Hi,
I have also had similar plans and assuming you will have a 4-axis driver card, this is fairly simple. I don't know about Mach3 or other CNC software, but at least with UCCNC this is possible because it has a 3D printer plugin. I designed and built a 3 axis CNC some time ago and planned to use it as 3D printer also, so I tested the concept but then I decided not to carry on because of several reasons...
- The Z axis is far too heavy for no reason (in this concept).
- The machine is too noisy for a several hours of printing pass.
- Need a special fixture for the heat bed.
- Temperature control of heat bed plus several extruders is not an option, especially if you want PID control.
- My CNC is very fast as CNC (maximum 9000mm/min on all axes) but still very slow as 3D printer.
- CNC is useless when the printer head is installed and the spindle is removed (and vice versa).
- 3D printing means many very small movements and that created a lot of rapid up/down motion which may tear the Z apart after a short time.
- The 3D printer is independent of the PC, if I want I can run it from an SD-card. The CNC requires the PC.
- Better, more advanced heat control. The 3D printer has PID for two extruders and a bed, the CNC has nothing of that.
The list can be made long.
...but if you want to print at snail speed then it is OK.
In the end and after some consideration I decided to buy a super cheap acrylic 3D printer kit to test and learn and to see if I want a better one later. It arrived a a few days later and after a few hours it was ready to print. I think considering the price and the used materials it is an excellent printer, but already before it arrived I decided to make some changes, so after less than a week I started with some serious modifications, still with the aim of learning. After the fairly simple improvements the printer is very accurate and is super fast, making 15000mm/min on X and Y and 6000mm/min on Z (using ordinary threaded rod with 1.25mm pitch). I decided to build a 3D printer from scratch and use this one to make some parts and as a test object. Improving this Geeetech is done, as far as I am concerned, it is as good as it gets. The building of my own 3D printer is in progress, frame is ready, 1204 ball screws for Z, 9mm wide 2mm pitch belt for X and Y, MGN12 linear rail guides for Z and Y and MGN9 rails for X. Will use the same electronics as the Geeetech, which is the
GT2560 board.
With the Geeetech as it is, I have rapids of 15m/min and my CNC has a maximum of 9m/min, so already here the cheap 3D printer wins over the considerably more expensive CNC. Noise level is extremely low compared to the CNC, I can run the printer for several hours in my home and sitting in the same room I can Skype, speak on phone or watch a movie without being disturbed and in the next room you gear nothing. With the CNC this is not possible even if the spindle motor is not running and the CNC is just milling air. This is because of the belt drive on X and Y axis. While ball screws run pretty quiet nothing beats belt drive when it comes to low noise and high speed. Another point against the CNC/3D printer combo is that you don't need to have a high mass on the gantry (if you have a moving gantry type) or the table (if you have a moving table type like I have). The table of CNC is very heavy, as opposed to the table of the 3D printer, which is very light. The 3D printer must only be able to move a few grams, while the CNC must shuffle around a heavy table PLUS a heavy vise as well as a work piece which can also weight a few kg so a ball screw is better and a 9mm wide belt would not be able to manage it.
Stiffness is an advantage for both, the stiffer machine produces better and more accurate work, but low mass is an advantage for 3D printer but a HUGE disadvantage for a CNC, high mass on moving parts is a HUGE disadvantage for 3D printers because speed and acceleration becomes low unless you spend a lot on it.
There are plenty arguments against, but I think if you have space it is better to buy a 3D printer than to modify a CNC. All I can say is that my decision of abandoning the idea of combining CNC and a 3D printer was right and have not regretted ever since I made my very first printing. I don't think it would have made a difference which board I'd use. Currently I am using UC300ETH and UCCNC for my CNC and GT2560 plus Marlin and Repetier-Host and Slic3r for the 3D printer and very happy with those.