(This reminds me of my wood routers, the cost of the router itself is “nothing” compared to the cost of all the accessories – like bits, jigs, tables, fences etc!)
From your picture I see one thing that caused me (your mileage might vary) problems in the beginning – the “egg-coupler” – whigh was always slipping. As a remedy I bought the “precision coupler” which at the time consisted of a 1″ toothed belt drive wheel. (See image further up in this thread) I removed the abrasive paper which got “slippery” as the abrasive wore off/filled with eggshell dust. In place of the abrasive pad I glued a rubber garden hose gasket (US) – after this mod the eggs are “self-aligning” and needs very little “tweaking” to get them centered. The new “precision-coupler” made of alu is very nice but the diameter is to small to give me the “precision” I want! I mount the eggs with the “butt-end’ towards the motor to increase the area in contact with the egg. To avoid using the “reverse motor direction” in the eggbot extension I have reversed the leads to one of the motor coils.
Two more changes I have made “pimping my bot”;
Added a drawer-pull knob to the spring loaded shaft, makes it a ‘breeze’ to “install” the eggs – with the motors disengaged the knob can be turned to check egg alignment as well. The knob is hot-glued to the shaft. Home-Depot sells the knob I used (US).
Next “upgrade” consists of feet made of erasers – this cuts the noise from the motors considerably. (try lifting the bot off the desk while it’s running to hear the difference). I have ‘mounted” the feet by hot glue as well, not a very good method as the hot-glue does not stick to the erasers.
Made a quick Inkscape drawing of the eggbot with a 42mm diameter egg and a pen height error of 2mm.
To compensate for the difference in pen height the pen arm has to swing 2.5° which in turn causes a pen alignment error of 1.46mm.
To me “bumping” the laser seems more likely than “bumping” the paperclip but the “imponator effect” of the laser are a couple of magnitudes higher!