Like many mill-drills, mine came with a depth stop that consists of a threaded rod (1/2-20) with a longitudinal flat on it. (See first picture below.) The actual "stop" consisted of two nuts; one acted as the stop and the other as a lock nut to hold the stop in place.
Calling it a poor design would be a compliment. The nuts had too little inertia to spin freely so one had to tediously turn them by hand, then repeat the same operation with the so-called lock nut. Precise adjustment was nearly impossible and the lock nut seldom stayed tight so the setting would drift.
I solved part of the problem by turning a heavy, elongated brass nut (see second picture) that had enough inertia to spin rapidly up and down the thread if given a healthy twist. Rather than fuss with a lock nut, I threaded the stop for five 8-32 setscrews. Tightening one of these against the flat holds the stop in place with no chance of drift. I used five screws because that makes for a convenient setting interval. (1/5) * (1/20) = 0.01" so each setscrew position represents a 0.01 inch change in depth relative to its neighbors.
I only use this stop when I need very fine control over the depth setting. For less demanding work, a clip-on commercial stop of the type shown in the third picture is quicker to use. It consists of a spring-loaded "clothespin" with a split hole threaded to match the depth rod. Repositioning it is done very easily and it doesn't slip but it does lack the fine position control of my design.