Shop OBEX P1 Docs P2 Docs Learn Events
Need a $13 Robot Arm? — Parallax Forums

Need a $13 Robot Arm?

ercoerco Posts: 20,255
Flash sale, whatever that means. Never again? EVER? Looks like a uArm style arm.

Pretty sure the four 9G servos are not included, but the hardware (screws & such) is. BYO controller.

I still like Hobby King's HXT900 servo, although metal gear servos might be best for at least the two shoulder joints (heavy loads).

Ken, I still say Parallax ought to sell an arm kit like this. Sure would look great on top of an AB or S3!


  • ercoerco Posts: 20,255
    Or get this metal kit (no servos) for $24. 6-axis, includes ball bearings.

    This beast definitely needs metal gear/shaft servos for the high-load joints.
  • wmosscropwmosscrop Posts: 406
    edited 2018-10-21 19:54
    erco wrote: »
    Pretty sure the four 9G servos are not included

    The servos are shown in the included parts (4th picture). I'd wonder about their quality, though, at this price.

    Edit:The servos are included for $17.99 (select the size on the right).

    Edit 2: I just bought one (with the servos). I selected the "US express" shipping option, so I should get it in about 3 weeks.

  • ercoerco Posts: 20,255
    Yes, the servos are not included with the $13 option. If you get 4 9g servos for $5 more that's a pretty good per-servo price. But quality is hit-or-miss. I plan to use Hobby King's HXT900 servos, they have a great reputation and are not expensive at $3 each now. Servos in this arm will be under a big load, so I like to build in the best quality I can from the start.

  • OK, got the arm on Friday, put it together over the weekend, in between several sessions making wood crafts for my wife to sell next month.

    The good:
    * The parts are sturdy and very well-finished (I didn't have to remove any burrs, but I did spend quite a bit of time removing the protective paper.
    * Extra screws and nuts are provided.

    The bad:
    * Instructions are very poorly done. Instead of diagrams there are pictures. The arm assembled in the instructions is made of a translucent plastic and makes it very difficult to figure out which part is which in some of the illustrations. Not to mention the very skimpy text.
    * Some of the parts provided are almost mirror images of the the parts in the instructions. Lots and lots of fun figuring this out!
    * Most of the parts go together by threading a screw into a part. Unfortunately, quite a few of the parts that were supposed to have the smaller holes necessary for the treading instead had the larger (pass through) holes. I ended up using quite a few of the extra parts (with some nuts I had on hand) to complete the arm.
    * As designed, the entire arm is supported by the servo gear. This causes multiple problems (severe oscillation, etc.). I modified the arm to slide on the base, which seems to fix the issue.
    * The arm, even without lifting anything, appears to be too heavy for the two positioning servos (very jerky operation even when slowly ramping the servos). I have verified that there are no binding parts.

    It took me about 4 hours to assemble.

    My recommendation: you get what you pay for. Spend some extra money and get a good arm. For my project this will suffice.


  • eBay always has good deals on open box Dynamixel AX-12A actuators. Make an offer here for $100 and I bet he will accept (I've done it once with this seller).

    That's $16 per actuator, excellent functionality, and very easy to use. You will need the connectors, and it doesn't hurt to have the programmer. That puts the price closer to $25 per actuator, still a good deal IMO.
  • The problem with Parallax buying and selling a product like this is a total lack of consistency.

    We might get 100 units that are mostly alike, but the next 100 will be totally different. Been there, done that. The only way to guarantee any quality and consistency is to make it ourselves, but that can't be done for this cost.

    For the one-off, buying a kit like this is the way to go.

    Ken Gracey
  • What about a free virtual arm?

    1. Make parts in Blender.
    2. Develop simulator in Unity 5.
    3. Interface sim with Blocky or simulate Blockly IDE in-sim.
    4. Release free on Steam and mobile markets.

    That would be a fun project. I wouldn't want to solo it, but could do a lot of the work.
  • ercoerco Posts: 20,255
    @xanadu: I just offered $100, will see if he bites.

    @wmosscrop: Thanks for that great feedback. Still haven't received mine yet! Sounds like that bottom servo and maybe the next one or two up the power chain should be metal gear/shaft servos.
  • ercoerco Posts: 20,255
    No bites on my $100 dynamixel offer. Just as well, they were probably sour anyway.
  • W9GFOW9GFO Posts: 4,010
    edited 2018-11-15 22:00
    Have you tried the LewanSoul LX-16A servos? They are $15 on Amazon.

    I bought one but have not yet tried it out.
    309 x 400 - 11K
  • ercoerco Posts: 20,255
    WOW, those look nice, Rich! Ordering some now. Funny, just last week I got four of these servos by the same manufacturer. I looked for a while on Amazon but I never saw these serial control LX-16As. Thanks for the heads up, they look great.

  • Well, I dropped my arm (about 2 feet, onto carpeting) and it fractured several of the parts. Not just cracked, but fractured (lots of little bits of black plastic).

    Being the cheap guy I am, I've stripped off the usable parts for future projects.

    In the meantime, if you've bought one of these kits and break a part, let me know. If I didn't break the same part I'll be happy to send you a replacement.
  • @erco Maybe people started buying them at the asking price. I must have made the offer when they were first selling them. That ad has been up for a very long time.

    @W9GFO Interesting find. It's strange that something with internal temp monitoring would have the "Please be sure to avoid locked-rotor when using servo, the locked-rotor will lead to internal current increased to more than 7 times and the temperature increases, servo will burn out." disclaimer. Hopefully that is just a precaution.
  • ercoerco Posts: 20,255
    edited 2018-11-22 18:27
    Too bad the LX-16A serial servo doesn't include brackets. Hopefully they are available. The LDX-218 does include brackets, but they don't fit the serial servos. Different form factor.
  • W9GFOW9GFO Posts: 4,010

    I just bought five more of these servos and half a dozen optical encoders for a project idea. I thought surely someone would have made an object to control these things in the last five years. I see that JonnyMac was working on it for a while but from my searching I cannot find evidence that it was completed.

    I'm wondering, has anyone managed to make these work directly with the Propeller?

  • W9GFOW9GFO Posts: 4,010

    I did see that, thanks.

Sign In or Register to comment.