As we remove material with end mills in the form of chips, a wear land forms on the cutting edge. When this wear land develops to the point of generating excess heat, vibration, roughness, or  produces a part that is out of tolerance, the end mill should be pulled.
On high volume operations, a predetermined number of parts minutes in the cut, power draw on the machine, or cubic inches of material removed can be established as the point to pull the end mill.
When an end mill becomes dull, we face a choice of whether to regrind or discard the end mill. This choice is basically an economic one.  Will it cost more to regrind then it will to replace it. End mills 1/4" in diameter and above can generally be economically reground if there are sufficient quantities to absorb setup cost.
As the diameter of the end mill is reduced through regrinding, there is a significant reduction in the hook or radial rake angle. The original diameter can generally be reduced by the following and still retain up to 80% of the original cutting efficiency.
.250 thru .500  15%
.562 thru .625  12%
.750 thru 1.00  10%
Over 1"             8%
End Mills that are badly chipped or severely worn to the point that the diameter will need to be reduced more than the above guidelines, will need to be refluted to restore the original flute form, and radial rake angle.

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