Heading machines perform a combination of forging, extruding and upsetting processes. This allows for a much greater range of diameter ratio reductions than other forming methods.
This method uses a specific type of steel known as cold heading steel. It can also be used for tempered alloy steels, and ferritic-martensitic duplex steels.
Wire Draw Box
The Wire Draw Box is a key component of a Heading machine. It takes round wire in coil form and converts it into fasteners in four steps: upsetting, extruding, pointing, and ejecting. It grips and ungrips the wire surface while oscillating on guide shafts to minimize damage to the surface and coating.
The block rotates evenly, pulls the wire at a constant velocity to avoid “snatching” which can weaken or break the wire, and draws the wire through a die at a controlled rate. The speed at which the wire is drawn varies according to the material and the amount of reduction required.
The wire is cleaned of contaminants immediately before it enters the drawing process to reduce wear and improve cage quality. An inline turkshead attachment can also be used to form deformations in the wire, allowing green hot toll wire to be utilized for both standard and deformed fasteners. This reduces material costs.
Wire Straightener
During the straightening process the wire is removed from the coil and passed through a set of rollers that remove any bends or irregularities left behind from the casting process. This is a critical step in the wire processing line as it removes any “cast memory” that might cause a defect in the end product.
The method of wire straightening consists in bending the material of wire 1 in two opposite directions simultaneously with stretching it, so that it achieves plastic deformation on the surface of a greater bend radius. This is achieved by means of a pair of rolls 77, 78 and 79, 80 kinematically connected to each other with a transmission ratio 1:1 in each of the pairs and 1:(1.0-1.4) between the pairs.
A number of different accessories are available to the wire straightener to increase both the ease of set-up and the “repeatability” of the settings. These include quick opening/closing options, roller position indicators and locking levers for the entry/exit guides pulleys. Also available is a Novo “Quick Set Reference Chart” that records each setup for future use.
Cut-Off Knife
All cold headers have a system for cutting off and measuring out a piece of wire coil. This is known as the cut-off knife. The blades are housed in a knife bar and have meshed gears that ensure positive drive. The regulating screws allow the blades to be fine tuned and a self lapping procedure ensures that they stay sharp.
Engineers have learned over the years that only a limited amount of material can be squeezed or upset at one time before it bursts or cracks. The limitation is roughly two percent of the diameter in each stroke. Many bolts or screws have more than this much metal inside, so they must be hit several times.
Upsetting requires a lot of power, and the ability to control this force is key for good results. Top alloy producers tightly control analysis and produce grades that perform well in heading. This allows higher speeds, better quality parts and reduced operating costs.
Punches
Punches are generally categorized according to their shape and points. The prick punch is the most commonly used and useful for marking reference points on metal sheets. It is also helpful for transferring dimensions from paper patterns directly onto the metal and for orienting them properly. It is usually made from high-carbon steel and can be utilised with a hammer.
Cold heading imposes severe impact stress on the punches and dies. These factors are largely responsible for the need for light wire drawing reduction (skin passing) in front of heading operations, which decreases costs and strain aging.
Solid dies are typically preferable for the heading of complex shapes. They consist of a cylinder of one material with a hole through the centre, which can be hardened by forced quenching from the die’s interior or, in some cases, with the inclusion of an insert of another material. The punch can be either solid or split into two pieces, allowing the use of both open and solid dies in the same machine.
