Most CNC CAD/CAM programs are subtractive tool paths. Where do I start in regards to creating my own CAD/CAM CNC post processor for additive toolpaths?
I know there are free ones out there but I need something that can handle a few functions only with a custom GUI.
Any thoughts?
Confused by your goal a little. A “post processor” is typically a small part of an existing CAM program to output G-Code of a certain dialect typically usable by a certain machine or G-Code interpreter (like LinuxCNC or Mach3 for example).
Do you want to make a full CAM program from scratch? This is potentially non-trivial 
. Do you just want to do 2D fills basically?
Could you fork an existing slicer for 3D printers as a start? Possibly just forking some of their code.
I assume all of this is kinda like the pancake printer you’ve compared your project to?
Yes, this is for the project I was talking about previously. Its a CNC cake decorator . It has 4 AXIS Movement. I plan to be able to take a cake and apply decorative frosting to it. Those tool paths will be “additive” to an existing volume ( the cake).
My plan is to create a custom (freeware) online 3D Decorator studio that is basic and with limited features for cake decoration. (all you really need is some color selection, nozzle selection and a handful of patterns)
Then simply click a button and the 3D model is converted to G-code file. No steps in between like a slicer or CAM conversion.
Just curious, what’s your 4th axis? I assume the first 3 are X, Y, and Z, but is your nozzle at a set angle and you intend to rotate around Z? On a Haas for instance the 4th revolves around X, but the cake would probably slip off the table 
It seems like you would almost need a scanner on this thing to get the cake properly sized and located etc. G-code is pretty dumb for the most part just says “go here”. Typically we have something like a vice square to the table and facing operations etc. to make sure every thing flat and where we want it. On cake, that seems quite a bit more variable (how much did it rise while baking, is it in the actual center of the table, etc.). If you probe or scan for instance, you would then typically alter the G-code to adjust for these things, which gets tricky.
Seems like a cool project if you ever see me up at the space and want to discuss it. I know G-Code pretty well and have been playing with some software to eventually one day (before the sun burns out) possibly make a CAM program for destructive milling/routing.
The 4th axis is a wrist for getting the sides of the cake as well.
I would like to talk G-code for sure. I am more interested in getting from the 3D model to the G-code. I will be there Monday maybe we can chat if your there then. I understand all the variables you described as well. I have controlled almost every aspect of the cake so it will be very predictable. Let’s assume there will be a near perfect cake every time. Other thing to consider is there is no need for close tolerance. After all we are talking frosting. plus or minus a 1/4 of an inch doesn’t scare me.
You might want to talk to @Chris_Wischkowsky since he’s building a 5-axis CNC with custom software.