Metal spinning is the operation of shaping thin sheets by pressing them against a form. The sheet is rotated at a very fast speed and blunt tool is used to force the metal flow in the desired form. Spinning is carried out on an ordinary lathe. The lathe is just like an ordinary wood working lathe with the only difference that in place of the usual carriage a tool rest is provided to hold the blunt tool.
The drive is obtained by pulleys and belts. First a form or pattern of the desired shape is prepared. The form might be made either from hard wood or cast or carburising steel depending upon the sheet material to be spinned and number of operations required from the same form.
When spinning of large diameter are to be produced and the working is performed on the outside periphery, then it is uneconomical to make forms entirely of steel. In such cases wooden centre body fitted with machined steel inserts at the outside edge, where spinning will take place, is used.
The form is provided with a necessary mandrel which is fixed in the headstock of lathe. The cast iron form should be given very good surface finish in order to avoid the interior imperfections and have better finish on the spinned product.
The steel sheet is cut in a circular shape whose diameter is calculated on the basis of the shape of the desired product. The sheet is held against a compression pad which is supported by tailstock centre. If the shape is too complicated and where deep drawing is required the sheet may have to be first given simple form by press operation.
The forming of the sheet to the desired shape is achieved with the aid of blunt hand tool which presses the metal against the form. The blunt tool is supported on a compound tool rest and pivoted about a pin fixed on the tool rest. Some means are also provided for supporting a trimming cutter or forming roll in the rear of lathe.
Most of spinning work done on the outside diameter but inside work is also possible. For spinning work, lubricants like soap, bees-wax, linseed oil are generally used in order to reduce the friction at tool-work interface. In spinning also, the materials are subjected to high stresses and, therefore, sometimes one or the annealing operations become necessary for greater amount of drawing.
Spinning of pitcher shape components is carried out by a small roller supported from the compound rest and pressing the metal out from inside against a form roller. The part is first drawn and given a bulging shape beforehand as spinning cannot be done near the bottom. Such spinning work can also be done by using collapsible dies which are made in parts, so that bulged shape can be formed on the form and also it can be taken out of spinned part by collapsing the form.
Spinning is limited to symmetrical articles of circular cross-section. This is a very cheap process and does not require much of investment. This very suitable for making aluminium utensils, reflectors, funnel bases of certain domestic appliances like mixers, lanterns, lamps etc.