Given fixed parameters for temperature, plating solutions, anodes, set-up, and current density, thickness is a function of time. Expect around .75 to 1.2 mil per hour of plating time.
I have plated up to 20 mills successfully at home - admittedly this was by accident because I was aiming for 3 mills deposit to refinish a piston! It had previously taken six hours using a particular chromic acid solution to deposit 3 mills of excellent chrome. I thought to shorten plating time I would increase the current density from 600 mA to 800 mA and the temperature of the solution was tweaked from 450 C. to 500 C. (1 13oF to 122o F). I then plated, with agitation, for five hours and wound up with an hour-glass shaped piston, due to a 13 mill chrome deposit measured at mid-skirt level and 21 mils on the edges (formed by the bottom of the skirt and the piston crown).
Let that be a lesson to all of us: Never change more than one parameter at a time. Subsequently, grinding of the same piston was successfully carried out; which attests to the excellent adhesion of the chrome to the base metal (steel) as prepared earlier.
Of course, the piston was then lapped to a perfect fit in the re-lapped bore (no rings involved in that 0.020 cu.in. engine). We'll come to the grinding and lapping notes later. Chrome will lap to a superb finish, to a degree of precision obtainable by no other method and limited only by the machinist's patience and skills.
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