ROBOTIC Arm in plexiglass

Robotic arm with four degrees of freedom, with bench-top, gripper for grasping objects and five servo controls

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Automation and robotics represent the most promising sectors in Italy in the industrial field and companies that create solutions such as automatic assembly machines and robots proliferate; surely the most appreciated product is the robotic arm, because it is capable of carrying out many operations, fixed vertically or horizontally to the supporting structure and equipped with very complex mechanisms that allow a high degree of freedom and considerable speed in executing the required movements.

The robotic arm is also something coveted by the makers, for the fascination it exerts as the first, perhaps most significant, a step towards the construction of a humanoid robot.

The robotic arm is also something coveted by the makers, for the fascination it exerts as the first, perhaps most significant, a step towards the construction of a humanoid robot.

The importance assumed in various sectors by this portion of robots makes it necessary to have, in schools and in research and development, skills, teaching and prototypical support to develop projects in the robotics sector. The arm that we propose in this article was designed precisely to take the first steps and experiment and develop robotic applications, clashing with issues such as motion control and related firmware; it is therefore aimed both at hobbyists and at technical schools, where it can become an indispensable teaching platform, and also to designers who need to prototype robotic applications, as well as develop and refine their software and firmware for managing more complex industrial automation systems..


Let us, therefore, explain better what the project consists of; it is composed of a mechanical structure in which the electric drives are integrated, as well as electronics that govern the latter to obtain the desired movements.

The arm is of articulated type (also called anthropomorphic) since all the joints are rotating, characterized by 4 degrees of freedom (basic rotation, shoulder movement, elbow, wrist rotation) which, together with the pincer on top, give a certain positioning and orientation ability for small objects.

Let’s start from the mechanical structure, which is the most important part because it is, in fact, the robotic arm; electronics is just the part that controls it. The arm has a structure made entirely of 3 and 5 mm thick laser-cut plexiglass parts and assembled by means of brass / ABS spacers, screws and nuts.

The arm has three joints for up and down movement (it is composed of an arm and a forearm to which a gripper is attached as the hand function) and can rotate on the horizontal plane thanks to a steel ring pressed to a circle of balls, in turn screwed to the base support of the whole; at this base the section containing the electronics is also fixed. The arm can rotate on the support base by 180 degrees and the fifth wheel joins the base of the arm to the support base. The arm and forearm length is 160 mm and the arm can extend in height for a maximum of 27 cm, which becomes 310 on the wrist (the joint on which the gripper rotates) and a good 400 mm considering also the gripper.

The base is large enough to support the extended arm, but if it has to lift heavy weights it must be fixed to the support surface; for this purpose, we have provided four holes for screwing it, for example, to a table, a workbench or a machine whose arm you want it to become part of. The gripper has 45 mm long concave jaws and toothed jaws, so as to facilitate the taking of objects of various kinds.

The arm is driven by two 13 kg/cm servos with metal gears: one controls the rotation of the first section and the second raises or lowers the forearm on the elbow joint. In correspondence with the latter, there is a rocker arm which, thanks to a connecting rod pivoted on the rotating base and to another pivoted on the clamp support, ensures that it remains horizontal regardless of how far it extends or retracts the arm.

The rotation of the arm is 180° and is controlled by a further 13 kg/cm servo with metal gears identical to the previous two; note that for kg/cm we mean the force exerted by the servomotor for a given length of the lever, considered at the end of the lever itself, therefore 13 kg/cm means that the servo exerts a force of 13 kg to one centimetre from the axis of its post, while at 2 cm the 13 kg halves (becoming 6.5) and so on. This is logical since the mechanical system consists of the servo shaft and the operating lever, a disadvantageous lever (power on the fulcrum and resistance at a certain distance).

The support base has a steel ring with one row of balls...

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  • 1
    Step 1


    So, having explained the arm we can move on to see how to build it: all the necessary parts are available in assembly kits, in laser-cut plexiglass, complete with accessories for assembly such as screws, bolts, etc. However, nothing prevents you from making them yourself, however, by using plastic material and maybe with 3D printing.

    In any case, once in possession of all the component parts, the mechanism is started by assembling the support base with the fifth wheel and adding to it the hub of the servo control which activates the rotation of the arm on the base. The plastic hub supplied with the servo must be fixed with 4 2.5×12 self-tapping screws complete with 3×6 flat washer (the screws must not be tightened so as to allow the hub to align correctly with the rotation servo pin that will be mounted later).

  • 2
    Step 2

    Once the support base has been put together, you can assemble the arm, starting with the rotating base: take the 5 mm plexiglass right shoulder, to which you must attach the 13 kg/cm servo with 4 M4x14 TCEI screws and 4 self-locking M4 nuts and then insert the cable into the indicated slot. Also, mount an analogous servo on the opposite shoulder (SX) (but interposing 4 4×4 mm ABS spacers between the two elements) securing it with 4 M4x18 TCEI screws and 4 M4 self-locking nuts. Also, in this case, you have to insert the servo cable into the special slot on the robot’s shoulder. Then assemble the lower part of the arm using the screws and the spacers with the hexagonal column, then fixing one side to the hub of the 13 kg/cm servo control mounted on the shoulder, obtaining the assembly seen in Fig. 2.

  • 3
    Step 3

    Now take the two “arm 2” elements in 3 mm plexiglass and connect them with M3x8 TB cross screws complete with 3×6 flat washers and 2 hexagonal F / F M3x15 spacers, obtaining the block seen in Fig. 3. You have thus made the forearm, which will then join the first portion of the arm and the return rod. Then take this 3 mm plexiglass rod and fix the lower end to the level of the corresponding servo, whose opposite side will be connected through the appropriate hub to the corresponding servo control (interposing 3×6 mm flat washers).

    Fig. 3

    At this point, apply the intermediate support of the rotating base and in it introduce the axis of rotation of the initial part of the arm, which will also support the hubs of the two opposed 13 kg/cm servos and the operating lever of the return rod which will command the forearm. The set will result as shown in Fig. 4.

    Fig. 4

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