May 10, 2011
It is almost unrealistic to expect some stamped part part features, such as hole position and flatness, to remain within a small tolerancing zone.
Editor's Note: This is Part II of a two-part article on part tolerancing. Part I, which appeared in the March/April 2011 issue, discussed the effects of variations in the incoming metal and the importance of consistent strain levels.
It is almost unrealistic to expect some part features to remain within a small tolerancing zone.
Hole position tolerance is a big one. In fact, I am very confident that as I am writing this article, within a 500-mile radius at least 500 QC inspectors are complaining about the dies being unable to hold a certain positional tolerance on the pierced holes.
To understand why it is sometimes very difficult to achieve and maintain certain positional tolerances, you need to understand the entire process and all of its variables. One of the least-understood concepts is the catalyst for this question: If all of the holes that have a small tolerance are pierced in the part in a single operation, why don't they maintain the same positional relationship with each other?
For those who don't understand the variables, it would seem logical that two holes exactly 200 mm apart in the die would measure 200 mm apart in the part. However, this is not true, especially if the parts have shape to them.
Remember, the incoming material is not perfectly consistent. This inconsistency in material, as well as other factors, results in different springback values for each part.
In a nutshell, each part made has a slightly different shape. While this difference might be well within the form or profile of the part's surface tolerance, keep in mind that as a rule, positional tolerances of features such as pierced holes typically are held to much tighter tolerances.
For the holes to be pierced in the part, the part must nest or fit onto a punch that matches the profile or the shape of the part. This punch usually is cut to the exact profile or shape of the designed part. Even though the punch is cut very accurately, it's unlikely the part will fit perfectly onto the punch—and every part will fit differently on the punch.
A spring- or nitrogen-loaded pressure pad, cut to fit the designed shape of the part, is used to hold the part tightly to the punch. When this pad contacts the part, it forces the part to take the ideal shape by pushing, twisting, and bending part surfaces back to their intended geometry (see Figure 1).
When the pad is fully engaged, the pierce holes get put in the part in the desired location. However, once the pad is removed, the part will return to the original shape that came out of the forming die, and position of the pierced holes will move with the surfaces of the part.
So why not fit the punch and the pad to the part that is coming out of the forming die? While this is sometimes attempted, the inconsistency of the formed part makes it hard to determine just what the shape should be.
Basically, when you specify a tight positional tolerance on two holes distanced far apart on a formed part, you are asking for the same tolerance on the shape of the part. Good luck with that one!
Of all the geometric tolerances that are difficult to achieve, flatness is one of the hardest. Many people believe that dies are responsible for part flatness. While dies sometimes can improve or diminish the flatness of a part, their main purpose is to retain a given flatness in the incoming material.
Part flatness is more a product of the coil straightener and tension leveler, and it is affected by the severity of the steel cutting deformation, the sheet material's mechanical properties, incoming material or coil flatness, metal thickness, and residual stress created in prior operations.
For example, specifying a flatness tolerance of 0.002 in. total indicator reading (TIR) over the surface of a 0.010-in.-thick stainless steel part that is 2 by 4 in. is insanity. Keep in mind that the coil this part is made from is wound over a mandrel and contains coil set. The amount of coil set changes with respect to how tightly the coil is wound. To remove the coil set, the material must be strained using a tension leveler and straightener.
To achieve a critical flatness characteristic, most die designers try to keep the part flat during cutting and also employ methods such as pressure blanking, compound blanking, cut-and-carry, fineblanking, and GRIP®flow.
Achieving certain tolerances can be very difficult. Only through a comprehensive understanding of the stamping process can you dimension and tolerance your parts accurately.
Until next time...Best of luck!
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