Nozzle size issue

Hi, everyone.

I have this Aurora Z-605 3D printer which I bought from local store. What the manufacturer states about the nozzle diameter is "normally it is 0.4 or 0.5, but we use a customized 0.3." I don't know what they mean by customized, but definitely they are instructing to use 0.3.
For a long time I followed their instruction, everything went well enough until I got myself a vernier caliper. I don't know how to measure the nozzle directly with this thing. All I can do is to measure the thin steel wire that they give along the kit to poke the extruder when it is jammed. This wire measures 0.3-0.35 mm and there is still some space to wiggle when I poke it into the extruder. So one thing for sure, this thing is not a dead 0.3 and then I assume that it is actually 0.4.

The problem starts here. After I changed the nozzle setting into 0.4, most of the time, my extruder skips back (or so what it is called, like there is too much pressure and the extruder motor rotates backward by some degrees). It skips back when the setting is still 0.3, but not as often, like only when doing solid infill but was easily mitigated by rising the temp. But now, the temp is already too high I think but the skips don't vanish already, even when just laying perimeter at 40mm/s.

Also, most instructions and people tell that nozzle diameter should be measured as accurately as possible. But why it has to be so accurate? I've used a wrong nozzle value all this time but there seems no problem.

You can can get sets of small drill bits from 0.2mm to 1mm in various steps.
With the heated nozzle insert the drill bits and when you get to one that does not go in easy, don't force it as it should be either the right diameter or slightly higher.
Best to check with a new nozzle as there is no plastic residue or burnt stuff messing with the diameter.

Do some calibration prints, like 10mm cubes and thin wall objects.
Check the dimensions with your calipers and adjust your slicing setting till they match.
If in doubt set the nozzle diameter to 0.3mm for all slicing jobs and do the calibration.
Once that is done change the diameter to 0.4mm and compare.

But if a 0.35 wire has room to play I would simply say it can not be a 0.3mm nozzle size.

As well as nozzel size, have you checked print and layer size options?

I've tried to print with both diameter set (one by one, of course) and I measured the resulting print. The measurement doesn't differs much, the only difference is that the wall is thicker with 0.4 mm in the setting ( tried it with single walled cube). However, both measurements is a bit off of what it should be, so I still don't know which diameter is more correct. But, still, why must it be so accurate?

About the skips, it was because of dirt since my home is currently under renovation. Those building guys cut ceramic tiles indoor and it really takes most of my breath.

UPDATE :
I've tried with a 20mm cube. 0.4 setting bears the sides of about 20.1~20.3 mm and 0.3 setting has around 20.3~20.5 mm. The surface is still rough and I don't know which one is actually closer for I haven't calibrated the dimension yet (and now it's even harder to do it since I don't know my nozzle size). Btw, looking at the top solid infill, 0.3 gives denser infill -- with some random gaps because of defects. On the other hand, 0.4 has it evenly spaced like a grating, Like it's trying to extrude with a wide width but actually cannot.

UPDATE AGAIN:
Whenever I try to have a layer height setting of > 0.3 mm (with 0.4 nozzle diameter setting, indeed), I can see the nozzle tip is floating above the layer and deposits and drops the filament onto the layer beneath, not deposits it while grinding the deposited filament with its tip like it should be.

Based on these, it should be clear that the nozzle tip is of 0.3 mm diameter. But the steel wire is still bugging me.

Edited 2 time(s). Last edit at 06/15/2015 08:20PM by Vashikovich.

You should never try to print higher than half the nozzle diameter for good results.
I would suggest to assume the 0.4 and base the settings on that.
The main difference for really accurate results would be in the filament settings as you have to compensate a bit here for the nozzle diameter.
In the real world you won't see much difference between 0.4 and 0.3mm anyway.
Accuracy is a must when prnting as otherweise every tiny error build up on bigger models.
A difference in final size on a 10mm test cube might only result in 0.2mm bigger or smaller, but if you scale this up to 15cm...
And no it won't be a difference of 1mm all of a sudden, just the fact the the big outside dimensions look close to perfect while smaller things like holes never match up in size.