How-to Forge a Skull Crushing Robotic Arm to Do Your Dirty Work.


Introduction: How-to Forge a Skull Crushing Robotic Arm to Do Your Dirty Work.

I recently was given 10 servos as a birthday present, so the question really was what to do with all of them? I started looking around at robotic websites that offered robotic arm kits. One of the sites sells robotic arms that are very cool, but they are also very expensive at prices upwards of $300. Then I came across a miracle on Lynxmotion's website, an older robotic arm known as the L6AC-KT. The whole construction was of lexan, so I thought to myself, hey, I have lexan as a resource. Now that the build is completed I hope to spread my ideas and hopefully inspire someone to build something equvilent.

Also, if you could please vote for this instructalbe in the Microcontroller contest , the Toy contest , and the Make it Move contest it would be highly appreciated ;D

Step 1: Design and Fab

For this project I used the lynxmotion arm as a large point of reference, but because the site does not actually give any CAD files or any dimensions of the older arm I was forced to make those decisions for myself. I used SolidWorks for the design of the whole arm assembly, this made everything go by so smooth since I could save the files as .STL or .Dxf and have a laser-cutter and FDM machine make the parts for me. Besides that the Items I used are:

1x 24x24 sheet of neon green lexan.
5x Hitec HS-422 servos
2x Hitec HS-645MG servos
1x box of 4-20 x 3/8 self tapping screws
1x box of 2-28 x 1/4 self tapping screws
some 8/32 x 1/4 socket cap screws

Check the next step to see what all the parts are used for...

Step 2: What Goes Where?

The picture here shows where all the materials are used on the arm.

Step 3: More on Fabrication

I never touched any tool on this project except a simple hand-drill, and yet the quality of this project is very high. My secret, I used a combination of laser-cutting, and 3D part printing. To print my 3D plastic parts I did work in SolidWorks creating the dimensions of the part which I saved in a .STL file and sent to an FDM machine which would drip plastic in a pattern in layers to construct a full plastic part. I mainly used this method to create supports between pieces of lexan that were not held by much other than servos. As far as using the laser cutter I used a similar process, but instead of saving the file in SolidWorks as a .STL, I saved it as a .Dxf which the laser-cutter could use as a path for cutting on flat sheets of material (i.e. lexan). One thing to highly note is that the base servos that lift the arm up, (the ones on the desk in the picture) will fight each other for position. In order to  stop those servos from over heating and causing failure you have two options. The first option is to slot the holes in the lexan where they are mounted to the servo horn, this will allow for adjustment of the structure against the servo horn so that both servos will both be in the same position at the same time. Another method is to program both servos to go to 90 degrees, but take the horn off one of them. When both servos reach the correct position, screw the horn back on and it should be all set. Also note that I added a spring in the final design to assist the forearm in lifting.

Step 4: All Your Base Are Belong to Us!

The base for this project was an old project box I found in my basement, nothing special, but it makes a great space for placing all the electronics I plan on using for this project. The base also has a Hitec HS-645MG to act as the whole arms rotation, not much more here, except an added switch and led to turn on the servos with external power.

Step 5: THE CLAW! and More...

I did not really feel like making the claw for this project, but i still may in the future. Right now I'm using a claw from lynxmotion, which in turn is okay, but the quality of the claw can really be improved on. I recommend designing your own claw if you wish to grasp larger objects, because the claw I have only opens to a width of 1.5inches, not that big at all. Also note that I used large HS-422 servos on the claw for rotation and the open/close function, please, if you have the money, purchase small servos that don't weigh as much as these, it will improve the movement of your whole robot.

Step 6: Electronics


UPDATE (6/20/11): The arm has been mounted to the base, only things left to do are the electronics and programming. Also, the controller I'm deciding to use is an Arduino Duemilanove. It has just enough PWM outputs to control all of the arms movements. Hopefully in the next update I can upload a sketch of my code for others. I will add a video when the project is 100% completed.

Now that the arm is completed I have decided to make it into a modular arm for a larger robot, but that is for another day.

Step 7: Finished Product.



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    23 Discussions

    Can you tell me how you joined the two parts together at the other end of the servomotor

    i need to build a robot under 11 volts and i was wondering what the voltage on this robot is

    that is possible, but look out that it isn´t to heavy.
    but if you use springs so the arms weight is near nothing.
    you would be able to make it out of alluminium with no problems

    Hi, i'm about to start this project but might there be any chance of sharing the drawings/codes? I have solidworks so no issues there,
    if so please send to

    1 reply

    hello can help me , i am searching about this design to save my time i have my final project so plz if can send to me as DXF file , my email is

    Very nice model.........have you used encoders?? Yes or or no, in both cases please make a tutorial on how to choose, use and connect encoders.

    1 reply

    Pretty cool! This neon green material looks great.
    I want to make a robotic arm to play chess, but I don't know what to choose, stepper motor or servo. Stepper motors are more precise, but more expensive, heavier and i would need more mechanisms.
    So my question is: do you think your robotic arm has precision enough to position pieces in a chess board?
    Thanks and sorry for bad english.

    2 replies

    The issue I had with stepper motors was with the control, you see I used an 8-bit arduino chip to control my arm, in order to control stepper motors I would need to make or buy an array of transistors, neither option was cheap. I would say servos could easily place chess pieces on a game board, plus you can easily program them to the degree. If you do take the servo route you may need to have stronger servos like the Hitec HS-645 series to have it move smoothly and precisely.

    Nice design. A video would be nice. I wonder: is it smart to have to servo's at the base in parallel? Are you sure they will move in the exact same way. Otherwise it will be a current drain, I gues, since the 2 servo's will try to rotate all the time.
    But again: nice job


    5 replies

    Good question(s). You are right, they the two servos at the base are going to try to fight eachother, but it can easily be fixed. First you can slot the holes where the lexan is attached to the servo, and second you can always take a servo horn off and reposition it. Both those methods should prevent the servos from fighting eachother for home position.

    OK. Slot the hole will work but then only one servo will do all the work or you get a sluggy respons.
    One servo will work, but you only have the strength of one.
    Reposition the horn: I am curious if it works and keeps working over time.
    Maybe it isn't that critical. Keep us posted wtih your experience.

    Sorry, what I mean is you can either slot the hole in the lexan the same diameter as the original hole inorder to secure the lexan to the horn. You should not see less performance from slotting the holes because they will still keep the screws thight to the lexan. The second option was moving the horn. Lynxmotion has their lexan slotted, but I think I may set both servos to 90° and adjust the horns to match. Hope that helps. I'll Include more info in the instructable when I have some time.

    OK. Seems like you know what your doing. Looking forward to see more results. I have made several project but no robots yet. Would like to but other family thing keep getting more priority. But have a look at the wooden aperture. I made a instructables for that, inspired (read:copied :-) by the cardboard aperture. Keep up the good work.

    I think you can use Futaba SR10 for the precision you want.
    See the link of the product