The process is called jointing the knives. Where a flat is created on the very tip of the knives. This flat can only be not more then 1/32", then the knives must be ground again.
On a planer with grinding attachment, after the knives are ground, a stationary stone is attached, then traversed along the "running cutter head" to establish the flat-and bringing the knives to exact height.
If the jointing has gone too far, the knives will begin to rub instead of cut.
On a jointer a stone is moved across the running cutter head by hand. There are those that still do this.
Of course, this is somewhat dangerous thing to do, and needs full attention!
But, it is a way to get exact even knife height on a planer or jointer without an expensive dial gage…and a quick way to touch up the edges between full grinding.