Cartesian Style MPCNC?

Title says the idea, but is it possible?
Got me thinking cause I only use my MPCNC for 3d printing and its mechanical nature is beginning to show. Its weight is putting it back from printing as fast or as tall as the lighter weight prusas can. I know about the MP3DP, but that takes away the build volume the mpcnc has.

I was thinking of modifying my mpcnc by transforming it into a classic cartesian style printer where the z axis is incoperated on the build plate itself and slowly descends as the print sticks. This allows the gantry to be much lighter without the weight of 2 sets of z axis pipes, a stepper, and other hardware. I may even try to modify the gantry to not include the z bearings for even more weight reduction. I realize it is still heavier than most 3d printers, but that would help. Also, without a z axis on the gantry, the rigidity reduction caused by the positioning of the extruder would be completely gone, since the extruder can be placed right where the rails meet.

With these changes, I can keep the big xy I like, but increase my z and my print speed a little.
Is this a good idea?

Not going to work out the way you want it to… Honestly if you want a fast printer with a large build volume, just use your MPCNC to cut out parts for a core-xy.

Also, just be aware of the physical limitations of printing. Really the best you’ll get is maybe 120mm/s with a top notch core-xy or delta (you can set it higher, but it won’t actually reach those speeds). If speed is your primary concern, printing larger layers is a much more reliable path to success. Get yourself a (cheap knockoff) E3D Volcano setup and you’ll greatly reduce print times.

By faster, I only mean’t a little bit. My current mpcnc can print at about 35mm/s max before I start noticing some weird artifacts. I ordered a .5mm nozzle from china so hopefully that will cut back my times just a smidge as I will be using a .3-.35mm ish layer height.

That is pretty slow… I’ve only used my MPCNC as a cutting tool, so I don’t know how well it prints… But having a CNC/printer opens up a lot of options for making a nice printer.

The MP3DP would round out your fabrication capabilities pretty nicely. Prusa i3 derivitives like the MP3DP are pretty solid from what I hear… Or build a delta if you want to print tall objects (though honestly deltas are a real pain to get working right)