Introduction to Metal Spinning
Metal spinning is one of the oldest techniques for the chipless production of circular hollow metal components. History records evidences that metal spinning was known to Egyptians of hieroglyphic days. Results of the craft appear in the histories of most countries since that time. The process was introduced into our country about 1840 and was first used almost exclusively for the production of fine gold, silver and pewter hollowware and chalices.
By the end of the century it was being used for making chandelier parts. Only the soft nonferrous metals were employed in industrial applications as late as the first World War. Around 1920 a few daring men of the industry began to experiment with tougher materials, heavier gauges, and larger diameters. Due to its versatility, metal spinning is often employed today for production of small, medium, and large lot sizes.
In metal spinning, a disc of metal is revolved at controlled speeds on a specialized machine similar in design to a machine lathe. Instead of the clamping chuck common on a machine lathe, a wood or metal spinning mandrel is used, the form of which corresponds with the internal contour of the part to be produced. The blank is clamped between spinning mandrel and a follower on the tailstock spindle. The mandrel, blank, and follower are then set in rotation. Spinning tools or spinning rollers are forced against the rotating blank by hand or auxiliary power. Employing a series of axial and radial (swivel) strokes, the blank is spun onto the mandrel causing the metal to flow to the shape of the desired part. Although the metal is usually cold when spun, it appears to flow somewhat like a piece of clay on a potter's wheel. With this forming technique, material thickness generally changes slightly from blank to finished component.
The blank diameter is generally larger than the diameter of the finished component, and its size is determined from the surface area of the finished part. Pressure on the spinning tool or roller may be exerted by hand, air or hydraulics. The availability of hydraulic spinning machines and automatic template-controlled devices has made metal spinning, normally associated with prototype and limited-production runs, competitive in the mass-production of many shapes. The use of precision templates and automatic machinery has also insured the repeatability of each part.
Because of the rotation, parts usually are symmetrical and circular in cross-section in the metal spinning process. However, many spun parts are cut into segments or welded into intricate assemblies that bear little resemblance to the original spun components.
Metal spinning, once usable only on the most malleable of metals, can now be applied to stainless steels or the most exotic workable alloys, such as Titanium, Inconel®, or Hastelloy®. Size restrictions of a few years ago are almost non-existent. Parts can be spun up to 26 feet in diameter and thicknesses of up to 3 inches for aluminum and 1 1/2 inches in ferrous alloys.
Hand or manual spinning is the simplest form of metal spinning. In this process, the metal is formed by hand pressure on a spinning tool or roller.
Metal spinning on hydraulic lathes is similar to metal spinning on manual lathes. Rollers follow curves approximately equivalent to the swivel motions of the hand-held tool.
As the spinning forces applied are higher in comparison io hand spinning, mandrels have to be used which are made from harder material, e.g. boiler plate, chilled cast iron, or hardened tool steel.
Automatic spinning lathes are very similar to hydraulic power spinning lathes. However, an automatic control system is included which determines the sequence of operations. These lathes are provided with tracer attachments for exact control of the spinning rollers.
Before work commences the spinning mandrel and follower are fitted (as is done on manual spinning lathes). The hydraulically powered compound rest holding the spinning roller is fixed at an angle with respect to the mandrel axis. The compound rest is also provided with a template tracing attachment. The stylus is attached to the drive mechanism which controls all intermediate passes. After the templates have been adjusted, the blank is located on the centering device and lathe cycle is initiated.
A variation of the spinning process is known as shear forming. This process achieves a deliberate and controlled reduction in blank thickness as opposed to limited reduction of blank thickness in conventional spinning.
Local metal flow is achieved by the shearforming roller, which applies a pressure on the blank against the support from the steel mandrel. Thus conical or contoured components can be produced in one operation. Special shear forming rollers effect the material flow, and they move the free material parallel to the axis of the mandrel. The remaining portion of the blank, which does not take part in the actual deformation, remains always at right angles with respect to the axis of rotation and does not change its external dimensions. Thus circular, square, or other blanks may be shear formed.
The material flow takes place in the axial direction. The wall of the component is produced from the reduction in blank thickness. The thickness of the initial blank is dependent upon the angle of the final component and finished wall requirements. The bottom and the flange maintain their original thickness. A particular advantage is that surface finishes can be achieved which are comparable to grinding or fineturning. The accuracy of the shape and the dimensional repeatability is excellent.
The mechanical working of the metal during the spinning process provides natural metallurgical benefits, with a refined and strengthened grain structure. The heavy pressures required to produce plastic flow of the metal during spinning cause an orientation of grain parallel to the principal axis, in much the same manner as a forging. Cold working of the metal also increases tensile properties appreciably. According to one test, mild steel with a yield strength of 33,000 psi was improved to 60,000 psi. Integrity of metal is another important advantage. Unlike a casting, a spun part that requires further machining will not be scrapped by a hidden blowhole or inclusion. This same integrity assures a higher degree of reliability on parts that have a structural function. Certain shapes lend themselves more readily to spinning than casting. In particular, those cast shells or forms that have a re-entrant configuration in which a detail tends to come back toward the center making it difficult to remove a core. A spun shell could be made in halves and welded to form a single unit without complex, expensive tooling. A quality weld, possibly further refined by planishing and inspected by Magnaflux, X-ray or another NDT (Non Destructive Testing) technique, can provide strength and uniformity comparable to the base metal.
With the current stress on conservation of materials, and with constantly rising material and labor costs, spun metal parts offer significant savings. Cast and forged items are usually made in thicker cross section to provide stock for a special contour and to prevent warping, then they are machined down to achieve the desired thickness and contour. The spinning process permits the contour to be formed with little or no added machining, avoiding the need to pay once for the extra metal and then pay again to convert the extra metal into chips.
To prevent warping, a cast part may have to be thicker as the diameter increases. For example, a 20 inch diameter part might be 1/4 inch thick, whereas a 100 inch diameter casting of the same type might have to be 1 inch thick just to hold its shape. The nature of the spinning process avoids this problem, with the result being lower cost and less weight for a spun part - in addition to the usual benefits of grain structure refinements and integrity of metal. Because metal spinning requires less complicated tooling, which can often be produced on the same equipment used to make the final parts, lead times are usually shorter than for castings or forgings.
Much of the accuracy of a spinning job depends on the workmanship and the quality put into the mandrel. Carefully selected maple, specially kiln-dried and expertly put together in properly glued laminations, is the most satisfactory material from which wooden mandrels can be made. Simple spinning jobs may require only one mandrel which is made to conform to the shape desired.
For prototype and limited production quantities using most common materials, simple wooden tooling is sufficient. When more difficult-to-form alloys or longer production runs are contemplated, steel or composition tooling is used. Even in these cases, the necessary mandrels are usually produced by simple turning and machining, and do not require the services of highly skilled, expensive tool and die or pattern makers. Low Cost Tooling is the metal spinners keynote!
A comparison between two identical parts, one made by press forming and the other by spinning, illustrates the step-saving advantages of automatic metal spinning over conventional metal forming for some applications. The standard method of press forming the part requires eight steps, as opposed to only three steps for spinning. An added benefit is that tooling costs for spinning are a fraction of the tooling costs for press forming.
For certain components, the best approach is often a combination of processes. The cross-section of a particular part might be heavy in only one or two areas. In this case, it might be possible to spin a piece of sheet stock to the required contour, then weld on readily available ring or bar stock and finish machine at a substantial saving in production time and material costs. Some changes in design might have to be made with this approach, but the important criterion is that the part fulfills the same function rather than has exactly the same appearance. For instance, a particular forging might have sharp edges and corners, whereas the same part made by spinning would require radii of 1 - 1 1/2 times the metal thickness as a rule of thumb.
Sometimes a casting can be refined by the spinning process. Especially where symmetrical thin cast shells are used, spinning can provide grain structure refinement through mechanical working of the metal, as well as dimensional accuracy and closer control of tolerances.
Spinning is an ideal process to use where other methods cannot be justified because the component shapes are too complicated or the production quantities are too small to amortize the necessary press tooling. Spinning may even be more economical for large quantities where extensive tooling is necessary to produce a given component by another method. This is most certainly true for large diameter parts.
Finally, for some component shapes, the most economical solution may be a combination of spinning and deep drawing. Quite frequently, parts of considerable depth can be produced very economically by using both processes. The preliminary draw operations can be performed on a standard set of draw tools, then the work piece is subsequently spun to final shape on a spinning mandrel.
Aside from special applications, the parent form for spun parts is usually a circular flat blank. To achieve some contours, a blank may be preformed by pressing or by welding developed segments together prior to spinning.
The important thing is to think of spinning as a metal forming process that can be integrated at any stage in the production of an item, rather than only as a total means of making it. Working closely with a metal spinning source will make it easier to achieve the most efficient combination of processes to produce a given component.
As previously pointed out, the spinning process is generally limited to symmetrical shapes of circular cross-section. However, welding and machining techniques can be combined with spinning to achieve a broad variety of configurations. Flowing contours are preferred since sharp corners and extremely small radii cannot be achieved without further machining, if at all. A good rule of thumb for designers is to allow 1 - 1 1/2 times the metal thickness as a minimum radius for spun parts. The cone, hemisphere, and straight-sided cylinder are the basic shapes from which many spun parts are developed.
In press forming operations, the cone is one of the more difficult forms to produce. Metal spinning readily forms it.
The hemispherical shape is more difficult to spin than the cone, but still ideally suited for spinning.
In the spinning of a sharp angle, the metal is exposed to a greater strain and requires more time and skill; consequently, the straight-sided cylinder is not as easily produced by metal spinning as the hemisphere or cone.
In many cases, press forming is used to form cylindrical shapes as a pre-form for subsequent spinning. Many times it is possible to achieve lower cost production by starting with a rolled and welded cylinder and finish with a spinning operation.
For some configurations, the most economical approach is to create a spun "envelope" that encompasses the finished dimensions. The finished part is machined after the spin-forming operation to achieve the cross-section shown.
Rough spinning dimensions are sharp in contrast to starting thickness and to the final machined unit. In this case, where welded construction could not be used, combined spinning and machining provided the most feasible answer to specifications.
Inconel is a registered trademark of Special Metals Corporation
Hastelloy is a registered trademark of Haynes International, Inc.
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