Coating requires space. Fasteners such as nuts and bolts that are coated for the purposes of corrosion protection need to have slightly altered dimensions. If a coated bolt is to have exactly the same dimensions, it needs to be designed to be slightly thinner. For a nut with coated thread it is necessary to enlarge the inner diameter slightly.
The problem is old
100 years ago, when the first bolts were galvanised, they no longer fitted into the nuts. Put another way, galvanised nuts no longer fitted onto the bolts. Since that time the initial dimensions of fasteners that are to be coated has been adjusted by manufacturers. This is currently a significant issue in the area of lightweight construction.
Calculation of the initial dimensions of fasteners coated with zinc flake is detailed by the International Organization for Standardization (ISO) in standard 10683. The current version dates from 2014. The German version of the standard states “DIN EN ISO 10683:2014, fastening element – non-electrolytically-applied zinc flake coating”. The data stated in the standard refers to worldwide standardised bolts with metric dimensions and a flank angle of 60°.
Firstly, some theory regarding size and manufacture of bolts and screws
- They are categorised according to the outer diameter of the thread. A category M6 bolt has an outer diameter of 6 mm, a category M10 that of 10 mm.
- Thread tolerance categories refer to the manufacturing precision of fasteners.
- This category designation can be supplemented by the lower case letter “h” if the thread goes outwards, as with bolts (the large case letter “H” is added where the thread goes inwards, as with nuts).
- The letters “g” and “e” added to a thread tolerance category indicate how many micrometres a bolt or screw has been designed in order to create space for a protective coating.
M10 bolts of the tolerance category 6h have a tolerance of up to 0.236 mm. This tolerance is subtracted from the zero line – the outermost dimension. The outer diameter of such bolts is therefore between 9.764 and 10 mm.
If such bolts are coated, their thread is slightly thicker on the base, the flanks and the tip. The counterpart to the bolt, such as a nut, is tighter as a consequence. It is possible to calculate just how tight. Decisive here is the height of the protective coating on the flanks of the thread: the flank diameter is increased by four times the coat thickness respectively. For example, if the coat thickness of a M10 bolt is 15 µm high, the flank diameter is increased by 60 µm. The counterpart to the bolt – i.e. the nut – would consequently have 60 µm less space available.
The technical solution is simple
- Bolts that are intended to be coated are designed in a way that provides space for the protective coat, They are somewhat slimmer, but in turn offer sufficient room on the thread flanks when coated. Bolts such as these are manufactured for very thin and slightly thicker coatings.
- The flank diameter of M10 bolts of the tolerance category “6g” is 32 µm thinner, the height of the protective coat on one flank may be up to 8 µm. The outer diameter of the bolt is around 16 µm smaller.
- The flank diameter of M10 bolts of the “6e” tolerance category is 67 µm thinner; the height of the protective coat may be up to 16.75 µm. The outer diameter of the bolt is around 33 µm smaller.
For each individual bolt size and tolerance category there are different values for g and e. These are compiled in tables.
Protective coats are usually unevenly distributed in threads – for example when zinc flake coatings are applied to bolts or nuts using the dip spin process. The coat is then somewhat thicker on the base of the thread than on the flank. In contrast, on the tip of the thread the coat is slightly thinner than on the flank.
The correct design of the thread can be checked using ring gauges in the case of bolts and plug gauges in the case of nuts. In the case of plug gauges, in 2012 the Deutscher Schraubenverband passed the special guideline “Inspection of threadability of nuts with zinc flake coatings”. The reason: due to production reasons, scooping parts such as nuts have an increased coating thickness. The plug gauges are designed in such a way that they take account of the thicker coat thickness at the base of the thread. Experience shows that the threadability of the nuts is not influenced by the thicker coat on the base.
Three thread diameters
- The outer diameter is the distance between the outer points of the thread.
- The core diameter is the distance of the inner thread points.
- The flank diameter is the distance between two opposing thread flanks, measured from the middle of one flank to the middle of the other.
All of these distances are measured vertical to the axis.