- Email: [email protected]
Magnetic Property Changes by Machining Ferromagnetic Materials
Available online at www.sciencedirect.com
ScienceDirect Procedia CIRP 46 (2016) 250 – 253
7th HPC 2016 – CIRP Conference on High Performance Cutting
Magnetic Property Changes by Machining Ferromagnetic Materials Kirsten Trappa*, Franziska Hertera, Dirk Baehrea a
Institute of Production Engineering, Saarland University, Campus A4 2, Saarbruecken 66123, Germany
* Kirsten Trapp. Tel.: +49-681-302-3727; fax: +49-681-302-4858. E-mail address: [email protected]
Abstract The higher the demands concerning the cleanliness of the component and the surface qualities, the more the change of the magnetic surface characteristics influenced by machining comes into focus. The influence by machining has to be examined under different aspects to meet the complexity of the subject matter. This paper focuses on the correlation between the process parameters, the batch of the material and the change of the magnetic surface characteristics. The findings concerning the correlation of magnetic characteristics of a workpiece and the machining lead to an optimized approach to the planning of process chains. © 2016 2016The TheAuthors. Authors. Published by Elsevier B.V.is an open access article under the CC BY-NC-ND license © Published by Elsevier B.V This (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the International Scientific Committee of 7th HPC 2016 in the person of the Conference Chair Prof. under responsibility of the International Scientific Committee of 7th HPC 2016 in the person of the Conference Chair Peer-review Matthias Putz. Prof. Matthias Putz Keywords: Bearing, Machining, Surface analysis, Magnetic properties
The higher the demands concerning the cleanliness of the component and the surface qualities, the more the change of the magnetic surface characteristics influenced by machining comes into focus. The revision of the VDA 19 shows clearly the growing focus on the field of the cleanliness of components [1]. It thus demands need for action. A related field of study is the adhesion of chips to components and tools, inter alia through the change of magnetic properties of the component. Figure 1 shows the dependence of the size of particles to the existing magnetic field on the surface of a workpiece, assuming idealized steel particles. Diverse boundary conditions influence the magnetic surface properties of a workpiece. This paper aims to demonstrate systematical effects and to analyze influences on these effects. For this reason, machined steel 1.7225 with varied predefined machining parameters was analyzed. Here, milling was used for its excellent repeatability properties, aiming to produce a preferably even surface. So it is possible to apply nondestructive test procedures to measure the magnetic properties. The possible influencing variables can be detected by a statistical Design of Experiments (DoE). Thus, qualified
measurements can also detect a possibly minimized change in the magnetic flux density. magnetic flux density B[mT]
1. Introduction
4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 0.1
1 10 100 cubic steel particle edge length a[μm]
Fig. 1. Required magnetic flux density B in mT for adherence of idealized cubic steel particle, edge length a in µm.
2. State of the Art The change in magnetic properties near to the surface of materials is a long-known side effect of machining that has been researched since the 1970s. Schreiber [2] investigated primarily the change in hardness and residual stress. Those
2212-8271 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the International Scientific Committee of 7th HPC 2016 in the person of the Conference Chair Prof. Matthias Putz doi:10.1016/j.procir.2016.04.006
251
Kirsten Trapp et al. / Procedia CIRP 46 (2016) 250 – 253
3. Effect Analysis and Experimental Procedure 3.1. Effect Analysis In a DoE a system is analyzed which can be influenced by various factors. In this paper, the system analyzed is the machining process. Exemplarily, Figure 2 shows the influencing variables of the workpiece and of the machine used. The variables induce changes in the considered quality features, which are defined as magnetic properties in this paper. They can be described as an impact of the machining on the workpiece. Other quality features as surface quality are not investigated here. The listing of influencing variables is not exhaustive and can be completed by multiple factors, for example the surrounding conditions or human impacts. In
order to give valuable input on the statistical DoE, some factors were chosen amongst the influencing variables and then connected with defined factor levels of the DoE. Influencing Variables Machining • Machining process • Parameter • Tool • Forces •Temperature
System: Stock Removal Process
Magnetic Properties
1
Influence Impact
Methodical and statistical analysis
Material • Type of material • Alloying elements • Heat treatment • Batch • Mechanical properties • Pre-Machining • Transport/ Stocking
Statistical Design of Experiments
values were mainly produced by thermal effects through machining. Besides Schreiber, also Byrne [3] investigated the effects of machining on magnetic properties. His research approaches the process parameter milling, turning and drilling. In his experiments he uses the one factor at a time-method. As a result, no interdependencies between single factors can be found. [3] Hence, it is rather difficult to compare the results. Subsequently, Eyrisch et al. [4] indicate the changes of magnetic properties while machining using their methods. They come to the conclusion that there is a low influence of machining to magnetic properties of the material. This influence is not strong enough to have a significant effect on metallic particles. Further research on the topic has been conducted by Bähre et al. [5,6]. There, they show the relation between influences of parameters and magnetic fields. Influencing factors are external magnetic fields like the electromagnetic fields of the machine drives, the terrestrial magnetic field or magnetic clamping. The latter has been researched by Grimm [7]. There, he describes the development of magnetic clamping systems, which include an electronic pole reversal control unit. Furthermore, Bähre et al. [6] assume that the ductile plastic deformation of a workpiece is an influencing factor on the change of magnetic properties. Moreover, the thermal influences on the workpiece during the machining process as well as the Villari-effect represent an important factor on the change of magnetic properties. Trapp et al. [8] also show the impact on the change of magnetic properties through certain parameters during machining. Examples for these parameters are depth of cut, feed speed and batch. The results depict on one hand the significant effect of the batches and the feed speed. On the other hand the paper identifies the effect of the depth of cut to be rather low. With its statistical DoE, this paper is a valuable continuation of [8], where a statistical base has not been applied. Furthermore, Trapp et al. [8] investigated the measurement distance between Hall sensor and surface of the sample. As a result, they stated that the distance between Hall sensor and surface of the sample should be minimized. This arrangement helps providing significant and reproducible values. Likewise, Su and Chen [9] conducted experiments on the distance between Hall sensor and surface of the sample. In their work, they also observe the most significant values with a minimized distance between Hall sensor and surface of the sample.
Measurement • Accuracy • Distance sensor-surface • measurement time • number of measuring points
Fig. 2. Influence and impact analysis between machining process, influencing variables, magnetic properties and statistical DoE.
3.2. Experimental Procedure The workpieces made of 1.7225 steel are sawed on a length of 200 mm from a rolled bar stock with a cross section of 40 mm x 60 mm. The main difference of the two batches are shown in the chemical analyses by sulfur (A:0.0072 wt%, B:0.022 wt%) and phosphor (A:0.0171 wt%, B:0.0131 wt%). The DoE defines the machining, using values, factor and factor level, from table 1. The milling process appears either parallel or perpendicular to the direction of rolling. The quality feature is characterized by the magnetic flux density B which is measured via a Hall sensor and is conducted over the whole sample in all three components of spatial dimension as 2D and 3D scan. The setup can be seen in Trapp et al. [8]. In this work, the Hall sensor metrolab THM 1176-LF with an accuracy of 1% of the measured value has been used. Table 1. Experimental setup parameters during milling. Tool
45° plane milling head with five cermet edges and 80 mm diameter
Milling conditions
non coolant fluid
Milling Direction to rolling direction
parallel II
Workpiece material
automotive steel 1.7225
Factor
Factor Level
perpendicular ٣
Rotation speed n
625 min-1
833 min-1
Feed per edge fz
0,05 mm
0,15 mm
Depth of cut ap
0,5 mm
1 mm
A
B
parallel II
perpendicular ٣
Batch Milling Direction to rolling direction
Kirsten Trapp et al. / Procedia CIRP 46 (2016) 250 – 253
4. Results
feed direction
batch
depth of cut feed per edgerotation speed
0.08
(a)
0.06 0.04 0.02
٣
II
A
B
0.5
1.0
0.05
0.15
625
demagnetized state Intensity B [mT]
average value of B
The DoE-software minitab 17 is used to calculate the effects of the single factors through the response variables. Using this software, main effect and interdependency diagrams can be created, as can be seen in Figure 3 and 4.
leads to the assumption that the change in the structure near to the surface is further increased by the milling process and produces a even stronger change of the magnetic properties. It can also be assumed that around the area next to the surface of the longer side is a different structure than in the inner material. Further it can be seen that a higher material removal has a higher impact to the changes of the magnetic flux density.
833
Fig. 3. The main effect diagram shows the average value of the magnetic flux density B over the test series by varying the different factors.
machined state Intensity B [mT]
252
Figure 3 shows the main effects. Here, it can be observed that depth of cut and batch indicate a much higher effect than the other factors. For a better understanding of the interdependency between factors, there is also shown the diagram for interdependencies between two factors, Figure 4. Using diagrams for interdependencies, it can be estimated if there exists an interaction effect. 0.5
1.0
0.05
0.15 625
(b)
833 0.10 0.05 0.00
batch
0.10
0.05 0.00 depth of cut
0.10 0.05 feed per edge
rotation speed
feed direction II ٣ batch A B
depth of cut 0.5 1.0
0.00 feed per edge 0.10 0.15 0.05 0.05
Intensity B [mT]
B
feed direction: II ap: 1 mm
Intensity B [mT]
A
II
٣
feed direction: ٣ ap: 0.5mm
0.00
Fig. 4. Diagram of interdependencies for the defined factors.
Workpieces that are milled parallel to the direction of rolling there can be observed anomalies at the areas of the entry and of the exit of the milling head. These anomalies lead to an increasing value of the magnetic flux density. Figure 5 show the graphical evaluation of two experiments comparing the demagnetized and the machined state using different depths of cuts and feed directions. The 3D distribution of the magnetic surface properties indicates a recognizable magnetic flux density after the process of demagnetization. Especially the X-direction in the areas of the entry and the exit of the milling head show high positive values on one side and high negative values on the other after milling. Observing the workpieces milled parallel to the direction of rolling, the areas of entry and exit can be identified clearly, Fig. 5(a). This phenomenon can also be observed with workpieces milled in perpendicular direction, Fig. 5(b). In these cases such influence is not clearly visible for entry and exit of the milling head. That means that the pre-machining, or rather the separation through sawing has left its traces. This
Fig. 5. 3D-Scans of two experiments comparing the demagnetized and the machined state using different depths of cuts and feed directions parallel (a) and perpendicular (b) for batch A with a rotation speed of 325 min-1 and feed per edge of 0.05 mm.
Figure 6 shows a comparison of two batches with different machining direction using the same process parameters. The above described phenomenon can also be found in the 2Dscans. For batch B, it is not such a strong effect as in batch A. The demagnetized state is for both batches with an overall value BA = 0.075 mT and BB = 0.066 mT nearly a similar initial state and also possesses the same process parameters. However, the overall value of batch A results with 'BA = 0.124 mT in a higher change of magnetic flux density than batch B with 'BB = 0.034 mT. Furthermore there is a difference between the value maximum for batch A BmaxA = 0.658 mT and for batch B BmaxB = 0.392 mT. This could lead to an adherence of steel particles between 1 - 10µm at the surface of batch A. For batch B the particles are smaller than 1µm.
Intensity B [mT]
Kirsten Trapp et al. / Procedia CIRP 46 (2016) 250 – 253
feed direction: ٣ batch: A ap: 1 mm fz: 0.15 mm n: 625 min -1
Intensity B [mT]
Å
feed direction ٣
Å feed direction II
feed direction: II batch: B ap: 1 mm fz: 0.15 mm n: 625 min -1
Fig. 6. Comparison of the value B to the workpiece ending by considering the machining direction and the batch comparison
Additionally, there exists another phenomenon occurring with workpieces that are milled in perpendicular direction. In these cases one can observe a change in the magnetic flux density in those areas with overlapping machining processes. Figure 7 shows in its upper part a drawing of the perpendicular machining referring to the diameter of the milling head. In its lower part it shows the 2D-scan in Y-direction of a perpendicular milled workpiece. The dashed line indicates the areas of overlapping machining and their accordance to the change in magnetic flux density. Those appear to be higher in the entry than in the exit areas. Furthermore, the effects on the edges of the workpiece that were described above can again be seen in this figure.
253
head seems to have a slight impact on the magnetic properties. This slight impact is to be seen in comparison to the areas that were milled only once. It can be assumed that multiple following machining steps with removal leads to an additional change of the magnetic properties.
5. Discussion and Conclusion During the application of machining processes workpieces made of ferromagnetic materials change their magnetic surface properties. The presented results of the statistical DoE show which process parameters have a significant influence on the changes of the magnetic surface properties. It is to determine that the change of magnetic properties during machining depends on many factors and can clearly be affected by varying process parameters. A great emphasis is put on influences of the batch, depth of cut as well as the interaction of both of them. This confirms results from earlier studies. The material removal in several steps or with a higher removal rate in one step is influencing the changes. Furthermore, the extent of the magnetic surface properties shows a recognizable magnetic flux density after the demagnetization process, which is shown in two illustrations of the 3D-scans. This characteristic basic orientation is caused by the material itself and the preparatory steps. The influence of the batch can be seen and has to be examined in further investigations. Along with that the reachable accuracy of the surface in comparison to the optimized parameters for improved magnetic characteristics will be a new focus. The findings concerning the correlation of magnetic characteristics of a workpiece and the machining lead to an optimized approach to the planning of process chains. References
feed direction ٣
Diameter Milling head
Intensity B[mT]
Overlap during machining
Fig. 7. 2D-Scan in y-direction of a perpendicular milled work piece in relation to the overlapping during machining.
Observing Figure 7, it is to be determined that the change in the magnetic flux density in these areas are not significantly high in relation to the areas that were milled only once. This leads to the assumption that these areas are the result of a second contact of the milling head after the milling process itself, but without removal of material. Merely the simple contact of the workpiece with the edges of the milling
[1] VDA Volume 19 Part 1. Inspection of Technical Cleanliness Particulate Contamination of Functionally Relevant Automotive Components. 2nd Revised Edition; 2015. [2] Schreiber E. Die Werkstoffbeeinflussung weicher und gehärteter Oberflächenschichten durch spanende Bearbeitung. In: VDI-Berichte; 1976; 256; 67–79. [3] Byrne G. Das Zerspanen von weichmagnetischen Werkstoffen. Phd; TU Berlin. 1989. [4] Eyrisch T . Dem Magnetismus auf der Spur. Werkstatt und Betrieb; 2009; 10; 70-72. [5] Bähre D, Becker D, Tschunky R. Analysis of Influences on Occurrence of Magnetism during Cutting Processes. In: Proceedings of the 4th CIRP HPC 2010, Vol. 1; 2010; 19-22. [6] Bähre D, Trapp K, Tschunky R. Influences on Occurrence of Magnetism during Cutting Processes. In: International Journal of Automation Technology; 2011; 5; 3; 294-299. [7] Grimm M. Magnetische Werkstück-Spannsysteme ergänzen innovative Zerspantechniken. In: Maschinenmarkt; 1998; 104; 31; 20-23. [8] Trapp K; Käfer L; Bähre D. Effects of Machining Ferromagnetic Materials on Magnetic Property Changes. In: 43rd Proceedings of NAMRC 43. 2015. [9] Su H; Chen M. Experimental and theoretical analysis of metal magnetic memory signals in the stress concentration area of 45# steel under tensile testing. In: International Journal of Applied Electromagnetics and Mechanics 2014; 46; 1; 271-280.
ScienceDirect Procedia CIRP 46 (2016) 250 – 253
7th HPC 2016 – CIRP Conference on High Performance Cutting
Magnetic Property Changes by Machining Ferromagnetic Materials Kirsten Trappa*, Franziska Hertera, Dirk Baehrea a
Institute of Production Engineering, Saarland University, Campus A4 2, Saarbruecken 66123, Germany
* Kirsten Trapp. Tel.: +49-681-302-3727; fax: +49-681-302-4858. E-mail address: [email protected]
Abstract The higher the demands concerning the cleanliness of the component and the surface qualities, the more the change of the magnetic surface characteristics influenced by machining comes into focus. The influence by machining has to be examined under different aspects to meet the complexity of the subject matter. This paper focuses on the correlation between the process parameters, the batch of the material and the change of the magnetic surface characteristics. The findings concerning the correlation of magnetic characteristics of a workpiece and the machining lead to an optimized approach to the planning of process chains. © 2016 2016The TheAuthors. Authors. Published by Elsevier B.V.is an open access article under the CC BY-NC-ND license © Published by Elsevier B.V This (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the International Scientific Committee of 7th HPC 2016 in the person of the Conference Chair Prof. under responsibility of the International Scientific Committee of 7th HPC 2016 in the person of the Conference Chair Peer-review Matthias Putz. Prof. Matthias Putz Keywords: Bearing, Machining, Surface analysis, Magnetic properties
The higher the demands concerning the cleanliness of the component and the surface qualities, the more the change of the magnetic surface characteristics influenced by machining comes into focus. The revision of the VDA 19 shows clearly the growing focus on the field of the cleanliness of components [1]. It thus demands need for action. A related field of study is the adhesion of chips to components and tools, inter alia through the change of magnetic properties of the component. Figure 1 shows the dependence of the size of particles to the existing magnetic field on the surface of a workpiece, assuming idealized steel particles. Diverse boundary conditions influence the magnetic surface properties of a workpiece. This paper aims to demonstrate systematical effects and to analyze influences on these effects. For this reason, machined steel 1.7225 with varied predefined machining parameters was analyzed. Here, milling was used for its excellent repeatability properties, aiming to produce a preferably even surface. So it is possible to apply nondestructive test procedures to measure the magnetic properties. The possible influencing variables can be detected by a statistical Design of Experiments (DoE). Thus, qualified
measurements can also detect a possibly minimized change in the magnetic flux density. magnetic flux density B[mT]
1. Introduction
4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 0.1
1 10 100 cubic steel particle edge length a[μm]
Fig. 1. Required magnetic flux density B in mT for adherence of idealized cubic steel particle, edge length a in µm.
2. State of the Art The change in magnetic properties near to the surface of materials is a long-known side effect of machining that has been researched since the 1970s. Schreiber [2] investigated primarily the change in hardness and residual stress. Those
2212-8271 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the International Scientific Committee of 7th HPC 2016 in the person of the Conference Chair Prof. Matthias Putz doi:10.1016/j.procir.2016.04.006
251
Kirsten Trapp et al. / Procedia CIRP 46 (2016) 250 – 253
3. Effect Analysis and Experimental Procedure 3.1. Effect Analysis In a DoE a system is analyzed which can be influenced by various factors. In this paper, the system analyzed is the machining process. Exemplarily, Figure 2 shows the influencing variables of the workpiece and of the machine used. The variables induce changes in the considered quality features, which are defined as magnetic properties in this paper. They can be described as an impact of the machining on the workpiece. Other quality features as surface quality are not investigated here. The listing of influencing variables is not exhaustive and can be completed by multiple factors, for example the surrounding conditions or human impacts. In
order to give valuable input on the statistical DoE, some factors were chosen amongst the influencing variables and then connected with defined factor levels of the DoE. Influencing Variables Machining • Machining process • Parameter • Tool • Forces •Temperature
System: Stock Removal Process
Magnetic Properties
1
Influence Impact
Methodical and statistical analysis
Material • Type of material • Alloying elements • Heat treatment • Batch • Mechanical properties • Pre-Machining • Transport/ Stocking
Statistical Design of Experiments
values were mainly produced by thermal effects through machining. Besides Schreiber, also Byrne [3] investigated the effects of machining on magnetic properties. His research approaches the process parameter milling, turning and drilling. In his experiments he uses the one factor at a time-method. As a result, no interdependencies between single factors can be found. [3] Hence, it is rather difficult to compare the results. Subsequently, Eyrisch et al. [4] indicate the changes of magnetic properties while machining using their methods. They come to the conclusion that there is a low influence of machining to magnetic properties of the material. This influence is not strong enough to have a significant effect on metallic particles. Further research on the topic has been conducted by Bähre et al. [5,6]. There, they show the relation between influences of parameters and magnetic fields. Influencing factors are external magnetic fields like the electromagnetic fields of the machine drives, the terrestrial magnetic field or magnetic clamping. The latter has been researched by Grimm [7]. There, he describes the development of magnetic clamping systems, which include an electronic pole reversal control unit. Furthermore, Bähre et al. [6] assume that the ductile plastic deformation of a workpiece is an influencing factor on the change of magnetic properties. Moreover, the thermal influences on the workpiece during the machining process as well as the Villari-effect represent an important factor on the change of magnetic properties. Trapp et al. [8] also show the impact on the change of magnetic properties through certain parameters during machining. Examples for these parameters are depth of cut, feed speed and batch. The results depict on one hand the significant effect of the batches and the feed speed. On the other hand the paper identifies the effect of the depth of cut to be rather low. With its statistical DoE, this paper is a valuable continuation of [8], where a statistical base has not been applied. Furthermore, Trapp et al. [8] investigated the measurement distance between Hall sensor and surface of the sample. As a result, they stated that the distance between Hall sensor and surface of the sample should be minimized. This arrangement helps providing significant and reproducible values. Likewise, Su and Chen [9] conducted experiments on the distance between Hall sensor and surface of the sample. In their work, they also observe the most significant values with a minimized distance between Hall sensor and surface of the sample.
Measurement • Accuracy • Distance sensor-surface • measurement time • number of measuring points
Fig. 2. Influence and impact analysis between machining process, influencing variables, magnetic properties and statistical DoE.
3.2. Experimental Procedure The workpieces made of 1.7225 steel are sawed on a length of 200 mm from a rolled bar stock with a cross section of 40 mm x 60 mm. The main difference of the two batches are shown in the chemical analyses by sulfur (A:0.0072 wt%, B:0.022 wt%) and phosphor (A:0.0171 wt%, B:0.0131 wt%). The DoE defines the machining, using values, factor and factor level, from table 1. The milling process appears either parallel or perpendicular to the direction of rolling. The quality feature is characterized by the magnetic flux density B which is measured via a Hall sensor and is conducted over the whole sample in all three components of spatial dimension as 2D and 3D scan. The setup can be seen in Trapp et al. [8]. In this work, the Hall sensor metrolab THM 1176-LF with an accuracy of 1% of the measured value has been used. Table 1. Experimental setup parameters during milling. Tool
45° plane milling head with five cermet edges and 80 mm diameter
Milling conditions
non coolant fluid
Milling Direction to rolling direction
parallel II
Workpiece material
automotive steel 1.7225
Factor
Factor Level
perpendicular ٣
Rotation speed n
625 min-1
833 min-1
Feed per edge fz
0,05 mm
0,15 mm
Depth of cut ap
0,5 mm
1 mm
A
B
parallel II
perpendicular ٣
Batch Milling Direction to rolling direction
Kirsten Trapp et al. / Procedia CIRP 46 (2016) 250 – 253
4. Results
feed direction
batch
depth of cut feed per edgerotation speed
0.08
(a)
0.06 0.04 0.02
٣
II
A
B
0.5
1.0
0.05
0.15
625
demagnetized state Intensity B [mT]
average value of B
The DoE-software minitab 17 is used to calculate the effects of the single factors through the response variables. Using this software, main effect and interdependency diagrams can be created, as can be seen in Figure 3 and 4.
leads to the assumption that the change in the structure near to the surface is further increased by the milling process and produces a even stronger change of the magnetic properties. It can also be assumed that around the area next to the surface of the longer side is a different structure than in the inner material. Further it can be seen that a higher material removal has a higher impact to the changes of the magnetic flux density.
833
Fig. 3. The main effect diagram shows the average value of the magnetic flux density B over the test series by varying the different factors.
machined state Intensity B [mT]
252
Figure 3 shows the main effects. Here, it can be observed that depth of cut and batch indicate a much higher effect than the other factors. For a better understanding of the interdependency between factors, there is also shown the diagram for interdependencies between two factors, Figure 4. Using diagrams for interdependencies, it can be estimated if there exists an interaction effect. 0.5
1.0
0.05
0.15 625
(b)
833 0.10 0.05 0.00
batch
0.10
0.05 0.00 depth of cut
0.10 0.05 feed per edge
rotation speed
feed direction II ٣ batch A B
depth of cut 0.5 1.0
0.00 feed per edge 0.10 0.15 0.05 0.05
Intensity B [mT]
B
feed direction: II ap: 1 mm
Intensity B [mT]
A
II
٣
feed direction: ٣ ap: 0.5mm
0.00
Fig. 4. Diagram of interdependencies for the defined factors.
Workpieces that are milled parallel to the direction of rolling there can be observed anomalies at the areas of the entry and of the exit of the milling head. These anomalies lead to an increasing value of the magnetic flux density. Figure 5 show the graphical evaluation of two experiments comparing the demagnetized and the machined state using different depths of cuts and feed directions. The 3D distribution of the magnetic surface properties indicates a recognizable magnetic flux density after the process of demagnetization. Especially the X-direction in the areas of the entry and the exit of the milling head show high positive values on one side and high negative values on the other after milling. Observing the workpieces milled parallel to the direction of rolling, the areas of entry and exit can be identified clearly, Fig. 5(a). This phenomenon can also be observed with workpieces milled in perpendicular direction, Fig. 5(b). In these cases such influence is not clearly visible for entry and exit of the milling head. That means that the pre-machining, or rather the separation through sawing has left its traces. This
Fig. 5. 3D-Scans of two experiments comparing the demagnetized and the machined state using different depths of cuts and feed directions parallel (a) and perpendicular (b) for batch A with a rotation speed of 325 min-1 and feed per edge of 0.05 mm.
Figure 6 shows a comparison of two batches with different machining direction using the same process parameters. The above described phenomenon can also be found in the 2Dscans. For batch B, it is not such a strong effect as in batch A. The demagnetized state is for both batches with an overall value BA = 0.075 mT and BB = 0.066 mT nearly a similar initial state and also possesses the same process parameters. However, the overall value of batch A results with 'BA = 0.124 mT in a higher change of magnetic flux density than batch B with 'BB = 0.034 mT. Furthermore there is a difference between the value maximum for batch A BmaxA = 0.658 mT and for batch B BmaxB = 0.392 mT. This could lead to an adherence of steel particles between 1 - 10µm at the surface of batch A. For batch B the particles are smaller than 1µm.
Intensity B [mT]
Kirsten Trapp et al. / Procedia CIRP 46 (2016) 250 – 253
feed direction: ٣ batch: A ap: 1 mm fz: 0.15 mm n: 625 min -1
Intensity B [mT]
Å
feed direction ٣
Å feed direction II
feed direction: II batch: B ap: 1 mm fz: 0.15 mm n: 625 min -1
Fig. 6. Comparison of the value B to the workpiece ending by considering the machining direction and the batch comparison
Additionally, there exists another phenomenon occurring with workpieces that are milled in perpendicular direction. In these cases one can observe a change in the magnetic flux density in those areas with overlapping machining processes. Figure 7 shows in its upper part a drawing of the perpendicular machining referring to the diameter of the milling head. In its lower part it shows the 2D-scan in Y-direction of a perpendicular milled workpiece. The dashed line indicates the areas of overlapping machining and their accordance to the change in magnetic flux density. Those appear to be higher in the entry than in the exit areas. Furthermore, the effects on the edges of the workpiece that were described above can again be seen in this figure.
253
head seems to have a slight impact on the magnetic properties. This slight impact is to be seen in comparison to the areas that were milled only once. It can be assumed that multiple following machining steps with removal leads to an additional change of the magnetic properties.
5. Discussion and Conclusion During the application of machining processes workpieces made of ferromagnetic materials change their magnetic surface properties. The presented results of the statistical DoE show which process parameters have a significant influence on the changes of the magnetic surface properties. It is to determine that the change of magnetic properties during machining depends on many factors and can clearly be affected by varying process parameters. A great emphasis is put on influences of the batch, depth of cut as well as the interaction of both of them. This confirms results from earlier studies. The material removal in several steps or with a higher removal rate in one step is influencing the changes. Furthermore, the extent of the magnetic surface properties shows a recognizable magnetic flux density after the demagnetization process, which is shown in two illustrations of the 3D-scans. This characteristic basic orientation is caused by the material itself and the preparatory steps. The influence of the batch can be seen and has to be examined in further investigations. Along with that the reachable accuracy of the surface in comparison to the optimized parameters for improved magnetic characteristics will be a new focus. The findings concerning the correlation of magnetic characteristics of a workpiece and the machining lead to an optimized approach to the planning of process chains. References
feed direction ٣
Diameter Milling head
Intensity B[mT]
Overlap during machining
Fig. 7. 2D-Scan in y-direction of a perpendicular milled work piece in relation to the overlapping during machining.
Observing Figure 7, it is to be determined that the change in the magnetic flux density in these areas are not significantly high in relation to the areas that were milled only once. This leads to the assumption that these areas are the result of a second contact of the milling head after the milling process itself, but without removal of material. Merely the simple contact of the workpiece with the edges of the milling
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