When, in order to provide improved corrosion resistance, it is required to deposit coatings
thicker than those that appear in Table 2, or alternatively, when it is required to apply
coatings to parts of pitches smaller than appear in Table 2, it is necessary to manufacture
threads to special limits and tolerances.
The minimum pitch limits of applicability in Table 2 can be lowered if for any particular
thread the normal tolerance is confined to the range near the minimum material limits
(external threads) or maximum
material limits (internal threads). This provides a greater fundamental deviation or, in the
case of tolerance position H, it provides a deviation which does not otherwise exist.
Alternatively, the whole tolerance zone can be displaced to provide a greater fundamental
deviation.
The minimum fundamental deviation required for particular pitches and deposit thicknesses is
given in below Table.
| Coating thickness | Minimum fundamental deviation, μm | ||||
|---|---|---|---|---|---|
| If local thickness measurement is agreed | If batch average thickness measurement is agreed | ||||
| μm | All nominal lengths | l ≤ 5d | 5d < l ≤ 10d | 10d < l ≤ 15d | |
| 3 | 12 | 12 | 15 | 18 | |
| 5 | 20 | 20 | 25 | 30 | |
| 8 | 32 | 32 | 40 | 48 | |
| 10 | 40 | 40 | 50 | 60 | |
| 12 | 48 | 48 | 60 | 72 | |
| 15 | 60 | 60 | 75 | 90 | |
| 20 | 80 | 80 | 100 | 120 | |
| 25 | 100 | 100 | 125 | 150 | |
| 30 | 120 | 120 | 150 | 180 | |
| NOTE: Since larger fundamental deviations for the thicker deposits may significantly reduce the thread engagement, their application should be agreed between manufacturer and purchaser. | |||||
Where a thick deposit is required on a portion of the fastener, for example the heads of bolts or blind nuts, it is often possible to use the procedure of selective electroplating. In such cases the thickness of the deposits applied to the different areas of the part should be specified.