M8 dome nut
thingiverse
wanted some M8 dome nuts to cover the rod ends on my new mendel build. found http://www.thingiverse.com/thing:27769 and added a dome with some clearance inside. Instructions printed in black PLA at 0.2 layer and 50% fill and came out looking great. not sure if its my printer but the thread did not come out great but it is very fine, I run an M8 tap in the hole and they work very well. I nipped them up with a 13mm spanner which is a very good fit and was surprised how much force they will take. If you bottom them out you will split the dome off the end but I am only using them to tidy up my build and not to hold any load.
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