December 7, 2004
Of all the geometric tolerances that are difficult to achieve, flatness is one of the hardest. Most stampers probably would much rather try to make a complex-shaped, thin, high-strength steel deep-drawn part than hold a small-tolerance part requiring a tight flatness.
Achieving part flatness is a function of many controlling factors:
To achieve a critical flatness characteristic, most die designers try to keep the part flat during the cutting operation, using methods such as pressure blanking, compound blanking, and cut and carry, as well as specialty stamping operations like fineblanking and Grip Flow®.
There is no doubt that retaining part flatness is the best way to achieve a respectable part flatness characteristic, but processing and financial constraints often make this impossible.
So how can you regain flatness in parts that have lost it? You can choose from a few methods that, while they may seem like industrial Band-Aid®s, have been proven to work.
To understand why these methods work, you first must understand how the flatness characteristic is lost.
Cutting. The cutting and piercing processes often introduce a lot of stress into the product. You might think that cutting does not actually deform the metal, but before cutting takes place, the metal first undergoes plastic deformation.
The selected cutting clearance affects the amount of internal stress created in the part. Depending on the metal type and clearance between the upper and lower cutting steels, this stress can be significant enough to promote part distortion. Severe cutting often results in large stressed areas within the product. The result is uneven springback, and flatness is gone.
Bending. Bending the metal or permanently deforming it in any way introduces stress into the product. To understand the effect that simple straight-line bending has on stress levels, consider what happens during the bending process.
The inside bend radius is in compression, and the outside radius is in tension. Excess metal in compression pushes outward parallel with the bend line axis, and the metal in tension pulls in the opposite direction. Figure 1shows bent Silly Putty® to demonstrate this concept.
Note how the "metal" in Figure 1 is pushed outward on the inside bend radius and sucked inward on the outside radius. Now imagine this happening along the entire length of a U-formed part with a short flange length. The result is upward bowing of the part. If the flange is long enough, it will be ridged enough to retain the part flatness; however, a short flange will not give the part its necessary stiffness.
Hit It Harder. Trying to achieve flatness by using a flat "spank" or flattening die may be somewhat successful with heavy, thick, soft metals, but it usually will prove useless on a thin, high-strength part unless the press has enough tonnage to coin the metal, induce outward plastic flow, and reduce the metal's thickness. Even with such a press, the surface area that can be coined is limited.
Unless the metal is soft and is more than 0.125 inch thick, the spank method should be avoided.
Overbend It in the Opposite Direction. Overbending the material in the opposite direction can improve flatness, but the process of determining how much overbend is required can take an extensive amount of time. Even if the correct amount of overbend can be incorporated into the die, slight changes in material properties can change springback levels, resulting in a loss of part flatness.
Stippling the blank–a process that addresses internal stress after it has already been created–involves coining a crosshatched or alternate pattern into one or both surfaces of the part after all cutting–and extensive metal deformation–is done./td>
Trapped stress is the result of permanent metal deformation, and it is the fundamental reason for a loss of flatness.
Trying to stop the stress from being created during the cutting or bending operation would be an attempt to defy physics. Since it can't be stopped, the alternatives are either to break it up or counterstress the metal. This involves breaking up the stress and placing the areas in tension in compression. Try to balance the strain levels so that they are more evenly distributed or broken up into manageable sections.
Stippling the Blank. Stippling the blank—a process that addresses internal stress after it has already been created—involves coining a crosshatched or alternate pattern into one or both surfaces of the part after all cutting—and extensive metal deformation—is done (see Figure 2). The stipple pattern breaks up internal part stress and destroys the part memory, allowing it to be rehit flat.
The depth of the stipple depends on metal thickness, the mechanical properties of the material, and the stress that was previously induced. Experimentation may be necessary to achieve the desired results.
By coining or placing a small dent in the outside radius of the bend, you can break up and often reverse the tensional stress that was created during deformation.
Stippling the Outside Bend Radius. This method works well for the bowing-up problem shown in Figure 1.
By coining or placing a small dent in the outside radius of the bend, you can break up and often reverse the tensional stress that was created during deformation (see Figure 3). This is because the coining action pushes metal outward and changes the stress from tension to compression. The depth of the dent is a matter of experimentation.
The best way to achieve flatness is to retain flatness, if retention is an option. Take a good look at why the parts are losing their flatness, and address the problem by making good data-based decisions. These basic principles have been used for years, but the key is to understand why they work.
Until next time ... Best of luck!
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