Having watched this I am at a loss to see what is being adjusted by the Allen key.
It is adjusting the stopper, that limits the depth being removed from the cutter being ground.
I spotted the stop after I posted. Lol but I was blinded by the fact he is grinding on a round wheel. But it's trying to get a flat on the end of the mill.
He's not trying to get a flat on the end of the cutter - he's grinding a concave facet.
There are tables giving all of the 'heights above centre' for different sized wheels - here is one from the Norton hanbook.
OMG looks like paying someone to sharpen a few for me lol
Anyone on here do it as a side line or retirement rates..
With that cutter you only really cut on the sides, unlike a slot drill. All he is doing is cutting clearance.
A chap advertises on Home Workshop from time to time. He donates the money to charity.
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