Short story from 50s-60s
War kills off all humans
Fighting machines are old and failing
Try to reproduce selves and fail
1) robots build body and 'brain' , but trying to solder the brain in place, they cause a short circuit and it melts
2) robot tries to touch a butterfly and crushes it
I have spent hours on the net searching and posting on Sci-Fi sites with no luck.
As I recall, it was in a large anthology-type book. Probably read this in the very early 1960's.