PSI - Issue 13

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

961

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However, when tapping threads with a tap, the feed rate is determined by the pitch of the thread and the depth of the cut by the difference in the nominal diameter of the thread and the diameter of the pre-drill hole and tool geometry. In this case, the problem is also the chips removal at the threading of blind holes and tool lubrication, since the calibration part of the tap has very small (10 ’ - 20 ’ ) or no back angle, and so it causes the intensive abrasion of the workpiece surface. (Lee and Poppke 2013); (Gawronska and Sczygiol, 2010) In the past, some work has been devoted to monitor tap wear in order to predict when the tap will break; Sha (1989), Liu et al. (1991) and Li et al. (2002), and as a result there are some commercial systems available for this purpose. However, currently only one known study Gil Del Val A. (2013) has been done related to the topic of on line diagnostics detection of thread quality in tapping. On-line monitoring of vibrations enables operator to control cutting process and to observe the level of tool wear without necessity to interrupt machining process caused by taking out the tool and measuring values of cutting wedge wear. (Vychytil and Holecek, 2010) The article deals with vibro-diagnostics of wear of selected taps – tools for inner thread making.

3. Conditions of experiments

The unalloyed steel C45 (STN 41 2050), which is usually used as a reference material for tool life tests according to the relevant standards, was selected as the machined material. The workpiece was clamped into the flat jaws during the experiments and the ends were supported by the restraints, as it is shown in Fig. 1.

Fig. 1. Clamped workpiece during the experiments

The holes were drilled by means of a drill tool  10.2 mm with catalogue designation DH 404 102 of YG-1 company. The diameter of the drill corresponds to the recommended hole pre-drilling for the M12 thread according to the taps manufacturer's recommendations. The 87 holes (29 holes in one line) were drilled into one steel workpiece. The JCK PS process media was used at drilling. Drill tools were considered as worn and subsequently replaced, when increased tool vibration appeared. The countersinking operation followed after drilling. An angle of countersink was 120° and tool shank was cylindrical. The cutting technological conditions used at the holes preparation are shown in Table 1.

Table 1. Cutting conditions used at the holes preparation Operation Revolutions per minute [min -1 ]

Feed rate [mmin -1 ]

drilling

2200

400

counterboring

500

40

The taps for M12 thread making with tolerance 6H and with helix angles of tool ω = 0°, 15° and 35° were used at the experiments. The functional length of threads at tool with helix angle 15° and 35° was 20 mm, what means that the tool worked to the depth of 25.25 mm of pre-drilled hole. For helix angle ω = 0° , the depth of the thread was reduced in 5 mm as it is recommended by supplier what resulted in a cut depth of 15 mm. It was done due to the possibility to remove chips from the bottom of holes, what in the opposite case could cause a damage of cutting tool. The tools were made from High Speed Steel (HSS-E) and they were uncoated. The experimental tools were clamped in the  10 clamps during machining. The taps used at the machining are shown in Fig. 2.