Quote:
Originally Posted by GeeTwo
Tapping 1/8" aluminum for machine screws is an exercise in futility - for any thing heftier than holding an encoder in place, they're almost certain to be stripped out, or rattle loose, or both. Always end in steel, or 1/4"+ aluminum. Self-tapping or sheet metal screws in 1/8" aluminum are not too bad, but can still be stripped out more easily than I'm comfortable with for the FRC team.
When we've used bolts and gussets with tubing or c-channel, we've usually gone all the way through, and used gussets on both sides to help distribute the load (minimize deformation of the tubing). Never use a single hex nut for any joint under load or vibration. Either get nylon locking nuts, or jam two nuts together to lock them in place. Sprocket-type lock nuts work when they're new, and being pushed against steel (they often mangle aluminum rather than grip). They don't work as well after they've been flattened - and how many teens will notice the difference?
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Quote:
Originally Posted by sanddrag
I have to respectfully disagree. I've tapped 1/8 aluminum quite successfully in every thread from #4-40 up to 1/4-20 and it works just fine if you're careful about it.
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I have to agree with GeeToo that tapping 1/8" aluminum is not a good choice. The general rule of thumb is that for a hard material like steel the thickness of the tapped material should equal the diameter of the fastener. For soft materials with a hard bolt the thickness of the tapped material should be 2x the diameter.
So for a #10 bolt you would want the tapped material to be ~3/16" if it is steel and 3/8" if you are tapping aluminum and you want the full strength of the bolt.
If you look at standard hardware that you'll find at your local store a 1/4" nut will be 1/4" high, a 1/2" nut will be 1/2" high ect. Note that is for a standard non lock nut, nylocks, stover nuts, kep nuts ect will be slightly taller to account of the locking portion. This holds true whether the thread is coars or fine and regardless of the strength/material of the fasteners since the assumption is that you will use the same strength/material for the nut and bolt.
Now that does not mean that you are guaranteed failure is the tapped material does not follow those guidelines, just that the ultimate strength of the combination will not be realized.
In regards to the original question I prefer a bolt and nut through just the gusset and one wall of the tube. That does mean that the order of assembly must be considered and held to since if you are joining two tubes at right angles you will block access to one of the tubes with the other tube. This method will result in the ultimate strength since you'll have double the fasteners and often with less weight. Yes it is more difficult to assemble.
The other option is to use a rivnut. But for many applications I prefer a high strength pop rivet, as others have mentioned.