The search for strong and cost-effective new materials for manufacturing is one of the hot button topics for design engineers and managers today. Developments in technology mean that cold forging, one of the older processes, now offers new benefits and scope for producing materials with a wide range of applications.
Specifying engineers sometimes overlook cold forging as a process, perhaps because it has not been widely used in recent years and they have not therefore been exposed to its benefits. But in these days of cost down and value engineering, cold forging can provide a cost-effective alternative to bar turned components.
Cold forging at its simplest level consists of cutting up raw material (usually wire in coil) and hitting it very hard. The process retains the grain flow of the raw material, making the strength of the finished product considerably higher than that produced typically by turning from bar stock.
Net or near-net shape parts are produced straight off the machine with no swarf as the process relies on the material being formed in dies, punches and around extrusion pins. Multi diameter components are manufactured by extrusion (forward and backward) upsetting, piercing or trimming in various combinations to suit the component.
New technology in cold forging is making it possible to create a much wider range of component shapes and is bringing the intrinsic benefits of the process back into focus. Modern multi die, multi blow transfer machines and clever tooling design ensure that shapes thought impossible a few years ago are now readily achievable.
For example, Aylesbury Automation recently provided a solution in cold forging for a customer in the security grille market who required a strong one-shot fastener. The rivets previously being used could be drilled out by an exceptionally determined and well-equipped burglar. The fastener had to secure the concertina lattice work to enable the grille to function correctly and also foil these attempts on its security.
Engineers from Aylesbury Automation Fastener Division worked with the customer to produce a cold forged, two part fastener which comprised a deep hole female rivet and a case hardened (to deter drilling) male rivet with a fast spiral U form thread on the outside which generated the thread form into the female rivet, when the two are brought together under pressure. This solution gave a pull out strength in excess of 3tons. It is tamper-proof and the overall appearance of the grille is significantly enhanced. As an additional benefit, the fastener does not need any capital equipment to install it.
Most common materials such as steel (both low and high carbon and boron), aluminium, aluminium alloy, brass,copper and stainless (430,304,316,394 typically) can be forged successfully, together with more exotic materials such as titanium and Monel. The material should be in wire form and have drawn tensile in the range 308Mpa to 540Mpa to ensure a good balance between forgeability and tool life.
In terms of product design, diametric tolerances of 0.025mm are readily achievable over typical batch runs of well in excess of 500,000. Composite dies and punches enable concentricity between shoulder and minor shank diameters of a similar order to be produced consistently.
Due to the shearing nature of the initial cut off, when the slug is cut prior to being forged, slightly more generous length tolerances are required on the more basic forging machines, eg single die, two blow, typically 0.127mm. But this can be overcome on multi station machines enabling the component to be coined prior to the completion of the forging cycle. The amount of material which can be upset varies by material and forging machine but three diameters is fairly typical up to five and a half with a sliding former.
Extrusions require a lot of forging pressure but this can be significantly reduced if the component can tolerate a 30 extrusion angle. Multi die machines enable this extrusion angle to then be coined in a similar way to the shear angle referred to above. All corners of a cold forged component are subject to natural forging radii due to the nature of the process.
This is a positive benefit in the shank to head areas, as sharp corners tend to set up stress points within the component. This can be a problem when the component fits into a tight tolerance hole as the head will sit proud of the base part because the underhead radius interferes with the edge of the hole. This is a common criticism but is easily overcome by a technique known as reverse countersinking. This maintains the head to shank radius but forms it inside the head, enabling the head to sit flush on the base component.
Manufacturing speeds vary depending on the component being produced but 350 parts per minute is fairly conservative on relatively simple components. More complex forgings will run at around 200 parts per minute.
It is this speed combined with the inherent strength of the component and the material efficiency (lack of waste in the form of swarf) that makes the economic case for cold forging so attractive.
The economic case becomes irresistible when cold forging is combined with automatic, parts fed, second operations machinery which enables non-forgeable features such as grooves, recesses,cross holes, and flats to be machined into the forging.
An example of this type of work is a product which used to be bar turned and is used on windscreen wiper assemblies. This is now cold forged and then has the grooves and points turned as second operation. The combination approach forging and second operation produces an endless range of possibilities for a wide range of components and design engineers are looking again at the benefits of this process.
Even the humble rivet has undergone a transformation. Fastriv, manufactured by Aylesbury Automation’s Fastener Division, utilises advanced end geometry techniques forged on a six station parts former to produce a self-piercing rivet. It is a substitute for spot welding, producing inherently stronger joints. Fastriv can fix together dissimilar materials such as steel and aluminium, which cannot be spot welded. It can also join pre-coated sheets without damaging the coating on either surface.