Heading machines transform a metal blank into intricately designed fasteners. They are an automated, high-speed method that is able to economically produce these parts.
Cold heading is a form of metalworking that can be used to create net shaped fasteners from the initial starting slug of material. The process does not create any waste and offers cost savings over machining.
The Die
Unlike metal cutting which produces a lot of waste, the cold heading process can produce intricately designed parts with very tight tolerances with only minimal material loss. It accomplishes this by making use of high-speed automated part forming equipment that is commonly known as a heading machine.
The process of cold heading transforms a defined volume of wire into an intricately designed fastener with very little loss of raw materials. It also produces much faster than other machining processes, producing up to 300 pieces per minute.
It is important to note that there is a limit to how much metal can be upset in one stroke (or blow). Engineers have determined this maximum amount to be roughly two percent of the diameter for each strike.
For this reason, it is not uncommon to use multiple blows to form a single screw or bolt head. This is why single-die, double-stroke heading machines are widely used. Increasing metallurgical knowledge also allows for tougher alloys to be used in the heading process. This results in better cold forming qualities and increased performance when coupled with subsequent machining operations.
The Punch
Unlike metal cutting, which is slow and generates up to 60% waste, cold heading is more efficient as it forces the material into shape without removing any material. Cold heading is the most common metal forging process, especially for fasteners, as it can take less time and energy to produce complicated shapes from a raw material.
The punch is a simple shaped hammer that strikes the blank with a great deal of force to form the head of the bolt. The force from the punch must surpass the elastic limit or yield strength of the material in order to plastically flow and form the desired shape.
As a result, the forming process is volume specific and requires blanks cut to consistent volume. This is why single-die, two-stroke machines (one die, one punch, two blows) are more commonly used compared to multi-station. Combining upsetting and extruding is also a normal practice, however, maximum deformation limits for both upsetting and extruding are based on the wire diameters initially fed into the machine.
The Bed
The bed is the portion of a machine that grips and ungrips wire stock as it moves across it. The gripping surface is designed to minimize damage to the wire surface and coating, as well as reduce snags.
A large portion of the bed is used to support a sliding cone-shaped heading tool (punch) and a slug that is formed in the die. A single punch can upset a length of metal up to 2.5 times its original diameter in one blow. Larger upsets require more than one blow, which requires a large, sliding, cone-shaped die cavity to accommodate the metal.
A slug is then transferred to the head of the die where it is pushed into the head by the punch. The slug is then ejected from the die by an ejector pin. A slug can be combined with extruding to form long shanks or heads. This is done on transfer heading machines, also known as bolt makers and nut formers.
The Head
Heading equipment is used to convert round metal wire into fasteners at a very high rate of production. The process involves four basic steps: (i) a length of stock or blank is cut from the coil, (ii) this slug is placed in line with a die and forced into a shape with one or more upsetting and/or extruding blows (Fig 5), (iii) the slug is then ejected.
It is possible to combine upsetting and extruding in the same operation; however, a maximum of a reduction in diameter of 80 % may be achieved. Also, to avoid buckling of the unsupported blank, the upsetting ratio must not exceed 3:1 or more, as discussed in Fig 5.
Advanced heading machinery now performs several operations in a single machine, such as piercing and trimming, allowing the production of complex fasteners at affordable rates. Increased metallurgical knowledge and information has also allowed the heading of much tougher materials, extending the range of products that can be produced.