Flow Forming and Shear Forming
Flow Forming has been used for decades as an economical and versatile method of producing seamless, hollow, circular metal parts thanks to its integration with CNC technology.
Like any other process, flow forming has its limitations. However, in certain applications it can provide advantages over other metal forming techniques, such as casting, forging, deep drawing and hydro forming.
What is Flow Forming?
Flow forming is one of several types of metal forming techniques. In spinning, all use the same basic tools to shape metal. These tools include: a lathe-type spinning machine, rollers and a mandrel or chuck.
The work piece, or blank, is a normally flat metal disc that is clamped between the chuck and the machine's tail stock. Rollers are used to apply pressure to the rotating blank forcing it, in successive passes to take the shape of the mandrel which is made to the inside dimensions of the part. Because the metal work hardens during shaping, it must be annealed between passes.
In conventional spinning, there is no intentional thinning of the blank. The starting blank diameter must be considerably larger than that of the finished piece. However, a type of spinning known as shear forming produces high compressive shear forces that reduce blank thickness up to 75 percent during shaping. This process is used to produce straight sided cones.
Shear Forming from a a flat blank produces a cone with a uniform wall thickness that adheres to the sine law. Wall thickness is equal to the thickness of the original blank multiplied by the sine of one-half the included angle at which the cone is formed.
Shear forming is done in a single pass of the rollers, eliminating the need for annealing. The finished diameter is virtually identical to the starting diameter. This reduces material requirements. With reductions in thickness, the need for machining is often reduced. But shear forming is limited to producing conical parts with uniform walls.
Flow forming elevates the process to yet a higher plateau. It differs dramatically from other types of spinning in that, rather than shaping the metal by bending or compressing it, the metal is shaped by being forced to flow along the mandrel. Because of the high forming pressures involved, temperatures at the roller-work piece interface typically are 1000 degrees or higher, causing the metal to soften and flow; hence the name flow forming. The temperature of either the work piece or the mandrel, or both, may be elevated further to facilitate metal movement when the work piece material, size, thickness or configuration requires it.
Flow Forming can be used to produce hemispheres, ogives and other curvilinear surfaces, as well as cylindrical parts, either with uniform or tapered wall thicknesses. Because the process changes the structure of the metal, drastic reductions in wall thickness can be achieved along with very fine accuracy.
When flow forming a contour from a flat blank, wall thickness varies throughout the contour - as the side angle is reduced, so is the wall thickness. Producing a uniform wall thickness requires the use of a pre machined blank and a tracer attachment or a CNC lathe.
CNC Enhances Capabilities
CNC (computer numerical control) technology has increased the uses of flow forming for many of the reasons that made it attractive for machining centers. Closed-loop control of the forming forces yields closer tolerances, consistent repeatability and elimination of human error. Standard CNC software includes features for contouring, spinning speed and axis speed orientation. Computer control permits more complex forming geometries than those available with tracing or copying templates.
Programmed sequences can be edited quickly to improve operating efficiency. Once programming is complete, the lathe can be used economically for either short or long production runs. In addition, CNC lathes can be interfaced with other equipment, including temperature sensing and pressure measuring devices, or even robots to automate loading and unloading procedures.
Virtually any ductile metal may be flow formed, from aluminum or stainless steel, to high-strength, high-temperature alloys. The diameter and depth of formed parts are limited only by the size of the equipment available.
With its broad range of capabilities, Flow Forming provides opportunities for solving a multitude of manufacturing problems. Rather than having to weld or assemble several components to produce a finished part, parts can be manufactured from a single blank and often with a single setup.
Flow Forming lathes also may be equipped to perform supplementary operations such as trimming and machining on the same setup. Lower cost sheet and plate stock often can be used at a fraction of the cost of forgings and castings.
Another built-in cost advantage is that less starting stock is required than in metal removal processes and scrap losses are virtually eliminated. Because you can process smaller, heavier blanks, less tooling is required that with other processes. Since spinning tools are relatively simple, they can be modified with minimum time and cost to accommodate design or material changes. This has made Flow Forming especially useful for developmental prototype work and for limited production runs.
Work hardening of the metal during Flow Forming refines and elongates grain structure of the work piece, resulting in increases in tensile and yield strengths and hardness of 40% or more. Thus stronger, lighter-weight designs often can be produced with less material, and in some cases with the substitution of lower cost materials. By controlling the roller tool configuration and feed rate, surface finishes as fine as 6 micro inches are possible. This can completely eliminate the need for costly secondary finishing operations.
Flow Forming is a versatile technology. It can reduce the number of operations as well as the time and cost required to produce metal parts. Flow Formed parts can also be used with great success in combination with other processes and components to make finished assemblies.