Practical Guide to Assembling a 4 DOF Robotic Arm (by Capitan Farloc)

From screws to motion: the allure of building with your own hands

Assembling a robotic arm may seem like a task for experts, but with a bit of patience and the right approach, it becomes surprisingly accessible—a perfect project for anyone who wants to get into robotics, improve manual skills, and discover how a system capable of moving and interacting with the world can emerge from a handful of components.

The 4‑DOF robotic arm found on AliExpress and other major online stores is one of the most affordable and widespread kits for those starting to explore robotics and mechanics, although, unfortunately, the assembly instructions provided are often sketchy or unclear.

In this tutorial, I offer a detailed, step‑by‑step guide to correctly assemble your 4‑Degree‑of‑Freedom robotic arm. From preparing the components to the final tightening of the screws, each stage is carefully explained and accompanied by illustrative images to help you avoid common mistakes and achieve a stable, functional result
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The 4‑DOF Robotic Arm

The robotic arm we’ll be working on today is a small, simple mechanism that allows a mechanical gripper to move and pick up lightweight objects within its surrounding space.

It’s important to note that this device is intended purely for educational purposes—to practise with this type of complex actuator and to understand how it can be controlled using a programmable microcontroller board such as an Arduino or a Raspberry Pi. It is certainly not a precision mechanism.

To give you a clearer idea, this photo shows the robotic arm after full assembly:

What does 4 DOF mean?

D.O.F. stands for Degree of Freedom, and is sometimes referred to as 4 Axes. In practice, it indicates how many types of movement the robotic arm is capable of performing.

On each axis where a movement occurs, a small motor must be mounted to enable that motion. In our case, we use RC servos—compact motors with significant gear reduction, commonly employed to control the movements of radio‑controlled aircraft, boats, and cars.

In the diagram shown later, the four Degrees of Freedom of our robotic arm are:

• DOF 1: Rotation of the entire robotic arm on its base
• DOF 2: Rotation of the first arm segment, allowing a semi‑circular forward/backward movement
• DOF 3: Rotation of the second arm segment, enabling a semi‑circular up/down movement
• DOF 4: Rotation of the joint between the two gripper claws, providing the opening and closing motion

The Robotic Arm Assembly Kit

I bought this robotic arm because I was fascinated by the potential of such a mechanism. As an Arduino enthusiast, I saw a range of possibilities—though, to be honest, I wasn’t entirely sure whether programming one of these “things” would be within my reach.

That’s why this seemed like the perfect educational robotic arm: a way to learn how to programme it, understand its limitations, and deal with the challenges of controlling multiple motors simultaneously—without spending too much money.

The cost of the robotic arm together with four RC Servo MG90S motors purchased on AliExpress in mid‑2025 (when I bought them) came to just over €10. If you prefer to buy from other online stores such as Amazon, the price is around €20, though delivery is much faster in that case.

Of course, you also need to add the cost of the control board (in my case an Arduino Uno R3) and a PWM driver for the servos, since the power draw of the MG90S motors is too high to run them directly from the Arduino board. These two components, which cost me about €7 in total (again from AliExpress), can naturally be reused for other projects or for more advanced robotic arms, so they’re easy to recycle for future activities.

The kit, as I purchased it, consists of laser‑engraved sheets from which the individual components must be manually detached.

The Kit Components

It’s worth noting that there are two versions of the same kit: a cheaper wooden one (which I bought to save money) and another, costing a few euros more, made of acrylic plastic.
With hindsight, I would recommend the acrylic version, since the wood is softer and the screws—which often bite directly into the fixing holes—tend to remain too loose and fail to hold properly.

That said, this guide also explains how I solved the various issues encountered with the wooden version. So if you’ve already purchased that one, don’t worry—it can still be assembled and made to work correctly. I’ve done it myself.

In the photo below, I show all the pieces already detached from their sheets. To make the following steps easier, I’ve numbered them in groups that correspond to the paragraph numbers explaining the assembly. The number will be written in brackets “(1), (2), etc.” at the beginning of each section title. I hope this will make the assembly process clearer..

The Mini RC Servos MG90S

As mentioned earlier, each movement of the robotic arm must be driven by a motor, which in this case consists of four MG90S Mini RC Servos.

These servos are smaller in size compared to standard servos, but their functionality is identical. They come in various models and price ranges. The most significant distinction is whether they use plastic gears—cheaper but more fragile and prone to breaking—or metal gears, which cost slightly more but are naturally more durable.

Bear in mind that these mini servos cannot be reused for a more advanced robotic arm (such as the 6‑DOF metal robotic arm I’ll cover in a future article), simply because of their reduced size and power. Hobby‑grade robotic arms generally use standard‑sized servos.

Each servo is sold with a small bag containing the fixing screws and the various arms used to attach the components to be driven.

Centring the Servos

RC servos normally have a rotation range of 180° (more precisely from –90° to +90°) and are controlled via a PWM (Pulse Width Modulation) signal—a square wave with a fixed frequency (typically 50 Hz) and a variable pulse width between 1 and 2 ms. A 1 ms signal moves the shaft to one extreme, a 1.5 ms signal places it in the central position, and a 2 ms signal moves it to the opposite extreme, with all intermediate positions reached by intermediate pulse widths.

The connector has three coloured wires:

• BLACK: Common negative, connected to ground
• RED: Positive supply, powered by a 5‑volt DC source
• YELLOW: Control signal, receives the PWM signal that sets the target position

Before mounting the servos onto the robotic arm, they must be set to the central position. This can be achieved in several ways, the three most common being:

1. Place one of the control arms onto the servo shaft and gently rotate it all the way to one end stop. Then reposition the arm perpendicular to the motor and rotate it 90 degrees in the opposite direction. This should approximate the central position. Note: this method is not recommended for servos with plastic gears, as manual force may damage the gear teeth.
2. Connect the servo to an Arduino (or similar board) and write—or download—code that sends a PWM signal of 1.5 ms to the yellow wire.
3. Use a servo tester like the one shown in the following photo. This tool allows you to test the servo’s range by turning a potentiometer, or (by pressing the dedicated button) set the servo directly to its central position. 

(1) Mounting the Rotation Control Servo (DOF 1)

The first step is to take the two pieces marked with the number (1) in the kit components image, two M3 screws from the small bag, and one of the mini servos 

The components should be assembled as shown in the following photo, paying particular attention to the smaller piece, which has a groove designed to make it easier for the servo wires to pass through:

Everything is then secured with the two screws, which should be placed into the designated holes

Note that the screws are normally meant to be tightened into the smaller lower holes. This probably works fine with the acrylic kit, but with the wooden version—given the softness of the material—the screws simply won’t hold. In most cases, you’ll need to add locking nuts to the screws, as shown in the photo.

If I remember correctly, a few nuts are included in the kit, but especially if you have the wooden version, I strongly recommend keeping a supply of M3 nuts and washers—you’ll definitely need them.

(2) Mounting the Fixed Base of the Robotic Arm

The base, marked with the number (2) in the kit components photo, is the part that supports the entire mechanism. To prevent the arm from wobbling excessively with every small movement, it is advisable—once assembly is complete—to secure it to a larger, heavier base.

This base must hold the plate with the rotation servo (DOF 1) of the robotic arm, which we assembled in step (1). To do this, four screws are used as “columns”; I replaced mine with slightly longer ones for greater stability.

Now insert the long screws into the underside of the base. I recommend fitting the nuts and washers exactly as shown in the following photo. Do not tighten the lower nuts, however, since the screws must remain free to rotate in order to be fastened to the upper plate.

Now the support base for the rotation servo, assembled in step (1), must be mounted onto the ends of the four screws fixed to this base, as shown in the following figure.

Unfortunately, it is not possible to fit locking nuts on the upper base because the space is too limited, and they would obstruct the correct rotation of the robotic arm. However, once the tips of the screws are tightened into the holes of the upper base, simply tighten the counter‑nuts on the fixed base (the ones we previously left loose) to ensure that everything remains securely in place.

(3) Assembling the Rotating Base of the Robotic Arm

Once the servo has been secured, we need to assemble the piece marked with the number (3) in the kit components photo, together with its corresponding arm and the screws included in the servo kit, in order to build the rotating base on which the robotic arm will then be developed.

The servo arm must be placed behind the piece and secured with the screws fitted on the front side.

Pay close attention to how the plate should be positioned in relation to the arm—make sure it is not placed upside down. The arm must have the splined hole (the one that fits onto the servo shaft) inserted into the hole in the plate.

Once the arm has been mounted, the assembly should look exactly as shown in the following photo:

Mounting the Rotating Base onto the Arm’s Rotation Servo

If we now take the result of the assembly described in paragraph (3) and place it alongside what we assembled in paragraph (2), we should obtain the following situation:

At this point, the rotating base must be positioned on top of the servo exactly as shown in the following photo, and secured using the small fixing screw supplied with the servo arm in the kit.

(4) Mounting the Arm Forward Movement Control Servo (DOF 2)

Now take the pieces marked with the number (4) and a second servo, as shown in the following figure:

Using the M3 screws, fix the servo onto its mounting plate, taking care to assemble it exactly as shown in the photo:

Here too, it is strongly recommended (especially with the wooden version of the arm) to fit locking nuts onto the protruding ends of the screws.

(6) Assembly of the First Part of the First Segment of the Robotic Arm

On the servo mounted onto its base in paragraph (4), the first part of the first segment of the robotic arm must be installed. This consists of the piece identified with the number (6) and the double servo arm, together with the corresponding fixing screws.

Here too, the hole in the servo arm with the splined fitting for the servo shaft must be placed inside the hole of the piece, as shown in the figure, and the whole assembly should then be secured with the supplied screws.

Using the small fixing screw supplied with the servo arm in the kit, secure the piece to the servo assembled in paragraph (4) as shown below.

(7) Assembly of the Second Part of the First Segment of the Robotic Arm

To complete the first segment of the robotic arm, we now need to assemble onto the first part the components marked with the number (7), together with the corresponding screws and nuts supplied in the robotic arm kit

The two fixing nuts must be slotted into the dedicated recesses located in the central part of the arm segment, as shown in the following photo

(5) Mounting the Arm Elevation Control Servo (DOF 3)

For the assembly of the servo that will control the elevation movement of the second segment of the robotic arm, you will need to take the components identified with the number (5) and a third servo 

The servo must be mounted onto its base exactly as shown in the following photo. Be careful to respect the positioning of each individual component.

Here again, it is advisable to use nuts to secure the structure and prevent possible loosening caused by vibrations.

(8) Mounting the Control Arm for the Second Segment of the Robotic Arm

The servo just installed must act on the second segment of the robotic arm. To achieve this, a mechanism consisting of two movable arms and a linkage is used to transmit the movement to the second segment of the arm.

To assemble the first part of this mechanism, take the piece marked with the number (8), the usual double servo arm, and the standard fixing screws.

The whole assembly must be mounted exactly as shown in the photo

(9) Mounting the Support of the First Arm Segment onto the Rotating Base

Now, to mount the first segment of the robotic arm onto the rotating base, we must assemble this support using the components marked with the number (9).

These three pieces must be slotted together exactly as shown in the photo (for now, no fixing screws are used).

The whole assembly is then slotted onto the previously assembled rotating base, exactly as shown in the photo.

Here too, it is still a press‑fit connection, and no screws are to be used.

At this point, take the base with the servo and the first segment of the robotic arm assembled in paragraph (7), and place it laterally against the support just mounted. Secure it using the screws, which will tighten into the nuts inserted in the dedicated slots.

The other part of the first segment of the robotic arm, the one not attached to the servo, must be connected to the central base using a screw with a washer placed in the centre to facilitate its movement

This joint must be left fairly loose; otherwise, the friction between the two parts could block its movement.

For this reason, the fixing nut must not be tightened fully but should leave about 1 mm of free space between the two parts.

To prevent the nut from loosening, a locknut must be added, which, when tightened against the nut, will secure it in the desired position.

At this point, I connected my servo tester to check the movement of the first segment of the arm, and I noticed that, at the lower end of its travel, it was hitting against the front part of the support.

I therefore marked with a pencil the point where the contact occurred and, using a mini‑drill (similar to a Dremel) fitted with an abrasive disc, I removed the excess material that was blocking the full downward movement of the arm.

Mounting onto the Rotating Base the Servo for the Movement of the Second Arm Segment

Now we will mount the base with the servo controlling the movement of the second segment of the robotic arm (the one assembled in paragraph (8)) onto the rotating base, on the opposite side to the servo already installed.

This component must also be secured with screws, which tighten onto the nuts inserted into the dedicated slots, as shown in the photo.

(10) Assembly of the Second Segment of the Robotic Arm

We will now assemble the second segment of the robotic arm. To do this, take the components identified with the number (10), as shown in the following figure:

Of these components, the first will later serve as the joint for transmitting the movement of the gripper, while the other three must be positioned as follows to build the actual segment of the arm.

The three pieces must be assembled using their own slots in the following way:

Fixing the Second Segment of the Robotic Arm to the End of the First Segment

Naturally, the second segment of the arm must be connected to the end of the first segment, which we previously mounted onto the rotating base. To do this, it must be positioned as follows:

This second segment must be free to move in relation to the first. Therefore, we will mount the screws with the nuts left fairly loose, and place a washer between the two parts of the joint (to ensure that friction does not block the movement). After each nut, a locknut must be added which, once tightened, will prevent any possible loosening.

The joint (which is the first of the four pieces shown in the photo of paragraph (10)) must be mounted on the right side of the connection between the two segments of the robotic arm, in the position shown in the photo.

Here too, it is advisable to place a washer between the two parts before inserting the screw, and then secure it with a nut and a locknut.

Mounting the Joint for Transmitting the Gripper’s Movement

This component (the first of the four shown in the photo at the beginning of paragraph (10)) is part of a series of linkages designed to keep the gripper in a horizontal position during any movement performed by the two segments of the robotic arm.

Specifically, this joint must be mounted onto the connecting screw between the first and second segments of the arm, on the right side. Therefore, we need to remove the screw fixed in the previous step, add an additional separating washer, and then place this joint. Once done, the nut and locknut can be reinstalled to secure it in place.

In the following photo, you can see the detail of the washers placed between the various moving components to reduce friction, as well as the locking of the nuts with the locknuts.

(11) Assembly of the Linkages for Transmitting Movement

The two components identified with the number 11 are identical in shape and size, but they serve two different purposes.

The first must be mounted between the hole on the base of the servo controlling the movement of the first arm segment and the joint for transmitting the gripper’s movement, which was installed in the previous paragraph.

Meanwhile, the second linkage must be mounted between the control arm of the second segment of the robotic arm (whose assembly is described in paragraph (8)) and the hole on the protruding arm of the second segment of the robotic arm, which was assembled in paragraph (10).

At this point, our 4DOF robotic arm should be at this stage of progress, as shown in the following photo:

(12) Mounting the Final Component of the Movement Transmission Linkage

The last component, identified with the number (12), is identical to the two previous ones, as can be seen in the photo.

This last linkage component must be fixed onto the remaining free hole of the joint that transmits the gripper’s movement.

(13) Mounting the Support for the Gripper Control Servo (DOF4)

Now we need to take the components marked with the number (13), which are intended to secure the servo that controls the opening and closing of the jaws of the mechanical gripper of our robotic arm.

For assembling the components, I recommend carefully following the assembly steps illustrated in the next photos:

1) Mounting the servo onto the first support

2) Mounting the spacer with the connecting arm to the linkage

3) Mounting the second spacer.

4) Mounting the Second Servo Support and Securing with Screws and Nuts.

(14) Mounting the Control Lever for the Gripper Jaw Movement

For this component, shown in the photo and identified with the number (14), the connection to the servo must be made using the single arm supplied in the kit, and not the double arm as used for the other servos.

With the servo positioned at the centre of its travel, the mounted arm must be placed approximately in the position shown in the photo.

(15) Assembly of the Mechanical Gripper of the Robotic Arm

Finally, we need to take the components of the mechanical gripper, identified with the number (15), and start with the two jaws of the gripper. A screw must be inserted into each jaw, followed by a round spacer and a metal washer to reduce friction.

The two jaws must then be screwed into the holes of the corresponding servo support in this way:

As usual, the two screws must be secured with two nuts, to which we will also add locknuts to prevent loosening.

Now we must connect the control linkage for opening the jaws of the gripper, and our robotic gripper will be complete.

At this point, we only need to attach the gripper to the second segment of the robotic arm and to the linkage in order to complete the assembly 

And finally, the assembly of the 4DOF robotic arm is complete.

One last piece of advice: since the weight of the gripper is not well balanced compared to the rest of the arm, the upward movement of the arm (DOF3) is rather difficult. Adding a counterweight to balance this uneven load greatly improves the smoothness of the arm’s movement. The correct balancing weight must be determined through testing. Personally, I achieved good results by using an M10 screw with two washers (which also help to secure it to the arm) and three nuts, as shown in the photo.

Well, my article is now complete. I hope it has been useful in helping you to assemble your 4DOF robotic arm quickly and without errors.
Thank you for following me this far, and I wish you lots of enjoyment in programming and operating this device, which I personally found truly fascinating.
See you soon with the next article...

Luciano (Capitan Farloc)