Wavy walls, got to be Z axis issue?

Blacksheep99

I'm looking for advice and/or ideas how to 'fix' an issue with wavy walls. They run in the Z axis and are consistent between prints. They are move evident on straight or curved prints. I've uploaded an example.

https://photos.app.goo.gl/JZdNDs4hgkVEo1ML6

The Z screw/thread is driven by a stepper connected via a small belt. I wondered if the belt is pulling the screw over slightly causing a flex. Pic...

https://photos.app.goo.gl/4RYSmvpfHd396mFQ8

I'm going to buy a duet which may allow me to change my printer slightly to improve this hence it would be good to get some ideas how I can upgrade it to resolve the walls issue.

Thanks.

grizewald

Are you saying that the outside of that print should actually be smooth?

If so, that's a huge amount of slop and seeing as the Z axis appears to be supported by two large ground rods, I can't see how the lead screw would be able to cause that much wobble.

What does the printer itself look like?

DocTrucker

Just precaution before getting to deep onto the fault finding remove the screw and make sure your axis is silky smooth. Lube if needed for the bearings.

Check everything is tight where it should be. Assumption is the mother of all muck ups and stung me again recently.

While the screw is out roll it along a flat surface and to make sure it isn't bent.

Blacksheep99

@grizewald said in Wavy walls, got to be Z axis issue?:

Are you saying that the outside of that print should actually be smooth?

If so, that's a huge amount of slop and seeing as the Z axis appears to be supported by two large ground rods, I can't see how the lead screw would be able to cause that much wobble.

What does the printer itself look like?

Yes, the wall should be smooth in the Z axis. I thought the build plate was pretty well supported in that configuration however I was trying to apply some logic to understand why this happens.

The printer in question here is a ST3Di Modelsmart 200

http://www.st3di.com/products/model-smart-pro-200

I am however using Cura (4.2) to slice the stl.

I also have a Polaroid Modelsmart 250s which is based on the same design (assumed they licensed it) and it exhibits the same issues.

The bed has auto-levelling using a micro-switch (5 points) so the Z is constantly moving if only minor adjustments. Could this introduce the issue?

Blacksheep99

@doctrucker said in Wavy walls, got to be Z axis issue?:

Just precaution before getting to deep onto the fault finding remove the screw and make sure your axis is silky smooth. Lube if needed for the bearings.

Check everything is tight where it should be. Assumption is the mother of all muck ups and stung me again recently.

While the screw is out roll it along a flat surface and to make sure it isn't bent.

I was going to do this but wanted to see what you chaps thought before I started to disassemble the printer further. I do appreciate what you're saying, assuming something and discounting it can prove a big error in trying to resolve an issue.

grizewald

@blacksheep99 said in Wavy walls, got to be Z axis issue?:

Yes, the wall should be smooth in the Z axis. I thought the build plate was pretty well supported in that configuration however I was trying to apply some logic to understand why this happens.

That's truly appalling. Any printer that makes that kind of mess on the Z axis is not fit for purpose. If it printed like that from the start, I'd say returning it for a refund would be the best idea.

Blacksheep99

It was £90 from ebay (as new condition) so that's not really an option. I figured that the components alone were worth that so hopefully I can find a way to improve things.

zapta

If you look at a single layer all around the cylinder. Is it 'out' on one side and 'in' in the opposite side (consistent with a shift on the X/Y plan) or is it 'out' or 'in' all around?

Also what is the vertical pitch between bolded layers, (measure over 10 interval and divide by 10)? Does it match the Z rod lead (which is the vertical distance per one full rotation)?

Blacksheep99

@zapta said in Wavy walls, got to be Z axis issue?:

If you look at a single layer all around the cylinder. Is it 'out' on one side and 'in' in the opposite side (consistent with a shift on the X/Y plan) or is it 'out' or 'in' all around?

Also what is the vertical pitch between bolded layers, (measure over 10 interval and divide by 10)? Does it match the Z rod lead (which is the vertical distance per one full rotation)?

Hi, sorry about the slow reply and thanks for trying to help. The layers are out and out on both sides rather than exhibiting a shift. Could this be caused by fluctuation in the hot end temp perhaps? Now you've made me think about that it doesn't follow what you would expect to see if the bed was shifting with the screw does it.

I'll have the measure the pitch and come back later on that.

droftarts

It looks like the steps in the print are at roughly the same frequency as your Z leadscrew. It's probably that the Z layer height you have chosen is not a good match for the steps per mm on the Z axis, though the constantly moving bed may not be helping. It's best to use a layer height that matches full steps on the Z: from Prusa's leadscrew calculator :
'Z axis isn't usually enabled during inactivity. If the axis is disabled during micro-step, axis jumps to the closest full step and introduce error. This effect is occurring to some extent even while leaving the Z axis motors enabled. This is most useful to machines with imperial leadscrews but also for unusual layer heights with metric leadscrews.'
For example, it looks like you've got a leadscrew of something like 8mm diameter, 1 start, and perhaps 4mm thread pitch (you'll have to measure to make sure). Only the thread pitch really matters, though, but you'll also need to know the step angle of your Z motor (usually 1.8°). Using Prusa's leadscrew calculator shows that the full step movement of such a leadscrew is 0.02mm, so multiples of this would be a good choice for step height, eg 0.1mm (5 full steps), 0.16mm (8 full steps), 0.2mm (10 full steps). Cura's 'Normal' setting uses 0.15mm layer height, which is a bad match. If you have an imperial leadscrew, this does get difficult!

Ian

Blacksheep99

@droftarts

Thanks for the information and links. I will try and do some measurements and calcs. I printed the part you see at 0.2mm .