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How the Heading Process Works
Heading equipment primarily takes round wire in a coil form and converts the wire into desired parts at a high rate of speed.
Four basic steps comprise the heading process (Figure 1):
1. A length or blank of wire is cut from the wire coil.
2. The blank is placed in line with a cavity or die.
3. The blank is forced into a desired shape with one or more upsetting and/or extruding operations called blows.
4. The part is ejected.
This heading process may be part of a sophisticated cold forming machine that has additional points or stations for further operations - trimming, piercing or pointing - after upsetting and extruding. Most headers, however, are of the single or double blow variety. Multi-station part formers can include up to seven die stations. The part moves from one die to another until it is completed. The typical arrangement is horizontal, though some multi-station formers are arranged vertically; the part progresses from the first die station at the top to the last die station at the bottom in this case.
Forming parts on a heading machine using upsetting or extruding is not merely a matter of hammering the metal blank until the desired shape is reached
The punch and die work together. The punch is a simply shaped hammer that strikes the blank on its end. This forces the other end into the die that produces, for example, a headed bolt (Figure 2).
In a typical heading machine the punch, carried on the gate or ram, moves toward the blank with a great deal of force, striking it with an impact of many tons per square inch.
Perhaps no operation in the cold heading sequence is more important than the wire cut-off to form the blanks. This is because the volume of the finished part essentially equals the volume of the blank from which it was made. Since part dimensions and part volume are interdependent, blanks must be cut to consistent volume.
In many instances the upsetting of the blank is controlled by the punch and takes place outside the die. However, the head can also be formed in the die, in both the punch and die, or between the punch and die, a technique called free upsetting (Figure 3).
Commonly, each die station in the heading machine has two punches that oscillate to form the fastener head. The first punch action partially shapes the head and is called coning, while the second punch finishes the head.
A heading machine includes either solid dies or open dies. Solid dies are more common; open dies are used when a fastener requires a very long shank that cannot be fabricated with a solid die. In solid die headers, the knockout (or kickout) pin is equally important to the interaction of the punch and die. The knockout pin serves as a support at the back end of the blank as the punch strikes the front end, and the knockout pin then ejects the finished part (Figure 4).
Different combinations of upsetting and extrusion blows are possible, but upsetting is generally the first blow, with an extrusion blow following. Upsetting and extrusion can take place in the same blow.
Knockout Pin Specifics
Knockout (or kickout) pins serve two functions. They stop blanks as they enter the die at the point where upsetting is to start. For this reason, pins must withstand some of the forming pressures. The second knockout pin function is to eject the headed part to clear the die for the next blank. The unsupported length of the pin should not exceed eight diameters. This is a good general rule, though some fabricators run parts with the knockout pin equivalent to 10 or 12 diameters unsupported.
When the knockout pin's unsupported length exceeds these diameters, a supported pin assembly (Figure 5) is suggested.
Lack of support isn't the only reason for pin breakage. Broken pins can result from running poorly coated wire, or wire with an incorrectly selected coating. Rough, rusted or uncoated spots on a wire make the parts more difficult to eject; this may result in pin breakage. Also, as the end of the knockout pin wears, it's possible for metal to extrude around the end of the pin. This may cause a tight spot in the die where the pin and workpiece overlap, resulting in sufficient additional pressure to break the pin. A similar effect can occur when the diameter of the pin is too large. As the pin stops the workpiece when it enters the die, it often absorbs part of the heading process. A pin that is too large, which means it fits the die too closely, may swell, bind in the die and break from the resulting pressure.
A back taper in the die (a smaller bore diameter at the die face than toward the kickout pin) may allow the metal to upset to a larger diameter at the end of the shank than under the head. In this instance the knockout pin must force the larger diameter through a smaller hole during ejection. Here, excessive ejection pressure may break the pin. Machining marks left in the die can also create excessive pressure. Die bores must be smooth and rust-free.
The third installment of this six-part series on heading basics will discuss controlled upsetting, extruding and contained (trapped) extrusion.
Figure 5 - When unsupported knockout pin length must exceed 12 diameters, a support pin assembly like this is recommended. The center support (A) reduces the unsupported length to less than 12 diameters on both sides of support.
Carpenter, based in Wyomissing, Pa., is a leading manufacturer and distributor of
specialty alloys and various engineered products. More information about Carpenter is
available at www.cartech.com.
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