Surface Area

On the deceptive nature of surface areas

I began zinc plating as a tool for my primary hobby: restoring an antique automobile. Being able to keep and reuse original parts is a big factor in judging a restoration, and I wanted to do it right.

Well at first I looked at a handful of small parts (nut, bolts, clips, etc) and thought "That looks like about 3-4 square inches". Boy was I ever wrong! Trying to judge surface area by the "mark-1 eyeball" method is very inaccurate.

Using the "dip basket" screen as an example: when I first thought about it, I figured a 3 x 5 inch piece would have about 5-7 sq-in. (3x5 = 15, but it's all full of holes, which don't count), but when I actually calculated it, it turns out to have 3 4- times what I estimated. I've had a few people tell me my figures must be wrong, but I can only reply "do the math".

So here's the math: Take a 1" square piece of the 16 mesh copper screen

• 16 mesh (16 wires per inch), gives you a total of 32" of linear wire (16 horizontally & 16 vertically)
• Wire diameter = 0.015", so wire circumference = 0.0471"
• 0.0471 x 32 = 1.51 sq-in of surface area per sq-in of sceen!
• 1.51 x 3 x 5 = 22.6 sq-in for a 3 x 5 piece of screen!

Amazing isn't it?

A flat 3 x 5 SHEET of copper (same thickness) only has 30.24 sq-in! So all those "holes" in the screen really only reduce the surface area by @ 25%.

My initial guess was 1.5 sq-in., but the math says 3.85 sq-in! The threaded portion of the bolt adds quite a bit of surface area.

• Consider a straight line 1" long. Obviously, it's length is 1".
• Now zig-zag it at 60º angles for 1" (like the threads in the bolt).
• What's the length along the zig-zag? 2 inches!

So whatever length ot threads you have on a bolt, you have to effectively double that length for surface area calculations!

Bottom line? Don't trust your "eyeball" to estimate suface areas.