temperature Different °C Rectangular 0,05 1,73 2 0,0289 0,64 0,02 3,41E-04 5,83E-08

6 Elasticity drift μm Rectangular 0 1,73 1,00E+10 0,0000 1,00 0,00 0,00E+00 0,00E+00

7 ABBE Error μm Normal 0 1,73 50 0,0000 1,00 0,00 0,00E+00 0,00E+00

8 Geometric Error of Instrument μm Rectangular 0 1,73 50 0,0000 1,00 0,00 0,00E+00 0,00E+00

9

Sums 0,002 1 0,000 0

Combined uncert, uc 0,045 4

Eff. Deg of freedom, veff 73

Cov. Factor for 95% CL 1,993463567 2

Expanded uncertainty, U95 0,09

REMARKS:

1. Repeatability is ISO 5725-2, basic method for determination of repeatability and reproducibility of standard measurement method, and is dealing with interlaboratory

experiments performed in order to obtain two measures of precision, repeatability and reproducibility. ISO 5725 assumes that the characteristic values are continuous and follow normal distribution.2. Instrument Accuracy taken from Maximum Permissible Error of the instrument MPE E1 = 0,15+L/2000 μm according to the Scale misalignment by laser.

3. Misalignment Error

Caused of error from wringing between surface contact of the flat tip or from spherical tip. In the observation found 0.00005 mm misalignment.

4. Coefficient Thermal Expantion (CTE)

The material of Base of machine made of Steel Casting with CTE = 10 x 10^¹º and the material contact tip and the accessories made of special steel generally with CTE = 11.5 x 10^¹º

5. Temperature Different between Gauge and scale is 0.2˚C. The different can be happenend because the gauge in the open space and the scale inside the cover.

1/2 CMC worksheet Precimar ULM E/Calliper 2003

So it will be make different themperature.

6. Elasticity of the steel material can be happened caused of the own weight and from the work table as well as from the accessories and the workpiece.

7. ABBE Error

The maximum accuracy may be obtained when the reference scale and the work-piece being measured are aligned in the same measurement.

8. The machine has Instrumental error following the geometric error.

9. Constan L = 1000 mm

10. Thus the Maximum Permissible Error is 0.7 + L/1000 μm.

2/2 CMC worksheet Precimar ULM E/Calliper 2003