Z Ribbing artifacts and the Z-cal factor

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Z Ribbing artifacts and the Z-cal factor

Postby RetireeJay » 2013-Feb-Thu-18-Feb

In an article in RepRapMagazine, page 30 http://issuu.com/garyhodgson/docs/reprapmagazine_issue_1?mode=window, Gary Hodgson argues that you are bound to get artifacts along your z axis (z ribbing) unless your z calibration and your layer height work out to an integral number of stepper motor full steps per layer.

The cause of the ribbing is the fact that if your layers are not an integral number of steps, then some layers will end up with one more (or one less) step than adjacent layers. It's the software rounding off a floating-point number into an integer number of steps.

On my PB Plus, I did an experiment using my measured calibration factor of 2269.56 microsteps per mm and a layer height of 0.25mm. I did, in fact, see slight ribbing every six layers. This agreed with a prediction I made using an Excel spreadsheet: Layers were 341, 340, 341, 340, 341, 340, 340, 341...

However, the way the math works out, I can use a z-cal factor of 2260.00 microsteps per mm and select layer heights in any multiple of 0.05mm. I tried that, and the ribbing went away. My PB has Kysan motors, and apparently the combination of my PB electronics and motors results in smooth microstepping. Gary Hodgson feels that the absolutely most reliable way to avoid z ribbing is to us an integral number of full steps, but you may find that your machine, like mine, does just fine with integral microstepping.

If you are using an 18 turn/inch threaded rod, your theoretical z cal is 2267.717. So if you use 2260 as your z-cal, you will theoretically have about a 0.3% error on your z axis.

So now you can make an informed choice: do you want to avoid slight visual artifacts and produce a work of art, or do you want to have the tightest possible calibration factor so that your parts will integrate with other mechanical parts that are made to tight tolerances?
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Last edited by RetireeJay on 2013-Mar-Fri-13-Mar, edited 1 time in total.
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Z Ribbing artifacts and the Z-cal factor

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Re: Z Ribbing artifacts and the Z-cal factgor

Postby plexus » 2013-Mar-Fri-02-Mar

i want both :D now to go work out numbers and try it. how did you work out the excel spreadsheet?
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Re: Z Ribbing artifacts and the Z-cal factgor

Postby RetireeJay » 2013-Mar-Fri-09-Mar

Here's the spreadsheet. On one page, I examine different step & microstep values to see which ones produce the greatest number of integer counts per layer for different layer heights.
On the other page, I demonstrate the consequence of integral or non-integral steps per layer by stacking up lots of layers and seeing where the round-off errors result in repeating patterns.
I did this spreadsheet for my own use, so it may take you a few minutes to play around with it and see how it works. But there's nothing esoteric; it's just brute force simple calculations repeated over and over (gotta love Excel for that!)
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Re: Z Ribbing artifacts and the Z-cal factgor

Postby gyronictonic » 2013-Mar-Fri-12-Mar

Are you saying that if one would to go with the no ribbing method (calculated z with no refinement), it's not possible to print parts with tight tolerance? I've been adopting this method using the calculated Z values as well as for the X & Y for a month now and my part's tolerance range from +/- .3mm. Whenever my parts starts to tread past this tolerance range, it's usually due to some mechanical issues such as a loose belt. I thought calibrating the extrusion width would give you tighter tolerance within your parts since there wouldn't be any overlap during the perimeter.
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Re: Z Ribbing artifacts and the Z-cal factgor

Postby RetireeJay » 2013-Mar-Fri-13-Mar

Not necessarily.

But what you have to do is find a combination of z-calibration AND layer height that yields an exact integer number of microsteps per layer.

The value of 2260 microsteps per mm allows you any layer height that is a multiple of 0.05mm, but it gives about a 0.3% error in the height of your object.
You can't select layer height and Z cal independently of each other unless you are willing to accept a little ribbing.
In reality, the ribbing is not very significant; we're talking about extremely tiny refinements here - but some people are looking for the ultimate. For most purposes most of the time, it's probably not that important. As my grandma would say, "It wouldn't be noticed on a trotting horse."
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Re: Z Ribbing artifacts and the Z-cal factor

Postby plexus » 2013-Mar-Fri-16-Mar

I dont know about that, grandma... trotting horse :)
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Re: Z Ribbing artifacts and the Z-cal factor

Postby RetireeJay » 2013-Mar-Sat-09-Mar

Plexus, touche :lol:

To make a clean summary of the conclusion: There are three things you might want from your Z-axis motion:

1) Perfect calibration (a 100mm tall object is exactly 100mm tall)
2) Perfect smoothness of objects along the z-axis (no "ripple" at all)
3) Perfect freedom to choose a particular layer height of interest to you

The bottom line is that you can pick any two, but you can't have all three. But you CAN get both (1) and (2) if you are willing to settle for certain specific layer heights. Or you CAN get a pretty wide selection of "rational" layer heights if you are willing to compromise on either (1) or (2).

Of course, if you are using a metric screw for the Z-axis it makes things easier.
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Re: Z Ribbing artifacts and the Z-cal factor

Postby gyronictonic » 2013-Mar-Sat-18-Mar

Since I'm using 1/4"-16 acme rods, I've been using .1905mm as my layer height and I don't see the benefit of going any lower. I made a simple excel sheet of different layer heights to choose from. One day, I'll try converting to M6 threaded rods as my lead screws but would need to print out some adapter to fit the M6 nuts.

https://docs.google.com/spreadsheet/ccc ... 0Unc#gid=0
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Re: Z Ribbing artifacts and the Z-cal factor

Postby holmes4 » 2013-Mar-Sat-20-Mar

gyrotronic, that's a great table. What makes a selection "first pick" or "second pick"?
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Re: Z Ribbing artifacts and the Z-cal factor

Postby gyronictonic » 2013-Mar-Sun-08-Mar

The first and second tier picks were mostly based on choosing a lower decimal value as the ideal layer height, keeping the error at a minimum. Although I think it should be fine to use any of those listed values.
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Re: Z Ribbing artifacts and the Z-cal factor

Postby drawcut » 2013-Mar-Sun-09-Mar

Very useful sheet Gyrotronic! I'm planning on changing to M5 brass thd rod similar to what Lincomatic did here with 10-24 steel thd rods: http://blog.lincomatic.com/?p=792. 10-24 rods would be much cheaper but when I add columns for 10-24 and M5 it is easy to see the nice even numbers for the M5 and I can't resist :D.

Your sheet doesn't seem to include microstepping, though. That would give 200*16 steps per revolution, I think. That might help give more layer height options for Imperial threads. Although I don't know if full steps might be more desirable than the micro steps.

Edit: I just reread the Reprap magazine article and it clearly says that full steps are more accurate than micro steps. :oops:
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Last edited by drawcut on 2013-Mar-Sun-09-Mar, edited 1 time in total.
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Re: Z Ribbing artifacts and the Z-cal factor

Postby gyronictonic » 2013-Mar-Sun-09-Mar

I thought about adding micro-steps into the sheet but I'm not sure how accurate the micro-steps can be. Probably best to start with full steps since the NEMA17 were built to have a single step angle of 1.8 degrees. Wouldn't a 10-24 thread pitch give you a running decimal value? I figured using values with repeating decimals would introduce errors in the long run.

(1/24) = .041666666666667 inch = 1.0583333333333 mm

1.0583333333333 mm / 200 steps = .00529166666666665 = single step size

*Edit
I went ahead and added a table for #10-24 and M5
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Re: Z Ribbing artifacts and the Z-cal factor

Postby drawcut » 2013-Mar-Sun-09-Mar

OK, so we're typing at the same time! I picked M5 rod since it will be the same diameter as the motor shaft allowing for the simplified plastic tube + zip tie coupling that Lincomatic is using. I'm thinking that M6 probably would work as well. You could likely get the M6 to thread into the same plastic tubing, just a little tighter fit.

Edit: I do see the repeating decimal factors in the 10-24. Not bad but still not as nice as the metric threads with their 0.1, 0.2, etc.

Edit2: I see now what you mean about repeating decimals (i.e not good). Pretty much going to happen if using inch value Z thd rods. Looks like the error can be minimized to the point of being imperceptible based on RetireeJay's post below as well as the info on Lincomatic's blog. To me: if buying new Z thd rods anyway, I'm going to get metric and eliminate one error possibility and get nice even numbers to pick for layer heights.
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Last edited by drawcut on 2013-Mar-Sun-18-Mar, edited 1 time in total.
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Re: Z Ribbing artifacts and the Z-cal factor

Postby RetireeJay » 2013-Mar-Sun-10-Mar

My test showed - at least for my machine - that microstepping is fine. I could see ribbing when I was using a non-integral ratio of layer height to microstep size, but the ribbing went away for integral microsteps.

EDIT: I was fooling myself; I really can't see the ribbing. viewtopic.php?f=20&t=4596

Also, I decided I was willing to accept a slight calibration error, so my 'bot is calibrated in "machine millimeters." In other words, instead of using 2267.72... microsteps per millimeter, I use 2260.00. This gives me lots of flexibility in selecting layer height. For example, every multiple of 0.05mm is integral. And the overall calibration error is only 0.3%.

I'm using brass 5/16 - 18 threaded rod, which seems to be very consistent; I don't see thread-caused artifacts.
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Printrbot Plus operational January 2013
Brass threaded rods (5/16" X 18) & nuts for Z axis
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E3D V5 Hot End, 0.4mm nozzle, also 0.8 and 0.25 in use occasionally
PB fan mount + 40mm fan -- using printed mount adapter, not the E3D supplied fan
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Re: Z Ribbing artifacts and the Z-cal factor

Postby drawcut » 2013-Mar-Sun-18-Mar

Another thought: If using Z lift, should we make sure that value is an even step number as well? Seems like that might introduce error if the software is Z lifting to an approximated value then dropping back down. Depends on how the software handles Z lift I guess. I think I'll set Z lift to an even step value just in case.
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Re: Z Ribbing artifacts and the Z-cal factor

Postby Mooselake » 2013-Mar-Tue-21-Mar

drawcut wrote: if buying new Z thd rods anyway, I'm going to get metric and eliminate one error possibility and get nice even numbers to pick for layer heights.

Know any US accessible sources for reasonably priced metric brass threaded rod? Trapazoidal (iirc the metric equivalent of acme) rod?

Kirk
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Re: Z Ribbing artifacts and the Z-cal factor

Postby mdfast1 » 2013-Mar-Tue-22-Mar

Awesome table gyronictonic! I've made myself a copy. Now for possibly a really dumb question. This chart really only applies if your steps/mm are not modified from the expected for the rod correct?:

Ex - 5/16"-18 rod with 1.8 degree motors: (((360/1.8) * (1/(1/16)))/1.41111) = 2267.71 (should be what your Z calibration is ideally)

Now if you messed with your Z steps/mm with M92 E00.00 wouldn't that make this table not applicable and really we should be looking at our Z calibration number and calculating the ideal layer height from that?
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Re: Z Ribbing artifacts and the Z-cal factor

Postby RetireeJay » 2013-Mar-Wed-09-Mar

You could try the spreadsheet that I attached several comments up in this thread (on 1 March). It allows you to play around with the z calibration.
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Re: Z Ribbing artifacts and the Z-cal factor

Postby gyronictonic » 2013-Mar-Wed-09-Mar

mdfast1 wrote:Awesome table gyronictonic! I've made myself a copy. Now for possibly a really dumb question. This chart really only applies if your steps/mm are not modified from the expected for the rod correct?:

Ex - 5/16"-18 rod with 1.8 degree motors: (((360/1.8) * (1/(1/16)))/1.41111) = 2267.71 (should be what your Z calibration is ideally)


Correct, the chart specifically applies to the calculated Z step based on what lead screws are used.

mdfast1 wrote:Now if you messed with your Z steps/mm with M92 E00.00 wouldn't that make this table not applicable and really we should be looking at our Z calibration number and calculating the ideal layer height from that?


Messing with the Z steps/mm may just land you back to the original problem, z ribbing artifacts but you may not see any difference above .2mm layer height. I think the goal is to limit the need for micro-stepping and rely on the Nema17s full step since that is what they were designed for. This method also applies to the X and Y steps/mm depending on what pulleys your machine uses. For example, a GT2, 16 teeth timing pulley would have a Step/mm value of 100 and should not be edited.

After hanging around the reprap forum for while, I learned that "re-calibrating" the X, Y, Z steps for tighter tolerance is a rookie mistake. Using the calculated steps/mm value should give you accurate parts right off the bat and we shouldn't be refining our steps/mm to make up for our machines shortfalls. Instead, we should be refining and maintaining the bots mechanical aspects; making sure the belts are tight, smooth rods are properly lubed, tightening loose nuts, tossing printed pulleys for machined pulleys, etc. Some reprap elitists seem despise the Printrbot's open frame style since it lacks frame rigidity, introducing vibration into the mix. They even named it the "wobblebot" because of that but they seem to hate everything but themselves.

Anyways, when I started using this method, I noticed the quality were far better than before. I was worried about my parts being inaccurate but they came out fine, about +/- .1mm. You can use Prusa's reprap calculator to give you the values you need.

http://calculator.josefprusa.cz/
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Re: Z Ribbing artifacts and the Z-cal factor

Postby RetireeJay » 2013-Mar-Wed-12-Mar

OK, I've beat the problem into submission using Excel. As I said above, you can have any two of the three: Perfect calibration, No Z ribbing, or Select-your-own Layer Height.
The below assumes you are using a lead screw of 18 turns/inch. If you are using a metric screw then you are golden; your choices are much more free.

If you want Perfect Calibration and No Z ribbing, then here are your possible Layer Heights. Entries shown with green highlight are an even number of full steps; entries without green use half-steps (8 microsteps). In other words, if you use the perfect Z calibration and want to use only full steps, you cannot ask for a layer height of 0.4mm. The nearest value would be 0.381.
TrueCalPossibleLayerHeights.PNG


If, on the other hand, you want "rational" layer heights and no Z ribbing, then you must accept a compromise on Calibration. The smallest error is if you use Z = 2260 microsteps per mm. And, it turns out using this value, that full steps don't work; it requires use of microsteps. You would have to accept a larger Calibration error in order to use only full steps. Although in theory the motors will do better with full steps, I have not noticed a problem on my PB Plus with Kysan motors.

I'm calling the layer height you specify "Machine mm." That's what you would put into Slic3r or KISSlicer. The Actual layer height is a little less, as shown.
Rational Layer Height.PNG
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Re: Z Ribbing artifacts and the Z-cal factor

Postby gyronictonic » 2013-Mar-Wed-13-Mar

Nice write up retireejay, gives us another option to choose from. So much grief for using SAE :(

drawcut wrote:Another thought: If using Z lift, should we make sure that value is an even step number as well? Seems like that might introduce error if the software is Z lifting to an approximated value then dropping back down. Depends on how the software handles Z lift I guess. I think I'll set Z lift to an even step value just in case.


Oh that's a good catch. I usually set my z lift to whatever layer height I selected but I forgot to change that recently.
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Re: Z Ribbing artifacts and the Z-cal factor

Postby mdfast1 » 2013-Mar-Wed-14-Mar

Thanks guys for the follow up, I agree, we shouldn't have to tailor the Z step/mm from the expected values. I'll try and set it back to expected and try a correct layer height to see how it goes.
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Re: Z Ribbing artifacts and the Z-cal factor

Postby RetireeJay » 2013-Mar-Wed-15-Mar

Oh, and by the way... Of course you can't directly enter the equation (200*16*18)/25.4 into your Z steps per millimeter. And the number when calculated out has repeating decimals... so what to do?

Well, if you want to keep that magical integral number of steps per layer all the way up to 200mm high, then you should enter the Z-cal value to at least three decimal places: 2267.717 Or you could gild the lily and go for 2267.7165. Of course, in the real world, screw threads are probably nowhere near this precise - but at least you'll have the satisfaction of knowing that with the layer heights pulled from the above table, you won't have any Z banding caused by roundoff errors in the software, and the calibration is about as accurate as you can get without a ton of laboratory instruments.
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PB fan mount + 40mm fan -- using printed mount adapter, not the E3D supplied fan
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Re: Z Ribbing artifacts and the Z-cal factor

Postby drawcut » 2013-Mar-Wed-20-Mar

Mooselake wrote:
drawcut wrote: if buying new Z thd rods anyway, I'm going to get metric and eliminate one error possibility and get nice even numbers to pick for layer heights.

Know any US accessible sources for reasonably priced metric brass threaded rod? Trapazoidal (iirc the metric equivalent of acme) rod?

Kirk


I got my metric rod from Mcmaster-Carr. M5 brass for $21 / meter. Not cheap but not horrible either. I think M6 would also be a good choice. SAE is definitely cheaper. And they only sell metric brass nuts in bulk pack. I don't use ACME - just standard brass threaded rod. Accurate enough IMO. Mcmaster does sell metric ACME rod but only down to M12 which is too big IMO for us and very expensive.
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Re: Z Ribbing artifacts and the Z-cal factor

Postby drawcut » 2013-Mar-Sat-12-Mar

Thinking about it some more, I think stainless threaded rod is cut more accurately than plain steel and is a lot cheaper than brass in metric sizes. Could go with a brass nut for less friction.
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Printrbot original. Major mods: Y axis extended to ~8", Z extended to ~8.5". 5mm SS Z threaded rods w/ flex couplings. E3D hotend. Purchased Acetel gears. Glass bed with Elmers' purple glue stick for most prints. Top of Z axis rods have added cross structure similar to a Prusia i3.
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Re: Z Ribbing artifacts and the Z-cal factor

Postby phil_roberts » 2013-Apr-Tue-10-Apr

RetireeJay- thanks for this suggestion. I tried running at 2260, with a layer height of 0.1- I think it reduced the ribbing. However, the print came out slightly taller.

Here's a silly question- if setting the z-cal at 2260 slightly changes predicted height, can't I just scale the model slightly (say 0.99 in the z axis) to compensate?

Another silly question- you mentioned the tradeoff between reduced ribbing and overall dimension quality- if you are printing mechanical parts, and they are all printed at the same z-cal- wouldn't they fit together fine?
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Re: Z Ribbing artifacts and the Z-cal factor

Postby halley » 2013-Apr-Tue-12-Apr

phil_roberts wrote: if you are printing mechanical parts, and they are all printed at the same z-cal- wouldn't they fit together fine?


Only if all parts were meant to fit with the same Z axis. Most machines have some kind of rotation involved. If the z-cal is not accurate, any vertical circle becomes an oval.
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Re: Z Ribbing artifacts and the Z-cal factor

Postby phil_roberts » 2013-Apr-Tue-21-Apr

halley wrote:Only if all parts were meant to fit with the same Z axis.

Good point.
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Re: Z Ribbing artifacts and the Z-cal factor

Postby PxT » 2013-Apr-Wed-11-Apr

gyronictonic wrote:Since I'm using 1/4"-16 acme rods, I've been using .1905mm as my layer height and I don't see the benefit of going any lower. I made a simple excel sheet of different layer heights to choose from. One day, I'll try converting to M6 threaded rods as my lead screws but would need to print out some adapter to fit the M6 nuts.

https://docs.google.com/spreadsheet/ccc ... 0Unc#gid=0



I just tried changing my layer height to 0.254 (I'm using ACME 1/4" rods) and I can't really say that I see much difference compared to what I was getting at 0.2.
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Re: Z Ribbing artifacts and the Z-cal factor

Postby rsilvers » 2013-Apr-Mon-19-Apr

This is good info. I am going to create an Excel sheet for my printer.
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Re: Z Ribbing artifacts and the Z-cal factor

Postby rsilvers » 2013-May-Thu-20-May

For 18 pitch, the following are some of the simpler full step z-height layer values:

0.0635mm
0.127mm
0.1905mm
0.254mm
0.3175mm
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Re: Z Ribbing artifacts and the Z-cal factor

Postby rsilvers » 2013-May-Sat-17-May

Doing pretty well on my Simple with 0.381 thick layers and M92 Z2267.720 ; calibrate Z.

Image
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Re: Z Ribbing artifacts and the Z-cal factor

Postby thawkins » 2013-Sep-Mon-12-Sep

rsilvers wrote:Doing pretty well on my Simple


Is that a beta or a v2?, i still cant work out what the specs are for the v2, i wish there was a data sheet somewhere that gave all the important details for each model.
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Zen Toolworks CNC/3d printer
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Flashforge 3d Creator Pro
Dual Extruder
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Photon, self designed printed printer.
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