Hi All,
Most of my tooling has been collected from auction sales and odd lot purchases.
While searching thru my motley collection of countersinks, I started wondering
about all of the different angles.

60 degree included angle matches lathe centers -- but I do not ever recall
using anything other than a combination center drill/countersink for this
application yet I have some single flute and multi-flute 60 degree
countersinks. Any other applications?

82 degrees included angle seems to match most flat head machine screws --
though I do not understand why the industry would have standardized on 82
degrees rather than some nice round number.

90 degrees included angle seems to match a few machine screws and is the
intuitive choice for deburring and chamfering. Would have seemed like a
logical choice for all machine screws.

100 degrees included angle -- I do not know why I have any of these

120 degrees included angle -- unclear on what these are used for as well.

Can anyone offer any insight? Seems like there must be some logic, but my
meeger collection of reference books is silent on the topic.

Thanks,
Mill