PSI - Issue 13

Peter Monka et al. / Procedia Structural Integrity 13 (2018) 959–964 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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Fig. 2. Taps used at experiments

Experiments were carried out using CNC milling centre Pinnacle VMC 650 with control system Fanuc. The experiments focused on investigation of taps´ durability were realized in two phases. In the first phase, the cutting speed 10 ms -1 was selected on the base of the supplier recommendation and based on company catalogues, where it is declared that the tool life is given by thread making at minimum 300 holes what corresponds to a 6 m length of thread. Feed per revolution 1.75 mm corresponds to the pitch of thread. As the criteria of tool wear have been considered either the changes in dimensions, which were out of tolerance field of M12 thread, or tool fracture. The wear of both cutting cone and calibration part of tool were also observed. It has to be said that all taps achieved required durability without tool wear in this phase of testing. Higher cutting speeds 20, 30 and 40 mmin -1 were selected for threading in the second phase of experiments. The criteria of tool wear were the same as it was in the first phase. The durability of taps was changed depending on the cutting speed. It has occurred to the abrasive friction at the flank of tool, the tip of tool deformed intensively and the built-up edge started to appear at the cutting wedge of tool. Simultaneously, the changes in dimensions out of tolerances at machined threads were appeared before fracture of the tool, but the process of tool wear up to brittle fracture (Fig. 3) was very fast. Due to this reason, it was very difficult to predict the moment of a fracture. So, authors decided to measure vibrations during the machining and compare the behaviour of both new and worn tool.

Fig. 3. Brittle fracture of a tool

Vibration measurements were done using vibro-diagnostic system with NI modules. The NI 9434 measuring module, which has 4 analogue inputs with +/- 5 V and 24-bit resolution, served to a signal modification. Records of vibro-diagnostic signals were done by means of TDG141-recorder-USB-4k equipment. Signals were processed and evaluated by means of software NI LabVIEW for tools with helix angles of ω = 15° and 35° at two cutting speeds 10 and 20 mmin -1 . Three piezoelectric accelerometers type PCB 607A11 with the frequency range of 0.5 - 10 kHz and weight 31 g were used for signals obtaining. The sensors were connected to the components by means of magnets, which natural frequency is around 6 kHz. The first accelerometer was joined to spindle of the machine (CNC milling centre Pinnacle VMC 650), the second two were joined to the workpiece in two different axes normal to each other as it is shown in Fig. 4.

Fig. 4. Positions of accelerometers at experiments