The First Part to Break on my Reprap Mendel

When I started a print today I was confused to see the first layer of plastic getting squashed down very flat, so that it was noticeably thinner at one end in the Y-axis. When I stopped this and checked what was happening with the extruder movement, it seemed as though the bed had lifted up at one end, which was confusing since there was no way for the sprung bolts supporting the bed to have unfastened with nylock nuts on them.
However, upon investigating a strange rattling noise that I was hearing on the opposite side of the printer, I found this:

y-bearing-180-outer-right

X-Axis Improvement Update

Having printed a few Prusa Mendel parts since modifying my x-axis, without changing any settings, I have now seen a very slight improvement in accuracy:

Left: Greg's Hinged Wade's Extruder Idler turned out the best I've printed yet, where the top-most section forming the hinge can sometimes be troublesome as it is such a small area, Right: Z-Motor Brackets have a slight blemish on the inner surface, but no layers going significantly out of line

Mendel X-Axis Upgraded

Difficult to see here: M3 nut torn away plastic around it, as its slot was too shallow to pull it out of the space the bar goes through, so put too much torque on it trying to squash plastic

So my original x-axis modification design didn't quite go to plan as I found a couple of weaknesses in the prototypes, which didn't print brilliantly in the first place, but after putting wider tolerances in places, for instance making the spaces next to the smooth rods for M3 captive nuts 1.5 times as deep as they should be, I arrived at something much more robust.

Mendel X-Axis Redesign

I was recently noticing a significant problem occurring in my prints referred to by makers as 'backlash', where some plastic deformation (permanent damage) or slack in the timing belts driving either of the X and Y axes, can lead to the extruder or printbed stopping at a slightly different real location for the same instruction, depending on which direction it moves there from.
This can be more technically termed a physical 'steady-state offset' in the way the extruder head, or any robotic part experiencing friction or loose parts for that matter, comes to rest after a movement.

Early indication: Note the ridges out of line on the near-left face; structurally insignificant here, but quite visible.

Why Didn't I Think of This Sooner?

Recently I'd been having an occasional problem when leaving my reprap unattended, especially if there was a big new reel of plastic on that barely fit, where the filament being unwound into the extruder would be close to the outer edge of my spool.
After a while of being pushed and pulled with the printhead movements, since the friction on the PLA bushings I used in the spool hub had such low friction, the filament would unwind itself loose enough for one loop to flop off one side of the spool, get caught on something and pull the extruder carriage to one side, skewing the whole piece being printed.

A lad on the reprap IRC channel going by the name DavideV tells me he uses some fancy piece of kitchenware called a Perfect Tear Paper Towel Holder to hold his spool, which apparently ratchets around in one direction providing some tension with a spring to stop kitchen paper rolls from unravelling. I looked around for other similar products to try and find a simple mechanism  that would keep my filament in place, and one was simply using this method of pressure:

SimpleHuman "Tension arm paper towel holder"

Which inspired me to create this:

My Bungee Cord Plastic Filament Retainer (BCPFR) needs to be on quite loose so that it doesn't create the same problem that it's intended to solve.

Glory be to the gaffer, the bungee and the methylated spirit, ramen.

Preventing Jams: Contradictory Constraints

Recently (okay, a couple weeks ago) after leaving my reprap printing while out buying food, I came home to find this calamity:

Extruder body print ended with some wispy filament and then a big gap of nothing.

Series Heating and Other Developments

After a long break spent graduating, moving furniture into/around our flat, and lots of other such messing around, I finally got around to permanently solving my heated printbed problem, while getting a sturdy desk to set the printer on.

First of all, I had a problem with the way my heated-bed power-supply was connected, as the circuit was fixed with solder, that meant that the adapter block was physically connected to my reprap by its cable, and that I couldn't easily change it for another power supply.
My solution to the problem of wanting to quickly connect/disconnect a DC power supply was to take some bits of electrical screw-terminal:

sometimes referred to as a 'chocolate block' connector, cheap in long blocks

...and bolt it to one of Nicoll's bolted PLA bushings for Prusa Mendel.