The tool is a key part of the frictionless process. Through modal analysis, parameters that are important for job safety can be measured and adjusted
When drilling, vibration can cause great problems. On the one hand, it will affect the quality. On the other hand, it may cause the tool to break and cause production interruption. In order to avoid such problems, tool vibrations should be measured and corrected in time. The results of the study indicate that laser scanning vibrometers are suitable for the determination of the relevant dimensions of single-edged deep-hole drilling.
Tool vibration is a major problem encountered during drilling. The adaptable adjustment system can detect and adjust in time when the tool vibrates. This requires an accurate knowledge of the vibration characteristics of the tool structure. The following describes how to determine the modal parameters of a single-edged deep-hole drill by the experimental modal analysis of a laser scanning vibrator. The most familiar method for dynamic system vibration analysis is modal analysis. This method, which is also considered as natural vibration analysis, can determine such parameters as the natural frequency, damping, and morphology of the system.
Modal analysis is based on the principle that each dynamic deformation of a structure is a weighted sum of its modalities, each model being derived from a structural vibration parameter and an exact degree of freedom. When the structure is subjected to vibrations, all modal reactions will be proportional to the movement of the entire structure. By summarizing the modal responses, the corresponding structural reactions can become specific excitation frequencies. The recognition of modal parameters can realize the description of the dynamic state of the system, and it is also an important basis for series exploration of model formation. Various models are based on simplified settings and are forced to a certain degree of uncertainty.
How the vibratory system behaves in practice is often clarified only on the basis of experimental exploration. Experimental modal analysis is one of the most important measurement methods in this area, and depends on the observation of the model of the structure. By measuring the transmission function of each measurement point of the system, the actual dynamic characteristics of the system can be obtained. In this regard, the corresponding excitation source is used to stimulate the structure of the investigation and the system's response at each measurement point is measured. The signal measured in the time range is converted into a frequency range by means of the Fast-Fourier transmission principle, and a system having a transmission function is obtained based on the reference signal. From this it is possible to measure modal parameters such as natural frequency, damping and own morphology. In the drilling process, a frequency range of up to 5000 Hz is significant. Therefore, the limited number of natural vibrations is enough to describe the dynamic characteristics of a structure, and for lower frequencies, the phenomenon of frequency resonance intensifies.
Special instrument for measurement
The laser vibrometer is a non-contact and optical type measurement method for the mechanical vibration process. The measurement principle is based on the movement of the optical frequency. When the measurement point of the vibration structure is discrete, the laser beam can be measured when the frequency moves. This method has no adverse effect and allows the application of sensitive structures and elements in a large number of application fields, while in the latter case, contact-type sensing elements cannot be used due to object characteristics and environmental parameters. In particular, a laser Doppler vibration measurement device was established as a non-contact measurement technique to measure the vibration of a structure. In this regard, the relative motion between the transmitter and receiver of electromagnetic waves and light waves can cause changes in the frequency or wavelength affected by the speed.
For plane vibration measurements, a scanning vibrator can be used. When this scanning method is used, scanning is performed at a plurality of measuring points on the surface of the object in a quick sequence (Fig. 1). A series of single points can be generated by scanning and highly spatial resolution of the laser beam on the surface of the object. Measurement results. From these vibration data, the vibration pattern of the work is measured and visualized either in the time-synchronized motion process or in the frequency range of the frequency band. Continuous measurements can be integrated under conditions where the vibration process can be accurately repeated. This can be achieved by a trigger signal generated on the oscillator in a specific frequency band range.
Figure 1 Scan point distribution for bending and torsional vibration measurements
Vibration measurements performed on the IWF can be performed using Polytec laser scanning vibrometers to measure the modal parameters such as the self-resonant frequency ωo and the yield N(ω) of the single-edged drill bit. As an experimentally-analyzed measuring object, a single-edged drill with a full hard metal head (C-shaped in circumference) having a diameter of 11.76 mm and a diameter of 7.22 mm was used. The drill pipe and tensioning sleeve (error: φ20 g6) consist of quenched and tempered steel, and the single edge drill has a total length of 335 mm. In order to be able to measure bending and twisting vibrations in an experimental analysis, it is first necessary to define the corresponding grid lines required for determining the measuring points. In this regard, the selection of the measurement points adopts a fractal distribution method, and the laser beams are sequentially scanned. In the absence of advancement, the measurement points directly distributed in a vertical array lie on a single plane; however, when the single-edged drill bit is subjected to vibration advancement, the drill bit is displaced. In the case of torsional vibration, the drill bit is twisted, and the results of stroke changes at the respective overlapping points detected by the laser beam are different. In the evaluation, this phenomenon is represented by the color change at each overlapping measuring point (Fig. 1).
Specific measurement
In order to avoid external vibration in the experimental analysis, a large-scale steel measuring table was used. First of all, the spindle and center of gravity of the single-edged drill must be calculated so that the direction of the longitudinal vibration of the drill can be directly aligned with the transverse direction of the center of gravity of the drill and both sides of the spindle. For the vibration in the longitudinal direction of the drill, the vibrator and the single-edged drill bit are opposed to each other and are clamped on a tiger table or a jig with a liner and a clamp. The free drill tip has a hardened vibrating tip with a Piezo load cell and an aluminum adapter at the tip. Under the effect of tension, these devices can be connected to the vibrator. The controller passes an amp to transmit a sinusoidal signal and a specific frequency band to a vibrator that oscillates at the other end against a single edge drill bit. The load cell feeds a force signal back to the controller via a loaded amplifier, and the controller calculates the modal parameters from the frequency-dependent trip change signal transmitted by the laser scanning vibrator.
Fig. 2 Frequency of flexibility in the direction of the first principal axis (34)
Fig. 2 shows the flexural frequency range of a φ11.76 mm single-blade deep-hole drill when subjected to vibration in the first main shaft direction (34), and the curve conforms to the characteristics of a conventional three-position vibrator. Since the three-position distribution involves drills, drill pipes, and tensioning sleeves for single-edged drills, this experimental method is appropriate. The 880Hz own pattern is easy to recognize, which is a bending vibration; and the other own pattern is 2000Hz torsional vibration. The third typical self-mode is still bending vibration with a frequency of 3300 Hz. For comparison, the softness frequency range of the single-edged drill bit when the drill shown in FIG. 3 is vibrated in the feeding direction. The first measured own mode is 580Hz and the second own mode is 850Hz. These are all bending vibrations. An atypical vibration amplitude observed in the direction of the first main axis is the first torsional vibration of 2000 Hz. Very typical are the 2600Hz, 3300Hz and 3860Hz own modes, these are bending from the vibration.
Figure 3 Frequency of flexing in the feed direction
On the whole, the higher the frequency, the stronger the softness of the single bladed cutter. Since the vibration force of the vibrator is maintained at a constant level, it can be said that the drill blade has a stronger deflection in a relatively large frequency range. In addition to the use of hollow drill rods, the ratio of length to diameter of the single-edged drill is also large, which reduces the rigidity of the drill and increases the risk of fracture and vibration amplitude increase. A detailed evaluation of the modal experimental analysis performed at each measurement point shows that different vibrations in the longitudinal and transverse directions of the drill spindle can also produce lower flexural frequencies in the 50 to 60 Hz frequency range. For a list of two different diameter single blade drills, a detailed list of the measured own modes and vibration modes is given in the table below.
The experimental results show that for different single-edged drills, the dynamic performance of the system can be clearly described. By measuring the knowledge of the bending and torsional vibrations obtained, an adaptive adjustment system can be developed in the future to adjust dangerous working vibrations. In this regard, special attention must be paid to the occurrence of the torsional vibrations that have been identified, because this phenomenon leads to a significant reduction in the service life of the tool and possible tool failures.
The modal parameters of a single-edged drill can be quickly and reliably measured by a laser scanning vibrator. The results of the study show that the distinction between self-oscillating and torsional vibrations can be achieved. For this purpose, it is necessary to select a matrix of measurement points on the surface of the drill tool. The self-generated frequencies that occur in the soft frequency range are almost the same in both directions of vibration. In addition to the ability to reliably measure modal parameters, laser scanning vibrometers can also visualize vibrational forms. Through this measure, the single blade cutter stuck on the hole wall can be clarified. In addition, it is possible to design an adaptive regulation system that can identify dangerous work vibrations and adjust them. In this regard, it is necessary to grasp the knowledge of the vibration of the work that occurs during the drilling process. The goal of the next phase of research is to identify the single-blade drill's own vibration pattern to understand the load and deformation of the tool during drilling.
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