Designing Deep Drawn Metal Parts for Low Cost
Deep drawn metal forming is similar to metal stamping. Parts made from this process are used in a variety of industries including electronics, medical, and consumer products. They are often the basic components of subassemblies for larger pieces of equipment, helping to reduce assembly costs and piece-part costs.
Deep drawing is popular because of its rapid press cycle times. Complex axisymmetric geometries, and certain non-axisymmetric geometries, can be produced in a few operations using relatively non-technical labor. And it is the process of choice for a large class of various geometrical configurations.
Deep drawn metal forming is particularly economical at high volumes, where reduced processing cost significantly lowers piece-part cost. At smaller volumes, the process can be more economical than progressive die stamping due to reduced tool construction costs.
From a functional standpoint, deep drawn metal forming produces high strength and light weight parts as well as geometries unattainable with some other manufacturing processes.
Free blanking is the distinguishing characteristic of this process. In free blanking, a punch forces a flat piece of sheet-metal into a die cavity. The formed part is then transferred to subsequent dies where the parts are forced through increasingly smaller die cavities of varying shapes until the final configuration is attained.
Whereas progressive die stamping removes the finished part from the material strip immediately before ejection from the press, the deep drawing process removes the material from the material strip at the first operation. This separation enables the deep drawing process to attain much larger drawing length-to-diameter ratios than progressive die tooling.
To take full advantage of these deep drawing benefits, parts should be designed with process capabilities in mind. There are many tools intended to assist engineers with component designs, generally classified as tools for design for manufacturability (DFM) or design for assembly (DFA).
However, most DFM systems either require a process knowledge beyond that generally possessed by a designer, or fail to alert the designer to difficult-to-produce features. With expert knowledge of such features, a deep draw job shop can provide redesign suggestions that can result in significant cost reductions Some of those features are discussed below.
Because deep draw tooling is a series of independent punches, dies, and transfer mechanisms, larger part sizes do not necessarily mean increased machining time or cost.
True, bigger parts require more material. As a result, they have a higher material cost. But the widest variety of deep drawn components are small, produced using light tonnage transfer presses. And material costs for these parts contribute less than 10 percent to the overall cost.
Smaller parts require a larger amount of fabrication and setup time. This is partially due to the smaller metal-working tools used in the machining process. Thin punches are much more apt to break than large punches. Thus, the tooling must be carefully adjusted before beginning production.
Also, smaller part sizes require much more fabrication detail. And the necessary finishing (grinding, polishing, etc.) operations of smaller tooling are more delicate than those of larger tooling.
Draw ratio is the length of a part section relative to the diameter of that section. Straight cylindrical parts have a single draw ratio, stepped cylinders have two draw ratios, and multi-stepped cylinders have more than two draw ratios.
Cost is significantly higher when a section with a high draw ratio forms the majority of a part's overall length, or when there is more than one draw ratio. There is a limit to the extent to which certain materials may be drawn before cracks, excessive part wall thinning, or other undesirable effects result. Those limits must be known by the designer.
Large draw ratios imply long, thin forming punches that tend to be very susceptible to breakage. This increases the maintenance costs included in the initial cost estimate.
The simplest part to draw is a straight shell. With this configuration, the tooling fabrication is straightforward. Cost is driven up as transitions are introduced, and as they sharpen. The primary reason for this increase is the additional detail that must be imparted to the punches, dies, and transfer mechanisms.
For example, punch and die surfaces can no longer be straight. They must have angled surfaces that form the steps required. As the step angle increases, these mating surfaces must be handled with greater precision. Further, the transfer mechanisms must have surfaces that mate with the stepped part. In addition, because deep drawing is a progressive process, several operations are often necessary to gradually draw the stepped geometry. Thus, as the number of steps increases, the level of die detail rises and, consequently, the tooling cost.
Another reason for this increase is the higher number of operations to qualify sharp step transitions. Smooth transitions can often be produced while drawing the attribute. However, one or more operations may be necessary to establish the required angular relationship.
The number, location, direction, and type of part features can all affect part cost by increasing engineering, machining, finishing, and set-up times. Each feature requires a separate operation. Increasing the number of features obviously increases the number of operations and, hence, the overall cost. There is also a relationship between part size and feature complexity. A small part with many features requires the features to be concentrated over a very small surface area. This concentration decreases the size of the feature along with the size of the tool necessary to impart that feature. Smaller tools have a propensity to crack in production, or distort during heat treatment. It is often preferable to put features on longer sections. This eliminates the problem of high feature concentration. Thin tool sections are avoided, and the probability off breakage is reduced. Placing features on or near a step transition is also a significant cost contributor because special tooling may be required. The direction of the feature is another cost driver. Transfer press technology allows features to be imparted parallel to the direction of the draw, as well as normal to the draw direction.
Parallel features can be imparted with the motion of the transfer press plungers. The tooling can be inserted directly into the machine and no auxiliary equipment is necessary. Such features include piercing, flanging bottom forming, and slotting.
Normal features include side stabbing, roll forming, and side lettering. They cannot be imparted with the motion of the press plungers. These features, commonly referred to as side action features, require special auxiliary equipment. For example, a side stabbing operation that is not cam actuated, may require an air cylinder to provide the necessary tool motion.
Because normal features are imparted using external equipment, tremendous setup time is required to synchronize the movement of this equipment with that of the transfer press.
It is estimated that one side action feature can cost more than four parallel drawing operations. The type of feature also plays an important role in the deep drawn part cost.
Small/thin features necessitate very thin tooling, which is subject to intense, cyclic shocks that can lead to rapid tool degradation and breakage. Also, prolonged set-up time is needed for this type of tooling. Because these features have an impact on virtually every aspect of build time, they are extremely costly to produce. Folds greater than 90 degrees are difficult to make and add to the cost. An internal bend requires a pin that moves through the die and into the piece, timed 90 degrees out of phase from the press plunger movement. An external bend requires a specially constructed and carefully finished punch. Depending on the dimension of the bend, this punch may even have a spring loaded mechanism. Such tooling significantly affects the machining, set-up, and tryout time necessary to complete the operation. Features on the inner diameter of the part are also difficult to produce. The machining and finishing operations necessary to make the punches for them are quite intensive. Further, the nature of the feature causes rapid tool degradation and results in high maintenance charges.
Protruding features also add significantly to cost. This is primarily due to these features being characterized by extensive plastic deformation normal to the part walls. Dimensional control of this deformation is extremely difficult and tryout time is significant.
Another costly feature is an oriented feature. An example of an oriented feature is a square, pierced hole.Another type is orientation between features, which occurs when two or more distinct features must be positioned relative to each other.
Producing such features requires careful press set-up and tool assembly to ensure that the punch and die components are properly aligned.
After the part material is separated from the strip at the first operation, it is transferred to successive operations by mechanical devices run by the motion of the press. Several part attributes impact the cost of constructing these transfer devices. The primary cost drivers are the number of operations, basic part configuration, draw ratio, dimensional relationships, and feature requirements.
Obviously, the number of operations plays a major role. As the number of operations required for a part increases, the number of transfer mechanisms also increases. The transfer of parts becomes increasingly complex as step transitions are added to the basic part configuration. Additional machining time is required to match the face of the transfer mechanism with the steps on the part. Parts that do not transfer properly can cause massive damage to production tooling, or can significantly impede production efficiency. Also, the unique shapes of transfer mechanisms often require that they be carefully finished by hand.
Tooling cost related to transferring shallow parts is more than that for transferring deep parts. Shallow part mechanisms require much more delicate finishing detail due to the reduced part contact surface area.
Parts requiring rotation, parts with surface finish requirements, and parts having distinct feature orientation also add to transfer complexity and higher cost.
The component design for parts requiring rotation as they are carried from one operation to another is extremely detailed. Their assembly and set-up are also extremely time consuming. Fortunately, such parts are quite rare.
Generally, transfer mechanisms are constructed from ordinary carbon steel and hardened to prevent premature wear. Transfer mechanisms for parts with surface finish requirements customarily have nylon or other nonabrasive materials in the area that actually contacts the part. Besides the increased design and build times associated with this method of construction, the excessive abrasion of the deep drawing process rapidly degrades the surface of these materials, increasing maintenance charges.
Feature orientation can also significantly increase tryout time. When orientation is required from one operation to the next, the transfer mechanisms must be used to position the part correctly so that successive features will be correctly located. This necessitates careful design of the transfer mechanisms along with special tooling components. Several factors must be considered when selecting deep draw part materials, including drawability, hardness, springback, and thickness. Material drawability affects cost in several ways. First, difficult-to-draw material requires carefully finished punches and dies. Great care must be taken to ensure that drawing surfaces are highly polished and free of any surface anomalies that may prevent proper material flow. This makes necessary the selection of morecostly die materials such as carbides or powder metals. Also, less material must be drawn at each operation for less ductile materials, increasing the number of operations. Hardness has a similar effect. Hard materials, such as stainless steel and cold rolled steel, quickly degrade the surface of tool steel, requiring the use of more costly die materials. These materials are much more difficult to machine than conventional steels, adding significantly to tool construction time. Springback is characteristic of hard temper aluminum alloys and stainless steels. These materials are difficult to draw or bend. Cost is increased because the try-out time necessary when working with these materials rises dramatically.
Although material thickness has a large effect on processing price, it also affects tool construction cost. As the material thickness increases, the difficulty of drawing the metal through the dies increases. It becomes necessary to design special sleeves to assist in driving the parts into the dies.
On the other hand, wall thickness much below 0.003 inch is extremely difficult to produce using conventional transfer press technology. These parts require tooling clearances that are difficult to maintain properly using available machining technology.
Drawing parts with wall thickness at this lower limit also requires special holding components to evenly distribute the drawing pressure and to control the flow of the part into the die.
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