A pantograph is a combination of links which are so connected and proportioned as to length that any motion of one point in a plane parallel to that of the link mechanism will cause another point to follow a similar path either on an enlarged or a reduced scale. Such a mechanism may be used as a reducing motion for operating a steam engine indicator, or to control the movements of a metal cutting. For instance, most engraving machines have a pantograph mechanism interposed between the tool and a tracing point which is guided along lines or grooves of a model or pattern. As the tracing point moves, the tool follows a similar path, but to a reduced scale, and cuts the required pattern or design on the work.
A simple form of pantograph is shown by the diagram, Fig. 12. There are four links, a, b, c and d. Links a and b are equal in length, as are links c and d, thus forming a parallelogram. A fifth connecting link e is parallel to links c and d. This mechanism is a free to swivel about a fixed centre f. Any movement of h about f will cause a point g (which coincides with a straight line passing through f and h )to describe a path similar to that followed by h, but on a reduced scale. For instance, if h were moved to k following the path indicated by the dotted line, point g would also trace a similar path.
( Franklin Day Jones, Mechanisms and Mechanical Movements, Elibron Classics, 2005, p. 19-20)
Toggle Joint
A link mechanism commonly known as a toggle joint is applied to machines of different types, such as drawing and embossing presses, stone crushers, etc., for securing great pressure. The principle of the toggle joint is shown by diagrams A and B, in Fig. II.
There are two links, b and c, which are connected at the center. Link b is free to swivel about a fixed pin or bearing at d, and link c is connected to a sliding member e. Rod f joints links b and c at the central connection. When force is applied to rod f in a direction at right angles to centre-line xx, along which the driven member e moves, this force greatly multiplied at e, because a movement at the joint g produces a relatively slight movement at e. As the angle é becomes less, motion at e degreases and the force increases until the links are in line, as at B. If R= the resistance at e, P= the applied power or force, and é= the angle between each link and a line xx through yhe axes of the pins then: 2R sin é =P cos é.
( Franklin Day Jones, Mechanisms and Mechanical Movements, Elibron Classics, 2005, p. 18-19)
Notch Wear
Notch wear is often attributed to the oxidation of the tool material from the sides of major and minor cutting edges, or to abrasion by the hard, saw- tooth outer edge of the clip ( for example, in hard machining). Notching is serious technological problem with workpiece materials that tend to have high work-hardening and generate high tool-tip temperatures, such as austenitic stainless steels and nickel-based superalloys. Notch wear can obviously lead to tool fracture and can be minimized by applying tools with chamfered edges, rounded inserts and avoiding small depth of cut.
( Wit Grzesik, Advanced Machining Processes of Metallic Materials, Elsevier BV. 2008, p.165)
Crater Wear
In metal cutting, the highest temperatures occur along some length of tool face. At high cutting speeds, these temperatures can be of the order of 1000 degree of C or more. There is a thermal softening and HSS tools wear rapidly. In carbide tools, solid-state diffusion at these temperatures can cause rapid wear.
A crater is formed on the tool face taking the shape of chip under side. This factor determines the life of the cutting tool. The cratering becomes very severe and the tool edge is weakened and fractured.
(R.K. Singal, Mridul Signal, Rishi Signal, Fundamentals of Machining and Machine Tools, I.K. International Pub., 2008, p.233 )
No comments:
Post a Comment