[seanc]

Ken:

I've never seen a "new" manual, so can't comment on why they would recommend a higher current density for cast iron. However, it's more realistic than the old manual. 20 amps/sq-ft comes out to 140 mA/sq-in, while the old manual said 25 mA/sq-in. I've done a few cast iron parts at 80-100 mA/sq-in and they came out fine. When I ran tests on steel at 150 mA/sq-in, it was worse.

Figuring out EXACT surface area of something like a caliper is near impossible, but here's what I've done for carburetor bodies (also got lots of nooks & crannies & weird shapes):

Take dimensions of each of the 3 major sides, like you'd see in a 3-view mechanical drawing. Calculate each side, add the 3 together, then double it. Plate it at the high end, 100 mA/sq-in. Any extra surface area added by the nooks & crannies should still keep your actual current in the 70-100 range.

Keep in mind that the amount of "frothing" you see depends on both the current density AND the size of the part.

For instance, a 1" piece will hardly produce any frothing at all, but a 100" piece will produce LOTS of it, even at the SAME current dentsity, simply because it's giving off 100 times as many bubbles.

If your anodes are too close, you won't get good coverage. You might try rotating the part, so different sides get better exposed to the anodes.

So just how big is the caliper, once you get a good estimate? And then how big are your anodes? If the anodes aren't at least as big as the part, you won't get good coverage either.

If your plating is cracking, then it's a good bet that it's being overpowered & burnt. Electricity will take the path of least resistance, and if the anodes are too close (how close are they?), then all the plating is going to the nearest portions of the caliper, leaving other parts uncovered.

You need a few inches of separation even for small parts. Larger parts need more for even coverage.

Sean