To better understand the targeted issues, it is therefore necessary to highlight and analyze and investigate the functionality of the main components related to the issues, in order to overcame all the mentioned problems and find out the best solution to develop the best wear detection system.
5.3.1 The isolation issue:
Many approaches have been done in previous researches for solving the problem of the isolation but it was without success due to the complexity of the systems. The reason why I decided to take this challenge and solve this issue definitely. Thus the first big target in this stage of research, was eliminating the polyester film placed between the workpiece and the vice of the CNC milling machine. Actually, this isolation was added to avoid the short circuit in the measurement system. Without this isolation it is not possible to detect the output signal. The location of this isolation caused an obstacle for the progression of the research in the case of cutting process under wet machining. The old measurement system was able to operate only under dry machining. Figure 35 illustrates the isolation that was used in the experiment.
60 As the current used in the measurement system is a Direct Current (DC), the purpose of this isolation was to separates two polarities as illustrated in Figure 36, the cable with a black color have a negative polarity, which connect the rotary mercury
Figure 35. The isolation that was used in the experiment.
61 contact to the vice of the milling machine and also to the measurement unit. The second cable in red color have a positive polarity, this conductor joined the workpiece with a measurement unit.
It can be observed from Figure 36, that isolation has to be designed to cover all the area that can have contact with a vice of the milling machine. In addition, it must be designed and renewed for every workpiece, which from the production point of view represent additional time consuming for the operator.
Figure 36. Location of the isolation in the experimentation setup
62 5.3.2 The Noise issue
During the monitoring we had faced a very difficult noise problems as illustrated in Figure 37, that affected the quality of the output signal. In addition, these noise represented also a serious obstacle during the signal processing. From the precision point of view, the selection of the adequate sampling period required for the EMF noise subtraction was very difficult, because it was short and unstable.
These noise made the measurement of the voltage drop and the calculation of the contact resistance difficult. A Visual Basic (VB) program has been designed to calculate
Figure 37. Signal output of voltage drop
63 the contact resistance directly from the voltage drop data, but the presence of the noise made the calculation of the contact resistance by the VB application difficult.
The reason why in the case of high noise calculation of the contact resistance has been done manually via observation
But in the case of a clear output signal, the VB application is very useful and fast.
Figure 38 illustrates the VB applications operations, which is the calculation average of sampling period at every waveform. After the data is uploaded, different options can be applied for the calculation of the contact resistance such as variating the average interval
Figure 38. Visual Basic application used to calculate the contact resistance
64 of the sampling period. The results of the average of sampling period and the calculated contact resistance can be seen in the right window dynamically.
The reason of the noise can be divided into three point which are as follow:
A) Implementation:
It represents the composition of the whole circuit. such as wiring, sensors and contacts including the measurement unit, that need to be considered. For this reason, a deep investigation has been carried out in this section to identify and analyze the probability that may generate the appearance of the noise.
B) Chip:
As the methodology of measurement in the old system was based on electrical contact resistance according to the wear area, every contact or disturbance between the tool and the workpiece can generate a noise. This mean that, chips have to be controlled.
But this phenomenon depends also on the characteristic of the tool itself. Thus improving this issue will need further investigation.
C) Chipping:
Same remark as the previous point regarding the chip, it has been already mentioned that, as our system is based on contact resistance, chipping may reduce the contact area between the cutting tool and the work piece. This action will theoretically raise the contact resistance value during the monitoring. In this issue also, tools have its characteristic including the good choice of the cutting conditions, which will increase the tool life and allow the measurement of the flank to take place.
5.3.3 Implementation
Finally, the implementation of the old measurement system on the CNC machines was complex. First, because its required a free access to the rotary shaft of the milling
65 machine, in order to install the rotary mercury contact, which is rarely accessible in the new generation of machines. Secondly, a customized support must be designed to carry on mercury contact fixation with a shaft.
In addition, two large wires were also required to connect this unit with a vice of the machine. Figure 39 illustrates the details of the mercury contact. There are some multiple components that was requested for the installation of the mercury contact that made the implementation complex and request time and cost. The issues related to the implementation are as follow:
a) Mercury contact:
The mercury contact was used for the transmission of the current between the measurement unit and the cutting tool. As it was difficult to transmit and receive the electrical signal between the cutting tool and measurement unit simply through the body of the CNC machine, because of two reason: The first one is the bearing, located between the machines and the rotating shaft that carry on the tool holder. And the second reason, is the noise that interfere the body of machine.
Figure 39. Specification of rotary mercury contact
66 Thus, the role of rotary mercury contact, was to avoid the mentioned obstacles and ensure a suitable transmission with a low resistance between the measurement unit and the cutting tool. Figure 39 shows the specification of the contact mercury used in the old system
Table 9 illustrates also additional specification of rotary mercury contact used in the first old developed system. The main criterion for choosing the model 105 was the low contact resistance value. It is very important to use a component with less resistivity to allow the perfect circulation to the flow of the output signal.
Table 9. Specification of rotary mercury contact
Specifications Model 105
Max frequency 200MHz
Amp rating 240 VAC 4A
Contact resistance 1m Ω
Rotation torque 10gm cm
67 a) Bearing:
Figure 40 illustrates the experimentation setup of the old measurement system.
The red zone highlights the location of the bearing and also location of all components that have a direct relation with bearing issue. The isolation film is located in the red zone and represented in blue color.
As it can be observed that, the current that was applied through the bearing was facing an important resistivity. This resistance is the resistance of bearing and also the CNC machines body, which is unknown and it can be variable.
This resistivity may disturb the output signal and slow it down. The reason why it’s very important to consider the resistance of bearing and find the solution that can avoid all this obstacle successfully.
68 Figure 40. Experimentation setup of old system with location of the
components related to the isolation issue.
69 c) Wires conductors
The rotary contact mercury has been fixed on the top of the shaft axle of the milling machine via an aluminum support unit. In addition, two large wires connectors has been installed to connect the mercury unit with a worktable of the milling machine as shown previously in the experimentation setup illustrated in Figure 40 , that the aluminum support unit was used to carry on the mercury contact as shown in Figure 41.
Table 10 shows the list of the mercury support unit components.
Figure 41. Mercury support unit
70 Numbe
Descriptions r
1 Wires connectors
2 Rotary mercury
3 Shaft of the milling machine
4 Support unit
Table 10. Mercury unit components
71