Wednesday, May 23, 2007

Fasteners vs. Adhesives

I'm looking at how to fasten six pieces of aluminum together to form a hexagon for the mirror cell. The rectangular sections have a 60 degree cut on each end. Ideally, these will be made from 1/8" thick aluminum (which is stiff enough to support the weight of the mirror without bending). Here's the problem: how do you fasten them together?

I had thought of drilling and tapping two small holes through adjoining sections, and then using two small screws to lock them together. The problem is that you need very small screws to do this. A 1/8" thick section cut at a 60 degree angle gives you approximately a .1975" face. A number 6 screw has a major diameter of .138"--not leaving much material on either side of the screw. (And yes, I would be locating and drilling these holes with a vertical mill.) A number 4 screw (about as small I want to go) has a .1150" major diameter--still not a lot of material on either side.

Two alternative strategies:

1. Use 1/4" Delrin instead. It will be about the same weight as 1/8" aluminum, actually slightly stiffer (because of the extra thickness). This way I have a .395" face, so a number six screw has lots of material on either side. I also don't have to blacken the Delrin (it is already black)--just sand it a bit to make it non-reflective.

2. Use an adhesive to bond the sections together. A friend tells me about a new miracle adhesive from Loctite that Ford is now using to hold its cars together not only because it ways less than bolts, but adds vibration absorption as well.

Can anyone suggest some other approaches that give great stiffness without adding weight or complexity?

UPDATE: A reader suggests making 120 degree angle brackets that would go on either the outside or inside of the hexagon. I could make these out of 1/8" aluminum, and because I am not using the inside corners of the hexagon (a round mirror goes in there), I could put them on the inside. This has two advantages:

1. I would have a total of 1/4" of aluminum that I could drill and tap. Since this isn't carrying much of a load, a number 6 screw would work fine.

2. I would not need to have thousandths of an inch accuracy on the holes. This speeds up the process, since I could use the drill press instead of the vertical mill.

On the other hand, putting the brackets on the outside puts clamping force in a direction that would help hold everything together more tightly, I think.

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