North East Wales Institute of Higher Education
University of Wales - Wrexham
Higher National Certificate
Olivier DURIEUX – email@example.com
Press tool design 3
Special Machining processes 19
Screw thread production 46
Assembly technology 58
NC Machining 72
Shot peening 90
Gear cutting 99
In the manufacturing section of the engineering industry, metal articles can be worked to shape either by metal cutting, or by metal forming.
Metal cutting can often be wasteful because, on the average, 40% of the original component material is removed byexpensive machining operations to become scrap. This scrap material will then on the average be worth 5% of its original value as raw material. In many cases machining operations can be more economically carried out by metal forming.
There has been a move in recent years to greatly extend the scope of metal forming, which includes such processes as hot and cold extrusion, hot and cold forging,and press tool work.
It is interesting to carry out a break-even cost analysis upon a manufacturing operation where metal cutting or metal forming are possible alternative processes. Often the forming process requires expensive dies and fixed costs are higher; material wastage is negligible, labour costs are low and hence variable costs are lower. The metal cutting process is often lower onfixed costs, but higher on variable costs. Hence, the forming process will be more economical when quantities required are large.
In this section we shall be concerned solely with the use of presses and press tools as a means of cold working metal objects into shape. Most of this type of work is carried out upon ductile metal in sheet or strip form of relatively thin section. It represents animportant part of the manufacturing industry, being used for the cheap production of large quantities of components, such as motor car bodies, electric motor parts, domestic electrical articles, etc.
Orthodox presses (usually vertical) are used, and may be hydraulic or pneumatic powered, or may be mechanical crank presses. For very light work, hand presses such as fly presses are satisfactory. Themetal if in coiled strip form, may be fed automatically into the press tool by power rolls or power slide, or may be hand fed by the operator. If the metal is in some other form, such as a sheet or partially formed shape, it may be located in the tool by mechanical hands which have gripping fingers, or locating pans which drop the metal part into the correct position. Again, an operator(s) may handfeed the part. The mechanical feed or location devices must of course be synchronised to operate every time the press ram lifts the top tool clear of the bottom tool. Where the press is hand fed, stringent safety precautions must be taken to ensure that the operator hands cannot be trapped in the press tool. Efficient guards must be provided which are completely foolproof, and it should always beremembered that a press is potentially a very dangerous machine.
In order to appreciate press tool design, it may help to briefly reconsider the elementary principles of metal plasticity. Standard tensile or compression tests which cold work the specimen being used are, an ideal means of obtaining data about the plastic range of metals.
Consider Figure 1 which shows the results of a tensiletest upon a relatively ductile material, such as low carbon steel.
The metal is elastic up to point A and will return to its original size if the force is withdrawn. If the force however, is increased to point B before being withdrawn, the force-extension graph follows line BC, parallel to line AO, as the force is removed. The test piece will then be permanently extended by amount OC, and will...