When two parts need to be precisely aligned. It’s tough to beat a two pins. One goes into a hole (that fixes X + Y translation), and the other goes into a slot (that fixes rotation).
It works great, but it’s always seemed a little sloppy to me. Then I was turned on to diamond pins.
Carr-Lane says it best
When a Round Pin and Diamond Pin are used together, the Round Pin is the primary, 2-axis locator, while the Diamond Pin is the secondary, 1-axis locator. Diamond Pins are relieved to locate only radially around the Round Pin, avoiding redundant location. This prevents binding while loading and unloading the part, yet still maintains high accuracy.
The first time I read this I saw “blah blah blah…. non binding…. blah blah blah”. Two pins in two holes can be such a headache, it’s nice to have this in your back pocket.
Also check this out for more than you ever wanted to know about locating pins.
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