As I recently built a CNC machine, it is often useful to draw patterns for testing purposes.
A difficulty/annoyance is that drawing devices were designed to be used by hands which can control the pressure against the surface, while the machine is designed to be rigid.
As such, controlling the Z-Depth of the machine to produce an even line as it travels over any even slightly uneven surface, especially when in the initial testing phase of using a new machine (where something like this is most useful) is difficult if not impossible.

There are some other instructables and guides out there to make a more long-term solution, many of which using felt tip pens. I was looking for something more durable than felt-tip that could be thrown together in a couple minutes using things most people can find laying around - and this is what I came up with.

In a nutshell, it is a spring-loaded ballpoint pen that will allow for minor variations in the writing surface while maintaining pressure.

Step 1: Parts Selection

For this device, I'm using the guts from two pens.

A fixed manual pen with a straight shaft
- these can be found just about everywhere in my experience.
A 'clicky' pen
- This is really the only part that will vary from person to person. The goal is to find a part that will fit over the ink tube of the fixed pen while still fitting into the tube. The purpose this will serve is giving the spring something to press against instead of slipping over the tube. A ball bearing or BB would probably also work well.

Step 2: Cut the Tube

Once the required parts have been gathered, it's time to start cutting things to size.

First you'll need to cut the tube. The goal here is to have it as short as is useful in your application. The longer the shaft, the more the tip will likely deflect as the machine moves it around. This will mostly appear when trying to make small movements.

For mine, I've cut it for use in a Jacob's chuck and just long enough to leave me some room for adjustment later.

Step 3: Final Assembly and Calibration

With the bulk of the pen done, we're ready to cut down the ink tube for our new device.

This part is the most critical of the assembly, but as you can find a similar sized ink cartridge in nearly every ballpoint pen - recovering from mistakes is relatively painless.

The tube should be cut long enough that the spring will put pressure on the tip once it's assembled into the pen.

Too much pressure and it will dig into the surface it's writing on - this will tear the surface, leave deep grooves that the pen will tend to follow on subsequent passes, or break.

Too little pressure, and it won't write evenly and it will be heavily influenced by imperfections in the surface.

With some tinkering, I hope you can find a pressure setting that works for you and your project.

Step 4: Notes

Ultimately, the usefulness of ballpoint pens in this context is limited.

Felt designs will give a much cleaner and more even result, however their tendency to bleed and deform doesn't make them very forgiving in early stages.

I find ballpoints give their best results in outline designs that don't attempt to fill in areas on the surface. The primary reason for this is the tendency for ballpoints to leave an indentation 'track' on the surface. Occasionally when the machine attempts to make a pass parallel to the first, the deflection of the pen in the tool holder, or even just the pen tip to the outer housing will allow the ballpoint to follow the previous track, leaving narrow white spaces in otherwise large filled in areas.

Ballpoints are also better suited to larger continuous lines. When trying to draw small details, the tip may not rotate enough to pull ink to the page and the design may have to be run again.

The strength of the ballpoint is that the same design can be run over and over in an attempt to fix any ink gaps or light areas because the ink won't soak or bleed like that of a felt tip.