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jleichne

Calibration Techniques and Printhead Mods

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I've been trying to decide on the best way to properly characterize the positional accuracy and repeatability of my printhead. My ultimate goal is replacing it with an alternate printhead for cell hydrogels so I'm not concerned about parameters relating to the fiber extrusion (although I'd like to work on a multi-spool feed system one day).

I came across a good post by Daid recommending attaching a pencil to the printhead - have any of you tried this and what have you found?

I'm trying to decide whether its necessary to install a positioning system to ensure that I can get below 20 micron positioning accuracy.

Any thoughts on this?

J

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A few questions:

Have you ever calculated the precise accuracy and long-term repeatability?

When you say 'Gearing on the Steppers', are you implying that if I move the XY steppers over laterally and make room for a certain size gear I can use gear ratios to improve my baseline 'microstep' size?

Is the microstepping of the motors embedded in the firmware or the software? I'm assuming that the microstepping capabilities are probably already maxed out as much as possible by talented electrical/software engineers so maybe I shouldn't play with that.

Another concern is that in modifying the structural loads at the printhead with a different extruder, I'd need to recalibrate the 'step to um' plot since I need to precisely speed up and slow down the motor per motion in order to make sure the printhead stops moving in the perfect spot.

I'm very tempted to hook up a linear encoder system so I can know exactly where the head is at all times. I'd probably need to rewrite all the software and firmware though to properly account for new Force-Distance curves, etc.

Other options include radio frequency modulated visible light sources or triangulation systems.

So nobody has every tried to supercharge their Ultimaker accuracy with an awesome positioning system?

Thanks,

Jared

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viewtopic.php?f=6&t=1429

^I believe this is what they are talking about in regards to gearing the motors...

If you were to gear down the steppers you could theoretically improve accuracy, though they are pretty accurate as it is. You would also be dealing with a slower print speed and would probably want to have additional cooling for your steppers as they would be working that much harder to complete the prints.

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In addition, I just read SnowyGrouch's excellent Direct Drive modification here:

viewtopic.php?f=6&t=1429

Lets say we describe my accuracy as a mathematical model, which goes like this:

Final Position (along one axis) = (Xo) + Etiming + Elong

Xo = what software considers the initial position

Etiming = Positional Error due to (1) Slip of timing belts and (2) Improper Orthogonality of Timing Belt to Long Belt

Elong = Positional Error due to (1) Slip of long belt, (2) Improper orthogonality of both long belts

(Also errors due to parts not being screwed in tight but is removed due to simplicity)

In a simplified model, can this be described as the primary source of error? I understand Snowygrouch's technique may introduce minor error relating to alignment of direct drive motor with axis, but we can neglect that too.

Does this make that machine substantially more accurate? I feel like we need positional accuracy testing metrics. The gear-ratio approach may include the extra source of error, but maybe by lowering my step size enough it is a viable method to achieve my desired resolution(?)

J

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The steppers would be working harder because if you were to dial down the gear ratio, you would increase the amount of travel it would have to incur to complete the same object. IE, if we double the gear ratio, thus making it theoretically more accurate, we would require twice the input from the stepper. Your steppers would be running nonstop, thus getting quite warm. Plus, you would be running at half the speed, thus taking twice as long to finish the print.

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A few questions:

Have you ever calculated the precise accuracy and long-term repeatability?

When you say 'Gearing on the Steppers', are you implying that if I move the XY steppers over laterally and make room for a certain size gear I can use gear ratios to improve my baseline 'microstep' size?

Is the microstepping of the motors embedded in the firmware or the software? I'm assuming that the microstepping capabilities are probably already maxed out as much as possible by talented electrical/software engineers so maybe I shouldn't play with that.

So nobody has every tried to supercharge their Ultimaker accuracy with an awesome positioning system?

Thanks,

Jared

sort of. at least 6 months of tweaking and fine tuning has led me to the conclusion that SG's direct drive mod is an important step forward and removing those belts is the next big step. that's most of the error (probably still be some left in the bearings, woodwork etc.)

you can get to 0.1mm repeatability but don't expect better unless you tweak the g-code.

fitting a big gear would change the gear ratio and might improve step size, yes, you'd want a gear without backlash of course, or it won't help as much.

the microstepping is a feature of the motor driver and it's not entirely guaranteed or accurate either but that's a discussion in itself.

I hear what you're saying - I want it to. finding ways to replace the belts is the best solution right now.

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I agree that direct drive is a crucial step in removing an unnecessary source of error.

How do you feel about the following two adjustments as well (in tandem with direct drive):

(1) Using Ground Ball Screws to drive two primary axes and the belts and pulleys to respectively drive the parallel two axes.

This may drastically raise print times but do you believe it will achieve a more reliable, consistent accuracy over solely direct drive.

(2) Adding two more vertical beams like those to the sides of the main vertical leadscrew at the front of the device to better stabilize the platform

J

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1) you'd have to draw a picture I think. it sounds like you added another axes which may lead to binding or other problems? ball screws are generally better yes, but also expensive (main reason people try to avoid them).

2) again, probably helps to draw a picture - it sounds like you're trying to eliminate wobble across the Y axis on the bed itself? I've seen little evidence of this and wouldn't the sprung bed counteract any improvement?

3) lots of work, but this might help slightly. perhaps a better (altho still lots of work) solution is to adopt corexy motion instead. the belts can (I think) be replaced with steel wire or similar to avoid stretching or at least be upgraded to re-inforced belts.

I think I'd like to see improvements in the corexy direction if possible - the firmware even supports this so it's the route of least pain for most gain.

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Hi,

The issue I see with corexy using a steel wire, is that your long term positional stability will

suffer since strictly speaking the cables are not indexing.

So start at xy=0, run the printer for 1 hour, and I would be amazed if it goes back to within even 0,5mm of xy=0

If you lost any tension at all the indexing would be ruined completely.

So I dont know, but I think that whatever you end up with it has to have some sort of positive indexing (teeth of

some sort basically). Either that or a feedback control system....which is I think just putting in alot of effort

to solve the wrong problem.

I want to use ball-screws but am a long way off working out keeping the inertia low enough such that it can

achieve high speeds without having to completely basically make a new machine with a more industrial

spec drive system and a proper metal frame etc.

C.

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yeah that's the problem - solving as much of the problem without completely redesigning everything.

you can get re-inforced belts which might be better? I read something about the belt type - GT (I think) as opposed to LX (or whatever the current one is) is supposed to be more suitable?

I haven't really spent the time to look into this enough but it's highish on my list of things to do.

direct drive and heated bed are next up.

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Okay, so, just to be clear, you are planning on reworking the slicing software anyways to ensure the high precision you are looking at, correct? And reworking the extrusion to a cell dripper, thus having the accuracy you want from your material application, correct?

If so, then I would say each of your three steps would improve accuracy. Though for step 1, I would replace the linear bearing on the drive axis with you ball screw gear and leave the linear bearing on the parallel side, thus not needing any belts (Assuming you would direct connect these axis like the z axis is done). I would say pursue this over changing gear ratios and keeping the belts. This would result in fewer moving parts that can wear/tear/generate inaccuracies/backlash opportunities.

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Alaris:

I didn't plan to add any extra axes. I will add a photo later when I have more time. My plan is to simply replace the particular X and Y axis that is timing-belt driven with a long ball screw. A similar belt system will pull the block on the opposing two axes. This will combine direct drive with a ball screw and I believe the two axes driven by belts will have much less of an impact on error than the ball screw.

Snowygrouch:

I'm happy to see that you've also considered this. I agree that its definitely not a speed-friendly approach, but for my purposes (small-diameter arteries), I don't need to go too fast. I feel like a ball screw would have incredibly more positional accuracy (the pricey ones at least), so a rotary encoder on the coupling should be unnecessary, right?

MSURunner:

Yeah, I will need to rework the slice software to automatically parse instructions to separate printheads and yes this is essentially a cell dripper. This is why my main concern now is my underlying XY positional accuracy (which I should hopefully be reliably testing soon). Are you saying that I can avoid belts by using the screw to push both blocks through the central bar? I was concerned about that approach because I felt that the belts would be needed to prevent any strange long-term axis deformation. I suppose I'll need to model it first.

I think that if users are going to make significant mechanical updates to the device, there should be a section of the forum for people to design replicable experiments to properly characterize improvements in the positional accuracy or mechanical improvements. This way we can over time build up metrics of proper device performance.

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Are you saying that I can avoid belts by using the screw to push both blocks through the central bar? I was concerned about that approach because I felt that the belts would be needed to prevent any strange long-term axis deformation. I suppose I'll need to model it first.

 

The way I'm seeing it, you would be essentially replicating SG's direct drive, but with a ball screw as one of your axis. if you were to try to connect the belts to the opposing axis, you would need to have a series of couplers with gear sprockets attached to axis sections at the beginning and end of your ball screw to maintain your identical gear ratios in the two axis. I only foresee that leading to inaccuracies. I think you need to replace the one drive axis with your ball screw and leave the other alone. Then you need to modify the carriage bar sections to have a lower friction bearing on the axis you left alone and a ball screw coupler on the drive side. That may mean you would want to bump up the thickness of the carriage bar section as well, which means you would need to swap out bearing in the print head and thus update the wooden box. That doesn't seem to be like too much of a hassle if you are already ditching the hotend. Of course this is all speculation here. You are the first person trying to do this with an Ultimaker, to my knowledge.

I think what you might be better served with doing, after looking at all that would need to be done, would be getting your hands on some acrylic/sheetmetal, your ball screws and some servos and building it from scratch. Really, all you are using at this point are half the axis rods, the servos, the outer box, the control board, the power unit and some other smaller parts. Since we already talked about reworking the box to something that will ensure less deformation around bearings and allowing for two additional axis shafts for the build platform, I think it's safe to say scrap that. Now we're left with re-engineering the build platform to have four points of contact with axis, which means scrap the platform at least, leaving you with two axis, the bearings and the ball screw.

My opinion, test to see how close you are out of the box (double check the squareness and alignment of your build). If the standard setup isn't going to cut it, you are better served getting the Arduino board and customizing your printer to meet more exacting standards. If you are buying all that stuff to build it up anyways, you are only a stones throw for a complete build, which would give you two printers. You could then either sell your Ultimaker or use it for enlarged modeling of what your custom one is doing.

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The way I'm seeing it, you would be essentially replicating SG's direct drive, but with a ball screw as one of your axis. if you were to try to connect the belts to the opposing axis, you would need to have a series of couplers with gear sprockets attached to axis sections at the beginning and end of your ball screw to maintain your identical gear ratios in the two axis. I only foresee that leading to inaccuracies. I think you need to replace the one drive axis with your ball screw and leave the other alone. Then you need to modify the carriage bar sections to have a lower friction bearing on the axis you left alone and a ball screw coupler on the drive side. That may mean you would want to bump up the thickness of the carriage bar section as well, which means you would need to swap out bearing in the print head and thus update the wooden box. That doesn't seem to be like too much of a hassle if you are already ditching the hotend. Of course this is all speculation here. You are the first person trying to do this with an Ultimaker, to my knowledge.

 

To make sure I understand you correctly, you are suggesting removing all the belts and making the two axes parallel to the two ball screws driven by the sheer force of the ball screw on the opposite side (meaning I will need to replace the opposite wooden blocks with something on a super frictionless bearing).

I feel that while this will better level the printhead, it will probably cause strange torque beyond a particular load I am applying at the printhead. (Still need to run simulations)

 

I think what you might be better served with doing, after looking at all that would need to be done, would be getting your hands on some acrylic/sheetmetal, your ball screws and some servos and building it from scratch. Really, all you are using at this point are half the axis rods, the servos, the outer box, the control board, the power unit and some other smaller parts. Since we already talked about reworking the box to something that will ensure less deformation around bearings and allowing for two additional axis shafts for the build platform, I think it's safe to say scrap that. Now we're left with re-engineering the build platform to have four points of contact with axis, which means scrap the platform at least, leaving you with two axis, the bearings and the ball screw.

 

This is more or less my current plan already - although since I'm going to be reworking the frame in acrylic and since I'll need to buy new stepper motors that can handle high humidity environments I'm planning to keep the Ultimaker on the side and use PLA to build little bioreactors.

I like the idea of just using motors with twice as many steps for my Ultimaker (although I still want ball-screw repeatability for my cell printer).

J

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[

To make sure I understand you correctly, you are suggesting removing all the belts and making the two axes parallel to the two ball screws driven by the sheer force of the ball screw on the opposite side (meaning I will need to replace the opposite wooden blocks with something on a super frictionless bearing).

I feel that while this will better level the printhead, it will probably cause strange torque beyond a particular load I am applying at the printhead. (Still need to run simulations)

 

Yep, that's kinda what I'd be afraid of too, but I don't know any other "convenient" way of building it :(

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