Though various factors (like Base metal thickness, Magnetic properties of the base metal, curvature etc) are responsible for plating thickness measurement errors (under the above principle), surface roughness is the main variable, as other variables are constant for a particular batch of components (as the raw material, design, machining, HT etc are common).

Mild steel tube of OD136 mm x ID122 mm x 210 mm length has been machined so as to have surface finishes of to (0.025 to 25 microns). Initially the surface was measured by using high precision equipment in Metrology lab. at specific locations. Measurements were also taken at the aforesaid locations by using "DELTASCOPE" (which works under magnetic principle). Surprisingly the Deltascope measurements are in line with the Surface finish measurements. Inconsistent readings were obtained in Deltascope measurements at the location where surface finish is > 9 microns (poor than ).

Cadmium plating was carried out with a aim to have plating thickness of 5 to 10 microns. After cadmium plating again surface finish was measured to ascertain the variation of surface finish if any, after plating. But the results confirmed that there is no variation.

Cadmium plating thickness was measured by using "Deltascope".

Hence plating thickness = D1-D2.

D1 = Deltascope measurements after plating.

D2 = Deltascope measurements before plating/surface finish.

As a supplementary to this analysis samples from few locations were cut and exact plating thickness was measured through metallography technique. Metallography results are found to be in line with the results obtained using the above formula.

Surface finish has got direct implication on the plating thickness measurements using magnetic principles. Hence the following conclusions are arrived from the above analysis.

• If the surface finish is (0.025 to 1.8 microns) or better - Direct measurements through ND method (magnetic principle) can be used as the max error would be only 2 microns.

• If the surface finish is (1.6 to 8 microns), cadmium plating thickness = D1-D2.

  D1 = Measurements of ND method (magnetic principle) after plating. D2 = Surface finish.

• If the surface finish is (poor than 9 microns) or worse, ND method using magnetic principle should not be used, as it gives inconsistent results.