Rule of thumb 10 to 20% of the diameter of the cutter. But as HowardT says the limit on a small mill is likely to be the power of the motor and the rigidity of the machine (weight, wear and adjustment), also how sharp the tool is, carbide vs HSS, whether you're cooling, the properties of the steel, and how firmly clamped the job is etc. I like my mill to sound as if it's working hard about 10% short of sounding in distress. Roughing out I normally go in at about 15% diameter and adjust the feed-rate to suit my 'no distress rule'. My hobby mill is capable of taking hard fast deep cuts when I'm in a hurry, but I'm sure it appreciates the lighter touch. No smoke or broken gears!  Owners of industrial machines could be more aggressive.

Dave

Edited By SillyOldDuffer on 22/06/2018 12:13:23

As Dave says on a 1/2" cutter about 20% depth is the most I would ask of my X3 and more than likely 15% would be what I would use more often than not. This is 2.5mm deep with a 12mm cutter so about 20%