Welcome to the reprap and 3D discussion forum.

You might want to share with us some pictures of your printer and the printed part.

Normally, if your walls has that layer on layer look then there is no issue there since that is normal for prints set to use low resolution layer height of 0.2mm or more.

I have seen this problem before and its either due to a wobbly z-axis threaded rod or the threaded rod is slightly bent off. If you put the part next to the threaded rod it should match more or less the pitch of the threaded rod.

If you can spend a bit more, try and replace your threaded rods with lead screws. Lead screws are meant for linear motion translation unlike threaded rods which is meant for holding work piece together.

Try looking back a few pages of posts in this topic since some of us have been posting up spare parts sources for 3D printers.

You could bring your own hack saw and cut the threaded rod there 

Is it possible to find such a long lead screw? One of my threaded rod is bent on arrival too put it didnt affect my printing that much. Perhaps you can try to straighten it a bit or swap it with the other threaded rod on the other side?

Normally lead screws comes in meter lengths and some retailers may cut the lead screw to size and chamfer the ends.
I think what you mean is to build a reprap with print volume of at least 16 inch on all sides. The source code for the firmware is open source and is available online.
Lead screws are intermediate level parts for repraps, it will be expensive but you should see a big difference in print quality. You can go pro with ball screws and its even more expensive. 

Trying putting a piece of glass on top of the heatbed. That should minimize your problem.

you should be ok with heated pcb > glass

no need aluminium plate anymore if it was bend

i also had problem with z wobble and i remix this http://www.thingiverse.com/thing:141479 design to fit my m8 threaded rods and i can say that the wobble on printed parts all gone now and it look nice and neat also really happy with the result

Guys!

I am exploring the software side of 3D printing. Trying to learn how to draw my own 3D object. Is there any good program which you would recommend for a beginner like me? Tried using Sketchup but all the measurement does not seems to add up once i export the file out as *.obj.

lol, sorry i think i used wrong word   i mean i redesign it in freecad to fit with my threaded rods and nuts

ABS is usually cheaper than PLA, any other way to swap it in a fair manner?

can i know where i can buy cheap abs