Help with fine-tuning perimeters please

[Bobjimbob]

IMG_20170914_185926652.jpgIMG_20170914_185933354.jpgIMG_20170914_185957757.jpgIMG_20170914_190024927.jpg

I have a QU-BD Two-Up that been significantly changed and modified. I've managed to work out all it's various problems to where it now prints very well, except for one issue that I just can't seem to fix. The external perimeters seem to have some slight overextrusion (I'm hesitant to use that word because I'm not sure that the problem is necessarily related to this) that makes some layers have some overhang. Basically, it looks like some of them almost get squished out and so I don't get a smooth or consistent finish on vertical faces. Looking at a print from below it isn't at all noticeable, but at certain angles from above it is clear that the walls are not flat and crisp. The inconsistencies and overhangs also seem to form a slight diagonal pattern moving up the faces, although it's very faint and I'm not sure how important this is. I've attached some pictures to try and help you understand what I'm describing, although I think it may be a bit difficult to see in the pictures. Please just let me know if you'd like more or any other information.

I've tried playing around with my settings for extrusion width, extrusion multiplier, and filament diameter but none of these seem to make any noticeable difference. I don't believe it's a mechanical issue with extrusion because I have a new e3D V6-lite hot end that seems to work very well and I'm using Hatchbox PLA which, although cheap, doesn't seem to give other people problems with inconsistent overextrusion. The slight pattern to this problem makes me wonder if it may be related to my Z axis movement (its equally prevalent in X and Y so I don't think either of those are a cause) but I really don't know how, why, or what I would do to fix it. Any suggestions would be greatly appreciated, thank you in advance for any help you can offer.

[curious aardvark]

looks like a slight wobble.
looking at the machine - no belts all screws - I'd have to say it's a function of the screwthreads and at least one of the threaded rods isn't exactly straight, or perfectly lined up.

Screw drives are all well and good, but don't have any of the built in give or tolearance, that a belt driven system has.

Either that or the threads are out of sync with at least one of the stepper motors.
that said - it's a cool pattern :-)
But I'd bet on it being something to do with the threaded rods.

[Bobjimbob]

Have you done any kind of extruder calibrations?

Not recently, no. I'll take care of that when I get home tonight and see if it helps. Thanks for the suggestion

[Bobjimbob]

looks like a slight wobble.
looking at the machine - no belts all screws - I'd have to say it's a function of the screwthreads and at least one of the threaded rods isn't exactly straight, or perfectly lined up.

Screw drives are all well and good, but don't have any of the built in give or tolearance, that a belt driven system has.

Either that or the threads are out of sync with at least one of the stepper motors.
that said - it's a cool pattern :-)
But I'd bet on it being something to do with the threaded rods.

Thanks for the input. I wouldn't be surprised if my threaded rod isn't straight because, as you may have noticed, the Two Up only has one rod and so the gantry sags on the opposite side and puts a lot of weight on it and could easily have caused it to bend. Does that mean my best bet is to just buy a new threaded rod or is there something I can do to counter the fact that it's not straight?
If this isn't the case however and the problem is that the threads are out of sync with the stepper motors, what would I do to fix this? Is that something to adjust in the settings or firmware or would I want to reseat the rod in the coupler? Not really sure what you mean by this or what I could do to investigate and possibly fix. Thanks

[curious aardvark]

Well, personally, I'd contact the company who made it and ask them.

I have zero experience with printers using threaded rods for anything other than the z-axis - and had no problem with those.
I'm now pretty much a total delta convert - belts all the way :-)

Just thought that pattern definitely looked like an interference pattern - which you would get with threads being slightly out of sync.
How you would fix that - not a clue. sorry.