Anıl Uzal, 030070012, 9th week

Peripheral Milling: (07.04.2011 ; 01:00)

Peripheral milling is a milling method which functions with horizontal tool axis. The cutting edges of the plain milling cutter are located at the tool’s periphery. Peripheral milling is subdividen into up- and down milling.

Up milling:

During up milling (Figure 11.1), the milling cutter rotates in a direction opposite to the feed direction of the workpiece. The feed motion direction (Figure 11.2) is characterised by the feed motion angle φ. If, over the course of a single tooth’s contact with the material (from the moment the tooth comes into contact with the material – tool entry - up to tool exit), φ remains less than 90^o, then it is an up milling procedure. During up milling, workpiece material is removed by the resultant force. There is the risk that the workpiece may be pulled out of the mounting or that the milling table will buckle. Specially designed clamping jigs and undercuts in the table guide-ways avoid damage to the workpiece or tool.

Down milling:

During down milling (Figure 11.3), the direction of milling cutter rotation is the same as the workpiece’s feed direction. The milling cutter approaches from the thickest part position of the chip. In down milling, the feed motion angle φ (Figure 11.4) ranges from 90^o to 180^o. The resultant force presses the workpiece against the base. In cases where the cutter arbour is insufficiently stiff, the milling cutter “climbs” onto the workpiece, and cutting edges break off.

During down milling the resultant force direction coincides with the feed motion direction. Thus, if the feed screw experiences backlash, the resultant force makes the lead-bearing flank at the feed screw changes at each start of the cut. Milling machines for down milling should have a feed drive with no backlash, cutter arbours and frame components of high stiffness.

(Heinz Tschätsch, “Applied Machining Technology”, page 173-174)

Torch Brazing: (07.04.2011 ; 01:55)

In torch brazing, flux is applied to the part surfaces and a torch is used to direct a flame against the work in the vicinity of the joint. A reducing flame is typically used to inhibit oxidation. After the work part joint areas have been heated to a suitable temperature, filler wire is added to the joint, usually in wire or rod form. Fuels used in torch brazing include acetylene, propane, and other gases, with air or oxygen. The selection of the mixture depends on heating requirements of the job. Torch brazing is often performed manually, and skilled workers must be employed to control the flame, manipulate the hand-held torches, and properly judge the temperatures; repair work is a common application. The method can also be used in mechanized production operations, in which parts and brazing metal are loaded onto a conveyor or indexing table and passed under one or more torches.

(Groover M.P., “Fundamentals of Modern Manufacturing: Materials, Processes, and Systems 3^rd Edition”, page 749)

Pulsed Arc Welding (07.04.2011 , 02:24)

Pulsed-arc welding is a controlled method of spray transfer welding requiring a more sophisticated power source, whereas the three types of transfer described previously can be obtained with standard power sources and wire feed units. In spray transfer, droplets of metal are projected from the wire tip across the arc gap to the molten pool at a constant current. In dip transfer, metal is transferred to the molten pool somewhat irregularly during the periods of short circuiting. Pulsed-arc welding enables droplets to be projected across the arc gap at a regular frequency, using pulses of current in the spray transfer range supplied from a special power source. Transfer of metal from the wire tip to the molten pool occurs only at the period of pulse, or peak current (see Fig. 5.6). During the intervals between pulses, a low “background” current maintains the arc to keeğ the wire tip molten, but no metal is transferred. Pulsed transfer means that the weld metal is projected across the gap at high current, but the mean welding current remains relatively low. The operator can vary the pulse height and the background current to obtain full control of both the heat input and the amount of metal deposited; however, in many modern power sources the pulse procedure is preset by the manufacturer to simplify use. Pulsed-arc transfer can be used on mild and low-alloy steels, stainless steel and is particularly useful with aluminum and its alloys on light to medium plate sections as dip transfer cannot be used on these alloys.

(P.T. Houldcroft, R. John, “Welding and Cutting: A Guide to Fusion Welding and Associated Cutting Processes”, page 80)

Piezoelectric Transducer (07.04.2011 ; 14:40)

If the dimensions of asymmetrical crystalline materials, such as quartz, rochelle salt and barium titanite, are changed by the application of a mechanical force, the crystal produces an emf. This property is used in piezoelectric transducers.

The basic circuit of a piezoelectric transducer is shown in Fig 21.13. Here, a crystal is placed between a solid base and the force-summing member. An externally applied force gives pressure to the top of the crystal. Hence, it produces an emf across the cyrstal which is proportional to the magnitude of the applied pressure.

As this transducer has a very good high frequency response, it is used in high frequency accelerometers. As it needs no external power source, it is called as self-generating transducer. The main drawbacks are that it cannot measuure static conditions and the output voltage is affected by temperature variations of the crystal.

(S. Salivahanan, N. S. Kumar, A. Vallavaraj, “Electronic Devices and Circuits 2^nd Edition”, page 770)