As with any electroplated finish, Zinc Nickel Coating adds material to the surface of the parts. If you are specifying or designing parts to be plated then it is wise to know the following information to avoid potential problems with dimensions of finished parts.
Typical Deposit Thickness
The typical thickness for Zinc Nickel Coating is in the range of 8-25 μm (0.3-1mm) with the average thickness for corrosion protection in the range of 12-15 μm (0.5-0.6mm). For the purposes of calculation to determine the impact of the coating on the overall dimensions of a part, it is best to double the thickness of the coating for a given diameter of a part (e.g. shafts or rods).
Shafts and Bores Behave Differently
Coating on a shaft builds up on all sides. The thickness of the coating therefore adds to the diameter of the shaft. The thickness of the coating is to be added to the internal bore of a hole, and this will reduce the clearance of the bore. This effect needs to be considered for close tolerance parts.
Working Back from Your Finished Tolerance
Machined parts that need to be within a few tenths of a mm can be machined to the lower tolerance before plating. Then, measuring the uncoated part and adding the appropriate amount of Zinc Nickel to check that the plated part still falls within tolerance.
If you are looking for Zinc Nickel Coating, see www.poeton.co.uk/surface-treatments/plating/zinc-nickel-plating.
Grinding After Plating
For tighter tolerance parts (better than 0.05mm), consider light grinding of the part after plating to achieve required part dimensions. For this approach, you would typically specify a heavier plating thickness (in the order of 20-25 microns) and then grind back to the required dimensions. The coating will still provide the required corrosion protection.
Using Masking Selectively
Masking, using caps, plugs or tape, allows you to coat around items such as threads, bearing seats or press fit locations. You must inform your supplier of items to be masked and provide them with the dimensions of the part(s) and the areas to be protected. Better to give them written instructions than verbal.
What to Tell Your Supplier
Supply finished part dimensions, the desired tolerance and any areas that will require masking. This will enable your supplier to establish the most effective process to meet your specific requirements, to prevent the majority of plated parts being scrapped.
Dimensional surprises can be avoided by a little preparation at the design stage.
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