13-11-2012, 05:34 PM
Screw threads and Gear Manufacturing Methods
Screw threads and Gear Manufacturing Methods.pdf (Size: 636.01 KB / Downloads: 132)
General Applications Of Screw Threads
The general applications of various objects having screw threads are :
• fastening : screws, nut-bolts and studs having screw threads are used for temporarily fixing one part on to another part
• joining : e.g., co-axial joining of rods, tubes etc. by external and internal screw threads at their ends or separate adapters
• clamping : strongly holding an object by a threaded rod, e.g., in c-clamps, vices, tailstock on lathe bed etc.
• controlled linear movement : e.g., travel of slides (tailstock barrel, compound slide, cross slide etc.) and work tables in milling machine, shaping machine, cnc machine tools and so on.
• transmission of motion and power : e.g., lead screws of machine tools
• converting rotary motion to translation : rotation of the screw causing linear travel of the nut, which have wide use in machine tool kinematic systems
• position control in instruments : e.g., screws enabling precision movement of the work table in microscopes etc.
• precision measurement of length : e.g., the threaded spindle of micrometers and so on.
• acting as worm for obtaining slow rotation of gear or worm wheel
• exerting heavy force : e.g., mechanical presses
• conveying and squeezing materials : e.g., in screw conveyor, injection moulding machine, screw pump etc.
• controlled automatic feeding in mass production assembly etc.
(ii) Classification Of Screw Threads
Screw threads having various applications can be classified as follows
Production of screw threads by machining
Machining is basically a removal process where jobs of desired size and shape are produced by gradually removing the excess material in the form of chips with the help of sharp cutting edges or tools. Screw threads can be produced by such removal process both manually using taps and dies as well as in machine tools of different types and degree of automation. In respect of process, machine and tool, machining of screw threads are done by several ways :
Thread cutting by hand operated tools
Usually small threads in few pieces of relatively soft ductile materials, if required, are made manually in fitting, repair or maintenance shops.
• External screw threads
Machine screws, bolts or studs are made by different types of dies which look and apparently behave like nuts but made of hardened tool steel and having sharp internal cutting edges. Fig. 7.1.1 shows the hand operated dies of common use, which are coaxially rotated around the premachined rod like blank with the help of handle or die stock.
Solid or button die : used for making threads of usually small pitch and diameter in one pass.
Spring die : the die ring is provided with a slit, the width of which is adjustable by a screw to enable elastically slight reduction in the bore and thus cut the thread in number of passes with lesser force on hands.
Split die : the die is made in two pieces, one fixed and one movable (adjustable) within the cavity of the handle or wrench to enable cut relatively larger threads or fine threads on harder blanks easily in number of passes, the die pieces can be replaced by another pair for cutting different threads within small range of variation in size and pitch
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• According to location
o external