Please help with Z axis quality issues

Hi there. I’m new to 3D, and new to the forum. I was hoping someone would be kind enough to help me with a Z axis problem. I’ve attached a photo of my first 4 prints.
Print #1 The 20mm box circle test, turned out much better than I expected, but it was a little out of gauge. Temp 200
Print # 2 Was going good but I ended up aborting it.
Print # 3 The .5 mm thin wall test box showed a drastic change in Z quality. It started good, but after 7 or 8 layers the steps look like they’re getting bigger.
Print # 4 I thought maybe there was a problem with the file, so I reloaded the first print. Z quality trended the same as print 3…. Aborted
All settings and parameters were the same on the first 4 prints. The only thing that changed was the print file.
I have since tried a few more prints with different temp settings, 185 deg back up to 200. All prints have the same bad results.
Printer Prusa I3
Marlin step settings #define DEFAULT_AXIS_STEPS_PER_UNIT {80,80,4000,629.55}
I don’t understand what could have changed.

Thanks in advance for any help you may be able to offer.

I ended up calling diytechshop from where I purchased my printer. They were very helpful. I had my layer thickness to high... .5mm. They advised to lower that value down to .3mm. Worked like a charm. I figured is was probably something simple....It usually is
Thank you DIY
But I still don't understand why the first print looked so good with .5mm and the others turned out the way they did. Any Ideas?

If they saw your picture, their awnser sounds strange to me. IMHO, it's not a Z axis issue but an extrusion one. It looks like your extruder gets warm.dirty and cloggs, or something like that.

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Collective intelligence emerges when a group of people work together effectively. Prusa i3 Folger (A lot of the parts are wrong, boring !)

It is because it needs to push a lot of filament to maintain 0.5mm layer height with a 0.5mm nozzle. There are dynamics at play here, so that means every part of the machine has it's own effect on the variable 'layer height'.

Good they could help you out so fast and pinpoint the problem

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http://www.marinusdebeer.nl/

Thanks for the replies. Yeah, after I we got it printing nice again, I printed the .5mm box to calibrate the thickness. I ended up lowering the extrusion multiplier 10%.