Z Banding on N1 with V2 Hot End

Topics around mechanical design, controller and electronics. Mods & hacks welcome.
mitchfx
Posts: 67
Joined: Thu Mar 24, 2016 12:46 am

Z Banding on N1 with V2 Hot End

Postby mitchfx » Fri Nov 24, 2017 11:12 pm

I'm finally getting around to using my N1 after buying it back in June (been primarily printing on my N2 Plus and Form2) and noticing some banding on my finished parts. I'm using the R3D premium white PLA and 0.1 resolution sliced with Idea Maker. Here are some photos, one to show entire part and the other held at and angle to the light to help show the problem more clearly. The spacing between these bands is around 1.6mm. I'm running some more tests now of a simple cylinder using different settings like no infill, vase mode, etc and will also try some other PLA. I'm wondering if it could be an issue with the Z-rod wobbling or maybe filament feed resistance (using the integrated spool holder with lid on).

Any ideas?
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Jetguy
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Re: Z Banding on N1 with V2 Hot End

Postby Jetguy » Fri Nov 24, 2017 11:56 pm

On any of the N series, we have multiple ways wobble can be seen in Z.
On all of the N series, the large Z axis ballscrew is constrained by ball bearings at BOTH ends of the shaft. This means that if any damage or defect to to the leadscrew as far as runout then means the ball nut riding up and down the shaft will wobble as the leadscrew rotates.
On the N2 and N2 Plus, the ball nuts are attached to the moving Z stage base with 4 capscrews and matching locknuts. They are intentionally loose on the N2 series so that if there is any wobble, in theory, the nut can move inside the hole of the Z stage and reduce (not eliminate!!!!) wobble transferred to the Z stage and thus your print.

On the N1, the nut is attached to the Z stage via 2 large capscrews. These are normally tight to the stage thus NOT allowing the ball nut traveling on the ball screw any room to move and slide in compensation.

On all of these, there are basically 3 ways to handle this.
Option 1= complain to Raise 3D and have them basically replace the printer because to change out the ball screw, you have to take nearly the entire printer apart. Go ahead ask me- I know and have done it.

Option 2= Ensure the screws holding the ballscrew nut to the Z stage are loose enough that it can wiggle, but not so loose the connection is sloppy. For N1, highly suggest you take those screws holding the Z nut out one at a time, apply threadlocking compound to the threads, add a washer, and then put the screw back but don't crank it down tight. This again, gives the main Z nut a way to move and not transfer wobble to the Z stage.
N1 Z nut capscrews.jpg


Option3= is by far the hardest to achieve, but is the mechanical opposite of option 2. Instead, you take nearly the entire printer apart to get to it, take out the ball bearing on the top of the ball leadscrew that is in a milled pocket in the top of all 3 models. This allows the leadscrew shaft to move in XY as required and thus with no bearing to transfer this wobble force back into the frame, you can tighten the ball nuts to the Z stage tight now.
N1 leadscrew top bearing.jpg

Top rear bearing.jpg


Now all the above is if the periodic wave of the wobble matches the pitch of the ballscrew.
If the error does NOT match the pitch, then the answer is more likely to be PID tuning of the hotend or (given recent diagnosis of the potential fault of what happens with a bad, loose, or failing ribbon cable to the extruder head breakout board and lack of modifying the mainboard to detect such a fault) you could have periodic temperature swings of the heater block and nozzle. And YES, this absolutely can cause what looks like Z wobble just from extrusion temperature varying. This is why I cannot stress enough, if Raise 3D is not going to publish information and fix this hardware known limitation, then you the users are going to have to pressure them. Again, based on what I have seen, I get a strong indication that many of these "weird" faults mid print that are NOT being shown on the LCD, not able to be detected by any firmware (Not even 1.1.6) are the root cause of many issues.

PID autotune exists in the latest touchpanel update viewtopic.php?f=3&t=1895&p=15591&hilit=PID+autotune#p15591

The ribbon cable discussion viewtopic.php?f=4&t=3524&p=19044&hilit=resistor#p18989
viewtopic.php?f=2&t=4348&p=20912#p20912
Anyway, probably too far in depth here but the issue is, the wiring for the heater power has a locking connector, the temperature sensing ribbon cable does not have a locking connector. In failure, what happens is the mainboard is totally unaware that the current temperature it is reading is not valid because the ribbon cable came unplugged. Again, the fault is, the temp or voltage that the actual processor pin is seeing stays the same when the cable comes unplugged. Without a change, there is no way code (firmware) can detect this state of error. If your PID is properly tuned, your system is printing and the last reported temp is perfectly normal but say 2C under the set temp, the heater will be in the on state because you are trying to reach the setpoint. If the cable comes unplugged in this state, the last temp keeps being read as if it's being updated even though the cable is physically not plugged in. As such, the heater cannot be controlled because the logic is being fed bogus information. This is why this requires a hardware fix- a set of resistors that change the voltage on the pin should the cable come unplugged.
Last edited by Jetguy on Sat Nov 25, 2017 12:47 am, edited 2 times in total.

Jetguy
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Location: In a van, down by the river

Re: Z Banding on N1 with V2 Hot End

Postby Jetguy » Sat Nov 25, 2017 12:04 am

One idea I was playing with on my N2 plus and have not yet implemented is to basically remove the 4 screws and nuts holding the ball nuts for the leadscrews on each side. But only after making a large flat stock piece of aluminum bent in a large "Z" shape, and then each ball nut has a tapped hole that originally was for a grease zerk that is M6 threads. My intent was to attach this bent relatively thin but strong bracket such that the nuts are connected to each other and cannot rotate. At the same time, now without any constraint, they can move in XY while still lifting and holding the Z stage. Since the Z stage weighs a good bit, no fear of backlash (heck, it already basically sits this way now with the 8 total capscrews and nuts being loose anyway. Again, My theory is that this bar attaching between the 2 ball screw nuts prevents them from rotating, but at the same time, the large Z bend will allow any wobble and thus slight change in distance between the 2 nuts. I feel this is superior to the loose capscrew setup, but obviously needs testing to bear this out.

Jetguy
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Re: Z Banding on N1 with V2 Hot End

Postby Jetguy » Sat Nov 25, 2017 12:57 am

There are other possible faults, but those are the most likely. Far less likely but still possible are that the drive gear on the extruder itself has filament stuck in the teeth. This would mean as it rotates, it has a different linear distance of filament pushed per degree of rotation. Since we push in 1.75mm nominal filament and comes out through a 0.4mm nozzle, this effect is amplified and significant changes in extrusion volume could occur. It would be periodic in nature but again, a factor that also does not likely match the leadscrew pitch. Another thing could be really bad filament that varies in diameter. For the same basic reason, this changes the volume VS linear rotation of the feeder motor and thus a period wave.

Another factor is when printing in vase mode with continuous extrusion, you can sometimes see shrinking and cooling effects of the fan duct interacting. This is because the ducts often blow from one orientation or position compared to the nozzle.

mitchfx
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Re: Z Banding on N1 with V2 Hot End

Postby mitchfx » Sat Nov 25, 2017 2:50 am

Hi Jetguy,

Thanks, as always, for the super informative and helpful response. I'm still running some additional tests at different speeds and resolutions and moving on to trying another PLA as well as feeding the filament from a low friction cradle above the printer (vs using the integrated spool holder).

Any thoughts on tests I can do to help narrow down where the issue is coming from? Being able to eliminate possible culprits would probably be a good first step, no?

Thanks again!

Jetguy
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Location: In a van, down by the river

Re: Z Banding on N1 with V2 Hot End

Postby Jetguy » Sat Nov 25, 2017 2:11 pm

Well again, on the N1, the axis is over constrained. You have the linear guide rods and then the Z axis ball screw with bearings at both top and bottom and a rigid (as shipped from the factory) connection to the moving Z stage. If there is any runout at all in the ball screw assembly the bearings top and bottom push in XY to flex and bend the Z stage from riding straight up and down the linear rails.
Now Raise 3D does use good parts and oversized parts (large 12mm shafts, huge 20mm ball screw shaft) but for all the same reasons again, if that ball screw has even an "in tolerance" defect, the system will as built and shipped transfer that defect to the moving Z stage and thus your print. So what I'm saying is, #2 loosening the 2 Z axis nut screws is a minimal modification that is just easy to use and test a print on. Again, I'm telling you and Raise 3D in general, that is a suggested change to how the factory should ship the printer. Heck, the N2 already does the same thing and it's in the FAQ, why should N1 be different with those screws being tight and thus causing a known over constraint?

The second part is, since we now have 2 updates of both Marlin firmware and the LCD touch with a few critical features, I highly suggest you use them. So step 1 is updating both to the latest, and then step 2 is using the PID tuning function (in the latest touch LCD update) to fine tune your temp control and eliminate that as a potential fault.

What I do as a test is take a 20mm test cube, then scale it to max Z and 40mm in XY. Then if you use the vase mode, and also set roofs and floors to 0 layers, then it creates the single wall vase mode. One of the older things on thingiverse but well proven https://www.thingiverse.com/thing:5573

The reason here for using a square object is that a cylinder is technically a polygon. Those short segments of the polygon have to be sent over the USB serial connection from the LCD pcduino to the marlin firmware and given high enough polygon count coupled with feed rate, I was able to choke the connection and get stuttering (story for another day). As such, the text object should be more straight walled so that you can see this Z axis error rather than potential system artifacts with completely different other causes.

mitchfx
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Re: Z Banding on N1 with V2 Hot End

Postby mitchfx » Sat Nov 25, 2017 5:45 pm

Jetguy,

I will run the 40x40xmaxZ test next. In the meantime here are some 20x20x20 cube test I ran in non-vase mode with 2 outer shells (3 floor and no roof layers). The white and red (both 0.2mm) exhibit the same pattern though it's harder to see on the red color. White used built in spool holder and red was fed from above with a low friction cradle but that didn't seem to make much difference.

23905622_10155181649332781_3860011495475490000_n.jpg

Jetguy
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Re: Z Banding on N1 with V2 Hot End

Postby Jetguy » Sat Nov 25, 2017 6:47 pm

There are 2 effects going on. The bad one I see is the periodic one seen on the 0.25mm that seems to match the pitch of the leadscrew. That same error is present in all 3, but the secondary pattern somewhat distracts you from it in the other cubes to the right.
Highly suggest you invoke fix #2 at a minimum. Going to #3 removing the bearing is not for everyone. But of course, definitely start a ticket in the system to cover you.

Secondary pattern may be a function of firmware and interrupts driving the steppers. Keep in mind what is happening, when you change layer height, now the volume of extrusion is less for that segment (length x width x layer height) so the extruder stepper is the one changing rate where XY is constant between all 3. Since we park at a layer, Z while yes is a different height- the error or ripple is not directly Z related (speaking of that diagonal ripple).

Again, if you were to compare a given line gcode segment between the 0.25mm layer height file and the 0.20mm layer height file. the change will be the E length extruded for a given XY length segment. This is because less linear distance of 1.75mm filament is required for the volume of the shorter (less tall) layer height. So in knowing that, knowing that the extruder feeds slower and less distance of filament per layer and this ripple comes in, if the current setting for the stepper driver for the extruder is not correct, or the inductance of the motor and the stepper driver are not a good match, this is where microstepping current regulation error invokes. This means that you could see a periodic too much too little plastic from the nozzle (at a very fine level here- keep in mind the variation we are talking is miniscule).

mitchfx
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Re: Z Banding on N1 with V2 Hot End

Postby mitchfx » Sat Nov 25, 2017 7:06 pm

I will try loosening the screws a bit and run a quick test. Do you reckon it is safe to do a short print after simply loosening them without going through the process of adding the threadlock and washers just to see how it affects things?

Also, are there any instructions online that walk you through how to update the Marlin and LCD Touch firmware?

Thanks again.

Jetguy
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Location: In a van, down by the river

Re: Z Banding on N1 with V2 Hot End

Postby Jetguy » Sat Nov 25, 2017 7:19 pm

Yes, safe to run a test print just by loosening.

Firmware for both versions (Marlin motion control and raise 3D touch are covered) viewtopic.php?f=4&t=3844&p=19531#p19531

Jetguy
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Re: Z Banding on N1 with V2 Hot End

Postby Jetguy » Sat Nov 25, 2017 7:20 pm

Oops, the original link on firmware versions viewtopic.php?f=3&t=1652#p13896

mitchfx
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Re: Z Banding on N1 with V2 Hot End

Postby mitchfx » Sat Nov 25, 2017 7:39 pm

Should the Panel and Marlin updates be run in any specific order? I have them both ready on different SD cards and ready to go once my "loosened cap screw" test is done. :)

Jetguy
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Re: Z Banding on N1 with V2 Hot End

Postby Jetguy » Sat Nov 25, 2017 7:43 pm

Not really a critical order. I would update the LCD firmware first, just because that is more critical since you need the features like PID tuning and then do the Marlin. LCD= Raisepack file. Marlin=*.firm file.

mitchfx
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Re: Z Banding on N1 with V2 Hot End

Postby mitchfx » Sat Nov 25, 2017 7:45 pm

Roger that. Any links to instructions on how to do the PID tuning? Thanks.

Jetguy
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Location: In a van, down by the river

Re: Z Banding on N1 with V2 Hot End

Postby Jetguy » Sat Nov 25, 2017 8:06 pm

PID autotune exists in the latest touchpanel update viewtopic.php?f=3&t=1895&p=15591&hilit=PID+autotune#p15591

mitchfx
Posts: 67
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Re: Z Banding on N1 with V2 Hot End

Postby mitchfx » Sat Nov 25, 2017 8:08 pm

Quick comparison of before (left) and after (right) cap screw loosening. While the banding is still there it's way less noticeable. I will try loosening up another 1/8 turn or so and run the updates.

24085011_10155182124032781_1021224064_o.jpg

Jetguy
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Re: Z Banding on N1 with V2 Hot End

Postby Jetguy » Sat Nov 25, 2017 8:08 pm

Found it further in that thread viewtopic.php?f=4&t=1505&p=15761#p15761

mitchfx
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Re: Z Banding on N1 with V2 Hot End

Postby mitchfx » Sat Nov 25, 2017 8:20 pm

Going to run a test print after the updates and slightly more loose screws, then do the PID tuning. Man, Raise3D really needs to add this bit of info to the downloads page...

These updates also go on the USB thumb drive and are labeled .firm and instead, you go to the print menu, choose the drive where it's stored, and "print" the file and because it's named a .firm, it knows to flash the file to the motion board.


:)

mitchfx
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Re: Z Banding on N1 with V2 Hot End

Postby mitchfx » Sat Nov 25, 2017 8:26 pm

Gotta run to the hardware store for some threadlock since I only have the high strength (semi permanent stuff) on hand. Do you recommend the medium or light duty kind for this?

mitchfx
Posts: 67
Joined: Thu Mar 24, 2016 12:46 am

Re: Z Banding on N1 with V2 Hot End

Postby mitchfx » Sat Nov 25, 2017 8:33 pm

And oh...flat or lock washers? Any suggested size? Thanks!


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