Buğra Çetinkaya 030070094 8th Week Definitions Part2

2)Positive Positioning (Fixture Quality)

Previous Answer

        A positive positioner (or positive-positioning relay) is designed to provide up to full main control air pressure to the actuator for any change in position. An increase in branch pressure from the controller moves the relay lever, opening the supply valve. This allows main air to flow to the relay chamber and the actuator cylinder, moving the piston. The piston movements is transmitted trough a linage and spring to the other end of the lever; and when the force due to movement balances out the control force, the supple valve closes, leaving the actuator in the new position. A decrease in control pressure will allow the exhaust valve to open until a new balance is obtained. Thus, full main air pressure is available, if needed, even though the control pressure mat have changed only a fraction of a psi.  The movement feedback linkage is sometimes mounted internally. Positioners may be connected for direct or reverse action. For large valves or dampers, main air to the actuator may be at higher pressure than pilot air.(Roger W., Douglas C. H., Control Systems for heating, Ventilating, and Air Conditioning, 2006 p.38)

New/True Answer

The speed, accuracy, and repeatability of modem machine tools are largely due to the qualities of the design and accuracy of modem fixtures. These must have the qualities shown in Fig. 4-1-2.

Positive Positioning: The part must be held securely enough to prevent any movement in the X, Y and Z axes.

(Stephen F. Krar,Arthur Gill, Exploring Advanced Manufacturing Technologies, p. 4-1-1)

3) Automated Pallet Cell (Manufacturing Process)
Previous Answer
The automated pallet cell(APC) is an ideal machining and workholding solution to maximize productivity and profit.The horizontal machining center with an APC allow parts to be machined in less time, reducing cost as well as reducing inventory.Workpieces are easily loaded and unloaded at an off-line station.Pallets can be manually indexed, allowing acces to all sides of the fixture for part load and unload.(Stephen F. Krar,Arthur Gill,Exploring advanced manufacturing technologies,p. 4-1-5)

New/Better Answer

A truly automated cell is one where all major functions are under computer control. Accordingly, the cell is capable of operating without manual supervision for a period of time for example, a second or third shift. This is why an alternative term for automated cells is "cells with limited manning." Cells are often called flexible manufacturing systems (FMS) if a large number of machine tools are connected by a material-handling system. The role of humans is usually limited to the loading and unloading of parts from pallet fixtures, and the ongoing monitoring of the processes. "Smaller" systems are sometimes called flexible manufacturing cells (FMC). For example, a cell with a robot connecting one or two CNC machines can be categorized as an FMC.

(Nancy Lea Hyer,Urban, Reorganizing the Factory: Competing Through Cellular Manufacturing, p. 36)

4) Power Chuck (Workholding)
Previous Answer
Power chucks are essential part of the automatic manufacturing process. Power chucks provide an even gripping force and their performance does not rely on the skill of the operator. The gripping power is provided by hydraulic or pneumatic cylinders that cause the chuck's jaw to increase their grip on the work piece before the machining force can cause the workpiece to slip out.(Stephen F. Krar,Arthur Gill,Exploring advanced manufacturing technologies,p. 4-1-6)

New/Better Answer

Power-operated chucks may be operated by two methods; the power-operated drawbar which is actuated by an air or hydraulic cylinder, and the power chuck wrench which is an external device, actuated by an electric motor. This latter device operates the screw-locking mechanism for opening or closing the jaws of a hand-operated chuck. In this chapter we will deal only with the first type which is widely accepted for use on high-production machines.

Power chucks are used on lathes, grinders, boring machines and other machines which are designed with a hollow spindle so that there can be a drawbar or drawtube connection between the actuating mechanism of the chuck and the rotating air or hydraulic cylinder.

(Harry L. Stewart, Hydraulic and Pneumatic Power for Production, p. 17-1) 

5) Pipe Defect (Manufacturing Process)

Previous Answer

Pipe defect is also known as tail pipe or fishtailing. In many cases, during extrusion the surface oxides and impurities are driven twards the cenre of the bilet , much like a funnel (called pipe). The length of the extruded part with this defect has to be discarded and cut off as scrap. The piping defect can be minimized by proper designing of the die so that a more uniform metal flow pattern may be obtained.(Kaushish,J.P., Manufacturing Processes, Eastern Economy Edition, pg. 407)

New/Better Answer

The metal flow in extrusion process sometimes tends to draw surface oxides and impurities towards the center of the billet, much like a funnel. This defect is known as extrusion defect, pipe, tailpipe or fishtailing. A considerable portion of the material can be rendered useless as an extruded product because of this, as much as one-third of the length of the extrusion can he rendered useless. Modifying the flow pattern to a less inhomogeneous one, such as by controlling friction and minimizing temperature gradient can reduce this defect. Another method is to machine the surface of the billet prior to extrusion to eliminate scale and impurities. The extrusion defect can also be avoided by using dummy block that is smaller in diameter than the container, thus leaving a thin shell along the container wall as the extrusion progresses.

(Surender Kumar, Technology Of Metal Forming Processes, p. 12)