Alistair Rae looks at some recent developments that are helping designers to reduce the cost and lead time for custom fasteners.
There are thousands of different standard fasteners available. For threaded fasteners alone, if you consider the possible combinations of different thread forms (with many offered in standard, course and fine pitches), lengths, head styles, materials and finishes, the extent of standard product portfolio soon becomes very large. However, there are various reasons why an alternative head form or non-standard length might be required, and it can be beneficial to incorporate location lugs, integral washers, O-ring grooves or other geometric features. Sometimes the 'fastener' can be redesigned so as to additionally perform the functions of another component.
Fastener manufacturers have been highly innovative in recent years, taking advantage of the important role their products can play in reducing bills of materials and assembly time, as well as providing reliable joints in adverse operating conditions. One example of this is Arnold Umformtechnik's screws that benefit from a patented Mathread dog point. These are designed to prevent the screws from being cross-threaded during insertion, which might otherwise result in increased cycle times and damaged assemblies, and a risk of injuries to operatives. As the threads of the patented Mathread dog point come into contact with the female thread, they begin to cam over the female thread, thereby ensuring that the two thread helices align correctly every time without fail (Fig.1). Compared with alternative solutions to this cross-threading problem, screws with Mathread dog point are claimed to be shorter, lighter and can be used in more applications. With innovative features like this available on standard fasteners, the message to designer engineers is that they should not hold back from asking fastener manufacturers to create new products to solve specific assembly problems.[Page Break]
Companies such as Non Standard Socket Screws (NSSS) can cater for much of industry's requirements for customised fasteners, with manufacturing facilities that include double-blow hot forging presses, CNC (computer numerical control) turning and milling machines, grinding machines and thread rolling machines, as well as on-site heat treatment.
Parts can be manufactured in various grades of steel, alloys, brass, aluminium, titanium and other exotic materials. For applications requiring enhanced resistance to corrosion, NSSS offers a comprehensive range of surface treatments, such as zinc plating, Geomet, Dacromet, Delta, galvanising and Xylan fluoropolymer coatings.
In addition, fasteners can be supplied with a dry threadlocking adhesive pre-applied to a portion of the thread so that potentially messy liquid threadlockers do not need to be applied during the assembly process.
Arnold Umformtechnik, which is part of the Würth Group, is finding that customers are increasingly making use of its Fastener Express service for rapid prototyping complex non-standard fasteners and small precision parts (Fig.2).
According to the company, prototype components can be supplied in one-quarter of the usual time and at a quarter of the cost of producing prototypes using conventional processes. Because the rapid prototyping process is so different to Arnold Umformtechnik's usual work, the company has formed a specialist team to ensure that orders are progressed quickly. First, the proposed design is assessed to ensure that it is technically feasible. Three-dimensional CAD data is then processed so that a suitably sized blank can be machined on automatic lathes and machining centres. If any other processes are necessary, such as heat treatment or electroplating, these can be completed in-house.
The resultant 'functional prototypes' have virtually the same characteristics as series-manufactured components, enabling customers to perform tests and assessments with a good degree of confidence that the results they obtain will be representative of the way the production items will perform in service (Fig.3).
Most of the time is saved by eliminating the need to design and procure special forming tooling, and other savings are achieved through having the finishing processes available in-house (Fig.4). There is a broad choice of materials, including case-hardened steel, quenched and tempered steel, stainless steel, brass, aluminium and copper, or specialist alloys can be specified. Customers can also choose whether to order a single item or a small volume.
For complex, multifunctional components, Arnold Umformtechnik has recently introduced its Conform process to help reduce the high costs often associated with non-standard fasteners. Compared with turning, components produced with the Conform cold forming process are claimed to be up to 80 per cent less expensive.
Precision and functionality are said not to be compromised by this alternative, six-stage process. Moreover, the level of geometrical complexity that can be formed may surprise some design engineers: external teeth, form-rolled parts, hollow and semi-hollow parts, eccentric elements and bearing journals can all be produced with tight tolerances (Fig.5).
Arnold Umformtechnik says that its customers often use a combination of the Fastener Express service and the Conform process. This provides the benefit of rapid deliveries of functional prototypes that are close to being fully representative of the production parts, together with lower costs once the product enters production.[Page Break]
For applications in which plastic fasteners are sufficient, Thomson Nyliner offers custom moulded fasteners that are described as durable and cost-effective. The company's custom moulded fasteners are often found in the automotive, medical, marine, office equipment and appliance markets, with typical products ranging from grommets and hole plugs, through to spacers and levelling feet.
Thomson Nyliner's rapid prototyping service enables custom moulded fasteners to be produced very quickly. Once the design enters production, the company can mould custom fasteners from hundreds of different materials to suit the performance requirements of the product and operating environment.
Given the current state of the art in rapid prototyping and CNC machine tools, it would be simple to find a company to produce metal or plastic prototype fasteners to a 3D CAD model or 2D drawing. However, if functional prototypes are required, it may be prudent to reduce the project risk by approaching a fastener specialist rather than a generic rapid prototyping bureau or machine shop, as the specialist is more likely to understand the true functional requirements for a fastener. This is particularly so when the project is likely to require high volumes of production parts, as in-service failures could be extremely problematic.