Fastener manufacturing by forging and rolling is the modern way off manufacturing with multi station bolt formers & part formers along with High Speed Flat Die and Circular Die Thread Rolling Machines allowing the production of all types of threads. A description of the methods and alternative ways of producing fasteners are detailed here.
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The most popular method of producing fasteners, cold heading, cold forging or cold forming are basically putting material in a die and hammering it. Unlike hot forging, where the metal is heated before forming, or casting, where the metal is melted and solidified, cold heading and other methods of cold forming cause the metal to deform at room temperature. Surface finishes can rival those achieved with machining, depending on the precision of the tooling. Cold forming can produce fasteners using the minimum of material, to close tolerances, and at the rate of dozens or hundreds per minute. In addition, cold-formed parts exhibit excellent strength, as the material grain flows into its final form, rather than being cut, as in a machined fastener.
All these benefits are not cheap, tooling costs for cold forming can be significant. Lead time for design and setup to run cold-formed parts is often measured in weeks, much longer than it would take to program and set up the same part on a CNC lathe. For small quantities, cold forming may not be cost effective and this is where machined or hot forging methods come into their own.
A machined fastener starts out with a chunk of material large enough to contain the desired part. Then you cut away everything that isn’t the part. This produces chips, which are wasted material. Cutting can also disturb the grain structure of the metal.
Today’s cold-forming technology has come a long way from the simple cold-heading process used in the fastener industry for over a hundred years. While cold forming is still “taking a piece of metal and hammering it,” the process can now produce complex, precision fasteners economically in large quantities, producing fasteners up to M36, double the size limitation of the 1990’s.
The entire process is performed on a “bolt maker” machine.
- Cut-off a length of “wire” that has sufficient volume to produce the final part. The only material lost during this sequence is that trimmed away when the hexagon is formed and a small amount cut from the thread end during the pointing operation.
- Create an upset. This “pre-form” step is necessary to insure that a strong and concentric head will form on the screw. Because the steel has a granular microstructure created during the annealing process, “upsetting” is necessary to allow uninterrupted and even grain flow into the final net shape. Without this step, grain flow would be interrupted, weakening the screw at the head to shank interface.
- Form a head from the material that was gathered in the upset.
- Extrude the shank, down to the “pitch diameter” of the thread that will later be formed on the screw.
- Trimming of the head. After this step the part leaves the “cold heading” (forming area) and is transported down a rail where it is feed into a cutting tool that cuts the chamfer on the extruded end. This is referred to as the pointing operation. More modern machinery crops and points as a single shearing operation.
- Finally, the part is run through a set of “roll dies” where the thread is rolled on the part.
Video courtesy of Non Standard Socket Screw
The use of heat to soften the metal to make it easier to shape is called forging and takes its name from the black smiths forge. This is the original method of producing fasteners and bolts; hot metals were beaten into shape with a hammer.
The idea of mass producing hot forged fasteners primarly bolts was originated with Micah Rugg, a town blacksmith. In 1818 in Connecticut USA, he invented a machine for making carriage bolts. Up to 1839 Rugg's made around 3,000 hot forged bolts, during 1839 he devoted his whole attention to bolts and nuts and in 1840, he took a partner Martin Barnes, constituting the firm of Rugg & Barnes. In the first year they produced over 100,000 hot forged bolts.
The big change is that the hammer is now a mechanical device and the forge has been replaced by an induction heater, The metal is pressed in to a high strength part known as a forging, the forging is much stronger than an equivalent cast or machined part; it is for this reason that forgings are used where reliability and safety is critical. When the piece of metal is being shaped into a bolt or socket form, its initial integral grain is deformed to fit the new shape, the result is the grain is continuous throughout the fastener, producing a piece with improved strength characteristics.
- Large diameters and exclusively above M36 are produced by hot forging.
- Extreme long lengths again lend themselves to hot forging.
- Short productions runs for diameters M5 and above make hot forging a prefered alternative to avoid the high set up costs for cold forging.
With the age of the CNC machine, bar turned fasteners have moved from the 19th and into the 21st century.
There are still fastener companies turning fasteners on lathes, meeting demand for very low volume.
CNC machines are capable of rapidly producing intricate turned parts by a mixure of Turning, Milling and Grinding.
Bright turned fasteners are still required when:
- Cold forging volumes are too high.
- Hot forging scale is not acceptable.
- Where extra precision is required.
- If the fastener is too large for forging or forming.
- When someone forgot to update the 19th century drawing.
Drilled or cut
Drilling Cutting Threading
Additional operations performed on fasteners are a regular feature in the fastener market place. There are a limited number of companies that have the facilities or skills to carry out this work. Drilling requires the accuracy and the ability to drill through high tensile steel, to hold the thread with out damaging it and to remove the bur after drilling. Cutting both unthreaded, threaded rods and rebar and the ability to thread rebar are both skilful activities performed on long lengths of product.