Back in the day, Tool tapers were set by the machine manfactruer and we just took what we got.
Then came along CATapiller, they said NO! they demanded a standard taper for all machines they purchased, today we have CAT 50, 40, and 30, and we have the Jap standard Taper, but I can't remember the name right now and I have a meeting in about 30 sec. so I'm trying to be quick.
the reason for the Notch, easier to get the tool out, if you have engagment full length the taper stuck.
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Be careful of what you wish for .....