Sunday, December 16, 2007

Big Bertha Rebuild: The Joys of Aluminum Fasteners

Big Bertha Rebuild: The Joys of Aluminum Fasteners

I mentioned yesterday
that I was going to buy some shorter bolts for the mirror cell. When I reached Home Depot, I was overjoyed to discover that they had a number of aluminum machine screws in the sizes that I needed. Reasons for switching from steel to aluminum for the mirror cell fasteners:

1. About 1/3 of the weight. I've knocked three ounces off the mirror cell this way. Final weight is almost certainly going to come in at or below four pounds.

2. If I get the cell anodized, I can get all the fasteners anodized as well.

3. Aluminum to steel interfaces means a bit more risk of corrosion (although Big Bertha is not going to be standing outside in the rain, and this isn't a moist climate). Aluminum to aluminum knocks this out completely. I'm told that using galvanized steel with aluminum helps; no surprise, because aluminum's electronegativity is 1.61, zinc's is 1.65, and iron (primary component of steel) is 1.83. Aluminum and zinc are so similar that their would very, very little electron flow, while aluminum and iron would have a bit more.

4. For certain applications, aluminum's lower tensile strength relative to steel means that I would have to go to much larger bolts. As I mentioned some weeks ago, two 1/4"-20 bolts should be many times more than sufficient to hold Big Bertha to the dovetail plate--but I will probably use four, just to be overcautious. But that's a load of more than 50 pounds. Here, I have a roughly 30 pound load distributed across six 1/4"-20 bolts in the mirror cell, and across three 1/4"-20 bolts holding the mirror cell to the tube. I may end up using galvanized steel bolts to hold the mirror cell to the tube--just to get the advantage of a dramatically higher tensile strength.

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