Trueness to gauge
So that a screw can be turned precisely inside a thread, both the thread measurements of the screw and those of the nut must provide enough ‘room’ for the process of being screwed together. To ensure this ‘trueness to gauge’ there exist in the industry tolerance systems precisely tailored to one another that incorporate different surface coatings. If a form of surface protection, e.g. a KTL or zinc flake coating, is to be applied, the thread on screws (the gauge) must accordingly be made smaller and on nuts bigger in order to ensure their functionality.
‘Gauging of threads’ function test
In order to ensure that they can be screwed together, screws and nuts are put through a special function test (‘Gauging of threads’). This determines whether parts to be connected together fit into one another. The result of the attributive test is a basic ‘OK’ or ‘Not OK’. Over and above that, qualitative tests can also produce concrete measurements, with gauging stipulated as the definitive criterion for the testing of threads and therefore obligatory.
How is gauging done?
In the gauging process precisely manufactured nuts (thread ring gauges) and screws (thread plug gauges) are screwed together to test how well they connect. At the same time the thread dimensions are checked – core diameter, screw thread diameter, outer diameter, thread pitch and thread angle. A thread is deemed to have tested ‘OK’ in the respective tolerance class if the thread go plug/ring gauge can be screwed on using the specified torque, while the thread no-go plug/ring gauge cannot. If any surface protection is to be applied to screws and nuts, ‘room’ for the coating must accordingly be incorporated. The gauging to be performed following the coating process makes it possible to check that the coating was not too ‘thick’. It is, however, also possible that the gauge may already have been made too small or that the whole thread could already be flawed prior to coating. Here it is advisable either to perform gauging in the condition supplied or to remove/strip off the surface.
Finding thread damage
Gauging also helps not least to identify any places where the thread has suffered a knock. These can occur especially in the case of low-strength materials as a consequence of impact loading during production and/or transportation. During or prior to heat treatment in particular it is also possible for deformation of the crests of threads to occur, making screwing in more difficult.
Specifics of thread gauging
When testing thread gauging using a GO gauge as per DIN EN ISO 1502, problems repeatedly arise in practice, which can, however, be avoided subject to taking into account a few tips:
The gauges used for pre-shipping checks should be ‘used’ gauges, as the dimensions of ‘new’ gauges are less than the thread maximum (see DIN EN ISO 1502).
The thread should be tested before application of the surface coating. If the thread ring gauge cannot be screwed on easily – for example in the case of scaled or blasted surfaces – it is allowed and helpful here to add a drop of oil.
In the case of zinc flake coatings it is possible for the surface to get scratched off or loosened by the ring gauge, which results in the ring getting clogged up. It is advisable in this case to clean it and put in on again.
A further problem arises when gauging internal threads: as nuts in particular are ‘scooping parts’ in the bath, this leads to an accumulation in the base of the thread. Gauging is thus sometimes not possible, although the nuts can definitely be securely fitted. DIN EN ISO 10683 2014-10 addresses this situation, enabling different procedures to be agreed between ordering party and coating company. In this connection the German Fastener Association has introduced an alternative test pin in its ‘Testing the screw-fastening capabilities of nuts with zinc flake coating’ guideline. This test pin is already in use and is set to be incorporated into a VDA regulation.
You will find detailed information on fasteners’ trueness to gauge and ease of fitting in DIN EN ISO 10683 2014-10, DIN EN ISO 4042 2001-01, DIN EN ISO 1502 1996-12 and on the German Fastener Association’s website www.schraubenverband.de