The True Effect of Coating Thickness

The true effect of the thickness of a coating layer (plating or painting) on fastener is much larger than most people think.

Take a M6-6g bolt (1mm pitch) and a M6-6H nut (1mm pitch) for example.

Will they still fit if a 5 micron zinc plaing is put on both of them? 

Let us check ISO 965-1 on tolerance and deviation of 6g/6H first.

  Grade 6 Tolerance  Fundamental Deviation 
Minor diameter of internal thread 236 micron H:+0 micron
Major diameter of external thread 180 micron  g:-26 micron

According to the table, when a bolt and a nut are made at maximum material condition, the gap between the threads will be 26 micron.

It sounds much larger than a 5 micron thick zinc layer, however, it might cause bad interference. 

There are two factors to make a superposition.


Firstly, not going to threads yet, consider a cylinder rod and a hole situation.

When a rod is "wrapped" by a 5 micron layer, the diameter actually increases by 10 micron, and this applies to hole features, too.

In the other word, a 5 micron layer on them will take out 20 microns gap between a rod and a hole.


Secondly, there is additional factor for thread features.

The 5 micron layer on 60 degree thread will actually leads to a 10 micron effect in radial direction by the following calculation:

(5/cos60 = 10) 

With superposition of the factors above, 5 micron layers will take out 40 micron clearance between inner thread and outer thread.


Going back to the initial question, a brief answer that Sun Through can provide here is:

it depends on the tolerance left by the maker for fitting, but they will interfere if the fasteners are made in maximum material condition.