Metal machining is usually performed using large amounts of cutting fluids and in 1996, 320000 tons of metalworking fluids (straight oils and water-miscible.
PERFORMANCE TESTING OF CUTTING FLUIDS
Walter Belluco
Ph.D. Thesis
Institut for Produktion (Department of Manufacturing Engineering) Technical University of Denmark
December 2000 Publication no. IPT.198.00 (MM00.63)
PREFACE This thesis has been financed in the context of a 3 year project, carried out at the Institute for Product Development (IPU), funded by the Danish Government, titled “Development of cutting fluids based on vegetable oils”. Thanks are due to project partners Henrik Reitz, Jørgen Eriksen, Mogens Nielsen and Paul Flengmark. I would like to acknowledge the work of Dr Dragos A. Axinte - the results in connection with turning tests were obtained during his stay in Denmark, which was sponsored by the Danish Research Agency under funding no. 9801683. I am grateful to Dr Valeria Carbone and René Sobiecki, for the assistance with the measurements on CMM. Thanks are due to Prof. Paolo Bariani, DIMEG, Universitá di Padova, for making available the force measuring equipment used in the drilling tests. I am also grateful to many research students at DTU, who took part in the experimental work within their M.Sc. projects and during course work. Thanks are due to the staff at the department DM/DTP/DSI, Renault, Direction de la Mécanique, where the author was warmly received during his six month placement and where some of the ideas put forward in this thesis were shaped, through many informal discussions with manufacturing engineers, cutting fluid manufacturers and end users. In particular, I am grateful to Patrick Seveno, François Trochu, Jean Paul Farina and Gérard Guise. I would like to thank Dr Martin Paul, University of Birmingham, for reviewing and proofreading this thesis, and Mario Lavezzo, for the assistance with the final printing. Finally, special thanks are due to all the staff at IPT and IPU, in particular to the supervisor and inspirer of this work Assoc. Prof. Leonardo De Chiffre.
iii
SUMMARY The importance of cutting fluid performance testing has increased with documentation requirements of new cutting fluid formulations based on more sustainable products, as well as cutting with minimum quantity of lubrication and dry cutting. Two sub-problems have to be solved: i) which machining tests feature repeatability, reproducibility and sensitivity to cutting fluids, and ii) to what extent results of one test ensure relevance to a wider set of machining situations. The present work is aimed at assessing the range of validity of the different testing methods, investigating correlation within the whole range of operations, materials, cutting fluids, operating conditions, etc. Cutting fluid performance was evaluated in turning, drilling, reaming and tapping, and with respect to tool life, cutting forces, chip formation and product quality (dimensional accuracy and surface integrity). A number of different work materials were considered, with emphasis on austenitic stainless steel. Cutting fluids from two main groups were investigated, water miscible (reviewed from previous work) and straight oils. Results show that correlation of cutting fluid performance in different operations exists within the same group of cutting fluids, for stainless steel. A possible rationalisation of cutting fluid performance tests is suggested. In order to select a set of basic tests and optimise them for use as general and standardised testing methods, an original approach to the evaluation of cutting force and tool life uncertainty is proposed.
v
CONTENTS
PREFACE
iii
SUMMARY
v
1. INTRODUCTION 1.1 1.2 1.3
Background of the project and state of the art Present thesis Organization of the work
3 8 9
2. CUTTING FLUID PERFORMANCE TESTS 2.1
2.2
2.3
2.4
2.5
2.6
Introduction 2.1.1 Overview of the tests 2.1.2 Fluids 2.1.3 Workpiece materials 2.1.4 Equipment Sequential hole making operations 2.2.1 Introduction 2.2.2 Experimental details and procedure 2.2.3 Results and discussion 2.2.4 Conclusion Drilling tests 2.3.1 Introduction 2.3.2 Materials and methods 2.3.3 Results and discussion 2.3.4 Conclusion Reaming and tapping tests 2.4.1 Introduction 2.4.2 Experimental details 2.4.3 Results and discussion 2.4.4 Conclusion Turning tests 2.5.1 Introduction 2.5.2 Experimental details 2.5.3 Results 2.5.4 Discussion 2.5.5 Conclusion Conclusions
13 13 14 14 17 20 20 20 24 32 34 34 34 40 49 50 50 51 56 63 64 64 66 70 76 77 78
3. COMPARATIVE ANALYSIS 3.1 3.2
3.3
3.4
Introduction Repeatability, resolution and cost of different tests 3.2.1 Introduction 3.2.2 Tool life tests 3.2.3 Cutting force tests 3.2.4 Surface finish tests 3.2.5 Discussion 3.2.6 Conclusion Correlation of methods for performance testing 3.3.1 Introduction 3.3.2 Performance tests 3.3.3 Results 3.3.4 Discussion 3.3.5 Conclusion Conclusions
81 82 82 83 86 88 89 90 92 92 93 95 104 109 110
4. UNCERTAITY EVALUATION IN MACHINING TESTS 4.1 4.2
4.3
4.4
Introduction Uncertainty evaluation in cutting force measurements 4.2.1 Introduction 4.2.2 Calibration uncertainty of metal cutting dynamometers 4.2.3 Uncertainty contributions in cutting force measurements 4.2.4 Turning 4.2.5 Reaming and Tapping 4.2.6 Conclusion Uncertainty evaluation in tool life measurements 4.3.1 Introduction 4.3.2 Uncertainty estimation for tool life 4.3.3 Conclusion Conclusions
113 114 114 114 118 120 132 148 149 149 149 154 155
5. CONCLUSIONS 5.1 5.2
Final remarks Further developments of present work
REFERENCES
159 162
Chapter 1. Introduction
“What am I doing here?” Bruce Chatwin
“La maladie principale de l’homme est la curiosité inquète des choses qu’il ne peut savoir” Pascal Pensées, I, 18
Performance Testing of Cutting Fluids
2
1. Introduction
1. INTRODUCTION
1.1
Background of the project and state of the art
Metal cutting, is a widespread manufacturing process of primary relevance for the economy of industrial societies, where it has been estimated that approximately 10% of all the metal produced is turned into chips [Trent, 77]. Wherever metal is used in any manmade object, one can be sure that it must have reached its final stage through processing with machine tools [Juneja-Sekhon, 87], and the costs associated with metal cutting operations have been estimated at about 10% of the gross national product in USA [Shaw, 84]. Metal cutting is indispensable for our lives, and even minor improvements in machining productivity are of major importance in our economy. Metal machining is usually performed using large amounts of cutting fluids and in 1996, 320000 tons of metalworking fluids (straight oils and water-miscible concentrate) were used in the E.U. [Europalub, 97]. Cutting fluids affect metal cutting performance: their action is to lubricate and cool the tool, thereby helping pursue optimal machining conditions. The use of cutting fluids not only reduces tool wear and increases tool life but also aids in obtaining the desired size, finish and shape of the workpiece. Cutting fluids have a negative impact on the environment and on social welfare and their use and maintenance is connected with high total costs, that have been estimated to up to 16% of total manufacturing costs [Klöcke-Eisenblatter, 97]. However, the main objective in use of cutting fluids is either reduction in total cost per part or an increase in the rate of production [Shaw, 84]. The most important characteristics of a cutting fluid, having high impact on other manufacturing costs, is performance. This thesis is concerned with performance testing of cutting fluids. 3
Performance Testing of Cutting Fluids In metal cutting, the present tendency is toward achieving increased material removal rates with higher degrees of automation and without human supervision. This requires a very reliable machining process and accurate predictions of surface finish, workpiece accuracy, tool life and chip control. Such predictions are made possible by modeling of machining operations, or by testing. The primary objective of modeling is to develop a predictive capability for machining performance in order to facilitate effective planning to achieve optimum productivity, quality and cost. A great deal of research is being dedicated to this field and in the latest years progress has been reported in the areas of physical modeling, numerical finite element modeling (FEM), as well as in Artificial Intelligencebased methods [van Luttervelt et al., 98]. However, to date, modeling of machining is still not reliable enough to be applied in practice by industry. In particular, modeling of cutting fluid action is still far from enabling the prediction of cutting fluid performance. The alternative to modeling of metal cutting is actual testing. Although the high costs and the deficiencies of testing according to the available standards (for instance [ISO 3685: 93] and [ISO 8688: 89], covering only tool life testing in turning and milling), this approach is still the most sound. The major drawbacks of the experimental approach are that the tests are time consuming, costly, and cannot cover the wide variety of different machining situations. Moreover, a further complication is the lack of reliability that is sometimes found in test results. When performance testing of cutting fluids is concerned, manufacturing engineers do not agree on how it should be done. In principle, a lot of different criteria for cutting fluid performance are possible (Fig. 1.1.1). In practice, performance evaluation of cutting fluids with machining tests is not carried out in a standardized way.
4
1. Introduction Independent Process Parameters
Dependent Process Varaiables
Cutting Data
Cutting Forces
Cutting Fluid
Cutting Temperatures
Tool
Contact Length, Chip Compression, etc...
Work Machine
Performance Criteria
Tool Life
Product Quality
Cutting Power Chip Formation
Fig. 1.1.1: Both Performance Criteria and Dependent Process Variables can be used to test the performance of Cutting Fluids.
The tightening of rules and regulations concerning manufacturing have recently introduced a driving force towards the reformulation of cutting fluid into more operatorsafe and environment-friendly products. The clearest changes are the reduction or even suppression of potentially harmful additives (halogen compounds for extreme pressure lubrication, triazines, some amines) and partial or total replacement of mineral basestock oils with more biodegradable products such as vegetable oils and esters. Therefore cutting fluids are a rising interest in research [Sheng-Oberwalleney, 97], for the great environmental impact that is connected with their use. Research is especially focussing in the fields of additive replacement, in the formulation of environmentally-friendly products and in the direction of minimizing or eliminating the quantity of lubricant required by the process (Minimum Quantity Lubrication, [Brinksmeier et al., 97], [Daniel et al., 97], [Wakabayashi et al., 98], [Braga et al., 99], [Brockhoff-Walter, 98]), (Dry Cutting, [Klöcke-Eisenblatter, 97], [Sreejith-Ngoi, 5
Performance Testing of Cutting Fluids 00]). However, despite many attempts in manufacturing research to avoid cutting fluids, the present state-of-the-art technologies do not seem to assure that cutting fluids will be entirely phased out in the next future [Pfeifer et al., 93]. That is also confirmed by the author’s experience, as resulting from discussions with manufacturers and users of cutting fluids, as well as by looking at the stable trend of metalworking fluid consumption in recent years. Some operations on certain materials exist which are very demanding with respect to lubrication and cooling (e.g. tapping stainless steel) and their success is largely dependent on the use of cutting fluids. Cooling lubricants should take part in any realistic approach to the forecast of the manufacturing scenario in the next years to come and the efforts of research in this field should go in the direction of both reducing their environmental impact as well as optimizing cutting fluid efficiency and quantity. In this context, cutting fluids reformulated to be based on vegetable oils and esters qualify as potential candidates to replace mineral-based products, because they are almost entirely biodegradable and well compatible with minimal lubrication technology. The use of vegetable oils as process fluids has been reviewed by Bisht [Bisht et al., 89], but to the knowledge of the author very limited information concerning the machining performance of vegetable-based cutting fluids existed in the literature. Most of the fluids tested in this work are of this kind. One could wish that the reformulation of cutting fluids took place without affecting performance, or in other terms, manufacturing economics. New products should thus be tested before being introduced into production. Currently, the efficiency of cutting fluids is evaluated in many different ways, characterized by a variety of different methods and evaluation parameters. Traditionally, cutting fluid performance is evaluated with basic tribological tests like “4 ball” [ASTM D 2783: 88], “Timken” [ASTM D 2782; 88], “Pin and Vee” [ASTM D 3233: 93] and others. It is generally agreed that basic tribological tests are poorly correlated with the behavior of a cutting fluid in actual machining, whereas 6
1. Introduction the closer we get to the manufacturing process, the better the expected correlation to actual production [Henshen, 68], [Webb et al., 74], [Blanchard-Syrett, 74], [Lorenz, 85], [De Chiffre, 80]. This approach is complicated by the difficulty of describing the variety of manufacturing situations with simple and possibly inexpensive tests, in which one or few parameters are measured.
Testing Cost €
FSPT
Tool Life Tests Product Quality Tests Cutting Force Tests
Trib.An.
Correlation With Performance Fig. 1.1.2: Alternatives for cutting fluid performance tests as a function of testing cost and “presumable” correlation with manufacturing. Full Scale Production Tests (FSPT) feature correlation with actual production, but at a very high cost. Laboratory Cutting Tests are performed in more simplified testing conditions and a priori lower correlation should be expected. Tribological Analyses (Trib. An.) are used in the development phase for their practicality and low cost, but are very far from actual manufacturing situations. The objective of test development is to reduce testing cost without loss of correlation with performance.
Some attempts have been made to define a standard procedure for cutting fluid performance evaluation with 7
Performance Testing of Cutting Fluids machining tests by ISO [ISO 3685: 93], [ISO 8688:89], ASTM [ASTM D 5619: 94] and NORDTEST [NT MECH 038: 97], [NT MECH 039: 98], [NT MECH 040: 98]. To date, confusion exists as to: (i) which machining tests feature repeatability, reproducibility, sensitivity to cutting fluids and low cost, and (ii) to what extent results of one test ensure relevance to a wider set of machining situations, see Fig. 1.1.2.
1.2 Present thesis The present work is part of a major effort at the author’s university, with the scope of assessing the range of validity of the different testing methods, investigating correlation within the whole range of operations, materials, cutting fluids, operating conditions, etc. The final goal is to select a set of basic tests in view of their relevance and correlation to actual manufacturing situations or to full scale production tests. In addition, the tests should be optimized in terms of repeatability, reproducibility, sensitivity, measurement uncertainty and cost, for use as general and standardized testing methods. The Institut for Produktion (IPT) (Department of Manufacturing Engineering) at the Technical University of Denmark (DTU) has already studied a number of different testing methods in which parameters such as tool life and wear, cutting forces and surface finish were used as performance evaluation criteria [De Chiffre, 77], [De Chiffre, 78], [De Chiffre, 80], [De Chiffre, 92], [De Chiffre et al., 94], [De Chiffre et al, 94]A, [Møller , 87], [De Chiffre et al., 94]B. By continuous enhancement of the theoretical and experimental background for carrying out the testing, a high level of confidence has been reached at IPT. During this project, these and new testing methods have been elaborated in terms of accuracy and uncertainty estimation. This thesis was financed in the context of a 3 year project funded by the Danish Government, titled “Development of cutting fluids based on vegetable oils”, in which the possibility of finding technical applications for biodegradable straight natural oils, 8
1. Introduction currently employed by the food processing industry, were explored. The results of this performance testing activity were valuable to the development of environmentally friendly cutting fluids based on vegetable oils.
1.3 Organization of the work This thesis is organized as follows. This Chapter is a general introduction to the work. Chapter 2 presents experimental methods and results of extensive campaigns of machining tests carried out within this project on stainless steel and other materials using straight oils. Chapter 3 discusses comparison and correlation of cutting fluid performance tests in different operations, based on a comprehensive review of experimental results, including previous work carried out at IPT. Chapter 4 presents an original model for uncertainty estimation in cutting force and tool life measurements, in particular when applied to cutting fluid testing. Finally, Chapter 5 treats conclusion and suggestions for further work. Only those references that are relevant to the experimental work have been included. More comprehensive literature reviews on cutting fluid testing, are available in [De Chiffre, 77], [Caratossidis, 98] and [Belluco, 00].
9
Chapter 2. Cutting Fluid Performance Tests
“There is nothing new under the sun. Is there anything of which one can say, ‘Look, this is new?’ No, it has already existed long ago before our time”. Ecclesiastes 1:9-10
“From causes which appear similar, we expect similar effects. This is the sum of all our experimental conclusions.” David Hume An Inquiry Concerning Human Understanding
11
Performance Testing of Cutting Fluids
12
2. Cutting Fluid Performance Tests
2. CUTTING FLUID PERFORMANCE TESTS
2.1 Introduction 2.1.1 Overview of the tests This chapter presents a description of the different experimental campaigns carried out during this project, with the goal of investigating the influence of different cutting fluids on basic metal cutting operations, using different workpiece materials and performance evaluation criteria. Different testing procedures were applied, comprising single as well as multiple operations. Measurements involving only single operations generally provide better control of all influence parameters for the test. This results in a smaller experimental spread, but may offer a somehow restricted picture. Multiple operations, instead, allow a more complete evaluation of lubricant properties as well as a closer simulation of a real production situation [De Chiffre, 78], though bigger data scatters should be expected as more influence parameters are introduced. An overview of the machining tests is presented in Table 2.1.1. In the following Sections the experimental setups, procedures and results obtained in the course of the experimental work will be detailed. Stainless Steel
Carbon Steel
Cast Aluminium xo
Brass
Drilling
xo∆
Tapping
x∆
x
x
x
Reaming
x∆
x∆
x∆
Turning
xo∆
x∆ x
Table 2.1.1. Testing operations and materials; x: cutting force measurements; o: tool life measurements; ∆: other measurements (e.g. roughness, part accuracy, surface integrity, chip formation).
13
Performance Testing of Cutting Fluids Section 2.2 is concerned with testing of cutting fluids using sequential hole making operations, where cutting force, part accuracy and surface integrity were used as performance criteria in drilling, reaming and tapping stainless steel. These tests were used as the basis for the preliminary screening of a number of different vegetable-based oil formulations, to select some fluids for more thorough investigations in single operations and using different workpiece materials. Section 2.3 presents the results of drilling tests on stainless steel and cast aluminium, where tool life, cutting forces and chip formation were measured. Section 2.4 is concerned with reaming and tapping tests on stainless steel, aluminium, brass and carbon steel, where cutting forces as well as part accuracy were measured. Section 2.5 deals with turning tests on stainless steel and brass, were tool life, chip formation and cutting forces were measured. 2.1.2 Fluids A set of base oils and formulated products as well as three commercial reference products, whose characteristics are described in Table 2.1.2, were tested. Most cutting fluids, all straight oils, were based on vegetable and ester oils. Additivated mineral-based cutting fluid RM was chosen as reference fluid, due to its diffusion in the Danish market. 2.1.3 Workpiece materials Workpiece materials from four different groups (stainless steel, carbon steel, cast aluminium and brass) were tested. The basic characteristics of workpiece materials are presented in Table 2.1.3.
14
2. Cutting Fluid Performance Tests Name
Type
S-add
P-add
RM
Commercial oil
Mineral oil based, heavy duty, 20cSt at 40 °C
Description
-
-
RV
Commercial oil
Vegetable oil based, 20cSt at 40 °C
-
-
RA
Commercial oil
Mineral oil based, 5 % lard
None
None
BA
Base oil
Rape seed oil, high oxidation stability
None
None
BB
Base oil
Oxidation stable coconut ester oil
None
None
BC
Base oil
Oil of Limnantes Alba
None
None
BD
Base oil
Rapeseed oil with high erucic content
None
None
A
Formulated oil
Based on E, for heavy duty, 20cSt at 40 °C
++
++
B
Formulated oil
Based on F, for heavy duty, 20cSt at 40 °C
++
++
C
Formulated oil
Based on E, for mild duty, 20cSt at 40 °C
+
+
D
Formulated oil
Based on F, for mild duty, 20cSt at 40 °C
+
+
E
Formulated oil
Blend of BA and BB, 20 cSt at 40 °C
None
None
F
Formulated oil
Blend of E and 10 % BC, 20cSt at 40 °C
None
None
Table 2.1.2. Basic characteristics of tested fluids, all straight oils. S-add and P-add columns represent the amount of sulphur and phosphor containing additives, detailed in levels high (++), low (+), and not available (-). Total weight percentage of additives for formulated oils: 10% max.
15
Performance Testing of Cutting Fluids Work Material Dimensions at delivery
Composition [%]
Work Material’s Mech. Properties
Stainless steel AISI 316L Ø 30 X 120 [mm]
C:0.016, Si:0.39, 258 HV20 (face) Mn:1.4, P:0.027, 177 HV0.5/30 Ni:11.21, Cr:17.31, (core) Mo:2.11, S:0.026, N:0.052.
Stainless steel AISI 316L 300 X 200 X 35 [mm]
C= 0.01, Si=0.3, Mn=1.5, P=0.029, Ni=10.5, Cr=16.7, Mo=2.6, S=0.001.
Stainless steel AISI 316L Ø 240 X 800 [mm]
C=0.016, Si=0.39, Mn=1.4, P=0.027, Rp =263 N/mm2 0.2 Ni=11.2, Cr=17.3, Rm=554 N/mm2 Mo=2.11, S=0.026, 147±1.5 HV20 N=0.052.
Rp0.2=276 N/mm2 Rp1.0=322 N/mm2 Rm=590 N/mm2 A5=50%.
Test (Section) Holemaking (2.2) Reaming, Tapping (2.4) Drilling (2.3)
Turning (2.5)
Carbon Steel C 45 Ø 30 X 3000 mm
Not measured
Cast aluminium A 390 200 X 300 X 35 [mm]
Al=76.6 , Si=15.7, Mg=0.57, Mn=15.7, 96 ± 1 HV 20 Mn=0.21, Fe =0.96, Cu=3.56, Zn =2.33.
Cast aluminium A 390 Ø 32 X 600 [mm]
Al=76.6 , Si=15.7, Mg=0.57, Mn=15.7, 94.5 HV20 (face) Reaming, Mn=0.21, Fe =0.96, Tapping Cu=3.56 , Zn =2.33. (2.4)
257 HV20 (face)
Reaming, Tapping (2.4) Drilling (2.3)
Brass CuZn39Pb3 Ø 30 X 3000 [mm]
Not measured
142.6 HV20 (face) Reaming, Tapping (2.4)
Brass CuZn40Pb2 Ø 201 X 500 [mm]
Not measured
99±1.5 HV20
Turning (2.5)
Table 2.1.3. Basic characteristics of workpiece materials.
16
2. Cutting Fluid Performance Tests 2.1.4 Equipment All the tests were carried out using a vertical milling centre and a lathe. Both machine tools are numerically controlled and had already been used at IPT for cutting fluid and machinability testing over the years. The vertical machining centre, used for drilling, reaming and tapping tests, is a Cincinnati Sabre 750 S CNC 7.5 kW. The lathe, used for turning tests as well as preparation of reaming and tapping test specimens, is a VDF Boehringer PNE 480 50 kW. Cutting force measurements were carried out using multicomponent dynamometers. A 2D load cell (torque and thrust measurement) and a 3D tool post turning dynamometer (measurement of orthogonal forces) were already available at the department [Dovmark, 83], [Frederiksen-Pedersen, 84], [Frederiksen-Pedersen, 86]. The 3D dynamometer was reconditioned and recalibrated [AxinteBelluco, 99]. In addition, a 6D load cell (measurement of orthogonal forces and torques) was designed, manufactured and calibrated, based on four 3D elements [Belluco-Bastianel, 99], [Belluco-Nicoletti, 99]. All cutting dynamometers were integrated measuring systems, equipped with piezoelectric cells, whose output charges were converted into voltages through charge amplifiers. The output voltages of the charge amplifiers were digitised using a PC equipped with 16 bit A/D converter and then transformed into loads using calibration data and laboratory software originally developed by the author [Belluco, 99]. In Table 2.1.4 the characteristics of the cutting force measuring systems are described, while Fig. 2.1.1 shows the elements of the measurement system in the case of the 6D load cell. Other test equipment as well as experimental conditions will be described in corresponding Sections.
17
Performance Testing of Cutting Fluids Measuring transducer (3D: 3 component turning tool post dynamometer measuring Fx, Fy, Fz) Charge amplifier (3D: 3 component turning tool post dynamometer, measuring Fx, Fy, Fz) Measuring transducer (2D: 2 component load cell measuring axial torque and thrust) Charge amplifier (2D: 2 component load cell measuring axial torque and thrust) Measuring transducer (6D: 6 component load cell measuring Fx, Mx, Fy, My, Fz, Mz) Charge Amplifier (6D: 6 component load cell measuring Fx, Mx, Fy, My, Fz, Mz) Digital measuring instrument
Laboratory software
4 Kistler piezoelectric cells type 9251A 3 Kiag Swiss type 5023 charge amplifiers Kistler 9271A SN76766 piezoelectric load cell Kistler 5001SN and 5006SN charge amplifiers Kistler Type 9366 AB 4 component piezoelectric dynamometer kit Kistler Type 9865 B 8-channel charge amplifier NI Inc. AT-MIO-16XE-50 16 bit A/D & D/A converter boardequipped with National Instruments BNC 2090 terminal. Original application developed with Labwiew 5.0.
Table 2.1.4. Cutting force measurement systems.
18
2. Cutting Fluid Performance Tests
Fig. 2.1.1. The experimental equipment, showing front panel of the data acquisition program, BNC cable terminal, charge amplifier. 6D load cell is mounted on vertical milling centre.
19
Performance Testing of Cutting Fluids
2.2 Sequential hole making operations 2.2.1 Introduction This Section presents the measurements carried out in sequential drilling, core drilling, reaming and tapping AISI 316L stainless steel with HSS-E tools, see also [Belluco-De Chiffre, 99]. The effect of different lubricants on cutting forces and power, as well as surface integrity, was investigated, see also [Belluco-De Chiffre, 99]A. Cutting forces and power at low cutting speeds are connected to the lubricating effect of a fluid: low cutting forces correspond to good lubrication and a process with good lubrication is preferred. The tapping torque test is a well-established method for cutting fluid evaluation [Faville-Voitik , 77], [ASTM D 5619: 94], [NT MECH 039: 98]. Drilling [Upton, 96], [Hann et al., 97] and reaming [De Chiffre et al, 94]B have also been studied. 2.2.2 Experimental details and procedure A set of base oils and formulated products as well as three commercial reference products, whose characteristics are described in Table 2.1.2, were tested. All operations were carried out on cylindrical specimens (∅30x30) of AISI 316L stainless steel, with hardness 176-178 HV0.5/30 (Table 2.1.3), using the vertical milling center mentioned in 2.1.4. The experimental conditions, taken from the tool manufacturer’s recommendations, are shown in Table 2.2.1. The specimen was not moved between subsequent operations. The positioning accuracy was better than 0.02 mm. The specimen was immersed in a reservoir, which contained three litres of cutting fluid and was clamped on a precision wrench mounted on the 2D load cell measuring vertical force (thrust) and torque (Table 2.1.4). The clamping fixture composed of a ø 30 mm (external) collar, designed to achieve the radial alignment between the specimen and the 20
2. Cutting Fluid Performance Tests dynamometer axes [Dovmark, 83]. The clamping fixture is shown in Fig. 2.2.1. Thrust and torque signals were acquired at a sampling speed of 100 Hz. The resolution of the integrated acquisition system was 0.01 Nm in torque and 1 N in thrust. Sampling windows were placed on a stable part of the torque diagrams and were held constant throughout the testing campaign. Thrust and torque were recorded for each operation, except when centering. One test consisted of ten repetitions with the reference fluid, followed by ten repetitions with the test fluid and then again by ten repetitions with the reference fluid. In each series, one specimen was stopped after reaming 1 and one after reaming 2, for dimensional measurements.
Tool
∅ (mm)
Centering
Form A, HSS DIN 333
Drilling
Type Alpha X-E, HSS-E
Step
Operation
Speed (m/min)
Feed (mm/rev)
1 2
3
Core Drilling Type N, HSS-E DIN 344
4
Reaming 1
5
Reaming 2
6
Tapping
Sampling window (points)
3.15
1
0.03
-
8.8
20.0
0.1
1000-1900
9.8
13.0
0.1
1200-3200
H7, HSS-E DIN 212
9.9
4.9
0.2
1200-4200
H7, HSS-E DIN 212
10.3
6.1
0.2
1200-4200
M12 HSS-E DIN 376
12
4.6
1.79
700-1200
DIN 338
Table 2.2.1. Machining cycle corresponding sampling windows.
and
cutting
data,
and
21
Performance Testing of Cutting Fluids
Fig. 2.2.1. Clamping fixture mounted dynamometer, with cutting fluid reservoir.
22
on
piezoelectric
2. Cutting Fluid Performance Tests The cutting fluid was the test variable, RM being taken as reference. Cutting fluid efficiency was evaluated as a ratio between the mean values of the parameters MR and MT, where MT is the average force (or torque, or power) obtained using the test fluid and MR equals the average force (or torque, or power) with reference fluid (average of two series). The experimental standard deviation of the efficiency was calculated with Eq. (2.2.1), where σMR and σMRT are the corresponding standard deviations for MR (pooled) and MT, respectively:
σ Eff =
σ MR 2 MT 2
+
MR 2 ⋅ σ MT MT 4
2
(2.2.1).
This procedure for data analysis is a better method when considering error propagation law, when compared to what is detailed in [ASTM D 5619: 94], where only the dispersion of the test fluid is accounted for. If the variability of data collected with the reference fluid is discarded, efficiency results will appear less dispersed thus resulting in an arbitrary narrowing of the spread on the resulting cutting fluid efficiency. Hole geometry was measured on a coordinate measuring machine. Three diameters in each hole were probed, at a distance of 5, 15 and 25 mm from the top face of the cylinder and using a 4-point measurement strategy yielding a measurement uncertainty of 5 µm. The average diameter was then calculated. Roughness measurements were carried out with a stylus instrument provided with a skid pickup and a 5 µm radius tip [ISO 4287: 1997]. Five profiles were recorded for each specimen, distributed at equal angles around the circumference. The profiles were taken over a length of 4 mm and with 0.8 mm ISO filtering [ISO 11562:1997] starting at a distance of 6 mm from the bottom of the hole, i.e. in a zone that was considered very sensitive to chip jamming and built up edge (BUE) formation. 23
Performance Testing of Cutting Fluids Subsurface microhardness inspection was conducted on the specimens that were used for geometrical measurements, roughness measurements, and on some tapping operation tests. Specimens were selected for preparation from the tests with fluids RM (reference), RA (unadditivated mineral oil) and B (additivated vegetable oil); the two test fluids had given in force tests best and worst results, respectively. Microhardness HV0.05/30 measurements in reaming were performed according to [ISO 6507-3: 1989] at distances of 15, 30, 60, 120 and 240 µm from the reamed bore. The first run after a fluid change was discarded from data analysis due to possible ‘carry over’ effects. Before the test, every tool was cleaned with light hydrocarbon solvent, checked for burrs or breaks on the cutting edge under a stereomicroscope and dried in air. All tools were run-in prior to each test by machining a series of three holes and a set of new tools was used for each test. The materials required to run one test were 33 specimens and a set of new tools (Fig. 2.2.2). 2.2.3 Results and discussion Efficiencies calculated from torque, thrust and total cutting power produced very similar results for each test. Torque efficiency was sufficient to characterise a fluid and was therefore that was used for subsequent work. Average torque values, as well as standard deviations for all cutting force measurements are presented in Table 2.2.2. In the following, the discussion will be focused on Drilling, Reaming 2 and Tapping results. Cutting fluid efficiencies obtained with these three tests are compared in Fig. 2.2.3.
24
2. Cutting Fluid Performance Tests 4
E CPR198
RM S310
RM S310
Torque (Nm)
3
2
1
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
0
Specimen #
Fig. 2.2.2. Example of drilling torque test measurements. Each box represents a single measurement, all measurement carried out with the same tool. White boxes discarded from analysis.
150%
Drilling Reaming
100%
Tapping
C
E
F
D
B
A
RA (2)
RV (2)
BD
BC
BB
BA
RA
RV (1)
50%
Fig. 2.2.3. Drilling, Reaming 2 and Tapping force efficiency results (relative to fluid RM). Error bars represent combined experimental standard deviations. Tool and cutting parameters according to Table 2.2.1.
25
Performance Testing of Cutting Fluids
RM
Drilling
Test FLUID A Av. [Nm] 3.53
(n=9)
s.d. %
BB 3.54
BC BD RA 3.56 3.55 3.62
F 3.53
D 3.54
B Av. s.d./Av. 3.47 3.54 1.9%
0.3% 0.6% 0.9% 1.2% 0.9% 0.9% 5.6% 0.7% 1.1% 1.3% 1.38
(n=9)
1.1% 2.6% 1.9% 2.1% 1.1% 3.6% 2.0% 1.2% 1.0% 1.3%
s.d. %
1.10
Av. [Nm]
15.22 15.34 14.90 15.12 15.12 14.94 15.10 14.77 14.65 15.18 15.03
1.4%
(n=7)
s.d. %
1.4% 0.6% 1.0% 1.8% 0.8% 1.0% 1.0% 1.2% 0.5% 1.0%
Drilling
Av. [Nm]
3.56
(n=9)
s.d. %
1.2% 1.9% 1.2% 1.5% 1.3% 0.6% 1.8% 1.4% 0.9% 1.2% 1.38
(n=9)
1.9% 2.0% 2.9% 1.5% 2.0% 2.1% 1.7% 2.0% 1.5% 1.0%
2.08
3.39
1.92 1.87 3.91
1.95
1.31
3.00
2.20
1.46
2.57
3.23
1.37 1.53 1.76
1.34
2.8% 6.4% 5.7% 1.3% 5.6% 3.6% 25.3% 5.0% 4.3% 18.5% 2.54
1.47
1.30 1.28 1.33
(n=9)
Reaming 2 Av. [Nm]
1.63
1.25
Reaming 1 Av. [Nm] s.d. %
1.14
1.27
3.49
Core Drilling Av. [Nm] s.d. %
1.20
3.55
2.43
2.10 19.5%
Tapping
3.85
2.33
7.7%
7.2%
3.98 3.90 4.19
2.66
2.72
1.28
2.43
4.24
2.29 2.27 2.39
2.33
1.47
14.0% 21.5% 5.3% 13.2% 6.3% 8.6% 22.8% 10.7% 8.6% 8.1%
3.44
2.38
2.23
1.30
s.d. %
3.90
2.26
1.75 1.88 2.18
1.29
2.72
2.49
1.86
1.28 1.27 1.29
2.2% 10.2% 15.4% 5.0% 8.1% 6.2% 8.8% 7.0% 2.4% 2.6% 2.79
1.78
1.21
(n=9)
s.d. %
1.52
1.24
Reaming 1 Av. [Nm]
(n=8)
RM
C 3.41
Core Drilling Av. [Nm]
Reaming 2 Av. [Nm]
TEST
E 3.63
2.21
2.19
1.93
3.81
7.5%
1.31
5.4%
1.78 24.9%
2.61 27.9%
(n=8)
s.d. %
14.0% 31.4% 7.4% 33.2% 7.9% 7.1% 52.3% 32.9% 23.3% 3.1%
Tapping
Av. [Nm]
14.97 15.29 14.92 18.08 15.58 15.03 16.82 14.90 14.75 15.13 15.55
(n=7)
s.d. %
2.3% 5.7% 1.1% 21.0% 1.0% 1.4% 9.5% 2.8% 1.7% 1.7%
Drilling
Av. [Nm]
3.56
(n=9)
s.d. %
1.0% 1.0% 2.1% 0.6% 0.9% 0.4% 1.0% 1.2% 1.0% 0.4%
3.65
2.81
2.9% 6.2% 11.3% 7.4% 11.5% 6.8% 19.5% 8.3% 9.3% 8.0% 2.66
3.00
2.22
2.33
2.23 2.09 2.32
2.39
2.16
1.46
(n=9)
Reaming 2 Av. [Nm]
2.15
1.30
3.51
Reaming 1 Av. [Nm] s.d. %
1.93 1.89 2.40
1.30
3.60
1.39
1.77
1.30 1.28 1.29
3.54
1.9% 1.7% 2.3% 1.8% 2.0% 1.4% 2.4% 1.0% 1.7% 1.2% 1.89
1.24
3.59 3.54 3.61
(n=9)
1.51
1.25
3.61
Core Drilling Av. [Nm] s.d. %
1.20
3.53
2.26
2.60
2.50
6.9%
3.57
1.2%
1.30
5.8%
2.11 18.9%
2.40 10.9%
(n=8)
s.d. %
23.5% 27.2% 4.8% 5.8% 5.1% 7.4% 8.0% 19.2% 9.6% 2.8%
Tapping
Av. [Nm]
15.25 15.54 15.08
(*)
15.46 15.15 16.71 15.00 14.91 15.09 15.35
(n=7)
s.d. %
2.3% 2.1% 2.6%
(*)
2.2% 0.8% 7.1% 1.6% 1.1% 0.8%
3.6%
Table 2.2.2. Torque results overview. Each column represents results of a test (33 specimens). N: number of specimens from which each average torque value was calculated. Av.: average; s.d.: standard deviation; (*): tool breakage.
26
2. Cutting Fluid Performance Tests An examination of Table 2.2.2 can provide useful information concerning repeatability, reproducibility and sensitivity of torque measurements. Each column presents the average torque results of one test, as well as standard deviations expressed as percentage of the mean value, obtained with one set of tools and one couple of test and reference fluid. Relative standard deviations characterise the repeatability of single measurements with a given fluid. Drilling, Reaming 2 and Tapping torque results obtained with reference fluid RM and different tools are examined by looking at the rows. In this case, relative standard deviations represent the reproducibility index of average torque values for a series of measurements obtained with the reference fluid. According to the results of the present investigation, drilling and tapping qualify as operations in which cutting forces can be resolved within one test when they differ by
