The mechanics (I am loosely condensing my old posts under http://www.metalforum-owl.de/viewtopic.php?f=21&t=2008#p51995) are where the proliferation takes place. As everybody uses a 5-pack „NEMA 17“ stepper motors, Arduino-based control units and Hot Ends made by the real geeks with their mills and lathes, this is the part to go on the rampage.
Seriously: don’t try to save money on the Hot End. They have to be good. You might think that it’s just a heating element combined with a sensor, so your controller can regulate the temperature, but designing and manufacturing a short and effective melt zone just above the nozzle and keeping everything cool above is an art.
Also: I have not seen many nozzles not between 0.35 and 0.5 mm. Your maximum layer height is ~80% of the nozzle diameter, so balance carefully how slow you want to go in favor of details. FFF-printers are mainly good for the raw stuff. This is the exception to the rule: http://hackaday.com/2011/10/13/ultimaker-quality-faq-is-like-porn-for-3d-printers/
The RepRappers have their own Bootstrap category, if you want to „Spax“ together some boards and drawer guides in order to print the parts for the next printer, before the flabby thing falls apart (or just develops too much play). I liked the WolfStrap in particular.
With tailoring from the hardware store and threaded rod you can get far enough (Prusa’s Mendel/i2 and i3). Many designs use proper extruded aluminium profiles (MendelMax / Lulzbot A101). The Eventorbot is made from welded square tubing. Once crowdfunding allows (the emerging company) using a laser cutter you often see plywood (Printrbot / Ultimaker).
Timing belts are used to drive the linear motion – some fear the inherent play of this, but with a 0.4mm nozzle you don’t have to.
In my humble opinion you should not move your object – only your Hot End. You mostly see this done with the „Rostock Delta“ approach. As a compromise you can move the bed in Z-direction (slowly and only down), which is done by the Ultimaker, Solidoodle und Sumpod (which also uses rack & pinion instead of belts).
My choice was to go for a „Delta“-setup, as I liked the XYZ-head and the symmetry: design once, print thrice – no need for different X-, Y- and Z-axes.
The Sanguinololu can be overwhelmed with calculating the trajectory, of course. An „Arduino Due“ with ARM Cortex-M3 µC won’t have any problems at all and the ATmega does it’s job most of the time, quick and complicated movements can slow down your print although the steppers have enough power…
The next disadvantage you encounter is the requirement to use a „Bowden Extruder„, which means you better use 1.75mm filament and at least the MK8 Drive Gear, as Airtripper suggests here. (I used a MK7 – not ideal. Not even with the highly praised E3D v5 Hot End.) In general seperating the Feeder from the Hot End has the advantage saving space on the print head, so the printer can print larger parts (or become smaller). Also more Hot Ends could be mounted, when the motors can be screwed elsewhere.
The classic „Wade’s Geared Extruder“ on the other hand has a „hobbed bolt“ with a much smaller effective diameter than any direct drive gear and the additional pair of gears. It is much more reliable and can handle 3mm filament, because the stepper motor will not have to use a marginal amount of torque to push the plastic string through the melt zone. The springs pushing it (normally with a ball bearing) against the teeth forwarding it can be even harder.
What really doesn’t matter is the biodegradability of PLA. This has to be done technically, not on a compost heap. But while your Polyimide („Kapton“) covered heated bed has to reach and remain at 100 °C for ABS and you still need to print with a „brim“ to prevent the part from warping away (and finally fall off – unless you smear it down again with some kind of hot knife) you can just heat some crepe tape to 50 °C (or not at all – if it is the proper one) and PLA will print fine. Additionally: ABS stinks, PLA doesn’t.
The cost to finally get startet? In 2013 (January) it was:
If you build a RepStrap:
=> Sanguinololu ca. 80,- €, RAMPS (Arduino + Shield) more than that (kits!)
=> 5er-pack stepper motors ~100,- €
…or maybe a „RAMPS + Motors“ package for about 180,- €.
=> You can buy complete extruders with hot ends for 100,- €.
…or, if somebody prints the feeder mechanism for you and you have a spare motor about 50…100,- € (Printrbot „Ubis Hot End“ / E3D / Lulzbot „Budaschnozzle“).
280,- € without slides, belts, bearings, frame, … – still quite a sum.
The cheapest Printrbot („Junior“) was 399,- $ (303,- €) – without power supply and heated bed (kit).
If PLA was totally sufficient: 499,- $ (~380,- €) for a Solidoodle 2.
…with „Heated Build Platform“ (and therefore reinforced power supply) it’s 599,- $ (~455,- €).
The then new Rostock MAX Delta-printer (kit) was at 999,99 $ (+ 50,- $ LCD + 100,- $, for acrylic instead of wood => ca. 872,- €).
The reference model (quality and speed) was the MK1 Ultimaker, kit price 1194,- €.
So I went ahead with the „RoStrap“…