Piercing Mandrels and Mandrel-Holders

The mandrel is cooled after every extrusion. This is mostly done by dipping the mandrel into a cooling pipe in which it is sprayed with air, oil or hot water. Thick mandrels - on account of the bad thermal conductivity of the alloy steel - tend to crack under cooling stresses these may be avoided to some extent by hollow-boring the mandrels. Mandrels of over 30 mm diameter are internally cooled, as illustrated in Fig. 134, when extruding at slow rates. 

For this purpose mandrel a and mandrel-holder h are provided with a central bore which accomodates feed pipes c and d. The cooling water is admitted to the hollow space in the mandrel through a number of holes drilled into pipe c and flows back through the mandrel-holder into chamber e of the piercer bar /. The water passage from the ram-holder g to the movable piercer bar / and vice versa takes place through two immersion tubes h arranged side by side, of w hich only one is shown in Fig. 134. 

Assmanx, H. Werkstoffeinsatz und optimale Leistungsausnutzung von PreB-dornen fur Metallrohrpressen. Metall 1948, H. 7/10. 

Connecting between the cooling-water pump and the movable ram-holder g is established by two hoses i and j. 

The wear resistance of the mandrels depends in the first place on the hardness and the smooth finish of the surface. They are therefore super-finished, i. e. their surfaces are lapped and polished after grinding. 

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