A Study Of Pre-Travel Variation Of Coordinate Measuring Machine With Different Stylus Configurations.

A Study of Pre-travel Variation of Coordinate Measuring

Machine with Different Stylus Configurations

AHMED SAEED ALI SAEEDAN

B051110303

UNIVERSITI TEKNIKAL MALAYSIA MELAKA 2015

UNIVERSITI TEKNIKAL MALAYSIA MELAKA

A Study of Pre-travel Variation of Coordinate Measuring Machine

with Different Stylus Configurations

This report submitted in accordance with requirement of the Universiti Teknikal Malaysia Melaka (UTeM) for the Bachelor Degree of Manufacturing Engineering

(Manufacturing Process) (Hons.)

by

Ahmed Saeed Ali Saeedan B051110303

03391942

FACULTY OF MANUFACTURING ENGINEERING 2015

UNIVERSITI TEKNIKAL MALAYSIA MELAKA

BORANG PENGESAHAN STATUS LAPORAN PROJEK SARJANA MUDA

TAJUK: A Study of Pre-travel Variation of Coordinate Measuring Machine with Different Stylus Configurations

SESI PENGAJIAN: 2014/15 Semester 2

Saya AHMED SAEED ALI SAEEDAN

mengaku membenarkan Laporan PSM ini disimpan di Perpustakaan Universiti Teknikal Malaysia Melaka (UTeM) dengan syarat-syarat kegunaan seperti berikut:

SULIT

TERHAD

TIDAK TERHAD

(Mengandungi maklumat yang berdarjah keselamatan atau kepentingan Malaysia sebagaimana yang termaktub dalam AKTA RAHSIA RASMI 1972)

(Mengandungi maklumat TERHAD yang telah ditentukan oleh organisasi/badan di mana penyelidikan dijalankan)

Alamat Tetap:

Tarikh: ________________________

Disahkan oleh:

DECLARATION

I hereby, declared this report entitled “A Study of Pre-travel Variation of Coordinate Measuring Machine with Different Stylus Configurations” is the results

of my own research except as cited in references.

Signature :

Author’s Name : Ahmed Saeed Ali Saeedan Date : 5th _{June 2015}

APPROVAL

This report is submitted to the Faculty of Manufacturing Engineering of UTeM as a partial fulfillment of the requirements for the degree of Bachelor of Manufacturing Engineering (Manufacturing Process) (Hons.). The member of the supervisory is as follow:

……… P.M. Dr. Mohd Rizal bin Salleh

i

ABSTRACT

Coordinate measuring machine (CMM) is one of the sophisticated, most powerful and versatile metrological instruments. CMM measurements are not accurate as they are always subjected to errors due to various causes. Nevertheless, quantifying an error and later uncertainty puts doubts on the validity of the measurement result. Such a probe system is much responsible for the CMM measurement accuracy andthe most important parameter characterizing probe inaccuracy is the pre-travel variation. This project studied the influence of using different stylus configurations toward the pre-travel variations and later identify the source of errors and the factors affecting the uncertainty of the measurement. Three factors were evaluated (stylus length, tip diameter of the stylus, and the approach distance) and a full factorial design was adopted for the experimental design followed by an ANOVA analysis to identify the significance of the factors toward the pre-travel variation and the uncertainty of the measurements. The experimental results showed significant effect of the three factors on both the pre-travel variation and the uncertainty of the measurement and there was an interaction between the stylus length and the tip diameter. It was also found that the stylus length has much influence on the results than the tip diameter and the approach distance was the least factor affected the measurements. Moreover, it was found that the increment of stylus length lead to a significant variations of pre-travel and uncertainties while the increment of the tip diameter at the smallest length of the stylus reduced the error propagation. Also, it was found that the smallest approach distance produced relatively higher pre-travel variation and uncertainties.

ii

ACKNOWLEDGEMENT

I would like to take this opportunity to express my heartily gratitude to my supervisor Associate Professor Dr. Mohd Rizal bin Salleh. This project would not have been possible without his guidance and support. I would like also to express my very great appreciation to all the lecturers and technicians who had provided me the guidance and help along the way to accomplish this project. I am grateful for the spirit support, cooperation and guidance given by my friends. I would like as well to thank my fellow course members in Bachelor Degree of Manufacturing Engineering (Manufacturing Process) of UTeM, for their helpful comments and caring along my study.

iii

TABLE OF CONTENT

Abstract i

Acknowledgement ii

Table of Content iii

List of Tables vi

List of Figures vii

List Abbreviations ix

CHAPTER 1: INTRODUCTION 1

1.1 Overview / Background 1

1.2 Problem Statement 3

1.3 Objectives 4

1.4 Scope 4

1.5 Project Organization 5

CHAPTER 2: LITERATURE REVIEW 6

2.1 Types and Constructions of CMM 6

2.2 Probing System8

2.2.1 Classifications and Categories of Probing System 8

2.2.2 Touch-Trigger Probes 11

2.2.2.1 Kinematic Resistive Probe Operation 13 2.2.2.2 Strain-gauge and Piezo Probes Operation 14 2.2.2.3 Touch Trigger Probe Performance 16

2.3 Factors Influence the Probing Accuracy of CMM 19

2.3.1 Stylus Length 19

2.3.2 Stylus Tip Diameter 21

2.3.3 Probe Approach Distance 22

2.3.4 Approach Speed of the Probe 23

2.3.5 Sampling planning 24

2.4 Errors in Measurement 25

iv

2.4.2 Types of Errors 26

2.4.2.1 Systematic Error 27

2.4.2.2 Random Error 28

2.5 Uncertainties of Measurements 28

2.5.1 Sources of Uncertainty 29

2.5.2 Model of Measuring Uncertainty 30

2.5.2.1 Standard Uncertainty 30

2.5.2.1.1 Type A Evaluation 31

2.5.2.1.2 Type B Evaluation 31

2.5.2.2 Combined Standard Uncertainty 32

2.5.2.3 Expanded Uncertainty 33

CHAPTER 3: METHODOLOGY 34

3.1 Overview 34

3.2 Experiment Definition 36

3.3 Experiment Parameters and Variables 36

3.4 Design of Experiment (DOE) 37

3.5 Conducting the Experiment 39

3.6 Data Collection 40

3.7 Data Analysis 40

3.8 Findings and Conclusions 41

3.9 Summary 41

CHAPTER 4: RESULTS AND DISSCUSSION 42

4.1 Overview of the experiment and Data Collection 42

4.2 Results 43

4.2.1Analysis of Variance (ANOVA) 46

4.2.1.1 ANOVA Model for Uncertainty 46

4.2.1.2 ANOVA Model for Pre-travel Variation 49 4.2.2 Comparison of Probe Pre-travel Variation 52 4.2.2.1 Comparison of Stylus Length effect on the Pre-travel Variation 53 4.2.2.2Comparison of Tip Diameter Effect on the Pre-travel Variation 57

v 4.2.2.3 Comparison of Approach Distance Effect

on the Pre-travel Variation 62

4.3 Discussion 71

CHAPTER 5: CONCLUSION & RECOMMENDATION 75

5.1 Conclusion 75

5.2 Recommendation 77

REFERENCES 78

APPENDICES 83

A Gantt Chart

B Stylus Kit Technical data C Data Collection

vi

LIST OF TABLES

2.1 Trigger Sensing Technologies 11

2. 2 Comparison of the Type A and Type B Evaluations 32

3. 1 The Independent Parameters and Variables 36

3. 2 Full factorial design 38

4. 1 Overall Results 45

4. 2 ANOVA table for uncertainty 47

4. 3 ANOVA table for pre-travel variation 50

4. 4 Experiment results of different stylus length

at 2 mm tip diameter and 5 mm react 53

4. 5 Experiment results of different stylus length

at 4 mm tip diameter and 5 mm react distance 55

4. 6 Experiment results of different stylus length

at 6 mm tip diameter and 5 mm react distance 55

4. 7 Experiment results of different Tip diameter

at 10 mm stylus length and 5 mm react distance 57

4. 8 Experiment results of different Tip diameter

at 30 mm stylus length and 5 mm react distance 59

4. 9 Experiment results of different Tip diameter

at 50 mm stylus length and 5 mm react distance 61

4. 10 Experiment results of different Tip diameter

and stylus length at 10 mm stylus length 63

4. 11 Experiment results of different Tip diameter

and stylus length at 30 mm stylus length 66

4. 12 Experiment results of different Tip diameter

vii

LIST OF FIGURES

2.1 Bridge Type CMM CONTURA G2 7

2.2 Classification criteria for probing systems 8

2.3 Sensors for coordinate measuring machines

classified by principle of operation 9

2.4 Basic tactile probing system in kinematic resistive design 10 2.5a one stage type with electro-switch transducer 12

2.5b two stage type with three piezo-elements 12

2.6 schematic of a kinematic resistive probe 13

2.7 The Mechanics of Kinematic Resistive Probe 14 2.8 Schematic of a Strain Gauge Probe Measuring the Contact Force 15

2.9 schematic of a Piezo Probe 15

2.10 Force Balance in a Touch-trigger 16

2.11(a) pivot point is further from stylus centerline in high force direction 17 2.11(b) pivot point is closer to stylus centerline in low force direction 17

2.11(c) force balance in a touch-trigger probe 17

2.12 Force Balance in the Z Direction 18

2.13 Pre-travel Variation Plot for a Kinematic Resistive TP6 CMM probe 18 2.14 Pre-travel Variation Plot for a Strain-gauge

TP7M Touch-trigger Probe 19

2.15 Errors of Touch trigger Probe of Different Stylus Length 20 2.16 Probe Performance of Different Stylus Length 20 2.17 The influence of the stylus length and the measuring force

on the one-stage probe characteristic 21

2.18 Range of styli with different ruby ball sizes 21 2.19 CMM dynamic error as a function of probe approach distance 22 2. 20 Three-dimensional probe uncertainty for different 23

2.21 Example results from a Probe Test 25

2. 22 Systematic and random error 27

2.23 Steps of measurement errors enter into the measurement uncertainty 29 2. 24 Error components that lead to uncertainties 30

viii

3.1 Project Flowchart 35

4.1 Histogram of residuals of the measurement results 44

4. 2 Normality test plots of residuals 44

4.3 Interaction plot of stylus length with tip diameter for uncertainty 48

4.4 Main effect plot for uncertainty 49

4.5 Interaction plot of stylus length with tip diameter

for pre-travel variation 51

4.6 Main effect plot for pre-travel variation 52 4.7 Pre-travel variation of different stylus Length

at a constant tip diameter of 2 mm and react distance of 5 mm. 54 4. 8 Pre-travel variation of different stylus Length

at a constant tip diameter of 4 mm and react distance of 5 mm 56 4. 9 Pre-travel variation of different stylus Length

at a constant tip diameter of 6 mm and react distance of 5 mm. 56 4.10 Pre-travel variation of different tip diameter

at a constant stylus length of 10 mm and approach distance of 5 mm. 58 4. 11 Pre-travel variation of different tip diameter at a constant

stylus length of 30 mm and approach distance of 5 mm 60 4. 12 Pre-travel variation of different tip diameter at a constant

stylus length of 50 mm and approach distance of 5 mm 62 4. 13 Pre-travel variation of different approach distance

at a constant stylus length of 10 mm and tip diameter of 2 mm 64 4. 14 Pre-travel variation of different approach distance

at a constant stylus length of 10 mm and tip diameter of 4 mm 65 4. 15 Pre-travel variation of different approach distance

at a constant stylus length of 30 mm and tip diameter of 2 mm 68 4. 16 Pre-travel variation of different approach distance

at a constant stylus length of 30 mm and tip diameter of 4 mm 68 4. 17 Pre-travel variation of different approach distance

at a constant stylus length of 50 mm and tip diameter of 2 mm 70 4. 18 Pre-travel variation of different approach distance

ix

LIST OF ABBREVIATIONS

ANOVA – Analysis of Variance

CMM - Coordinate Measuring Machine CMS – Coordinate Measuring System CNC – Computer Numerical Control DOE - Design of Experiment

GUM - Guide to the expression of Uncertainty Measurement ISO - International Organization for Standardization PTV – Pre-travel Variation

RSS - Root Sum of the Square TP - Touch Probe

UTeM - Universiti Teknikal Malaysia Melaka

1

CHAPTER 1

INTRODUCTION

This chapter presents a general overview of coordinate measuring machines (CMMs), states the problem of the project, and identifies the objectives as well as the scope of the project. Generally, this project is carried out to study, evaluate and analyze the probing system of CMM which is highly contributes to the uncertainty of CMM measurement and reduce its accuracy.

1.1 Overview / Background

In relation to the continuous advances in science and technology, measurements role has increased over the years in fields like manufacturing, information systems, electronics, nuclear, space, etc. According to International Vocabulary of Basic and General Terms of Metrology (VIM), it defines measurement as a process of experimentally obtaining one or more quantity values that can reasonably be attributed to a quantity (VIM, 2012).

Measurements have become a tool of science. Metrology, however, is the science of measurement. It comprises all theoretical and practical aspects of measurement, such as units of measurement and their standards, measuring instruments and their field of application.

The list of metrology instruments and their capabilities have grown through the years. Coordinate measuring machines (CMMs) are one of the sophisticated, most powerful

2 and versatile instruments CMM is an advanced quality control system. It used for many purposes to make inspection meet modern production requirements. Its primary function is to measure the actual shape of a workpiece, and compare it against the desired shape and evaluate the metrological information such as size, form, location, and orientation.

CMM is traced back to the year 1959 where it first appeared commercially at the International Machine Tool exhibition in Paris, exhibited by the British company Ferranti. The key element of the development of the CMM was the availability of an accurate, long-range, electronically compatible digital measuring system (Hocken & Pereira, 2012).

CMM is used in assembly and manufacturing processes to test a part or assembly against the design intent. It precisely records the X, Y, and Z coordinates of the target. Then, it generates points and analyses them, through algorithms, for the construction of features. These points are collected by using a probe that is positioned manually by an operator or automatically via computer control. The part for measurement is placed on the table of the CMM and aligned along the coordinate axis of the machine. The coordinate system of the part is created by the software and transferred to the coordinate system of the machine.

The economic success of most manufacturing industries is critically dependent on how well its products are made, a requirement in which measurement plays a key role. Coordinate Measuring Machine (CMM) is one of the most adequate measuring machines to meet the requirement. Due to high sensitivity and speed, CMMs have to be in a controlled environment to yield accurate results. Even with the advent of the CMM digital equipment readout and software packages to compensate errors and improve measurements, these measurements are not accurate because they are always subject to errors that are caused by either humans or materials. Quantifying an error to later quantify an uncertainty proves that the validity of the measurement result is doubted.

3

1.2 Problem Statement

There are many factors affect the uncertainty of CMM measurement. These effects that interact might be due operator when selecting a different probe or stylus. Other issues that can highly reduce the accuracy measurement of CMM are those errors related to temperature variations, vibrations, and even dirt. Different errors related to CMMs performance could be geometric errors due to the limited accuracy of the individual machine components such as guideways and measuring systems, errors related to the final stiffness of the moving parts, and thermal errors related to heat and cooling source in the environment, like lighting, air conditioning, ventilation, humidity, people around the machine and heat generated by the machine itself.

Quantifying uncertainty of CMM measurements is a very complex object, however minimizing an error factor will contribute to the accuracy improvement of CMM measurements (Ali, 2010). Such a probe system of CMM is much responsible for the coordinate measurement accuracy. It includes stylus and stylus tip which have its own dynamic characteristics during the measuring process. The probe stylus tip is a key element of coordinate measurements; it contributes to the overall performance of the CMM.

The most important parameter characterizing probe inaccuracy is the pre-travel variation which can be determined using a master sphere of known radius (Woźniak & Dobosz, 2005). Desai and Bidanda (2006) mentioned that the type of probe used for measurement is an important factor that influences the accuracy of CMM; such a touch-trigger probe, it has several factors related to its motion (e.g. approach speed, and probe acceleration) and configuration (e.g. probe stylus length) which influence its performance. Therefore, this study was carried out to compare the influence of using different stylus configurations toward the pre-travel variations and later identify the source of errors and the factors affecting the uncertainty of the measurement in order to minimize errors and increase the overall performance and measurement accuracy of the CMM.

4

1.3 Objectives

The objectives of this project are:

i. To identify the probing factors affecting the uncertainty of measurement. ii. To compare the pre-travel variation of touch-trigger probe using different

range of stylus configuration and extensions.

1.4 Scope

The accuracy of CMM measurement can be affected by various errors sources. In order to improve and obtain more accurate results, the source of errors should be eliminated or controlled. However, this study is focused on the performance and the evaluation of uncertainties of touch-trigger probe utilizing different stylus configurations and extensions which is highly related to reduce the accuracy of CMM measurements. The scope of this project will ensure that the project is conducted within its intended objectives and it should be eventually achieved. The scopes of this project are listed in the following points:

i. Literature study on the related theories and most updated researches will be reviewed.

ii. The parameters to be used which are highly related to reduce the CMM accuracy are probe stylus length, different tip diameter and probe approach distance.

iii. The measurements will be done using a master sphere of a known radius. iv. All results will be compared, and errors, uncertainties and pre-travel variation

5

1.5 Project Organization

This project is divided into two phases. The first phase comprises of three chapters. The first chapter contains a general overview of coordinate measuring machines (CMMs), states the problem of the project, and identifies the objectives as well as the scope of the project. In the second chapter, a literature review will be provided to investigate, analyze and compare the various and most updated research on the area related to the project. While the third chapter provides a methodology to accomplish the project, where it explains and illustrates the direction and the flowchart(s) of the project. The second phase will then start after all studies being carried out which includes chapter 4 and chapter 5. The results and discussion will be provided in chapter 4, while the last chapter will enclose the project with conclusions and suggestions. The Gantt charts of phase one and phase two are provided in appendix A.

6

CHAPTER 2

LITERATURE REVIEW

Many researches were conducted on evaluating the performance of CMM in general. Some of such researches are on probing system where as others were on identifying the dynamic errors of CMMs and other likely studies. The main purpose of this chapter is to investigate, analyze and compare the various and most updated research on the area of CMM probing systems. This chapter basically presents the literature reviews on the areas of the research. A brief review about the types of CMM will be presented. A classification of probing system and its types will also be reviewed as well as the mechanism and the performance of touch trigger probe. In addition, a brief overview about uncertainties and errors in CMM measurements and their sources will be covered. Finally, the most factors influence the accuracy of CMM probing system will be discussed.

2.1 Types and Constructions of CMM

The general basic components of any CMM consists of three linear axes( x , y, and z) mechanical frame, a probing system, a control unit, and a computer system which are integrated to enable readout and analyze the distances of the part being measured (Hocken & Pereira, 2012). The mechanical frame provides a guide way that enable precise movement along a straight line. Each of these guide ways (x, y, z) has a carrier that moves along by which it enables the second carrier to move along a straight line based on the first guide way. Each of three linear axes is fitted with a precision scale that records the position of the carrier measured from a reference point. The probe system is fitted to the carrier on the third axis, and the object is being measured by the

7 probe touches. Subsequently, the measurement system records the position of all three axes (Ratnam, 2009).

There are several physical configurations of CMMs. Each of these configurations plays an important role in its performance aspects such as accuracy, flexibility, cycle time, and life time cost. Although there are many commonly used configurations of CMMs but they can be grouped into five basic types as follows:

1. Cantilever type, 2. Bridge type,

3. Horizontal arm type, 4. Column type and 5. Gantry type.

This study will be carried out on a moving bridge CMM since it is the most widely used type. Figure 2.1 illustrates the components of bridge type CMM.

Guideway-side column Measuring table

Ram cover X-bridge

Ram Drive-side

Probe with stylus Guidway on the drive side

Control console Guideway

8

2.2 Probing System

The probe is one of the most important elements of coordinate measuring machine (CMM), it integrates the entire measuring system and forms a link between the machine and the work to be measured and it is responsible for the overall accuracy of a measurement (Weckenmann et al. 2004). It is the sensing element that makes contact and causes readings to be taken.

2.2.1 Classifications and Categories of Probing System

There are a verity of probes exist, these can be categorized into two different types of probe: contact (tactile) and non-contact probes which are practically used in modern CMMs (Hocken & Pereira, 2012). The type of probe to be utilized however is an important aspect to take into account when the precision of a particular CMM is going to be evaluated (Puertas et al., 2013).

Ali (2010) noted in his research that contact probes can also be classified into two specific families of manual hard probes and touch-trigger probes. However, Woźniak and Dobosz (2003) stated that contacting probes are categorized into tough trigger and measuring systems. Figure 2.2 calcifies the huge variety of probing systems to distinct application-oriented aspects, and Figure 2.3 classifies the probing systems according to principle of operation.

Figure 2.2 Classification criteria for probing systems (Hocken & Pereira, 2012). Probing systems for

coordinate metrology Kinematics

Mode of operation Displacement measurement Size Contact detection Moving during probing Operational

9 The literature of Weckenmann et al. (2004) has explained all of the above mentioned types of contact probes. In the same literature the authors also stated that the contact probes have at least the following components (Figure 2.4) to fulfil their task as the linkage between CMM and part to be measured:

i. A probing element to establish an interaction with the workpiece surface. ii. A transmitting device to transfer information about the interaction from the

touching element to the sensor (e.g. stylus shaft)

iii. A force generating and controlling element to produce a defined probing force (e.g. spring).

iv. A sensor to sense the interaction of the touching element with the workpiece (e.g. electric switch, force sensor).

Figure 2.3 Sensors for coordinate measuring machines classified by principle of operation (Christoph & Neumann, 2007)

10 v. An output (interface to the CMM) transmitting the information for triggering a length measuring device (e.g. scale) or for further processing (e.g. correction of bending, taking into account qualified tip ball radius, evaluation in instrument’s software).

The hard probes are used with manual CMMs for low and medium accuracy requirements, because their repeatability quality depends upon their operator touch. Recently the touch-trigger is the common used probe in CMM, it gives a higher quality of measured data compared to the hard probe type by eliminating the influence of operator touch, as it can be fitted on direct computer numerical control (CNC-CMMs) and manual CMMs system.

Many studies were conducted to describe the optical and large range probing and measurement systems (Estler et al., 2002; Schwenke et al., 2002), others were to evaluate and improve the optical probing measuring accuracy of CMM (Chao et al., 2011; Carmignato & Savio, 2011). Because of the huge variety of implementations, and the general nature of the task of probing systems, this study will focus on the touch-trigger probes.

Force generating element (spring)

Transmitting device (stylus stem)

Probing element (tip ball)

Sensor

(electric switch)

Interface to CMM

(electric connection)

5 1.5 Project Organization

This project is divided into two phases. The first phase comprises of three chapters. The first chapter contains a general overview of coordinate measuring machines (CMMs), states the problem of the project, and identifies the objectives as well as the scope of the project. In the second chapter, a literature review will be provided to investigate, analyze and compare the various and most updated research on the area related to the project. While the third chapter provides a methodology to accomplish the project, where it explains and illustrates the direction and the flowchart(s) of the project. The second phase will then start after all studies being carried out which includes chapter 4 and chapter 5. The results and discussion will be provided in chapter 4, while the last chapter will enclose the project with conclusions and suggestions. The Gantt charts of phase one and phase two are provided in appendix A.

6

CHAPTER 2

LITERATURE REVIEW

Many researches were conducted on evaluating the performance of CMM in general. Some of such researches are on probing system where as others were on identifying the dynamic errors of CMMs and other likely studies. The main purpose of this chapter is to investigate, analyze and compare the various and most updated research on the area of CMM probing systems. This chapter basically presents the literature reviews on the areas of the research. A brief review about the types of CMM will be presented. A classification of probing system and its types will also be reviewed as well as the mechanism and the performance of touch trigger probe. In addition, a brief overview about uncertainties and errors in CMM measurements and their sources will be covered. Finally, the most factors influence the accuracy of CMM probing system will be discussed.

2.1 Types and Constructions of CMM

The general basic components of any CMM consists of three linear axes( x , y, and z) mechanical frame, a probing system, a control unit, and a computer system which are integrated to enable readout and analyze the distances of the part being measured (Hocken & Pereira, 2012). The mechanical frame provides a guide way that enable precise movement along a straight line. Each of these guide ways (x, y, z) has a carrier that moves along by which it enables the second carrier to move along a straight line based on the first guide way. Each of three linear axes is fitted with a precision scale that records the position of the carrier measured from a reference point. The probe system is fitted to the carrier on the third axis, and the object is being measured by the

7 probe touches. Subsequently, the measurement system records the position of all three axes (Ratnam, 2009).

There are several physical configurations of CMMs. Each of these configurations plays an important role in its performance aspects such as accuracy, flexibility, cycle time, and life time cost. Although there are many commonly used configurations of CMMs but they can be grouped into five basic types as follows:

1. Cantilever type, 2. Bridge type,

3. Horizontal arm type, 4. Column type and 5. Gantry type.

This study will be carried out on a moving bridge CMM since it is the most widely used type. Figure 2.1 illustrates the components of bridge type CMM.

Guideway-side column Measuring table

Ram cover X-bridge

Ram Drive-side

Probe with stylus Guidway on the drive side

Control console Guideway

8 2.2 Probing System

The probe is one of the most important elements of coordinate measuring machine (CMM), it integrates the entire measuring system and forms a link between the machine and the work to be measured and it is responsible for the overall accuracy of a measurement (Weckenmann et al. 2004). It is the sensing element that makes contact and causes readings to be taken.

2.2.1 Classifications and Categories of Probing System

There are a verity of probes exist, these can be categorized into two different types of probe: contact (tactile) and non-contact probes which are practically used in modern CMMs (Hocken & Pereira, 2012). The type of probe to be utilized however is an important aspect to take into account when the precision of a particular CMM is going to be evaluated (Puertas et al., 2013).

Ali (2010) noted in his research that contact probes can also be classified into two specific families of manual hard probes and touch-trigger probes. However, Woźniak and Dobosz (2003) stated that contacting probes are categorized into tough trigger and measuring systems. Figure 2.2 calcifies the huge variety of probing systems to distinct application-oriented aspects, and Figure 2.3 classifies the probing systems according to principle of operation.

Figure 2.2 Classification criteria for probing systems (Hocken & Pereira, 2012). Probing systems for

coordinate metrology Kinematics

Mode of operation

Displacement measurement Size

Contact detection

Moving during probing

Operational

9 The literature of Weckenmann et al. (2004) has explained all of the above mentioned types of contact probes. In the same literature the authors also stated that the contact probes have at least the following components (Figure 2.4) to fulfil their task as the linkage between CMM and part to be measured:

i. A probing element to establish an interaction with the workpiece surface. ii. A transmitting device to transfer information about the interaction from the

touching element to the sensor (e.g. stylus shaft)

iii. A force generating and controlling element to produce a defined probing force (e.g. spring).

iv. A sensor to sense the interaction of the touching element with the workpiece (e.g. electric switch, force sensor).

Figure 2.3 Sensors for coordinate measuring machines classified by principle of operation (Christoph & Neumann, 2007)

10 v. An output (interface to the CMM) transmitting the information for triggering a length measuring device (e.g. scale) or for further processing (e.g. correction of bending, taking into account qualified tip ball radius, evaluation in instrument’s software).

The hard probes are used with manual CMMs for low and medium accuracy requirements, because their repeatability quality depends upon their operator touch. Recently the touch-trigger is the common used probe in CMM, it gives a higher quality of measured data compared to the hard probe type by eliminating the influence of operator touch, as it can be fitted on direct computer numerical control (CNC-CMMs) and manual CMMs system.

Many studies were conducted to describe the optical and large range probing and measurement systems (Estler et al., 2002; Schwenke et al., 2002), others were to evaluate and improve the optical probing measuring accuracy of CMM (Chao et al., 2011; Carmignato & Savio, 2011). Because of the huge variety of implementations, and the general nature of the task of probing systems, this study will focus on the touch-trigger probes.

Force generating element (spring)

Transmitting device (stylus stem)

Probing element (tip ball)

Sensor

(electric switch)

Interface to CMM

(electric connection)