Hi Dan!
Well, a few things, I spent many months working this stuff out. In this case I used black polystyrene painted white. Mostly because its what I had on hand. Ideally you would want to use white material painted black. This will greatly reduce the number of dots you need to mill to produce an image. Black with white paint means you need to invert the B&W image. Which means you need to mill out the black. White with black paint you can use the image as is and only mill the highlights Vastly reducing the number of dots to mill. I want to say that image I milled had 70,000 dots to mill. 9 hour run I think. Depends on the look you are after. You could mill black on white and not invert and end up with a nice inverted image. Or if you use colors other than black and white you can really do whatever you like and call it abstract.
Dots are by far the most time intensive. You can use lines and move along pretty quick.
The software I used is Halftoner.
https://jasondorie.com/page_cnc.html Its free and works pretty well. Basic but does work.
Things I learned:
Flatness is key. Dots or lines, if the distance between the bit and the work varies by more than .2mm (about 8 thou) you will ruin the job. I'm surrounded by failed attempts at that image because of that. Mostly because there is a lot of detail in that image. Variations really pop.
Mill your surface before the run. Not the night before. Assuming the surface is wood. You want to run the job before the surface has a chance to dry or absorb moisture and change shape.
Don't clamp if the materiel is thin. It will deform. I used Elmers mounting spray. It sticks but the material can be carefully removed. Don't use the 3M 77 stuff. Trust me. That being said spray the whole surface of the back. Spots with no glue will raise above the 0.2mm limit. Then pound the hell out of it with your palms to adhere the glue to the milling surface.
Use scraps of the same material to do a few small one or two minute tests on different parts of the image before a big run. It will save you lots of time.
If using dots remember not all v-bits have a point. You need a point. I used a 90 degree bit.
If you think your controller enclosure does not have a heat issue you will discover that it does when running a 9 hour dot halftone.
Give it a little more air than usual. Lost a job to that as well.
Wow that is a lot of stuff. No wonder I had a hard time at it.
Hope that helps. Reach out if you need more.
Jack.