I'm also happy that the coupling appears to have helped you. I haven't touched mine since the install I posted on this thread and am still band-free. Of course I've been making numerous other tweaks to the SD2 but that's half the fun of owning a 3D printer.

MFernandez: How were the results with the 5/16 threaded rod? Did you try the 8mm rod to see if it would improve?

Anyone: Where do you find replacement boss nuts for the print bed or how do you attach the print bed to the 8 mm threaded rod?

I'm pretty happy with the 5/16" replacement, though that may change with time.**  Now I've actually been going around replacing prints I'd given to people when I still had the horrible banding problem with the much improved prints I'm getting now.  The replacement rod is actually somewhat bent as well, but with the coupler absorbing much of that, the two vertical guide rods keep the bed in a straight line.  I still have some wobble, but as I'd mentioned previously, the variance in successive 0.1mm layers was much smaller than with the stock rod.

I am now a big believer in $65 stainless steel helical couplings.

** I assume it will never end; there will always be things to improve.  I still have some banding, but the quality is improved enough that people ask if I've used some sort of acetone vapor treatment when touching my new prints.  I'm no longer obsessing about the banding; just getting the added structural integrity from having equal-height layers makes things come out very solid.  I still may go for an 8mm rod, but I have a vague feeling I'll be obsessing about adding a second extruder first...

Anyone: Where do you find replacement boss nuts for the print bed or how do you attach the print bed to the 8 mm threaded rod?

Also know as PEM nut. See distributors list.
http://www.pemnet.com/fastening_product … r_new.html

You are looking for SP (standard profile) or CLA (specific to aluminum mounting)
This data sheet has pretty much everything you need to know and also a bunch of stuff you don't.
http://www.pemnet.com/fastening_products/pdf/cldata.pdf

Anyone: Where do you find replacement boss nuts for the print bed or how do you attach the print bed to the 8 mm threaded rod?

Unfortunately, my extruder had previously been getting worse and worse, and I ended up taking that apart and unclogging it.  I'll just summarize that as "Mike's Fiery Kitchen Adventure." Here's something useful I learned: you can just heat up the barrel while pushing a piece of ABS through the feed, and as soon as the barrel gets hot enough, the entire plug of solidified crap will just shoot out of it, speeding things up immensely.

In any case, calibrating the z axis wobble resulted in a much lower variance.  For the delta between successive 0.1 mm turns, the distance in mm between the points (at a distance of about 12 feet) was:
7    6    6    7    7    6    7    7    6.5    7    7    6.5    8
... whereas with the stock rod, my previous data from about a month ago was:
12    14    11    9    8    6    5    4.5    5.5    4.5    5.5    8    10    11    12
The differences in magnitude between the different sets is meaningless, but the variance within each set is immediately apparent.

Of course, after unclogging the extruder, there are a lot of things that need calibration, but after a long while, here's a photo of the results (a 25mm cube). 
[I'm not allowed to post links, so I'm trying to attach the picture.]
[The preview doesn't show that the attached picture is attached, so here's link without the prefix:
h t t p colon slash slash img547.imageshack.us/img547/9418/7wsu.jpg ]

Anyway, now I've got my outrageously expensive stainless steel helical coupler in hand (the $66 Ruland one from Amazon) , my 5/16 rods and the 8M rods, so it's time to dive in.  Both the 8M and 5/16 fit into the coupler, though the 8M should have less play.  I'm going to start with one of the 5/16 rods, because just using the coupler may solve some of my problems, and not require that I replace the nut on the print bed.

(At this point, it's looking like the nut-and-washer trick is just to basically provide a platform a better normal to the rod's axis than if only the side set-screw were keeping it in position.)

I'll report back ...

After reading [redacted because I'm not yet worthy to include links]
...
Figuring I was on the right track I changed the aluminum piece to a much stiffer stainless steel piece.
...
In addition, I used the nut and washer trick to get everything lined up as well as possible.
...
Anyway, I hope the above helps people out and I'm happy to field any questions.

What's the nut and washer trick?  It looks like the nut would lock the rod to the coupler; does that effectively force the rod and coupler into better alignment because the nut and the coupler are acting on different "sides" of the threads on the rod?  (Sorry; I'm not familiar with some of the terminology.)

Here is one for $29 and free shipping and they will machine the end for you
.
http://www.aliexpress.com/item/Ball-scr … 38830.html

i buy this one yesterday and its already shipped.... really fast for an asian shop...

do you take the 200 or 300 mm?

200mm

It will need shortening very slightly I think.

and

http://www.ebay.com.au/itm/111047323409 … 1127wt_952

I realise I will have to machine one end to 8mm for the coupler. Other pre-machined versions were available, but they were about 60mm before the 'thread' started, so they wouldn't get the entire Z travel.

That was the only seller on ebay I could find with 12mm instead of 16mm too.