With ABS part I use “same stuff” or Ambroid “ProWeld” to fuse surfaces together. Keying if necessary. I got that tip from a tour of Stratasys back in the mid 90’s
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The software 3D-COAT has a feature that will split an .stl, and automatically add in alignment pegs and holes.
Yup. That’s more-or-less the process I use. I wet the mating surfaces with Acetone before I stick em together with ABS/Acetone slurry though, just to soften the surface and ensure the dry surface doesn’t absorb the Acetone out of the slurry to quickly. A good sanding afterwards finishes the job. Spot the joint…
Cheers!
AndyL
Fantastic responses, keep them coming!! 
‘Keys’ seem like a great idea particularly for alignment. Some of the techniques still sound a bit messy and dare I say it not very user friendly. Is anyone out there trying chemical free solutions?
Solvent welding is the best solution I have found. Despite what community trends may lead you to believe, acetone is not ideal for dissolving ABS. MEK (butanone) works far better and is commonly used when assembling plastic model kits. If you don’t have a perfect fit between pieces solvent welding will not produce a strong void-free seam, you need to dissolve some ABS in the solvent to create a glue that can fill the gap. I have also experimented with friction welding (ABS filament in a handheld rotary tool) but I don’t think it’s ideal for most parts as the weld is only on the surface. It has applications, but is more limited than solvent welding.
8 days later
Or you modify your printer a bit…
https://ultimaker.com/en/stories/view/97-printing-out-of-the-box
Thank you @stephan_5 I think this project is really great and shows that innovation can be achieved by questioning, adapting and experimenting with what we already have available.