2 identification of critical subsystem

    Failure is defined as the termination of an item's ability to perform its specified function. Data on failures is considered as a "health report" of any machine. It was difficult to collect field failure data on machine tools several years ago because users hardly kept adequate and complete maintenance records beyond the warranty period. In this computerized maintenance environment,

users feel encouraged to keep the systematic maintenance records of their machineries. The data for analysis were extracted from the maintenance reports collected from a manufacturing industry

for the period of five years on 13 lathes. It contained the following information:

Product name, model and size、Product code、Reported time and date of failure、 Failure phenomena、 Cause of failure、Repair process、Repair time、Downtime、 Date of handover、Model, size and number of breakdown components、Number of service engineers or repair engineers、Site of machine tool.

3 Experimental investigation

  In this investigation, experimental studies were carried out on a precision tool room lathe in a laboratory environment at Machine Dynamics Lab, Industrial Tribology Machine Dynamics and Maintenance Engineering Center (ITMMEC) at the Indian Institute of Technology (IIT) in  Delhi. The deliberately contaminated grease mixtures used for bearing lubrication were prepared at the Center for Polymer Science and Engineering, IIT Delhi.

3.1 Equipment

  The main spindle of the precision tool room lathe is held in a pair of specially matched, pre-loaded taper roller bearings in the front end and a heavy bush bearing of ample capacity at the rear end. A foolproof lubrication arrangement has been provided to avoid any chance of the spindle running dry. The step pulley is mounted on the ground surface of the spindle over suitable bearings. The spindle is run by a 1.5 hp electric motor through V-belt drive. The spindle end is threaded and has a special taper bore to accommodate the chuck. The measuring instruments used for this study are:

  Piezoelectric accelerometer (Briiel & Kj~er, Type 4370)

  Preamplifier (Br/iel & Kj~er, Type 2635)

  FFT analyser (Ono Sokki, Type CF 3200)

  Nordmann SEM 68000 tool monitor with signal emitting

  processor (SEP)

  Taly-Surf 6 (Taylor-Hobson)

 The schematic of the measurement setup on the precision tool room lathe is shown in Fig. 4. The test bearings used in all the measurements were SKF taper oller bearings with a size code of 30209. The bearing had an inner diameter 45 mm, an outer diameter of 85 nun and a width of 19 mm, and consisted of 18 rollers and a pressed steel cage, Tolerances and radial internal clearanceof the beatings were of the normal class, according to the bearing manufacturer's specifications [29]. A mild steel workpiece was used during the experimentation. The size of the workpiece was 25 mm in diameter and 150 mm in length. The overhang was kept at 80 mm to avoid heavy vibration during the cutting process. The details of cutting conditions, tool inserts and tool holder specifications are given in Table 1.

Table 1. Cutting conditions, tool holder and cutting inserts details

Fig. 4. Schematic view of theexperimental set

3.2 Experimental procedure

  Accelerometers were mounted at locations 1 and 2 to receive the spindle bearing vibration. The overall vibration levels and vibration spectrums were recorded in FFF Analyser (ONOSOKKI) for all the cutting conditions, as shown in Table 1.The acoustic emission hydrophone SEH receives the acoustic waves through a jet of cooling lubricant which is connected directly to cutting zone, as shown in Fig. 4. The measuring jet was made as short as possible since this has better characteristics with respect to ground noise and damping. The flow quantity was also kept as low as possible to minimize the impact noise.