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Multistage cold forming: less stock, more product.

Multistage cold forming: Less stock, more product

Cold forming is especially suitable for small, complicated components of less than 45-mm OD, and the materials processed may include carbon steel, alloy steel, stainless steel, brass, and aluminum. Most workpieces of the type discussed here have traditionally been machined from bar stock with the consequent loss of half the material as discarded waste. The greater the waste, the more advantageous becomes the use of cold forming. Also, production is much faster than turning or other metal-cutting operations. The process is considered worthwhile if it can show a 70 percent reduction in total machining time.

Jyoto Machinery Co, Ltd, manufactures cold formers in Japan, and engineers there are engaged in cold forming various components on a trial basis. They are always pleased when cold forming succeeds the first time a new component is tooled up. But often there are surprises. For instance, the complicated component shown in Figure 1 is easy to cold form, while the apparently simple one in the same illustration requires a lot of time and expense. These unexpected happenings can be annoying, and extensive research has been aimed at reducing the number of surprises.

Cold-forming technology

There are almost limitless possibilities for cold forming, a technology that has grown up with the horizontal transfer press. This machine has developed from a simple header for nails and bolts to a multistage former for nuts in the fastener industry. The improved multistage former has produced pieces that are more and more complicated, some of which may be called components rather than fasteners.

The standard multistage cold former has five forming stages and one cutoff stage. These are simultaneously operated in one stroke of the creankshaft. The usual forming stages are: reducing, forward extrusion, backward extrusion, upset, and trimming and piercing.

At the first stage, bar stock continuously supplied from a coil of 1 to 2 tons is automatically cut to length. Cut pieces, which are called blanks or slugs, are suquentially transferred from one stage to the next stage by a multichuck transfer mechanism (Figure 2). The pieces are gradually formed into a final required configuration in one machine. Its production speed is generally 60 to 150 pcs/min, but some special formers can operate at speeds up to 500 pcs/min.

Cost effectiveness

In spite of the advantages of cold forming, most components today are still machined, in which case the outer diameter of the bar stock becomes that of the finished product. Generally, 30 to 60 percent of the raw material is discarded as scrap. For example, the automotive tubular wheel nut in Figure 3 weighs only 52 g when finished, but is machined from bar stock weighing 143 g. If successfully cold formed, the weight of raw material is reduced to 52 g, to entirely eliminate the waste material and machining time.

Cold forming depends upon the formability of raw material. The materials used for dies and tooling recently developed can provide longer operating lives. Furthermore, raw materials developed especially for cold forming have been introduced. The standard value of backward extrusion now becomes 60 percent--in some cases, even 85 percent. The operational life of many punches is 50,000 to 100,000 pieces.

In setting up cold-forming processes one or two points of unstable operation will probably occur. Such cases are often encountered even by experts with many years of experience. Theoretically, on paper, the process should work, but various parameters such as lubrication, tooling wear, material hardness, and increasing forming resistance may prevent operation according to plan. Deep holes, for example, may require surface-treated punches not normally specified.

In quoting a job, if a 70 percent success is predicted, the customer is advised to order. But if it is below 70 percent, Jyoto will set up the operation in its own factory to confirm whether it can be cold formed. During the trial, the engineers try modifying the dies and tooling of changing the processes by trial and error. In this area, the company points out that products that require much money and time for cold forming are as safe from imitation as patented or proprietary ones.

A set of trial dies and tools costs $2000 for bar stock of 8-mm dia, $5000 for 20-mm dia, and $8000 for 30-mm dia. When trying to change to cold forming, managers must have a pioneer's courage, backed up by technical theory, information, knowledge, and data.

The more, the merrier

As the number of stages increases, cold forming becomes easier, not harder. One of the reasons the determination is based on the success probability of 70 percent is that the multistage cold former has several stages (usually five standard). If trouble occurs in one stage, it may be solved simply by interchanging stages as shown in Figure 4. For example, if the simultaneous forward and backward extrusions do not go well in the third stage, the forward extrusion can be left in the third stage, and the backward extrusion can be done in the fourth stage.

Among various arrangements, the engineer can select the most suitable one by trial and error. It is not easy to determine how a complicated configuration can be formed, but if the possibility of modification is taken into consideration when designing the tooling system, it will save money and time.

Contrast this with a single-die vertical press that does not have such allowance. Because the design is likely to be only one pattern, there is not room to modify it if trouble occurs. Thus, vertical-press cold forming may sometimes have to be abandoned.

Forming analysis

Whether cold forming will go well or not is dependent upon forming-process analysis, or analysis of forging-process diagrams. The diagrams detail the exact processes in forming any part, and, if it looks good on paper, then a successful switch to cold forming is likely. In this procedure, the main cost elements such as applied model, material cost, tooling cost, and production quantity must be investigated.

Next, the engineers sketch a rough plan or blank layout, as shown in Figure 5. At the same time, the blank volumes in the respective stages are calculated. Accordingly, the major dimensions and materials of the dies and related tooling are gradually determined. The approximate dimensions are also available, because cold-forming machines permit adjustment of the respective stages after the tooling is mounted. Finally, the formal drawings are drawn and then the dies and tools are manufactured for trial operation. For more info, circle E57.

Photo: The Jyoto five-die cold former Model NS5-10P has special OCC or Quality Control, Center.

Photo: 1. Cold forming sequences for easy-to-develop and hard-to-develop parts. In spite of polygonal holes, the part in the top row at left ran smoothly the first time at 100 pcs/min. It is a chrome-vanadium-steel socket wrench 18-mm dia 26 mm. The chrome-steel chain roller below, left, looks simpler, but it caused problems of short tool life and eccentricity at the third stage, running at 120 pcs/min. Also, it took much time to develop because cracks appeared on the thin wall.

Photo: 2. Cassette-style gripper chucks make quick changeover feasible. The transfer fingers mount side by side to form a multichuck transfer block.

Photo: 3. Automotive tubular wheel nut is, shown machined, left, and cold formed, right. Machining requires a blank weighing 143 g. Cold forming requires a slug of only 52 g. Actual part is only 52 g, so cold forming saves 91 g of wasted stock.

Photo: 4. The use of five forming stations or stages and interchangeable multichuck cassettes allows easy process modifications. Photos in top row show original setup with simultaneous forward and backward extrusion at Stage 3. A modification moved the backward extrusion to Stage 4. Also, piercing at Stage 4 required a sliding die whose short life required slowing the entire machine cycle time. This operation was moved to Stage 3 to solve the problem.

Photo: 5. Plan sheet for flanged component to be run at 100 pcs/min. Production quantity is 100,000 pcs/month or 300,000 pcs/lot max.
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Title Annotation:aerospace vehicle components
Publication:Tooling & Production
Date:Feb 1, 1984
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