2.2.1. Portable Helical Milling Equipment
Targeting at the demands for high-quality hole-making of stacks made of difficult-to-machine materials in the aircraft and aerospace field, as well as the problems of poor environmental openness of the machining location, the portable helical milling equipment (Dalian University of Technology, Dalian, China) in this study was developed independently. As shown in
Figure 2, the equipment has the technical advantages of miniaturization and lightweighting, is able to provide the motion required for helical milling, and all the motion accuracy indexes meet the requirements of aerospace component assembly and hole-making.
The center of the self-designed and built control system for the portable helical milling equipment is the MELSEC-FX3U series PLC (MITSUBISHI ELECTRIC, Tokyo, Japan), which controls the pneumatic motor (Ober, Italy), revolution DC motor (FAULHABER, Schönaich, Germany), and feed stepping motor (Oriental motor, Tokyo, Japan) by sending commands to the E/P regulator (SMC, Tokyo, Japan), motion controller, and motor driver, thus achieving the rotation motion, revolution motion and axial feed motion in the helical milling process. The equipment performs the functions of center cooling, automatic tool setting, multi-functional and multi-step automated hole-making, and so on. The hole-making accuracy, quality, and machining efficiency can satisfy all requirements, and the machining adaptability is excellent. The principle of the motion control system is shown in
Figure 3.
The spindle speed real-time feedback monitoring module of the equipment was designed and developed, and the operation process is shown in
Figure 4. The pulse signal is sent from the rotary encoder (OMRON, Kyoto, Japan) to the PLC. Then, the current spindle speed value is obtained with arithmetic processing, transmitted to the output module in real time, and displayed on the developed human–computer interface. This module can realize the real-time reading of the spindle speed and the monitoring of the machining status of the equipment, providing an important basis for the identification and judgment of the machining interface.
2.2.3. Interface Identification and Adaptive Machining Principles
Due to the significant difference in the material characteristics and machinability of CFRP and Ti, the cutting forces and torques applied to the cutting tool in the process of the helical milling of CFRP/Ti stacks are different when the cutting tool is machining at the CFRP entry interface, the CFRP/Ti transition interface, and the Ti exit interface. The pneumatic spindle of the self-developed portable helical milling equipment has the characteristic that the spindle speed decreases with an increase in torque when the input air pressure is certain, so the spindle speed of the equipment will change in different magnitudes when machining at the different material interfaces. Based on the above analysis, this paper proposes an adaptive machining method for the helical milling of CFRP/Ti stacks based on interface identification.
Firstly, a single-parameter helical milling experiment for CFRP/Ti stacks was carried out on the self-developed portable helical milling equipment. The experimental material is a CFRP/Ti-laminated flat plate, in which the fiber filament grade chosen for the CFRP is T800, and the material thickness is 5.5 mm; the titanium alloy is chosen as Ti-6Al-4V (TC4), and the material thickness is 6.5 mm. The cutting tool used for the experiment is the 4-edge end mill, the tool material is cemented carbide, and the target hole diameter is 12 mm. The initial machining parameters are:
ns = 1600 r/min,
np = 22 r/min, and
fa = 10 mm/min. The experiment was repeated four times, and the average value was taken as the spindle speed reference value. Next, the designed and developed spindle speed real-time feedback module was utilized to extract the relationship curve between the spindle speed and the change in the axial feed position during the machining process using data operations and processing, as shown in
Figure 6.
The helical milling state corresponding to each machining stage in
Figure 6 is shown in
Figure 7. Using analysis, it is found that the whole helical milling process can be divided into seven stages. In stage
a, the equipment is started and fed forward according to the set parameters; at this time, the cutting tool has not yet contacted the workpiece. In stage
b, the cutting tool processes to the CFRP entry interface, the torque applied increases, and the spindle speed consequently generates a significant decrease. In stage
c, the cutting tool machines the CFRP, and the torque applied is stable; therefore, the spindle speed is relatively steady. In stage
d, the cutting tool processes to the CFRP/Ti transition interface, the torque applied increases further, and the spindle speed decreases considerably. In stage
e, the cutting tool machines Ti, and due to the difficult-to-machine property of the material, the spindle speed has some fluctuation but is overall smooth. In stage
f, the cutting tool processes to the Ti exit interface and the torque applied decreases; therefore, the spindle speed increases. In stage
g, the cutting tool completely cuts out of the Ti, and the hole-making process ends.
Within stages
b,
d, and
f, the threshold values of spindle speed variation,
nCe,
nTe, and
nTo, are extracted using computational analysis and are used as the basis for the identification and judgment of the CFRP entry interface, CFRP/Ti transition interface, and Ti exit interface. Due to the fluctuation in the cutting forces and torque applied to the cutting tool during the machining process, the real-time spindle speed also fluctuates. In order to eliminate the influence of spindle speed fluctuation and improve the accuracy of the selected threshold values, a fitting model for the relationship between spindle speed and axial feed position is established. Firstly, the spindle speed–axial feed position curve is linearly fitted in stages using the least squares method, as shown in
Figure 8, in which
B1,
B2,
D1,
D2,
F1, and
F2 are the starting and ending points of the spindle speed change when the cutting tool processes to the CFRP entry interface (stage
b), the CFRP/Ti transition interface (stage
d), and the Ti exit interface (stage
f), respectively.
n0 is the reference minimal value of the spindle speed in stage
a,
nC is the reference minimal value of the spindle speed in stage
c, and
nT is the reference maximal value of the spindle speed in stage
e.
The expression of the linear fitting model for machining stages
a~g is given below:
The parameters of the model for each machining stage are shown in
Table 1.
Figure 8 shows that if the threshold value is selected too close to the starting point of the spindle speed change at the interface (
B1,
D1,
F1), the spindle speed change is not obvious at that time, and it may not be able to accurately identify the interface position. If the threshold value is selected too close to the ending point of the spindle speed change at the interface (
B2,
D2,
F2), the identified interface position will have a large hysteresis compared with the actual position. In addition, the determination of the identification threshold value for each interface should also satisfy the following mathematical relationship with the spindle speed extreme value of its previous stage:
nCe <
n0,
nTe <
nC,
nTo >
nT. Based on the above constraint conditions, and considering the influence of interface identification accuracy and hysteresis, the spindle speed value at 0.02 mm backward from the reference extreme value position of each machining stage is finally selected as the threshold value for the identification of each interface. The set reference threshold values are obtained by substituting them into the linear fitting model, as shown in
Table 2.
The algorithm process of the adaptive machining method for helical milling of CFRP/Ti stacks based on interface identification is shown in
Figure 9. First, the initial machining parameters (
nsc,
npc,
fac) are input. At this time, the machining parameters used are suitable for the stable machining of CFRP, and the equipment starts to work. Next, the cutting tool machining is executed in stage
a. At this time, the cutting tool has not yet touched the CFRP, and the spindle speed is fed back and output to the PLC in real time. When the spindle speed
ns ≤
nCe, the cutting tool is considered to have processed to the CFRP entry interface (stage
b), at which time the current feed position
PCe of the cutting tool is recorded. Then, the cutting tool machining stage
c is executed, with the machining parameters remaining unchanged. When the spindle speed
ns ≤
nTe, the cutting tool is considered to have processed to the CFRP/Ti transition interface (stage
d), at which time the current feed position
PTe of the cutting tool is recorded. Then, cutting tool machining stage
e is executed, and at this time, the machining parameters are adaptively adjusted to the parameter combinations that are suitable for the stable machining of Ti (
nst,
npt,
fat). When the spindle speed
ns ≥
nTo, the cutting tool is considered to have processed to the Ti exit interface (stage
f), and at this time, the current feed position
PTo of the cutting tool is recorded. Finally, the cutting tool machining stage
g is executed until the tool completely cuts out of Ti. At this time, the adaptive machining algorithm process based on interface identification is finished.