Chapter 9 - Metal Spinning

Metal spinning is a forming process which produces hollow parts that are typically circular in cross-section. The basic spinning process starts with a flat metal disc (blank) which rotates on a lathe. This rotating blank is pressed against a tool (mandrel, chuck) which duplicates the interior of the part. This pushing action over the tool results in a formed part.

The basic metal spun shapes are the hemisphere, cone, cylindrical shell, and venturi as well as others, depicted in Figure 1 below.

Figure 1. Typical shapes produced by the metal spinning process. Products produced by spinning are utilized in many industries: aerospace, agricultural, computer, ventilation, lighting, marine, food service, automotive, etc.

Metal spinning used to be associated with prototypes and low volume production only. However, with the introduction of automatic lathes, spinning is now a cost effective option for both medium and high volume production. The relatively inexpensive tooling price for spinning still makes this forming method a cost effective one for fabricating prototypes.

The Metal Spinning Process

The diagram below shows a basic setup for a horizontal spinning lathe. The tool (mandrel, chuck) is mounted to the headstock of the lathe. A follower block (tail block) is mounted to the tailstock. A circular blank is then clamped to the tool by advancing the follower. The tool rest and pin provide a support system for the lever arms, applies pressure to the blank via a roller or other forming tool. The movement of the roller across the blank is called a pass. A series of passes, which ultimately forms the completed part, is achieved by repositioning the lever arms incrementally.

Figure 2. Manual spinning. The sequential development of the finished part in several passes is shown.

Metal Spinning Equipment Metal spinning lathes can be grouped into three broad categories: manual, power assisted, and automatic. Each paragraph below provides a brief outline of the equipment and typical applications.

• Manual Spinning lathes typically accommodate blanks ranging from 0.25 in. (6.35 mm) to 72 in. (1.8 m) in diameter. Larger manual lathes are available to accommodate blanks of up to 160 in. (4.1 m) in diameter.
• Power Assisted Spinning is manual spinning augmented by hydraulic cylinders which provide additional force on the workpiece rather than only human force. This additional power is useful when forming strong metals such as stainless steel and exotic alloys.
• Automatic Spinning is based on Computer Numerical Control (CNC) and Programmable Numerical Control (PNC). For PNC, the lathe is programmed through a "teach" mode where the first workpiece is manually spun while the computer records the movements. Once this initial workpiece has been spun, the lathe is put into "playback" mode for production. Blanks are positioned in the lathe by the operator and the playback cycle is initiated. The workpiece is then formed automatically, exactly duplicating the movements of the manual spinning.

  Automatic spinning is a cost effective option for medium and high volume production since it offers high repeatability and fast cycle times.

Basic Operations of Metal Spinning

In forming a part by spinning, a combination of processes may be utilized to complete a part. These processes include: preforms, conventional spinning shear spinning, edge treatment (trimming, beading, curling, hemming) as well as secondary operations.

• Preforms. A preform is a partially formed part which is used to increase the efficiency of the next forming process. Preforms may be in the shape of cylindrical shells, cones, and more. These preforms can be achieved through spinning or other forming operations such as drawing.
• Conventional Spinning. The spinning process usually involves a series of passes to complete the formed part. During each pass the metal is stretched thus thinning out the material. This thin-out characteristic is typical of the conventional spinning process. If necessary thin-out can be minimized.
• Shear Spinning. is a variation of conventional spinning. Shear spinning refers to the formation of a part in just one pass. This process allows for an accurate prediction of finished material thickness. Shear spinning is typically associated with conical and cylindrical shapes.
• Edge Treatment. The edges of a spun part can be finished in many ways. Parts can be trimmed for a straight edge, hemmed for a folded edge, or beaded for a rolled or curled edge. These edge treatments can be performed in a spinning lathe or on a separate machine.
• Secondary Operations. Often spun parts require secondary operations. This may range from piercing holes, to heat treating, to powder coating, to laser cutting. Many metal spinning companies perform these operations in-house or will subcontract them for you.

Tooling Composition

Tooling for spinning can be fabricated from various materials. Many factors determine which material is most appropriate: production volume, finish, tolerancing, metal, etc. The three basic alternatives are wood, plastic and steel.

• Wood tools are made from a variety of materials: maple, fine grain particle board, etc. Wood tools are typically used for parts where tolerancing and/or finish is not critical.
• Plastic. A paper-based plastic is also used for tooling. As compared to wood, plastic tooling is generally more durable, provides a superior surface finish and will maintain closer tolerances.
• Steel tools are mainly used to form shapes fabricated from stainless steel or other strong metals. Due to the relatively hard, smooth surface of steel tools, parts spun on steel tooling can achieve a superior surface finish and maintain close tolerancing. Steel tools can be fabricated from both mild steel and tool steel. The life of the tool can be increased through heat treating.

Tooling Design

Spinning tooling can be grouped into three broad categories: male, female, and collapsible

• Male Tool. A male tool is the most common type of spinning tool. It duplicates the interior dimensions of the part. See Figure3.

  Figure 3. Relationship of male tool, finished part and starting blank

• Female Tool. A female tool conforms to the exterior dimensions of the part. This type of tool is often used to form flanges and returns.

• Collapsible Tool (Segmented). A collapsible tool is required when the diameter of the part becomes smaller as the part is formed. If a male tool is used this smaller diameter or neck prevents the part from being removed from the tool; therefore, a collapsible tool is required. A collapsible tool has a removable center core which keeps the perimeter pieces in place during spinning. After the part is spun, the core is removed which permits access to the perimeter pieces. Note that the use of a collapsible tool involves assembling and disassembling the tool for each piece spun.

Advantages and Limitations of Metal Spinning

The metal spinning process has both advantages and limitations.

Advantages:

• Spinning tooling is relatively inexpensive due to its simplicity and composition.
• This simplicity translates into short lead times for new parts
• Design changes can usually be made at a minimum of expense again due to the inexpensive nature of the tooling.
• The factors above combine to make spinning ideal for prototypes.
• Spinning is typically a cold working process; therefore, spinning increases the tensile strength of the material.
• The spinning process can accommodate very large parts in excess of 120 in. (3.0 m) in diameter as well as parts requiring thick material such as 0.500 in. (12.7 mm) mild steel.

Limitations:

• Extreme tolerancing requirements may dictate the use of secondary operations.
• Manual spinning is more labor intensive than automatic spinning or other forming processes such as drawing.
• The uniformity of a manually spun part is closely associated with the skill of the operator.

Pre-Design Basics

The following design guidelines for metal spinning can affect quality and cost.

• It is preferable to specify the inside diameter (I.D.) and associated tolerance since the outside diameter will vary due to material thin-out. If necessary, a specific outside diameter (O.D.) can be maintained.
• If uniform wall thickness is required, identify the portion of the part which is affected. Additional operations may be required to achieve this uniform wall thickness.
• Corner radii should be specified at 2 to 3 times material thickness. Tighter radii can be achieved on thicker material and through secondary operations.
• If concentricity is critical, specify the total indicated runout (TIR) and indicate if this applies in the restrained or unrestrained condition of the part.
• Working closely with your spinning supplier during the design phase may significantly improve formability and reduce cost.
• If tight tolerancing is required in a small area only, specify that area. The erroneous assumption that the tight tolerancing applies to the entire part will dramatically increase the price of the part.
• Surface finish is affected by the material, thickness, tool condition, forming speed, and other factors. If the specified surface finish cannot be achieved through spinning, secondary operations can be performed.
• Any formable metal can be spun. The stronger the material the more difficult the spinning.

Tolerances

The tolerancing guidelines for spinning are shown in Table I.

Excerpt taken from Design Guidelines for Metal Stampings and Fabrications -- 2nd Edition copyright © 1995 Precision Metalforming Association

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