Musings and Experiments on the Art and Science of 3D Printing

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Delta Arm Joint Musings

By Michael Hackney Thursday, April 6, 2017
I see (and participate) in a lot of discussions about delta arm joints (or rod ends) - the connections between the ends of the arms and the effector at one end and the carriages on the other. An example appears in the photo below. These joints have the unenviable task of allowing unconstrained motion in three dimensions while not allowing play or backlash in the mechanism. Twelve joints are required for a delta printer. Four joints for each of three sets of parallel arms. The photo shows the six joints (three sets of parallel arms) at the effector end. The other end of each pair of arms is attached to the carriage that controls positioning of the effector.


First I'll present a bit of history, then I'll give my assessment of the strengths and weaknesses of each.

A little joint history

In the early days of delta printers, most builders used Traxxas rod ends – they are inexpensive and they work. When SeeMeCNC released their first Rostock MAX delta printer in 2011 they used their own "Universal Joint" made of aluminum and injection molded parts. The in 2013 a poster, Werner Berry, on SeeMeCNC's forum showed his all-custom delta printer with magnetics and metal balls for the end joints. To the best of my knowledge, this was the first public post showing magnetic ball joint ends. This was not a new idea, magnetic joints like this were used in other fields but Werner certainly pioneered their use on delta printers. The results were obvious and spectacular (more later). More recently (late 2015 I recall), SeeMeCNC released an all-new "Ball Cup" joint on several models of delta printers.

This makes up the four primary families of joint types used for delta printers, to recap:

  1. ball rod ends like the Traxxas rod ends
  2. universal joint rod ends
  3. magnetic ball rod ends
  4. ball cup rod ends
Now let's look at these in more detail.

Joint review

1) Traxxas ends (the name brand) are functional and inexpensive but they are not manufactured to high tolerances. New ends often have excessive play (or lash or backlash). They also wear relatively quickly, again introducing play. By "play" I mean the ball is not held firmly in its socket and can wiggle around easily. This wiggle – or backlash – results in imprecise movement during a print and leaves tell-tale print artifacts like visible layer shifts and overshot layers at sharp corners. This is their primary weakness and they have no built-in ability to eliminate this backlash. There are higher precision ball rod ends but, of course, they cost significantly more. Precision costs, how much do you want to spend?

2) SeeMeCNC's universal joint ends required a fair amount of tweaking and tuning to eliminate excess friction while not introducing backlash. Over a few years of iterations the molding technology and design improved and they were serviceable. I put them on par – arguably a little more reliable and longer lasting – with Traxxas ends.

3) When the delta world was introduced to Werner's magnetic ball ends it was like the veil of backlash had been lifted from our eyes. Werner's early print photos were spectacular. And there was good engineering reason for this – magnetic balls have built-in backlash prevention. The design uses a magnet mounted in a hemispherical cup – a type of ball-in-cup joint. The magnet attracts the steel in a precision bearing holding it in the cup. The ball is free to move – not much unlike your hip joint (which is a type of ball-in -cup joint) – but the magnet holds it tightly in the cup so there is no excess play. It's brilliantly simple.

4) More recently SeeMeCNC introduced their ball cup ends. These are another type of ball-in-cup joint and they also eliminate backlash. Interestingly, the ball cups are mounted perpendicular to the axis of the rod arm whereas magnetic ball ends are in line with the arm. This allows for a spring to pull the cups into the ball as shown in the photo above. Like magnetic ball end, ball cup ends give excellent print results.

The nitty-gritty

So now the Big Question "which type is best?" The answer, of course, is "it depends." I won't be so cruel as to end here, I'm going to give you my opinion and testing results on this. I've owned and tested delta printers with all of these end joint types so I assert I'm in a good position to make comparisons.

First off, the original SeeMeCNC universal rod ends (#2 in the list) have been replaced with much better alternatives and for good reason. Now we're down to three. The ball rod ends (#1) are actually quite good if you purchase precision parts. The Traxxas brand ends are fine for small Mini Kossel types of delta printers where high print resolution and high print speeds are not a requirement. They are easy to make or inexpensive to purchase and are a great beginner option. The higher end rod ends are quite nice but at the end of the day, they do not have built-in backlash prevention and they do wear. But, the one thing ball rod ends have going for them is they are secure – they hold the rods to the carriages and effector so they can't dislodge. This means that if the print nozzle hits a bump in the print or jerks quickly, the ball rod ends will hold everything together. We'll see why this is important next...

Magnetic ball rod ends (#3) result in much better print quality due to their ability to eliminate backlash. They are a little more difficult to fabricate the arms, effector and carriages but these parts are available and there are many STLs available to print your own. The one criticism about magnetic ball rod ends is that they can separate. Sudden jerks, hitting a plastic bump on a printed part, printing with a heavy(ier) direct mounted extruder, or simply working on the hot end like polishing the nozzle tip or removing plastic drool can result in inadvertant spontaneous separation. This is their one weakness. Some have tied the carriage to the effector with elastic strips to pull them together in an effort to prevent separation. I haven't tried this myself.

This leaves us at solution #4, ball cup ends. I really like them, especially the red after-market cups that Trick Laser manufactures. To me, they have the best characteristics of magnetic ball end (backlash elimination) and ball rod ends (secure fastening) with none of the negatives of either. They are well suited to large deltas with effector mounted direct extruders like the E3D-Online Titan Aero or Bondtech BMG extruders (more on these in the next few posts). The one "weakness" they have is they are single sourced from SeeMeCNC – at least the molded balls. The cup ends with carbon fiber tube arms are available from Trick Laser made to length and the injection molded "barbells" can be purchased cheaply ($6 for an entire set). I predict this will change in the not too distant future as more people become aware of them and their benefits.

On my delta printers I use either magnetic ball rod ends or the Trick Laser ball cup ends. I do have. preference for the Trick Laser ball cup ends and carbon rods, they are well made, secure and have no backlash.







11 comments to ''Delta Arm Joint Musings"

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  1. I have tried everything but the magball type. I like the tricklaser CF and red ends, but I will be installing the effector and truck adapters Tony K. designed and 713maker is machining them for me this weekend. I bought the 304mm magball arms from ultibots.com. I also will be installing your FSR plate from tricklaser. I am really looking forward to the results, once I get it tune in I would like to print one of your reels for my son's fish fishing trip! Your Blogs are the best information I have found and I very much appreciate your sharing with everyone.

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  2. Make sure to post photos of those Joshua. Matt (713Maker) does great work (as does Brian at TrickLaser). Excellent aftermarket upgrades. Thanks for the feedback too.

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  3. I had my Delta printers at MRRF last month using magnetic ball ends. Actually, the rods have steel balls, and the magnets are countersunk disk magnets glued to receptacles on the carriages and the end effector. As you mentioned in your text above, without a separate means of holding the magnetic assembly in place, the rods can come loose if the extruder hits a bump, or if the arm is shocked. I added retainers consisting of extension springs and spectra fishing line to the centerline of each arm assembly. The springs absorb shocks, and keep the magnets in place.
    Here is a video describing my printer:
    https://www.youtube.com/watch?v=jT24ClLlQKo
    I discuss the retaining springs at 3:50 into the video.
    The design is open source:
    http://www.thingiverse.com/thing:2139033

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  4. The other place where magballs can be problematic is if you move to one of the new direct drive extruders like the E3D Titan Aero or the Bondtech BGM. The UltiBots micro extruder (also direct drive) works well though on their D300VS delta that has magball arms. I have not weight all three of these options but I am planning to do that soon.

    That's a nice solution to hold magball arms together, thanks for the post and link.

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  5. Hello Michael, thanks for sharing your insight and experimentation, I always learn something new from this blog.
    Looking forward to your review of the Titan Aero. I'm considering installing it on my Rostock Max as a direct drive extruder, but the weight of it scares me a little. Also, I've been told that jerk and acceleration would need to be adjusted... and there, I'm lost :P

    Excuse my english.
    Cheers, Martin.

    PS: Is there a way to subscribe to this blog?

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  6. Hi Martin, I am in the process of updating my theme and the subscribe was missing. I've added it now to the upper right panel!

    Like all things, its a tradeoff between the extra mass of a direct drive extruder vs the hysteresis of a Bowden system. I've done enough testing and printing now with the Aero, Bondtech BMG and the UltiBots Micro Extruder (all direct) to appreciate that on large format deltas and form my requirements, direct drive gives much better print quality - significantly reduced stringing and blobbing. I don't print at 120mm/s but I do routinely print at 80mm/s and all of these are fine. You do have to have a well set-up machine (arms, joints, etc) but that's pretty easy with mag balls or SeeMeCNC's ball cup joints. With Duet setting the jerk and acceleration is really simple. I don't recall if Repetier requires these to be compiled in or if they can be set in EEPROM.


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  7. Done, subscribed.

    My standard print speed is set at 60mm/s with ext perimeters and details at 35mm/s (when feeling bold I bumpit up to 80mm/s). So great, I'm glad that considering the extra mass, those values will work fine.
    As for arms and joints, I have the ball cups but still stuck with the old Rambo board. Apart from the vertical banding issue, it prints fine.

    Yes, Repetier lets you change them in EEPROM. Stock values:
    Max Jerk 32mm/s
    Acceleration 1850mm/s^2
    Travel acceleration 3000mm/s^2

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  8. Ive been meaning to make use of these for a while now and Im just now finally getting around to it; they're Traxxas #4933X. They are hard-anodized, Teflon-coated, 7075 aluminum ball studs with nylon or delrin sockets included. Ball diameter is about 10.49mm and stud is m5. They seem perfect for this use and Im surprised that Ive never seen them used before (at least not the 4933X version; the 4933 version is discussed in a thread on the SeeMeCNC forums, which is how I came across these). Here are some pictures I took:

    http://i747.photobucket.com/albums/xx119/tylerjmast/RoMax%20v2/traxxas4933x-34.jpg

    http://i747.photobucket.com/albums/xx119/tylerjmast/RoMax%20v2/traxxas4933x-74.jpg

    http://i747.photobucket.com/albums/xx119/tylerjmast/RoMax%20v2/traxxas4933x-70.jpg

    http://i747.photobucket.com/albums/xx119/tylerjmast/RoMax%20v2/traxxas4933x-25.jpg

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    Replies
    1. Tyler, these are indeed interesting and worth taking a closer look at. I see your sketch on mounting these with a holder at http://i747.photobucket.com/albums/xx119/tylerjmast/RoMax%20v2/traxxas4933x-15.jpg. The commercial ball cup joints (including SeeMeCNC and TrickLaser) put the cup in-line with the rod. If they are offset like your drawing, there is risk that the arm/cup can rotate around the ball as it is not a stable configuration. Have you tested this or considered this potential issue?

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    2. No I haven't tested it out yet; my printer is currently down ( finishing up an upgrade to Duet WIFI). I think youre definitely right about the offset causing rotation, and I designed (with my very limited CAD skills) a cup holder that puts them at the end of the rods with the center axis parallel to and in line with the center axis of the rods (like most mag ball setups; of course this would require tension between the carriage and effector rather than between the rods, like the Cherry Pi III). I posted the STL file here: https://www.thingiverse.com/thing:2370865

      Assuming mounting isn't an issue, do you think one way is better than the other as far as parallel vs perpendicular goes? It seems the SeeMeCNC style would be much easier to (re)mount after removing the effector... and also, with the Cherry Pi method, attaching the tensioner/spring exactly in the center of the carriage and effector is critical to maintaining equal tension throughout the range of motion. But thats all I can think of and Im not sure if one is preferable in terms of kinematics/articulation.

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  9. That's a good question about morning direction. The axial mounting for mag balls works best due to simplicity - its easy to put the magnet in line embedded in the arm (CF tube). With these you could do it either way. Both designs would minimize slop - the main advantage to this style of joint. But installing them axially would require more effort for the tensioning spring or springs as they would have to be coaxial also. With the SeeMeCNC cup arrangement, the spring placement is very simple. I'd go with the proven simpler approach.

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