UNIVERSITI MALAYSIA PAHANG
BMM3643 (Sem II 2012/13)
a) What distinguishes machining from other manufacturing processes? b) What is a machining center?
c) How important is the control of cutting-fluid temperature in operations performed in machining centers? Explain. (8 marks)
a) In machining, material is removed from the workpart so that the remaining material is the desired part geometry. b) A machining center is a CNC machine tool capable of performing multiple types of cutting operations involving rotating spindles (e.g., milling, drilling); the machine is typically equipped with automatic toolchanging, pallet shuttles to speed workpart changing, and automatic workpart positioning. c) The control of cutting-fluid temperature is very important in operations where high dimensional accuracy is essential. As expected, the fluid heats up during its service throughout theday (due to the energy dissipated during machining), its temperature begins to rise. This, inturn, raises the temperature of the workpiece and fixtures, and adversely affects dimensionalaccuracy. Temperature-control units are available for maintaining a constant temperature in cutting-fluid systems.
a) What are the primary considerations in tool selection? b) What is the advantage of a helical-tooth cutter over a straight-tooth cutter for slab milling? c) What are the consequences if a cutting tool chips?
a) Primary considerations in tool selection are
i) What material is going to be machined
ii) what process is going to be used,
iii) what are the cutting speeds, feeds, and depths of cut needed, iv) what is the tool material,
v) what are lubricants going to be used.
b) Helical toothed cutters enter the workpiece progressively. There are a number of advantages, but the main advantages are associated with the fact that there are always multiple teeth in contact, and the transition in contact from one tooth to another is smooth. This has the effect of reducing impact loads and periodic forcing functions (and associated vibration) from the cutting operation. c) Tool chipping has various effects, such as poor surface finish and dimensional control of the part being machined; possible temperature rise; and cutting force fluctuations and increases. Chipping is indicative of a harmful condition for the cutting tool material, and often is followed by more extreme failure.
a) Why might it be desirable to use a heavy depth of cut and a light feed at a given speed in turning? b) Explain the reasoning behind the various design guidelines for turning. c) In drilling, the deeper the hole, the greater the torque.Why? (8 marks)
a) The material removal rate is a function of speed x feed x depth of cut. Assuming speed is kept constant, heavier depths of cut or heavier feeds will reduce the number of cuts or passes that have to be made, which reduces the number of adjustments or resettings of the tool which have to be made. Increasing either feed or depth of cut will increase the cutting force. Increasing depth of cut will have less effect on tool life than increasing the feed. Either way, the total machining time is usually less. In addition, the surface finish is usually better with the lighter feed. b) The design guidelines are mostly selfexplanatory, such as the need to design parts so that they can be easily fixtured. However, some examples of some reasoning are as follows: * Sharp corners, tapers, steps, and major dimensional variations in the part should be avoided. It’s easiest for a lathe to be set up to perform straight turning, thus unnecessary dimensional variations make the lathe operation much more difficult. The difficulty with sharp corners,...