Challenge Europe discuss the future for Imperial and Metric Screws

Sorting out the multiplicity of screw thread designs has been such a big issue for so long that there are lots of guides to give approximate conversions, although naturally there are no actual like-for-like equivalents. Our team here at Challenge Europe recently discussed a little history and some future-gazing.

Imperial threads – as it may be expected come out of the ins./feet standard developed in the blossoming of the industrial revolution when Imperial meant British Empire and then into the dynamic drive for mass production seen in the USA. These imperial threads were in their heyday at the end of that era which was approximately in the early part of the 20th century. When the Empire broke up under the stresses of two world wars and the rise of international power blocs together with demands for independence they continued in general use since the momentum of these newly grown industries carried them forward. Metric on the other hand comes from the millimetre/centimetre measurement system so popularised by Napoleon in the continental European theatre and subsequently established in the rebuilding and regrowth of the European manufacturing industries after the devastation of the wars that took place there.

They are not mutually compatible of course but may have approximate size replacements. The two standards essentially run alongside each other with metric (ISO) being the one that is replacing imperial across the world – it is the defacto global standard.

Consequently, metric screws are more readily available, e.g. ex-stock, more standard sizes, more easily sourced, more development in new products, more widely used and continuing to replace imperial globally on new projects. Challenge Europe are one of the few specialists who stock both standards in depth.

Increasingly tool kits are solely metric – although even today new spanners, taps and dies are available for Whitworth and BA – but are expensive so that they are often sourced by hobbyists and renovators as second hand items at auctions and car boot sales.

Since Imperial standards derive from the early days of the industrial revolution and industries like ship building and armaments that were important to the days of Empire and grew immensely at that time.

The first national standard was Whitworth – devised and specified by Joseph Whitworth in 1841. Followed by William Sellers who developed the Sellers thread of 1868 later developed into American Standard coarse and fine.

Well known to instrument and model makers is the British Association screw thread standard – known as BA these are small screws with the largest being OBA = 6mm dia. There continues to be a low level demand  for older applications, e.g. old instruments – or for rebuilding of older vehicles.

Many countries have run both types alongside each other but all are in the process of moving to ISO metric standards if they have not already done so. The USA has probably held out longest due to the inertia of its huge market, but globalisation has led to sourcing of cheaper metric products for mass production and this has driven the conversion. The US/American market is therefore becoming more restricted – imperial products used to be the mainstay of general sales in the US, but not so now as US customers are more comfortable with metric specs. Availability of metric equipment in the US has now completely overtaken imperial screws in a relatively short time – roughly coinciding with the rise of Chinese manufacturing as the component supplier to the world.

New thread design development continues as specialist companies and engineering enthusiasts seek to explore niche areas, for example, “total surface contact” and bone screw fasteners – which may one day find application outside the medical/surgical field.

One particularly interesting arena is that of alignment correction whereby a misaligned screw will self-correct and pull itself back to its true axis.

Further developments are aimed at improving the performance of screws subjected to transverse loads which can lead to self-loosening or fatigue failure.

However, in the mass manufacturing market we do not at present see any further new thread designs on the horizon since metric forms seem to have adapted well to all general requirements. Elsewhere, other designs of fastener have been extensively developed from push-fit plumbing connectors to pop-in electrical and electronic fittings and this approach to specialist fastenings has very much taken off as a new way of dealing with often completely new problems.

While the transition from “old” to “new” thread forms is almost complete and many threaded fasteners have simply not been specified where they might previously – because now more cost-efficient fasteners of specialised design are available. However, it does seem that pockets of demand for imperial threads will continue for a long time yet.

Self-drilling/self-tapping screws stocked in depth for metals and plastics at Challenge Europe

Self-drilling/self-tapping screws for metals and plastics – ex-stock from our team at Challenge Europe are often used in the construction industry, e.g. for fixing sheet metal in place on frameworks and can come with bonded washers to prevent water ingress in outdoor applications. Self-drilling screws come in a variety of formats but all are based upon the principle of drilling and tapping in one operation. They are generally used in steel, softer metals or plastics and there is a range specially designed for use in wood/composite materials.

Major benefits in using this type of fastening include increased speed of installation and efficiency provided by combining the drilling and tapping operations as well as a functional improvement in alignment, which otherwise could require re-setting the screw for correct fitment.

Self-tapping screws are stocked in depth as hardened and plated carbon steel and stainless steel self-tappers for use mainly in sheet steel products. These screws form or tap their own thread so saving time and providing a strong clamping action with the ability to remove and replace if required. Countersunk, Pan, and Flange heads are commonly available, the flange head having the advantage of spreading the load well and minimising deformation of the sheet thus eliminating the need for an under head washer. The traditional slotted drive is still available, but the modern preference is for cross recessed or multi-splined (star) drives to suit modern insertion equipment. Self-drilling and self-tapping screws are available in plated steel and stainless steel for engineering, construction and general assembly purposes.

Specialist Construction screws are often thought of as a separate genre but principally operate in the same way as their more standard cousins and tend to be designed to perform more specific functions. These functions range from having integrated sealing washers, for the fixing of cladding, through to Bugle head designs for use in fixing plasterboard. There are a variety of drives available, often linked to the intended function of the fixing, and can include designs such as Hex, Star, Pozi, Phillips and even Square. Some fixings are available in a Collated format where speed and handling is important.

Largely because of their wide utility this group of self-drilling/self-tapping screws offer an extensive range of different head and drive types. Head types include the following and may often be specified with a drive configuration of choice:

  • Pan head: which has a thick rounded, outer top with a large surface area
  • Button or dome head: which is cylindrical head with a dome shaped top
  • Countersunk or flat head: An inverted cone head with a flat outer surface
  • Raised countersunk head: Combines a countersunk head with a rounded upper surface for decorative purposes
  • Bugle head: which presents a smooth increasing transition from the shank to the angle of the head
  • Flanged head: Can be any head style incorporating an integrated flange so that it does not require a washer

Also wafer head – which is very thin – for use where a low profile is required.

In some instances they can also be offered with anti-vandal features, such as pin hexagon, pin star (multi-splined) and clutch head – contact us for more information.

The Challenge Europe difference between Thread-forming screws and Self-tappers

We are delighted to hold in stock both metal thread-forming screws, e.g. for castings etc where they secure components by forming a machine thread – also purpose designed screws for thread forming in plastics.

The team point out that thread forming tri-lobular screws are used for fixing into metal components – a form of self-tapping screw, they use the tougher tri-lobular design which assists the grain flow as it effectively deforms a thread into thicker material than could be managed with a standard self-tapping screw. This process results in a thread geometry that is more accurate than a standard self-tapper and so provides a stronger fixing which is more resistant to loosening under vibration. Almost a machine screw in concept these thread forming screws leave a tapped hole when removed and may be replaced with a standard rolled thread machine screw if required – for example at servicing or repair and thereby provide a much higher torque loading capacity than a standard self-tapping thread. They are typically used in steel, aluminium, aluminium die castings, zinc die castings, magnesium and other light alloys.

Heads and drives include countersunk, pan, and hexagonal/flange types with cross recess, multi-splined (star) or hexagonal formats. Hardened steel is normal for most applications and is suitable for use in a variety of metals providing the correct diameter pilot holes are in situ. Stainless steel thread forming screws are also freely available but tend to be used in softer materials such as aluminium and zinc.

Thread forming screws for plastics have become more and more important in the assembly of everything from electronic equipment to furniture, as they are easily inserted into drilled or moulded holes in thermoplastic materials where they deform the material to provide optimum fastening force superior to standard self-tappers. Thread forming screws are especially useful in the manufacturing production environment where an automatic feed can be employed to greatly speed the assembly process, by eliminating the need for nuts or inserts, so reducing piece part costs, labour costs and process costs.

There are a range of thread geometries associated with this type of screw, this is to allow for their use with different component materials and design features. For example, if required for insertion into thinner walled, harder plastic mouldings it might be advisable to try a tri-lobular thread variant which tends to reduce the risk of burst out. Of course, the choice of hole diameter can be critical and the team at Challenge can advise on both screw selection and suitable hole diameters.

Head formats for use with plastics include countersunk, pan, and flange types with cross recess or multi-splined (star) drives. Typical materials include hardened steel with a zinc & clear passivate finish or in some instances black finishes can be available, also stainless steel. Again, proper installation requires a pre-formed hole of the right diameter to properly form or cut the right size thread and to take advantage of the benefits of this design which include low radial stress, no material to jam the thread, maximum resistance to substrate relaxation and no material stress through plastification.

Also see here for self tapping screws.

See here for self drilling screws.

Challenge Europe announce ex-stock star/multi-splined (TORX) drive screws

Our team are pleased to offer ex-stock Hexalobular Internal Drive screws (generically of the TORX style) as part of our manufacturing supply service for delivery under normal production protocols, e.g. Lineside supply, JIT, Kanban or similar.

Often referred to as star drive or multi-splined screws, they are available in metric standard sizes and are designed for automation assembly in that they feature a positive drive with reduced slip-out and are especially suitable for controlled torque insertion – using bowl feed equipment.

Hexalobular Internal Drive screws are popular for use in the auto, motorcycle and bicycle industries, as well as instrumentation, electronics, computers, brown goods and construction projects.

A wide range of screw types in steel and stainless steel are now available with this type of recess to ISO 10664.

Security or tamper-resistant variants are available with centre pin requiring a special driver – also external Multi-splined bolt heads are available providing many of the benefits associated with the aforementioned internal drive version.

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