Several years ago I got this nifty Mill Drill. It's been a powerful and helpful addition to my shop. 
Although it came with a depth gauge-the one thing it lacked was a proper depth stop.
The gauge let you know how far you had gone down in the Z axis but there was no stop for repeatable operations.
I decided to build a stop. 
Here are some renderings of the design
I started by replacing the plastic cover with a shop made aluminium cover but I kept the scale from the original cover.
I made the fixed stop bracket to mount to the cover.
The new cover protruded out further from the Mill than the original plastic cover which meant I need to make an extension for the depth wheel .
The original interior threaded rod was 1/2 -13. There was nothing to keep the depth gauge straight as it moved up and down as there was some slop in the threads. 
So I replaced the interior 1/2-13 rod with a 1/2" acme threaded rod.
Here is the original rod with a captive coupling that I wanted to use for the new rod. I cut it apart on the lathe.
I used a new 1/2 Acme Threaded rod for the new piece. I turned one end to accept the captive coupling.
Here is the new interior rod installed.
I wanted to make a new depth stop "nut" that would ride along the inside edges of the mill cavity and keep the stop stable as it travelled in the "Z" motion
Here is the new piece. The "Nut" is threaded for a 1/2 Acme thread with a tongue that will protrude through the scale on the aluminium cover. The nut is wraped with a yoke that extends to ride along the inside edges of the Mill opening.
An exterior 1/2 Acme threaded rod gets mounted to the tongue of the interior nut.
Knurled nuts allow adjust ability for the depth then are set together to fix the stop setting. 
The new inside assembly is installed
The Drill Wheel gets the extension installed.
After the cover is installed the exterior threaded rod and yoke get mounted to the tongue from the inside "Z" nut.