Quote:
Originally Posted by SerpentEagle
I am using an aluminum block. Is it really that bad with aluminum? I've seen teams use aluminum blocks with threads before, and we've done it before ourselves without many problems.
The coupling nut in the block is an interesting idea. Have you guys tried it before?
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We haven't put one inside of a metal block spacer like this, but we did use 1/4-20 coupling nuts with machine screws on both sides to hold some flat angle brackets (used as gussets) alongside some 1-1/4" square x 1/8" thick c-channel for a practice chassis last summer. If we were building this today, we'd probably go with two or three times as many holes and rivets - you can achieve similar strength with much less weight that way. Actually, as I look at your picture (which I can see now that I'm at home), coupling nuts wouldn't help you much as they usually run about 4x the screw diameter, or 20 coarse threads.
Even though I can now see the pictures, I'm not sure what the left horizontal bar for (nor any of the others) or how the loading will proceed. That said, securing the two long horizontal holes to the vertical tube will not add very much to the situation. With the two holes above and the two below, the two in the middle will only contribute strength if things are being pushed right to the edge on the outer four. Therefore, I would suggest tapping the center holes in the aluminum block and putting bolts into those threads only from the horizontal tube side. Unless you have some unusual loading, you can probably get away with tapping all six holes, running four bolts from inside the vertical tube for the upper and lower holes, and two longer bolts from inside the horizontal tube shown in the first pic.
Finally, FWIW, five threads and the nominal shaft diameter are essentially the same number for coarse threads; there are usually six or seven threads per nominal shaft diameter for fine threads. I suspect that maximum safe loading is actually somewhere between the number of threads and the number of diameters. This is because the frictional forces increase for finer threads, but the shear strength of the threads themselves is greater for coarse threads.
There is a loss in strength for a tapped hole vs a lathed hole, but just based on geometry considerations, it's probably more like 15-25% than 50%.