Heading Hints: About Cold Heading (part 3)
This is the third in a six-part series on heading basics. This installment discusses controlled upsetting, extruding and contained (trapped) extrusion.
Carpenter Technology Corporation, whose Specialty Alloys Operations produces hundreds of stainless steels and specialty alloys, published the booklet "Heading Hints – A Guide to Cold Forming Specialty Alloys" to suggest proper cold-forming fabrication techniques. Request a free copy of "Heading Hints."
|Figure 1 - This simplified diagram shows how diameters of wire are calculated to determine upsetting limits. |
|Figure 2 - Extrusion is either open (left) in which the blank is forced under pressure into a smaller diameter die hole, or it is trapped where the blank is totally confined within the die prior to extrusion (right). Trapped extrusion permits reductions in area of up to 75%, while open extrusion allows only 30% area reduction in one blow. |
|Figure 3 - These are the elements of the basic ground rules for open extrusion.|
|Figure 4 - This diagram illustrates the radial extrusion die and its key dimensions.|
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 single-die, double-stroke (two punches) headers are more widely used.
Upsets are calculated on the basis of wire "diameters" (Figure 1). The length of the blank is divided by the wire's diameter. Thus, a 5'' blank of ½'' 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 2-¼ 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 1-¼ diameter range. With a two-blow heading sequence, up to 4-½ diameters can be upset.
At the moment of contact between the punch and blank, the part of the blank to be upset extends out of the die unsupported. If this unsupported length is too long, or greater than 2-¼ diameters, the blank will simply bend over on itself when struck, which produces what is known as a cold shut defect. With our 5'' blank, 1'' unsupported can be upset in one blow; 2'' unsupported can be upset in two blows.
If an attempt was made to upset 3'', it could not be controlled since this equals 6 diameters.
There are, of course, exceptions to the rule. A sophisticated header with a sliding punch that supports more of the blank allows two-blow upsets of 6-½ diameters. Also, in multi-station headers the number of diameters that can be upset is limited only by the available dies.
This relationship between diameters of wire and upsetting is critical. Improper calculation can mean mismatching the diameter of the feed wire with the machine's capabilities.
Many cold-headed parts are also extruded. Forward extrusion occurs when the metal blank is forced to enter a die diameter smaller than itself. Length is increased, while diameter is decreased. Backward extrusion involves subjecting the blank to pressure from an angular punch. Because it has no place to go, the metal literally squirts along the outer perimeter of the punch, flowing backward. Forward extrusion is used to produce bolts, screws or stepped shafts; backward extrusion is useful in forming a variety of cylindrical shapes such as nuts, sleeves and tubular rivets. Like upsetting, extrusion simply rearranges the shape of the blank and there is no loss of material.
Extrusion can be in an open or trapped (contained) manner. Open extrusion means the blank is forced into a die; trapped extrusion means the blank is totally contained within a die prior to extrusion (Figure 2).
While controlled upsetting is based on diameters of wire, extrusions are governed by the area reduction of the blank (calculated as a percentage) and the angle of extrusion. The basic ground rules for open extrusion, which is more widely used than trapped extrusion, is that the percentage of area reduction in one blow cannot exceed 30 percent. The extrusion angle (the angle the shoulder of the extrusion makes with the original blank) cannot exceed 30 degrees (Figure 3).
Area is defined as the cross-sectional area of the blank using the standard circular area formula A = pr². A blank with a cross-sectional area of 1.00 sq. in. extruded to a cross-sectional area of 0.75 sq. in. is a 25 percent reduction in area. Remember that diameter measurements, both before and after extrusion, are not directly used to calculate area reduction. The actual cross-sectional areas must be calculated by using the above formula. Successful extrusion practice also requires that the blank extend at least ?'' (land) into the die for proper guidance as the punch strikes.
These rules do not apply to trapped extrusion that typically allows for area reductions as high as 75 percent in one blow.
Contained (Trapped) Extrusion
This practice is responsible for allowing headers to produce more complicated and multi-shaped parts formerly made on automatic screw machines. It's especially applicable for extruding larger diameter wire to the required shank, a method that makes it possible to increase the ratio of the head diameter to the shank diameter.
The radial extrusion die (Figure 4) is generally preferred for contained extrusion of headed parts. This type of die reduces die pressures and improves the flow pattern of the wire as it is pushed through the die.
There are general rules to follow when using the radial extrusion die:
1. The blank to be extruded should be a minimum of 0.002'' smaller than the die entrance.
2. The entrance length of the die should be a minimum of one-quarter blank diameter. Maximum depth is determined by the volume of stock in the upset portion and geometry of the finished part.
3. The radius is usually equal to "C" in Figure 4. This varies with the percentage area reduction.
4. The extrusion land "B" is usually 10 percent of dimension "A." This varies with die material.
5. Extrusion relief is usually a half percent of "C."
6. Break corner "D" with about a 45-degree angle leading into the extrusion land.
With any contained extrusion die, best results are often obtained by forming wire that is somewhat larger in diameter than the finished shank. Wire size is selected so that the upset dimension of the head will be about twice the original wire diameter. This wire is extruded to the shank diameter and upset to the head dimensions in the customary manner.
The fourth installment of this six-part series on heading basics will recommend practices when combining upsettings and extrusions and discusses warm and hot heading techniques.