V. Replacing Brute Force

As larger and larger components of stronger and stronger metals are in growing demand for forming processes at room temperature, even the largest available equipment of conventional design, for bulk deformation by static loading, is no match for the job. Finesse must replace brute force. Some of the measures are old and still usable today. Inertia forces may replace static loading as in hammering. Today the repeated beatings may be applied through ultrasonic exitations of the tooling. Another concept calls for the application of the load locally, to affect minute changes at a spot, and transversing the spot throughout the entire workpiece, several times over, if needed. One class of processes is called rotary forming, and spinning is the example presented here. The local load is much smaller and therefore so is the equipment. Usually the processing time per part is higher, but large components are not produced in as many numbers as smaller ones. The mechanics of the process of spinning are similar to those of pottery-making a clay vase. Only the materials of both workpieces and tools are different.

A mandrel shaped to conform to the interior of the product is clamped to the rotating head of a spinning machine (See Fig. 33). For ages a stick of hard wood, with or without a copper tip, has been manipulated against the blank, back and forth, progressively laying the thin gauge blank against the mandrel. Holes, at intervals on the bed of the machine, are pegged to provide

 

FIG. 33. Conventional manual spinning.

support and leverage for the operator. When necessary the leverage mechanism is made more complex. The operator may strap himself to the frame of the machine, like a window washer on a tall building. Two operators may assist one another, and heavier gauges of harder material can be worked by heating the workpiece. The friction between the tip of the stick and the workpiece is large. In later models, especially in power-mechanized spinning, the solid tip of the stick is replaced by a roller to minimize friction losses, replacing sliding by rolling friction.

The proper manipulation to achieve good results was an art gained by experience. Because of springback (which is a major factor in spinning with manual tool manipulations) and the closeness of the operator to the operation with no safeguards, all "old-timers" in spinning carry scars caused by close encounters with the edge of the rotating blank.

Cooling pots and frying pans of aluminum and copper and like products were made by conventional manual spinning. They still are in some developing and under developed regions of the world. Precision of size and wall thickness and repeatability are not critical for such products and cannot be achieved by hand spinning. The process is slow, labor-intensive, and unsuitable for mass production. But the tooling can be made simple and inexpensive.

Occasionally prototype development on a limited production basis is produced by hand spinning, even today, and even in advanced technological societies. Changes are easy and inexpensive to introduce. Some experienced spinning machine operators can produce vessels of complex shapes without the support or backing of the mandrel to define the desired shape. Changes in shape and narrower sections can be improvised on the spot.

Next, spinning of cones and vessels will be presented, and then tube spinning. When mechanized, spinning is more economical for the production of small numbers of pieces than deep drawing because of the low setup time and costs of spinning. The pattern, for example, can often be made of wood or aluminum, thus saving tooling expense.

For the last thirty years the trend has been toward mass production with power spinning. The advantages of replacing manpower by mechanical power are the same for spinning as for any other industrial process. However, mechanical power requires a control system, and controls require prediction of the forces and motions to which the machine has to be set. In spinning operations, as they are performed today, this means the following things: (1) the tool is usually no longer manipulated back and forth, but performs the deformation in one pass; (2) the rotational speed, the feed, and the head-in pressure have to be fixed and preset before spinning is started.

Most recently, numerically controlled (NC) machines have been offered. The following advantages are claimed:

  1. They have the ability to produce and execute extremely complex programs to make extremely complex parts, impossible hitherto.
  2. They have the ability to make minor program adjustments during initial runs for optimization of machine and material utilization.
  3. They have the ability to make short runs and store programs for reuse.
  4. There is a high degree of repeatability.

In the early days of mechanized spinning, the manual application of the tool to the workpiece was replaced by two hydraulic piston and cylinder assemblies pushing the tool carriage against the workpiece and in the feed direction. The path of the tool was dictated directly by a template. With the further advance of hydraulic control systems, the guidance of the tools was delegated to a closed-loop feedback system whereby a stylus that followed the template activated, through valves, the pistons that controlled the motion of the tools. The template could be scaled up or down, and in more sophisticated methods a drawing could replace the template. The spinning could be performed by several passes of the tool, changing the position of the template, or the drawing, for each pass.

With the introduction of the stylus control, manual manipulation of the tool could be reintroduced. While the power is supplied through the hydraulic pistons, the positioning of the tool can be delegated back to manual manipulation and the tool can be applied repeatedly as shown in Fig. 33. Brute force, applied by the machine, is delicately controlled manually. While this option is feasible, it is infrequently applied, because the last generation of skilled manual operators is fast disappearing. In Fig. 34b, for power spinning of a cone, the tool(s) advance only once, in a motion parallel to the surface of the mandrel from top to bottom, pushing the flange ahead downwards while laying the workpiece under the roller flat onto the mandrel.

FIG. 34. Multiple spinning heads.

 

The strong head-on pressure applied during power spinning with a single tool causes high bending moments on the mandrel and asymmetric loads on the machine. Mechanized spinning machines are designed with two symmetric tool heads (see Fig. 34) and sometimes, in vertical tube-spinning machines, with three heads. The individual heads are usually arranged in tandem. For example, in tube spinning (Avitzur, 1983) one head may take half of the reduction in thickness while the second head follows by several revolutions and takes the second half of the reduction.

In tube spinning (also called "cylindrical flow forming"), a heavy-gauge tube is mounted over a rotating mandrel. The tool holder, with the roller pressing against the tube, advances slowly in the axial direction as the workpiece rotates under the tool. The wall thickness decreases locally under the pressure of the tool while the tool gradually advances through the entire tube surface. During tube spinning the thinning of the wall results in elongation of the tube in the axial direction with no change in the nominal diameter.

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