When a part is returned from the plater after hard chrome plating it is likely to have changed in size. Understanding how the thickness of a part is specified and controlled will help to minimise any potential problems.
How Thickness Is Measured
Thickness is measured on chrome plated components using magnetic induction gauges on steel parts and eddy current gauges on non-ferrous components. These methods of thickness measurement are non-destructive and measured in microns. It is essential to establish which method of thickness measurement the plater is using against which substrate, prior to agreeing dimensions for measurement on your components. Thickness is measured at all exposed surfaces on a component and hence is sensible to measure at several locations on a part.
Allowing for Added Stock
When machining components pre-plate the surfaces that are to be plated (after masking) must be sized to allow for the material to build up on unmasked surfaces. This is known as the ‘allowing for added stock’ rule. For example, a bore of 50.000mm diameter post-plate may require the pre-plate bore to be machined to 48-49mm diameter depending on the intended deposit thickness and number of sides to be plated.
As-Plated Versus Ground Tolerances
As-plated tolerances are typically rather loose (±25 micron or worse), however dimensional parts are typically ground dl_September 26 Poetons Article 13 – Chrome Plating Thickness and Tolerance Limitsto final size after plating to produce dimensionally accurate components (tolerances of a few microns). Hydraulic rods and bearing surfaces are typical examples of components that are ground after plating. More on Chrome Plating can be found at www.poeton.co.uk/advanced-treatments/apticote-100-hard-chrome/.
Typical Thickness Ranges by Duty
Functional wear coatings typically run between 25-75μm (although this can vary depending on application). The thicker decorative coatings for example can range from 0.3-1μm and are generally not wear resistant. Larger thicknesses are used for dimensional restoration to very heavy build-up which is subsequently ground back to size.
How Geometry Affects Uniformity
Sharp edges can plate up to twice the thickness of flat surfaces of similar area and corners can plate up to three times the thickness. Internal corners and recesses will generally plate a thinner layer than external edges and high points. Flag up any critical geometry to the plater before commencing work on a job.
What to Check on Return
It is essential that a report of the dimension of the part after plating is provided, with thickness recorded at each of the specified locations. Additionally, check the edges of the masks for any staining and that none of the plating has escaped toSensitive areas such as threads and bearing surfaces that were masked off prior to plating, surface roughness and local pitting.
Getting the specification right for hard chrome plating before you start, saves a lot of time in the long run.
