Common across a significant range of fastener products are metric thread forms, so much so, that for example an M5 thread is the same irrespective of the head type or screw style. There are fine thread, extra fine thread and constant pitch series, but if possible these are to be avoided as their stocked range is not as comprehensive as it is for more general coarse pitch threads.
Challenge (Europe) Managing Director Kevin Moorcroft explained that: “The coarse pitch series of threads will meet most general engineering requirements so these are the standard production and ex-stock items. The more specialist threads such as an M10 fine pitch socket head cap screw, may be stocked by specialists such as ourselves here at Challenge Europe but of course the finer pitch thread forms can come at a premium price simply by reason of economies of supply”.
Moorcroft continued: “Similarly for ‘old’ imperial threads such as the B.A. range, once commonly used within electrical equipment, which has seen its availability diminish during the last recession leading to enforced redundancy in many applications. Although demand still exists for the range, only those sizes/types with viable volumes tend to be stocked and available at reasonable pricing. Sizes not commonly stocked can be small batch produced but again at the risk of premium pricing.”
Other thread forms are slightly different – for example self-tapping screws which have a coarser pitch – and were conventionally used in sheet metal work with variants later developed specifically for use in plastics, the latter becoming more commonly known as thread forming screws.
Thread forming screws also exist for use in metals and create a finer machine screw type thread by material deformation during insertion, the theory being that a thread forming screw could be removed if required and replaced with a machine screw. Thread cutting screws also exist having a lead in design feature that specifically aids the cutting/removal of material in a similar way to a thread tap.
Thread forming and cutting screws tend to be smaller sizes typically M2 up to M8 – possibly M10 depending on the drive – since security of the drive determines the amount of torque that can be transferred to the screw.
Specials, e.g. self-drilling screws, are another variant available in a several drill point and thread configurations suitable for use in a diverse range of applications and industries.