Please Help ! Uneven layers and i can't find why

Hi everyone !
thank you for reading my post
I have my prusa i3 since december 2014 and i have an annoying problem that i can"t seem to fix ... My layers are uneven (see attached files) kinda like a Z wobble but the pattern goes : one thick layer, one thin layer...

Here's what i tried with no change at all :

-Bought new smooth rods (those from my kit were of the wrong diameter which produced a lot of play on all axis)
-Bought a new hot end (E3D Lite 6) instead of the crappy chinese jhead
-Switched from 3mm to 1,75mm and from ABS to PLA
-Autotuned the PID of both the hotend and the bed
-Added a bowden extruder to have a better extrusion
-Printed on a cold bed
-Added a bracket between the frame and the base structure
-Tried different temperatures of extrusion
-Checked belt tension
-Checked feedrate
-Checked hobbed bolt quality (tried with 2 different bolts)
-Check filament quality (tried with 3 Different filament spools)

Overall the structure is really sturdy and i can't find what is wrong ...
some people on french forums said that my threaded rods were the source of the problem, they are 5mm rods. Do you agree ?




Thanks for your help

It looks pretty random to me. Try wiggling the extruder carriage. Any movement?
Find the source of any movement- check that the bearings are not sloppy and that the guide rails aren't flexing.
What are the actual temperature variations you're getting after the PID autotune?

Make sure the Z axis motors have adequate current and make sure there's no binding in the Z axis due to screws/guide rails being out of alignment.

People like to blame problems like this on the screws, but I'm not so sure. Bent screws should be replaced with screws that aren't bent, but I don't know how everyone seems to be getting bent screws- maybe the screw makers set aside all their bent ones for the 3D printing community...
The Z axis bearings should limit the movement of the X axis boom to the Z axis direction (established by the Z axis guide rails). If a bent screw is moving the X axis around I think the Z axis bearings are too sloppy and/or the Z axis guide rails too flexible.

Look for slop in the bearings on the X axis- if the extruder tilts a little each time the movement reverses direction it could cause problems.

I have just been getting my Prusa i3 working. The layers are coming out pretty well.
You have to match your set layer height with the motor steps and threaded rod pitch. Is your threaded rod a metric M5 rod?
My Z-axis threaded rods are M5. They have a pitch of 0.8mm. With my steppers having a pretty typical 200 steps per revolution, each step raises the extruder by 0.8/200 = 0.004mm. What layer height are you using? You want to have it as a multiple of 0.004. So a choice of 0.2mm layers would work, but 0.15mm layers would not. The steppers can't do it. I think this mismatch in settings vs hardware results in interleaved thick and thin layers.
Just to note, I have read this somewhere, I haven't experienced it. And I realise it doesn't account for microstepping... maybe microstepping is not used in the Z-axis.

Hi thanks for helping me
No movements at all on the extruder carriage, or on the printer itself
Here are my values for the PID :
dflt kp : 35.05
ki : 3.96
kd : 77.52
i have no play on the Z axis
How to tell if my Z motors have enough current ?
I tried moving the carriage up and down and had no tilting of the hotend

Hi, i'm using M5 thraded rods too and the pitch looks similar to yours, i print ,with 0.2 and tried a multitude of layer heights with no change...
I have no idea for the number of steps per revolution on my stepper motors, how can i tune all that together ?
thank you

your M5 isnt the problem Unless they a captive at the top, they should be able to be free to move it allows it to wobble freely if they slightly not straight, your extruding too much material check the width of extrusion on the thin walled box to see if it matches up with your slicer settings

---

Check my rubbish blog for my prusa i3

up and running
[3dimetech.blogspot.co.uk]

Axes are programmed in firmware in microsteps/mm. M5 threaded rod has a pitch of 0.8 mm per rev. Your motors are probably run at 16:1 ustepping, so 3200 usteps/rev. (assuming a 200 steps/rev motor). 3200 usteps/rev * 1 rev/0.8mm= 4000 usteps/mm.
If you don't use microstepping, then it's 200 steps/rev*1rev/0.8mm= 250 steps/mm

Quote
chris33
your M5 isnt the problem Unless they a captive at the top, they should be able to be free to move it allows it to wobble freely if they slightly not straight, your extruding too much material check the width of extrusion on the thin walled box to see if it matches up with your slicer settings

Where does this idea come from? I've seen it repeated 1000 times and can't understand how controlling the lateral motion of the screw is going to cause a problem. 99.9% of industrial linear positioners have bearings at both ends of the screw. If there was something wrong with doing that, why would all those industrial machines be made with the lateral motion of the screw constrained? If a screw is bent and the end is constrained, it may cause binding. To say that you shouldn't constrain the screw is wrong- if you have a bent screw, it should be replaced with a straight one.

because industrial lead/ball screws are engineered to be straight but m5 threaded bar isnt, thats why you see in industrial lead/ball screws can be captive, i should know i work with cncs for my job, and had my i3 for 2 and half years way before all the different variations was out trying to improve it and digital dentist you will never find a straight piece of m5 at all so there will be wobble thats why its not captive ask josef prusa who designed it too he would tell you why its not captive at the top

---

Check my rubbish blog for my prusa i3

up and running
[3dimetech.blogspot.co.uk]

what is micro stepping ? I'm sorry i am a bit lost ..
Are these values to write in my firmware ?
thanks for you help guys

The motor driver uses microstepping to smooth out the motion of the stepper. On a RAMPS board there are jumpers under the stepper modules that set the microstepping ratio. Most boards come populated with jumpers set to 16:1, so a single rev of the motor, which normally takes 200 steps, will take 3200 microsteps to complete 1 revolution. The firmware has to know how many microsteps it takes to move the axis 1 mm.

Quote
chris33
because industrial lead/ball screws are engineered to be straight but m5 threaded bar isnt, thats why you see in industrial lead/ball screws can be captive, i should know i work with cncs for my job, and had my i3 for 2 and half years way before all the different variations was out trying to improve it and digital dentist you will never find a straight piece of m5 at all so there will be wobble thats why its not captive ask josef prusa who designed it too he would tell you why its not captive at the top

What is the logic that leads one from "you'll never find a straight piece of M5 threaded rod" to putting M5 threaded rod into a printer? If the stuff is so awful, don't use it. If it gets bent easily because it's so thin, use a larger diameter rod. If larger stuff is also lousy, use Acme screws.

Given all that, the bearings on the guide rails should determine the motion in the Z axis, not the screws. Or are the bearings are as bad as the screws?

Quote
the_digital_dentist

Quote
chris33
because industrial lead/ball screws are engineered to be straight but m5 threaded bar isnt, thats why you see in industrial lead/ball screws can be captive, i should know i work with cncs for my job, and had my i3 for 2 and half years way before all the different variations was out trying to improve it and digital dentist you will never find a straight piece of m5 at all so there will be wobble thats why its not captive ask josef prusa who designed it too he would tell you why its not captive at the top

What is the logic that leads one from "you'll never find a straight piece of M5 threaded rod" to putting M5 threaded rod into a printer? If the stuff is so awful, don't use it. If it gets bent easily because it's so thin, use a larger diameter rod. If larger stuff is also lousy, use Acme screws.

Given all that, the bearings on the guide rails should determine the motion in the Z axis, not the screws. Or are the bearings are as bad as the screws?

My understanding is that the Z axis threaded rod's job is only to raise or lower the carriage. It isn't intended to keep everything lined up nice and straight. That's the job of the smooth rod. So as long as it isn't really badly bent, the threaded rod spins and wobbles a bit on its free end but the smooth rod prevents that from having any effect on the lateral stability of the carriage. If you hold the threaded rod captive on both ends, there is a chance that those wobbles will be transmitted to the carriage through both the carriage and the frame as vibration or even by laterally displacing the carriage enough that the smooth rod can't hold it perfectly straight.

If a little wobble in a 5mm dia screw causes the frame to flex, then the frame is poorly designed/built, too.

Jeez, it's no wonder these things aren't ready for consumer use. Instead of getting more reliable, they're getting cheaper and harder to operate because you have to worry about this sort of nonsense created by stripping out quality in favor of the cheapest way to do things.

I don't think he'll like it but the answer to the OPs question is: You bought a cheesy printer, what did you expect?


Anyway, back to the problem at hand. Your extruder needs to be calibrated and the proper steps/mm needs to be programmed into all axes in order for your printer to provide its best prints. If you have an LCD panel and encoder on your printer you can probably adjust the steps/mm that way, otherwise you'll need to edit the config.h file and recompile the firmware. Read up on calibration here and do what it says: [reprap.org]
After you've got the machine tuned up try a print and see if the problem has gone away.

Quote
the_digital_dentist
If a little wobble in a 5mm dia screw causes the frame to flex, then the frame is poorly designed/built, too.

Jeez, it's no wonder these things aren't ready for consumer use. Instead of getting more reliable, they're getting cheaper and harder to operate because you have to worry about this sort of nonsense created by stripping out quality in favor of the cheapest way to do things.

I don't think he'll like it but the answer to the OPs question is: You bought a cheesy printer, what did you expect?


Anyway, back to the problem at hand. Your extruder needs to be calibrated and the proper steps/mm needs to be programmed into all axes in order for your printer to provide its best prints. If you have an LCD panel and encoder on your printer you can probably adjust the steps/mm that way, otherwise you'll need to edit the config.h file and recompile the firmware. Read up on calibration here and do what it says: [reprap.org]
After you've got the machine tuned up try a print and see if the problem has gone away.

The acrylic frames definitely have some flex to them although you can add braces to firm things up.

Quote
the_digital_dentist
If a little wobble in a 5mm dia screw causes the frame to flex, then the frame is poorly designed/built, too.

Jeez, it's no wonder these things aren't ready for consumer use. Instead of getting more reliable, they're getting cheaper and harder to operate because you have to worry about this sort of nonsense created by stripping out quality in favor of the cheapest way to do things.

I don't think he'll like it but the answer to the OPs question is: You bought a cheesy printer, what did you expect?


Anyway, back to the problem at hand. Your extruder needs to be calibrated and the proper steps/mm needs to be programmed into all axes in order for your printer to provide its best prints. If you have an LCD panel and encoder on your printer you can probably adjust the steps/mm that way, otherwise you'll need to edit the config.h file and recompile the firmware. Read up on calibration here and do what it says: [reprap.org]
After you've got the machine tuned up try a print and see if the problem has gone away.

Yeah i knew that it was a crappy printer but it was very cheap so i bought it anyway, i don't mind fixing problems it helps me know my printer better. And thanks to you guys my "cheesy" printer might become a great one
Alright thanks to you all i am going to try this as soon as i can

I have a Bowden extruder, how can i calculate the e steps for that since i dont have a gear reduction ?

e_steps_per_mm = (motor_steps_per_rev * driver_microstep) * (big_gear_teeth / small_gear_teeth) / (hob_effective_diameter * pi)

Do i replace "(big_gear_teeth / small_gear_teeth)" by 1 ?
In that case my steps per unit for the extruder would be 22.22 ... That is very different from what i hat before : 97.6
is that correct ?

You can measure the diameter of the filament drive gear. Multiply by pi to get the distance moved in one rev. If it's 10mm dia, the filament should move 31.4mm per rev. If there's 3200 usteps/rev then 3200/31.4= 101.9 steps per mm. Use that as a starting value for calibration.

Ok thank you other than that everything was accurate.
But from 97,6 to 101,9 is it thath big of a difference ? could that be my problem ?

I doubt it's the entire problem, but it could be a contributor. Keep in mind that measuring the drive gear is only approximate- the pinch wheel mashes the filament against the drive gear and the teeth sink into the plastic a bit so the effective diameter is a little smaller than you measure.
When chasing print quality issues you have to look at every possible contributor. Your slicer settings can be another contributor. The speed and acceleration settings that are set in the firmware. Motor currents. You have to get it all right.

Ok so i did everything in the guide and my prints are exactly the same as before

Try a different slicer. Slic3r goes carzy once in a while and causes weird problems. Try Cura.

I tried with cura and it is even worse than before

Are the bed and hot end temperatures stable within a degree or so?
Are you using quality filament - does the diameter vary a lot?
What layer height are you trying to print?
How big is the nozzle?

Yes it seems pretty stable, i'm using E3D filament printing at 0.2 mm layer height and with a 0,4 nozzle

I'm out of ideas. I think you have a mechanical problem and you'll just have to keep picking at things until you locate it and figure out a fix. Something is moving in a way it shouldn't or binding.

Check the article on Z axis artifacts here: [s3-eu-west-1.amazonaws.com]

So I think it must be one of these issues:
1. The layer heights are varying from layer to layer, or
2. The extrusion amount is varying from layer to layer, or
3. There is some mechanical movement from layer to layer (such as the bent Z-threaded rods already considered).
Can it be anything else? Maybe:
4. The extruder X/Y movement speed is changing from layer to layer.

For 1. Can you see how much it is moving up/down each layer? If you are printing at 0.2mm per layer, and your Z-axis threaded rods are M5 (thread pitch = 0.8mm), then they will do a complete rotation every four layers. You could use the grub screws in the motor/rod couplers to see how much they rotate each layer. Do they seem to be rotating 90 degrees each layer? Do they come back to the same spot every four layers?

For 2. If you have verified that the Z-axis shift is consistent from layer to layer above, check if the same amount of filament is going through the extruder each layer. You could somehow measure how much goes through for one layer, then mark the same length along the filament a few times, see if each layer is using the same amount of filament. Not sure how accurate you can be, but it might be worth a try.

For 3. It looks to me like you have spent some time on this already.

For 4. Not likely. You say you have tried different slicers... But who knows.

I don't know how helpful this is. If you find your problem, make sure you let us know.

thanks guys i'll try that out