Coating allowance
Anodising grows about half its thickness out of the surface and half into it; plating sits wholly on top. A diameter changes by twice the growth. Pick the coating, enter the finished size and its deviations, and get the size to machine to before coating. The coating’s own thickness variation comes out of your tolerance, so a tight size with a loosely specified coating may leave no machining band at all.
- Machine to, largest
- Machine to, smallest
- Deviations from 25
- −61.0 / −90.6µm
- Machining band
- Size change
- +40.6 to +61.0µm
- Growth per surface
- 20.3 to 30.5µm
- Above the original surface
- 50%
- Coating's share of the tolerance
- 41%
How it’s calculated
Each coated surface moves out by g = s·t, the share s of the thickness t that stands above the original surface. A shaft Ø or two faces change by +2g, a bore Ø or a slot by −2g, one face by +g. Machine the low limit to the finished low limit less the least change, and the high limit to the finished high limit less the most.
Coatings
| Coating | t, µm | Above | On a shaft Ø, µm |
|---|---|---|---|
| Anodise Type I, IB, IC, IIB · Class 1, 2MIL-PRF-8625 | 0.5 to 17.8 | 50 % | +0.5 to +17.8 |
| Anodise Type II · Class 1, 2MIL-PRF-8625 | 1.8 to 25.4 | 50 % | +1.8 to +25.4 |
| Anodise Type III · Class 1, 2MIL-PRF-8625 | 40.6 to 61.0 | 50 % | +40.6 to +61.0 |
| Chromate conversion Type I, II · Class 1A, 3MIL-DTL-5541 | per drawing | 100 % | — |
| Cadmium plate Type I, II, III · Class 1AMS-QQ-P-416 | ≥ 12.7 | 100 % | — |
| Cadmium plate Type I, II, III · Class 2AMS-QQ-P-416 | ≥ 7.6 | 100 % | — |
| Cadmium plate Type I, II, III · Class 3AMS-QQ-P-416 | ≥ 5.1 | 100 % | — |
| Passivation Method 1 Types 1–8; Method 2 (Citric)AMS 2700 | no layer | none | 0 |
| Electroless nickel Classes 1–4AMS 2404 | per drawing | 100 % | — |
| Electroless nickelAMS 2405 | per drawing | 100 % | — |
| Chromium plate Class 1AMS 2460 | per drawing | 100 % | — |
| Chromium plate Class 2AMS 2460 | ≥ 50.8 | 100 % | — |
| Zinc plate thickness per drawing | per drawing | 100 % | — |
How much does hard anodising add to a diameter?
A Type III hard anodise with no thickness on the drawing is 0.002 in (50.8 µm) nominal, ±20 %. About half of it stands above the original surface, so each surface moves out about 20 to 30 µm and a diameter grows about 41 to 61 µm. A bore shrinks by the same. Machine a shaft that much under its finished limits, and a bore that much over.
The coating’s spread matters as much as its size: the 20 µm between the thinnest and thickest Type III coating on a diameter comes straight out of the size tolerance, which is why a tight size is often coated first and finished after, or the coating thickness is specified more tightly.