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Surface Area Calculations for Rough Castings

Q. Does anyone know how much more surface area a rough casting has than a machine finish?

I know there are many differences in machined finishes as well. But for plating calculations, there is a big difference in something as smooth as a hull cell panel compared to a rough casting.

Are there any general multipliers for rougher surfaces to account for the extra surface area? Any good rules of thumb?

AJ Calderone
- Greensboro North Carolina
January 2, 2024

A. Hello AJ,
Without the proper measurement instruments it's very hard to arrive at an exact area calculation. If you know the machine finished area you can estimate a percentage gain that the rough casting has. Normally you will have a current density range (ASF) anyway. This range should be listed in your technical data sheet. Without seeing the difference between rough and finished castings it's hard to tell for readers. If the difference is minimal you should be fine adding a percentage of area or work within your current density range.

Mark Baker
Retired - Cazenovia, New York USA
January 4, 2024

A. Hi AJ. Exactly what goes on in the boundary layer is for larger brains than mine, so the difference between what will go on in a straight vs. a wavy boundary layer, and how wavy the boundary layer will be on a rough casting, is far beyond my intellect. But I agree with Mark that the ASF difference is probably minimal despite the roughness, and just a small adder should suffice.

But the question of surface area of things comes up frequently in electroplating; some readers will find this ramble in the woods helpful and others will find it vacuous, but here goes --
A meeting room's 'capacity' can be viewed in terms of how many seats it has, but if an event that is being held in that room has extremely constrained entrance & exit times, the size & number of doorways might be a better measure of its capacity than how many chairs it has, due to the limited rate of passage through those doors. When we are talking about current density issues, the current must also pass through doors, such that the dimensions of an anode basket can be used in determining the anode:cathode ratio, rather than having to estimate the surface area of its partially dissolved anode balls. I suspect that when you are plating a cast surface, if the roughness is reasonable, a similar situation applies.
Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey

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