In this article we will discuss about:- 1. Meaning of Metal Spinning 2. Advantages of Metal Spinning 3. Metal Spinning Speeds 4. Types 5. Design Considerations.
Meaning of Metal Spinning:
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.
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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.
Factors governing the selection of spinning process for shaping metals should be based on the thickness and type of metal to be used, tolerance permitted, quantity to be produced, cost of the tooling, and time element. The three basic spun shapes are the cone, the hemisphere, and the straight walled or cylindrical. Of these, the conical shape is the easiest to produce.
Hemispherical shapes are somewhat difficult to obtain by metal spinning. In the case of spinning of cylindrical shapes the metal is placed under considerable strain, the production of cylindrical shape by spinning is less economical and requires a highly skilled spinner.
Advantages of Metal Spinning:
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Unlimited variety of circular shapes can be produced by manual spinning on solid or collapsible forms of wood or steel and even ‘on air’. It is ideally suited, most economical and most satisfactory for production of one or two non-standard shapes. Repetitive production is quick.
Forming tools are of simple construction. Hardwood forms are low in cost and can be readily reworked for new shapes. Sizes of parts may vary from 10 mm to 5 metres in diameter. In conclusion manual spinning is quick, economical process for manufacture of small volume and special parts, where tool and die costs for other forming methods are prohibitive.
For large-volume manufacture, automatic tracer controlled power spinning and mechanical spinning are used.
Metal Spinning Speeds:
Rotational speeds for spinning are governed by size or diameter, thickness, and temper or workability of the sheet of metal to be spun. Slower speed is used for larger or heavier blank. Approximate peripheral spinning speeds are 600 to 1800 smpm for 7.5 to 22.5 cm blank diameter, 1200 to 2400 for 22.5 cm to 45 cm blank diameter, and 1650 to 3000 for 45 cm to 125 cm blank diameter.
Types of Metal Spinning:
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i. Flame Spinning:
It utilises oxyacetylene flame to heat the portion to be formed. It is developed to facilitate making pressure tight tubing ends, for partially closing ends of tubes, for reducing diameters at various points. This method permits rapid forming operations on parts where size or wall thickness eliminates the use of cold spinning.
ii. Heavy Spinning (Dishing and Flanging Machines):
These are used for spinning of extremely large and heavy wall parts. These can spin hot material upto 13 cm thick and cold material upto 2.5 cm. Cold material can only be flanged white hot material can be spun into flanged, dished, spherical and conical shapes.
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iii. Shear Spinning:
It is used to reduce the thickness of the starting blank, or shave to produce the final form—conical or cylindrical by point-extrusion process. Shear spin machines are large and rugged and provide high spinning forces upto 100 tons per tool slide. Such machines feature multiple rollers with individual tracer control and automatic operation.
Design Considerations in Metal Spinning:
Material thickness, bend radii, depth of spinning, diameter and steps in diameter and workability of material deserve attention in designing parts for conventional spinning. Easiest shape to produce is conical.
Hemispherical shapes are somewhat more difficult to produce, especially in full depth parts. Cylindrical or cup-shaped pieces are the most difficult of the simple shapes. For deep-spun parts, spinning operation has to be broken into 3 or 4 steps with intermediate annealing.
Intricate shapes or ‘closed’ parts which cannot be spun manually on an ordinary open chuck, can be produced on a two piece, or a sectional or an off-centre roll chuck. (Refer Fig. 6.14(B). The work is first spun to a plain cylindrical cup shape and then mounted on this machine for final inside spinning of bulging. Necking operation can be done by mounting a form roll on offset tail spindle and mounting spinning roll on outside.
During processing, the stock area increases slightly due to thinning. This increase in area usually provides sufficient metal for trimming, flanging, rolling or wire beading. To compensate for thinning out of the stock, the gauge of the blank should be approximately 30% greater than the thinnest section allowable on the finished part.
Usually outside bead is provided where strengthening bead or wired roll over the edge is required. (Refer Fig. 6.15).
Streamlined or smooth curves and large radii are an aid both to manufacture and improved appearance. Minimum of 3 mm radius should be provided.
The minimum thickness for spinning is as follows:
Tolerances of ± 0.15 mm in thickness can be specified and held.