As with all aspects of the plating process the thickness of hard chrome deposits can vary from application to application, often more so than the chemistry involved.
Functional Versus Decorative Thickness
Functional chrome thicknesses range from 25 to 500 microns or more, whereas decorative chrome thicknesses typically are in the range of 0.25 to 2.5 microns. The latter is used for appearance only.
How Tolerances Are Specified
e. a tolerance applied in one direction only). On hard chrome plated parts, the most typical method of specifying a tolerance is with a unilateral plus tolerance. This is because it is generally an easier task to remove plated material than it is to add it back. The achievable tolerance for commercial chrome plating is typically in the range of plus or minus 12 to 25 microns, with work of a higher quality being held to a tolerance of plus or minus 6 microns or better. Hard Chrome Plating is covered in more detail at https://www.poeton.co.uk/advanced-treatments/apticote-100-hard-chrome/.
Chrome Plates on the High Side
A typical chrome deposit is highly non-uniform, often greater on edges and raised surfaces than on adjacent flat surfaces. A number of methods exist for finishing plating to exacting tolerances, typically specified off the plated dimension with an allowance of 25 micron or so per surface for grinding to the final dimension.
Calculating Stock Allowance
When designing a part for hard chrome plating it is necessary to determine the amount of stock required for grinding after plating. A rule of thumb for the stock allowance for grinding is to allow 25 microns per surface (i.e. both sides of a shaft). The stock to be removed for grinding after plating would be calculated by subtracting the plated thickness from the finished dimension of the part. For example, a shaft is to be plated with 100 microns of chrome. After plating the entire surface would be ground to the finished dimension. The stock to remove for grinding would be 150 microns per side (100 x 2 = 200 and then divide by 2 = 100 and then add 50 more for allowance). Therefore the shaft would be machined 150 microns undersize on both sides before it is plated.
Geometry and Current Density Effects
Complex shapes, long holes, large surface areas, recessed areas and external corners all have an influence on the even distribution of chrome. As a rule of thumb, areas of lower current density receive less chrome, whereas external corners receive more. This must be taken into consideration when defining close tolerances on features. The only way to achieve this would be by conforming the anodes to the workpieces, or by applying a plating mask.
Knowing these numbers before you even start to draw up a specification for hard chrome plated components can save a lot of time and money later down the line.