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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.
ManualSpinning 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.
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Table I. Tolerances for metal spun parts which
are typically found practical for most applications
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Diameter of Finished Part
Up to 24" Diameter
(600 mm)
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Commercial Applications
+/- 0.015" to 0.031"
(0.38 mm to 0.79 mm)
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Special Applications
+/- 0.001" to 0.005"
(0.02 mm to 0.13 mm)
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25" to 36" Diameter
(600 mm to 900 mm)
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+/-
0.031" to 0.047"
(0.79 mm to 1.19 mm)
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+/- 0.005" to 0.015"
(0.13 mm to 0.38 mm)
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37" to 48" Diameter
(900 mm to 1200 mm)
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+/- 0.047" to 0.062"
(1.19 mm to 1.57 mm)
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+/- 0.010" to 0.030"
(0.25 mm to 0.76 mm)
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49" to 72" Diameter
(1200 mm to 1800 mm)
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+/- 0.062" to 0.094"
(1.57 mm to 2.39 mm)
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+/- 0.015" to 0.045"
(0.38 mm to 1.14 mm)
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73" to 96" Diameter
(1800 mm to 2400 mm)
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+/- 0.094" to 0.125"
(2.39 mm to 3.17 mm)
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+/- 0.020" to 0.060"
(0.15 mm to 1.52 mm)
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97" to 120" Diameter
(2400 mm to 3000 mm)
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+/- 0.125" to 0.156"
(3.17 mm to 3.96 mm)
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+/- 0.025" to 0.090"
(0.64 mm to 2.29 mm)
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Go to the Design Guidelines Overview
Go to the Glossary
Excerpt taken from Design Guidelines for Metal Stampings and Fabrications -- 2nd Edition copyright © 1995 Precision Metalforming Association
Purchase the new Third Edition of Design Guidelines for Metal Stampings and Fabrications copyright © 2004 Precision Metalforming Association at Marketplace today!
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