There is a limit to the amount of material that can be upset in one blow under controlled conditions. Forming a more complex part in which more metal is moved farther is better accomplished in two stages, or blows, which is why singledie, double-stroke (two punches) headers are more widely used.
Upsets are calculated on the basis of wire “diameters”. The length of the blank is divided by the wire’s diameter. Thus, a 5” blank of 1/2” wire is 10 diameters long; a 10” blank of 1” wire is also 10 diameters long. A rule of thumb is that in a single blow on a solid die header, the maximum amount of wire that can be upset under control is 21/4 diameters. Theoretically we could use approximately 1” of the 5” blank to upset into a fastener head. Most single blow heading, however, is within the 1 to 11/4 diameter range. With a two-blow heading sequence, up to 41/2 diameters can be upset.

Dear Sisi,

建议你以后注意提问方式，不然，如果我不在，你会错过其他朋友的帮助。因时间紧，我简单地翻译一下，供参考！

在受控条件下，一次冲击中能够锻压的材料的数量是受到限制的。为制造一个复杂的工件，需要把很多金属移送到远处，最好分两个阶段或者两次冲压完成，这是广泛使用单模和双冲头（两台冲床）的原因。锻压是根据金属丝的直径为基础计算的。空留长度是由金属丝的直径相除计算出来的。1/2英寸的金属丝要留空5英寸就是10个直径的长度。1英寸的金属丝要留空10英寸也是10个直径的长度。根据经验，在实心底模上单击，在受控条件下，所能锻压的金属丝最大数量为21/4个直径。从理论上讲，我们大约可以锻压5"空留长度中的 1"(5个直径长度)(到扣件的头部。然而，大多数单冲击机的头部锻压量是在1到11/4个直径长的范围内。 用双击机序列，锻压量可达到41/2 个直径长度。