Quote:
Originally Posted by JamesCH95
The number I know is 3 complete turns if the threaded material is as strong as the fastener. Because high-grade aluminum can be about 1/2 as strong as an alloy steel bolt (this is subject to wide variances) 6 turns would suffice.
Now, if the application actually requires the full strength of a selected fastener is a whole different matter...
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And thus we risk diving into an ongoing debate.
The guidance I have received from a number of sources is that 3 threads is the minimum to get the appropriate geometry for the threads in the hole. If you think about what having a thread or half a thread means...well that's not very reliable.
As for strength, there are definitely places on a robot where the strength of steel is needed, so there steel should be used. But for joints in compression (like say the corners of the frame, or a gusset plate in the corner of a frame), you probably don't need the full strength of steel in the fastener. Most of the time, the load will be carried by the interface between the two pieces being joined, not by the fastener itself (assuming it's properly tightened). Joints in tension are another story, where the load IS carried by the fastener.
And, as I mention, the required strength of the connection is a factor. The design guideline we use on my team is that all of the sheet metal structural connections get PEM nuts, so that we have a steel nut with a steel bolt for strength. Plates 1/4" thick and over are tapped without the PEM nut. We do, however, tap 1/8" aluminum sheet metal for control system mounting holes (jaguars, PDB, sidecars, etc) using #8-32 or #10-32 cap screws.
Regarding fabrication, if you get the correct PEM nut (that would be a self clinching nut in the S, SS, CLS, or CLSS series...each series denotes a different heat treatment or finish), they can be installed using a press or even an impact driver (though I wouldn't recommend the latter). We've used 1/4"-20 fasteners in the past, but the nuts are substantially harder to clinch than the #10 or #8.
There are other very interesting options from PEM. For example, they have PEM studs, which are threaded connections that stick out of the sheet. I find these quite appealing for the aforementioned control system mounts. There are also floating fasteners, which are intended to account for variations in manufacturing. They basically "take up" the tolerance, and allow for a slightly fudged hole to still properly fasten. For example, using a floating fastener is probably a good idea if you are going to bolt down all four corners of the PDB.
Yes, there was a time when I thought lock nuts and cap screws were all there were to fasten things. Then, I did what good engineers do...sought more experienced help. That's lead to a much more enjoyable build and less maintenance at competition...which is good for all.