Any guesses as to discrepancy in arm length vs. measured arm length?

  • So it looks like using around 307mm for my arm length gets rid of the "ridges and valleys" first layer issue on circles over 200mm in diameter on my print bed.

    Why would there be such a large difference between actual arm length (300.15) and arm length in the config file?

    Could it be some sort of bug or glitch in the delta kinematics?

    The X/Y scaling is definitely thrown off and comes out 2.5% to 3% smaller, so the only solution to that is scaling it up in the slicer as the XY scaling config command seems to leave gaps in the print infill.

  • administrators

    Several SeeMeCNC Rostock users have reported this effect, so I am wondering whether it is something to do with that design. Do the effector and the carriages use the same part to define the bearing spacing? If not then it could be caused by the rod spacings not being quite the same at both ends.

    I can't think of any reason why using XY print scaling would affect the infill.

  • With 300.15 arm length, I'm guessing you have the trick laser arms. Use softer springs than supplied with them - stiff springs cause tilt, and throw your calibration off. It will measure the arms being longer than they are until you resolve that. (They'll still measure as longer after, but not as badly)

    I recommend using the stock arm length as sold, and using only an S6 calibration on a Rostock, especially with the trick laser arms.

  • dc42 - the current carriages and effectors both use Ball Cup "barbells" for spacing, so it's even on all of them.

    I still haven't quite narrowed down what's going on with this oddity, but I'm getting closer. Waiting on some new parts to be machined to test out something else.

  • Again, lucky that this has been brought up already as it saves me the trouble of starting a thread out on my own! This is because I've recently observed some really bizarre behavior relating directly to this that I'd been meaning to discuss due some time.

    I, too, am using the 300.15mm diagonal arms with the red cup endings, each pair bound by two springs, one at each end of the arms pairs. Here's what I have found that's tripping me out:

    I have long been questing for an all green bed height map as I've seen some users here post one they have their flavor of Z-probe setup,though I've had lots of difficulty obtaining that result. Additionally, I happen to have really admired tricklaser's game design (the max metal delta frame) and so purchased a pair of them and outfitted them nearly identically and currently utilize them in such a way that I try to keep one working well, while I use the other as a test platform for new ideas and technologies – that way I've always got one working printer in the event that a new idea details me on the other one.

    Here's where it gets strange -- my reliable metalmax (I call it Metalmax_DuetWiFi_I) is the first one I noticed the or phenomena which is that if I leave my M665 with statically entered Length value of 300.15 and solely use S6 (six factor calibration after probing), oddly I can only achieve deviations of no better than 0.125 or so on a great day, which is not great as I know my bed is seriously flat considering the makeup of the base covered by 4mm Boro glass. The biggest advance I've had recently was when, in my frustration, I reduced the probing radius by a good percentage (which has negative effects of printing larger than that value), but in a nice change my numbers improved to deviation after probing of approx. 0.085. Still not great but better...

    I made an interesting discovery while tinkering about after setting my probing calculations to utilize 9-factor calibration routine, after which I get the kids off values I had expected when setting up probing to begin with -- in the vicinity of.03, which initially looks great, but upon closer inspection, I find that the values auto-determined are so far out that it greatly effects the scaling of prints in a huge way -- since, when I'm getting 0.03 deviation, my M665 "L" value gets calculated off of actual length by a considerable amount -- example values of like 387.85mm -- clearly inaccurate, the result of such are parts that are much smaller than they ought to be.

    Now there's another bizarre thing taking place which is: considering both of my MetalMax frames being populated by nearly identical components, one would logically be led to find that the calibration values determined by probing ought to be pretty similar, though the most bizarre thing is happening even prior to auto calibration -- even something as fundamental as the effector height at home position is vastly different from the MetalMax v.I and the second (MetalMax v.II)! Oddly, ask other things being the same (trick truck mini carraiges, along with the CF tricklaser 300.15mm arms and nearly identical effectors), the M665 H value on version I it's approximately 375mm while it's cousin (version II) needs a value of H nearly 75-85mm higher to grip a sheet of paper when traveling downward in the Z direction from homing position. Now most intriguing thing I'm finding on this version II with the increased apparent Z height, is that if I go ahead and run a 9-factor calibration on it, I obtain the ever so sought after value after Calibration deviation of 0.001, which at first was mind boggling! Until I checked the auto determined values entered into the M665 values such are so far out of reality that accurate prints are obviously impossible -- recently when I got that deviation of 0.001, the calculated diagonal rods length are entered in as mid 400mm lengths! Damn it! I just can't seem to get this to reflect reality while also obtaining a green height map. It's very frustrating, the two issues, one which is that two identical component- built frames should have such vastly differing values of foundational values such as height, but then also that the calibration results are terrible if only allowing for 6-factor calibrations, and the accompanying height map in those situations looking like god-damned mogul runs at a ski resort (for both printers side by side), but then if allowed to utilize 9-factor calibrations I can finally realize deviation values is been hoping for and expecting this entire time (0.003-0.001,at long last) but only at the expense of the numbers being even closely based upon reality. This entire thing is very frustrating and I'd love to know why this is.

    To that end, the suggestion posted ago email regarding the springs being far to stiff on the tricklaser diagonal arm configurations is the first time I've heard any advice that makes sense and I will certainly try that at my next convenience, though it is difficult to imagine that the spring tension could account for either the differential of lengths when comparing two nearly identical machines side by side having such strangely differing values, of the incredible altering calibration results between 6- and 9-factor calibration getting better to the tone of improvements from a shitty 0.254 deviation to the long sought after results of 0.003 - 0.001. This is giving me a headache and I'd be fidget indebted to anyone who can help me to resolve this wild discrepancy I'm realizing through my multiple hundreds of tries over a wide range of probing hardware, which I've finally settled on DJDemonD 's wonderful precision piezo hardware setup. Note to find out the reason for the strange results and ultimately how to resolve these inconsistencies!!

    Thanks for listening.


  • This might go some way towards explaining why you've had trouble with the sensors also.

    It seems this particular printer's geometry might need to be investigated. What do SeeMeCNC and Trick Laser have to say about it?

  • I calibrate at a 0.030 consistently at the moment. The best I could get on stock arms (with a 713maker effector) was 0.040. The best I ever got from Trick laser arms with the stock springs was 0.950 (usually in the 0.120 range, though). When I swapped in weaker springs I could bring it down to a 0.080, but no lower.

    My current configuration is a magball setup, with Haydn magball arms, and a custom effector. Trick laser carriages, but custom arm adapters (I've posted about them before, 713maker made them for me, and will sell them).

    I've calibrated down to a 0.010 with this setup, but not consistently. A lot of that can be altered with belt tension, and I suspect the carriages shift slightly in the grooves based on how high up I tension the belts. I have some ideas I want to test, but I'm waiting on some new parts before I can do so.

    In addition to the deviation, though, check for the "Taco Bowl" shape I've posted about before. Run a calibration at S6 a few times, then do an S-1. It will report the offset at each probed point - you'll find it's high at the towers, low between them.

    I've improved on that with my current config, but the oddity persists. I have not yet determined exactly why, but hopefully the new parts I'm going to test will tell me.

  • Its possible to get down to calibration deviation of 0.004 as I am doing so, this is with linear rails, 2040 kossel frame and ally corners, aluminium carriages, aluminium effector, Haydn's mag ball arms/carbon fibre/delrin cups, tooling plate bed. This is if I probe the inner 200mm, if I probe to 300mm it goes up to 0.008.

  • This oddity is specific to a rostock max. I've gone back and forth with dc42 and several others about what's going on, and I still have some suspicions.

    I do notice the calibration improves if you only probe the inner 200mm of the bed as well, but still haven't gotten it down to those levels with the actual arm lengths.

    If I let it do an S7 calibration, I can get to 0.01 or less, but I know those results are false, as the scaling of objects is incorrect.

  • I will switch out my older 325mm Rostock V2 arms to my new printer and see if that changes anything.

    I know the arm lengths have varied on the Rostock Max design from 269mm, 290mm, 300mm, and 325mm across different modifications of it. I'll check my old parts collection and see what sort of arm configurations I can swap out for testing.

    What length are the Haydn's mag ball arms? Are they exhibiting similar issues at all to the ball cup design?

    I did actually loosen a set of springs with pliers at one point and that didn't seem to change much, but I did not do sufficient testing, only a quick visual check with a print. I also ran a pair of arms though with small rubber bands from hair elastics, and those did not seem to improve anything that I could notice on the edge calibration, but I could take a closer look.

    I wonder if the software is suggesting that the arm length needs to be longer? Especially since the errors only come into play when the arms are reaching the farthest points from the middle of the bed.

    My latest probe deviation is about 0.02 or less, mostly with taking probe points at 140mm on the three towers, and then doing 6 to 12 points at a 75mm radius around the middle of the bed to rule out any sort of bed tilt.

    I'll continue to post any results I come up with and try to spreadsheet calibration information and record specific information for future reference.

  • saffi- Haydns arms are IMO the best delta arms currently available he will make them to length you specify, if I was going to be very picky, but then thats delta owners after all, (mine are 360mm) they are a little too bendy in 6mm carbon rod, 8mm or 10mm carbon tubes would be better.

    kraegar - if with your best calibration you then use M556 axis compensation for x and y, can you print correctly scaled objects at any peripheral coordinate as well as centrally? If you can I'd say the calibration is correct. I don't do this, I've tweaked rod lengths manually to give accurately scaled objects centrally printed using 8 factor calibration, and I can print an even-height spiral across the whole 300mm but I haven't tried the peripheral object test as above. Maybe I will. I'm instinctively against using M556 but then everything else on a delta requires computational correction to make them work, its just one more set of calibration parameters to shove in config.g.

  • I didn't see any arms from SeeMeCNC themselves above the current 292mm arms. I thought they used 269 for the old style, and 292mm for the current ball cup ones. Anything else (300, 325) was aftermaket (tricklaser).

    My Haydn's arms are 304mm, and do show the effect, but it's lesser. They're magball, not ballcup, and don't require any springs, etc between them.

    There was some question if the steps/mm was not truly 80, but several of us tested that and found it was spot on, or at least close enough to know that it's not causing the issue. If you only probe near the center, and not the edges, the issue also vanishes, or if you tell it your arms are longer than they are.

    I started a discussion here: but it fizzled out.

  • I'd like to test it with some longer and shorter magball arms, as well, but can't afford to buy them just for testing this.

  • I think the 80 steps versus 80.5 depends on belt tension if tight its not an issue.

    I wonder if a suitably rigid mechanism could be devised like a turnbuckle to lengthen/shorten rods as needed?

  • I tested belt tension from what I'd consider fairly loose, to as tight as I'd normally go, and saw far less that a .5mm discrepency in steps/mm. less than 80.1 consistently. Perhaps I'm not going as loose or tight as others might in their tests, but it seemed a good range, and didn't account for the problem. (also, I could measure it, see I was under 80.1, but only fix it by going up to 80.75 or so, which threw my height of objects way off)

  • I also did some more testing yesterday and used a 20x60 extrusion and the fsr sensors to be able to measure two different heights as proposed by dc42.

    I also have the issue of crazy wrong rod length proposed by s7 calibration and for me, the measured height difference between measuring the 20mm vs. the 60mm side of the extrusion is 39.98, so I guess I can say that my steps/mm are close to perfect.

    What I can do furthermore is replace the duet with a smoothieboard to see if this changes anything and I will also print a larger effector to see if effector size plays a role in this game. Glad that tightening the belts is already out of the equation, one step I do not need to do now. ..

  • I've changed to a much larger effector, didn't make much difference.

    I highly suspect that if I went to longer arms (around 325mm) with the magballs, the issue would vanish, or nearly so, even when probing to the edge of the bed.

    First I want to rule out the carriage system entirely, which I'll be doing in the not too distant future I hope.

  • Hmm… I have quite long arms, 360mm for a 300mm bed, sorry to tell you I see the issue 😉

  • That's helpful! This is a rostock max? What effector and carriages? What arms?

  • It's a homebrew printer, but I use the same rostock max effector (I liked the design) and 360mm tricklaser rods with ball cups. The carriages run on MGN-12 rails.

    The distance between the middles of the white bar-bells on the carriages is approx. 330-335mm

  • I'd be more inclined to rule it out if the arms didn't have springs between them for tension. That's why I want to test with longer magball arms.

    I'd be curious to see the output of a S-1 calibration on your printer (after you've done a few rounds of S6).

  • Btw, the reason I'm against the springs between arms (at least as far as diagnosing / testing this issue goes) is simple - they change your calibration. If you put the "heavy" springs on, and calibrated your printer, look at the outputs of M665 / M666 and an S-1 probe… then swap to rubber bands or lighter springs, repeat an S6 (or several, so it "settles), and compare the new outputs of M665 / M666 and an S-1, you'll find things change quite a bit. That means there's no way the springs themselves aren't having a measurable effect on your calibration values.

  • I had another look at data I collected yesterday and collected some more data today.

    Yes, I agree, there is an influence of the springs, but I guess it is not very relevant.

    What I did yesterday was checking probing speed / motor power vs. deviation of testresults.

    So I did a log of G32 runs with S-1 to collect the data. I varied probing speed and current for the motors, I saw a big influence of the motor current with the default probing speed of F120 when choosing 1.5 or 2A current, but with the default of 1A there was not much to see at different probing speeds.

    What is relevant in this context is that I measured the deviation of the results of each probing point, if springs have a huge impact that deviation in the points should be higher than the measured 5-7.5 Micrometers, shouldn't they? unless of course that because of the quite repetitive movement pattern the effect of the springs is always the same, but I kind of doubt that.

    Today I also did some more measurements with moving the head arround and then probing x0 y0, here the numbers are also quite close:

    	Probe with moves on Bed	
                    Probe with moves on 60mm	
                             Probe without Moves 60mm	
                                     Probe without moves 20mm
    	0.07	59.914	59.939	19.97
    	0.077	59.914	59.927	19.977
    	0.07	59.914	59.933	19.977
    	0.07	59.895	59.92	19.97
    	0.064	59.914	59.914	19.97
    Avg	0.07	59.91	59.93	19.97
    Dev	0.00	0.01	0.01	0.00

  • The effect of heavier sprints is to pull up on one corner of the effector at the point when it's between towers. This is easily observed - if you take a set of barbells off your printer and put your arms on them (with springs in place) you'll find the springs pull things into a square naturally. If you push it into a parallelogram, the springs will pull it back square. The weaker the springs, the less the effect. When your effector is beween towers the arms are in this parallelogram position, and the springs are fighting it. Something has to give. It flexes the outer arm - the longer your arms are, the more they flex, and it pulled up on that corner of your effector.

    The effect actually is very repeatable and measurable.

    If you build one of the effector tilt measurement probes I made, you can observe this effect in real time, and how much it tilts the effector on your delta. Info on that here:

    That said, the springs are NOT the cause of this issue, they just muddy it up and amplify it some. I'm running magball arms which don't need linkages or springs, and still see the "taco bowl" shape to my probe output.

    Here's a brief list of things I've tried to diagnose this:
    Seemecnc stock effector, 713Maker aluminum effector, custom aluminum magball effector
    Seemecnc 292mm stock ball cup arms, Trick Laser 300mm CF ball cup arms, Haydn Magball arms (304mm)
    Seemecnc Accelerometer probe, dc42 IR probe, Piezo Probe
    Seemecnc injection molded carriages, Trick laser aluminum carriages (current version)

    And then I've also made the electronic tilt measurement, so I could measure and eliminate tilt in my effector to rule that out. (I have no measurable tilt at this point, either digitally with the accel/gryo, or with a bubble level).

    I have loads of spreadsheets of data gathered, but have yet to find anything other than increasing the arm length that gets rid of the "taco bowl" shape.

    Big clarification on that last statement: Several of these have reduced the issue. Anything that adds tilt to your effector seems to really amplify the incorrect measurement of the arm length. The biggest improvement so far was going to magball instead of arms with springs between them)

  • administrators

    All, please bear in mind that I do not advise the use of 7 or 9 factor calibration. If you can't resolve the problem by using weaker springs or whatever, then I suggest you use mesh bed compensation to handle the residual deviation.

    FWIW, my large Kossel with PCB effector and carriage adapters and Haydn's magnetic arms calibrates to a deviation of about 0.025mm using 8-factor calibration and AFAIR 13 probe points.

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