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It is often said that metal spinning is one of the oldest forms of metal forming.
Yes and no. That is true for manual, or hand, spinning. But that traditional form has been significantly enhanced over the years, first with power assisted spinning, and then by automatic CNC spinning. Spinning is no longer limited to low volume applications and prototypes. Power assisted and automatic spinning economically produce much higher volumes.
Metal spinning is a seamless method to form cones, cylinders, hemispheres, and similar shapes. Some advantages of the process include low production costs for many applications, low tooling costs, short lead times, close tolerances, smooth finishes, and improved mechanical properties, such as tensile strength.
Bob Stratton, President of Spun Metals Inc. in Phoenix, Arizona, pointed out, "The metal spinning process actually improves the raw material metallurgically by realigning grain structures. Tensile strength can be improved, allowing thinner materials to be used. Part integrity may be better with spinning than with other processes."
Additional benefits were pointed out by Chris Roemlein of Alta Industries, Inc. of Fall River, Massachusetts. Chris says one of the benefits of spinning is, "Parts that may cost thousands to tool for stamping can be produced for several hundred with spinning."
Other benefits cited by Chris include that large part diameters can be produced for aircraft, communications, tank heads, or turbines. Also, a high degree of surface finish is achievable for parts such as lighting reflectors. Shapes made by spinning are well balanced, important for parts such as reels, pulleys, or wheel rims. And parts with complex radii, multiple steps, returns, beads, and flanges are all well suited to the spinning process.
Alta Industries is well positioned to understand the benefits of metal spinning. The company specializes in the process, manually spinning blanks up to 56 inches in diameter. But the company also does CNC automatic spinning, vertical shear forming, stamping, deep drawing, machining, welding/fabrication, and finishing, including polishing, automatic buffing, satin finish and painting.
Metal spinning is often used in conjunction with other metal forming techniques. For example, preliminary forming may be done by deep drawing, with spinning used to add intricate contours or shapes such as flanges, rolled rims, cups, cones, and double-curved surfaces. Or, it can add finishing touches such as trimming, beading, and necking. Examples of parts made by this method include light reflectors, tank ends, covers, housings, shields, and components for musical instruments. The technique is used extensively to produce aircraft and aerospace parts.
Any metal soft enough to be cold-formed can be spun. Aluminum, popular because of its ductility, can be spun in thicknesses of an inch. Though most metal is spun without heating, some hard-to-form metals, such as beryllium and magnesium, are heated to increase their ductility or to reduce their strength, enabling a thicker piece of metal to be spun.
These pieces of metal start out as circular metal blanks. The diameter of the metal blank can be increased as the thickness of the material is reduced. For example, the thickness of low-carbon steel that can be spun without mechanical assistance is about one-eighth inch. At that thickness, the diameter of the blank can be up to about six feet.
Similarly, metal thickness can be increased as the ductility of the metal is increased, or its strength decreases.
Besides thickness and ductility, the size of the blank can be limited by the capability of a spinner's equipment. The easiest way to determine if your part can be spun efficiently is to consult with a metal spinning job shop.
Fast, Low Cost Tooling
Spinning is performed on a lathe. A mandrel, or chuck, corresponding to either the inner or outer shape of the part, is attached to the end of the lathe and the metal blank is fastened over the chuck. Mandrels conforming to the inner shape are used to form the body of a part. Mandrels conforming to the outer shape are used to form flanges or finished edges on parts that may have been produced by spinning or another process.
The simplicity of producing mandrels for spinning is a key reason for short production lead-times and low tooling costs. It also permits design changes to be made quickly and at relatively low cost. Spun Metals' Bob Stratton says, "Typical lead times for finished parts are four to six weeks. This is generally significantly shorter than lead times for just the tooling involved with stamping."
Another advantage of the tooling was pointed out by Alta Industries' Roemlein, pointing out that different metal types and thicknesses can all be formed over the same mandrel.
In traditional manual spinning, mandrels are usually made from wood, or wood combined with metal. In today's high volume parts on power assisted or automatic spinning equipment, most mandrels are machined from metal. Wood remains a low cost method to make a mandrel, and is ideal for prototypes and low volume requirements. Also, wood is still frequently used to spin aluminum parts.
Some shapes need several spinning operations with a series of mandrels, each used to form the shape a bit more. These mandrels are referred to as breakdown mandrels. Mandrels can also be made to collapse. These are used to spin parts that have a turned shape smaller than the largest diameter of the workpiece. A collapsible mandrel consists of several pieces held together by one key section. When the key section is removed, the mandrel collapses and the remaining sections are individually removed from the workpiece.
In some cases, such as on prototypes with re-entrant sections, mandrels can be made with a low melting-point material. After spinning, the part and mandrel are heated and the mandrel melts away.
Various sizes of standard horizontal spinning lathes are available to spin blanks ranging from one quarter inch to 140 inches in diameter. There are even some special lathes that can spin blanks of 182 inches in diameter.
Rotation and Force
Metal spinning combines rotation and force to shape metal. In traditional manual spinning, while a lubricated metal blank is being spun by the lathe, an operator presses a tool against the surface of the metal, forcing the metal to conform to the shape of the mandrel. As the blank rotates under the tool, it gradually conforms to the shape of the mandrel.
When the blank has been spun completely to the shape of the mandrel, it is removed as a 'shell' or preform. The process is then repeated with the next mandrel breakdown, if needed.
Surface tools for traditional manual spinning came in several shapes, depending on the particular aspect of the part that was to be formed. They may have had a round nose or a ball nose, or they may have been shaped like a tongue. Sharp edged tools were used for trimming.
When scissors-type levers were developed to increase manual force, rollers started replacing those tools. In today's power assisted and automatic spinning, the surface tools are almost always rollers with different edge shapes.
Though manual spinning is cost effective for low volume and prototype parts, it is very labor intensive, with the uniformity of the spun part depending a great deal on the skill of the operator. That has been one of the disadvantages of manual spinning--it depends so much on the skill of the operator, and they are getting harder to come by.
Over time, various devices, such as scissors-levers or hydraulic pistons, have been developed to increase the control and the force an operator can apply to the blank. Today, spinning can accommodate parts of much thicker material, such as three-quarter inch thick mild steel.
Though manual spinning is still used in many shops, power assisted spinning and CNC automatic spinning are now widely available. Power assistance refers to the use of hydraulic pistons to increase the operator's force. But the greatest assistance is provided by the latest CNC equipment, and is often referred to as automatic spinning.
There are several advantages of automatic spinning. One is that it removes the uncertainties of operator skill and operator-to-operator variations, making spinning highly repeatable and accurate. After a CNC machine has been programmed or 'trained,' it automatically 'plays back' the instructions, hydraulically applying predetermined forces for predetermined lengths of time on precise areas of the blank, creating identical parts. Such machines can automatically shape the part, trim or otherwise finish the edges, and eject the finished part.
Not all CNC playback spinning lathes are purchased new-some are retrofits. For example, Hi-Craft Metal Products in Gardena, California recently retrofitted a metal spinning lathe with automated controls to handle their specific requirements. This CNC lathe is programmed using a computer program, a PC, and a mouse (the computer kind-not the four legged kind).
The operator can program the spinning lathe either of two ways. He can use the mouse to draw spinning passes on a computerized diagram of the part, or he can plot coordinates into the specialized software, which will generate the spinning passes necessary to make the part. The operator then runs the program to form a part, easily making adjustments to the program necessary to fine tune the part.
One of the advantages of automated spinning is the program can be saved on a floppy disk and stored for future runs of the same part. Another advantage is that it can be easily modified to produce similar parts. And a highly skilled spinner is not required to develop the program--though the programmer must be knowledgeable about the intricacies of metal spinning, as well as the software.
Usually, large diameter parts are not produced in large volume runs. Therefore, the cost of purchasing new equipment or retrofitting older equipment to produce large parts is difficult to justify. For that reason, automation is most often applied to smaller parts.
The increased level of automation is one of the reasons that metal spinning has become even more economical. Though the metal spinning cycle can be relatively long, the level of automation in the latest equipment frees the operator to do other things while the machine tool is forming a part. Consequently, spinning lathes can be surrounded by secondary equipment in a 'work center' approach, common among spinners to improve the cost effectiveness of spinning.
Other considerations include quantity and shape. As quantities increase, stamping or deep drawing can be more cost effective than spinning, especially if the stamping tools are not too complex. As part diameters increase, metal spinning is often the more economical choice.
Hialeah Metal Spinning, Inc. of Hialeah, Florida does both metal spinning and deep drawing. The company has manual, semi-automatic, and fully automatic spinning lathes to address both prototype and high volume production requirements. Hialeah also has both mechanical and hydraulic presses for deep drawing and stamping.
Hialeah Metal Spinning often uses combination forming--switching between spinning and drawing to maximize the benefits and cost effectiveness of each forming method. As an example, the company spins a large stainless steel part to add strength, follows with a drawing operation that gives the part its detailed profile and final height, the finishes with spinning to bead the edges.
With this versatility, Hialeah is well qualified to advise customers on the best option--spinning or drawing. According to Karla Aaron, President of Hialeah Metal Spinning, the company typically uses production volume as the factor that determines whether to use spinning or drawing. However, part requirements sometimes dictates a specific method. Aaron says, "For very close tolerances and extreme repeatability, drawing is a good choice. For a very cost sensitive part, spinning may be the best option in spite of volume since it uses less material than drawing, adds strength to the part, and typically tools up for less."
In a process similar to conventional metal spinning, called shear forming, there is a controlled reduction in blank thickness. In this process, one pass of the rollers moves a portion of the blank's thickness lengthwise down the mandrel, actually elongating the metallic structure of the blank. It's a good idea to consult with a metal spinning job shop if a design calls for a conical or hemispherical component with varying wall thickness. Also, the Precision Metalforming Association in Richmond Heights, Ohio can offer helpful guidelines when designing a part that may be produced by spinning techniques.
Many shear forming machines have two rollers, but others have three or more. On machines with two rollers, stepped cones and compound conical parts with two or more angles can be shear-formed in several passes, using one or both rollers to form each conical wall on stepped or separate mandrels. Sometimes, one roller is set to spin the first cone and the second roller is used to spin the second cone. This method, however, requires a stepped mandrel. In another method, one or both rollers traverse the entire length of the part over a stepped mandrel.
Tube spinning is used to reduce the wall thickness or increase the length of tubes without changing their inside diameters. There is also an improvement in strength due to plastic deformation. The process has been used to reduce wall thicknesses up to 90 percent and increase length up to 800 percent without annealing between passes. This method follows a purely volumetric rule. Limitations depend upon the amount of reduction the metal can withstand without annealing, the percentage reduction necessary to make the metal flow, and the force of the machine.
Shear forming and tube spinning are just two examples of how the metal spinning craft has been enhanced over the years. Metal spinning, like most metal forming processes, has turned toward advanced technology to improve productivity and capability. But unlike other processes, many metal spinners still combine the equipment and craftsmanship of yesteryear with today's increased high volume efficiency and flexibility.
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