Work in Progress Design Thinking: 3D Printed Modular Pencil Holder for FANUC ER-4iA Robotic Arm (with Tolerance Iteration)

We are two students from the ITS Academy Foundation in Borgomanero, and during our Design Thinking training we faced a very practical challenge: enabling the FANUC ER‑4iA robotic arm to draw precise and repeatable lines. To achieve this, we needed a pencil‑holder tool that was low‑cost, 3D‑printable, easy to assemble, and above all adjustable, allowing different pencil angles during the exercises.

We approached the project by applying the Design Thinking method, moving from the analysis of the laboratory’s needs to idea generation, and finally to CAD modeling and rapid prototyping. The result is a four‑piece tool designed in Creo Parametric, capable of offering up to 20 different inclination settings thanks to a star‑shaped pivot.

The first printed prototype revealed some issues with tolerances and hole precision, which we corrected manually in order to assemble and test it on the robot. Despite these imperfections, the support proved functional and allowed us to document the process and capture useful photos for this guide. The next iteration will include improved measurements and more accurate component fitting.

In this Instructable, we will walk through the entire process—from the initial problem to the prototyped solution—sharing files, steps, mistakes, and improvements so that anyone can replicate or further develop the project.

To create this pencil‑holder tool, we used a combination of digital and manual tools. The entire design phase was carried out in Creo Parametric, which allowed us to model the four components of the tool, verify the assembly, and generate the STL files for 3D printing.

For the prototyping phase, we used a 3D printer with PLA filament, chosen for its rigidity, ease of printing, and overall convenience. After printing, several manual tools became essential to correct the tolerances of the first prototype: precision files, a cutter, sandpaper, and a mini drill to refine the holes and adjust the star‑shaped pivot.

The project was developed to be mounted on the FANUC ER‑4iA robotic arm available in the ITS laboratories, so the required materials also include a standard pencil and the screws or bolts used to attach the tool to the robot’s flange.

The decision to divide the tool into four separate parts was not accidental, but driven by practical needs related to printing, maintenance, and functionality. A single solid piece would have required a large amount of support material during 3D printing, increasing time, filament consumption, and the risk of deformation. By splitting the model into separate components, we were able to orient each piece in the most efficient way, minimizing waste and improving surface quality.

This modular approach also makes the tool easier to maintain: the part that grips the pencil is the one most exposed to wear, and it can be replaced quickly without having to reprint the entire flange attachment for the robot. Finally, dividing the tool into multiple elements made it possible to integrate the 20‑position star‑shaped pivot system, which allows the pencil’s angle of incidence on the sheet to be adjusted—a mechanism that would have been much more complex to implement in a single block.

Overall, this modular structure not only optimizes the printing process but also makes the tool more versatile, repairable, and better suited to the educational needs of the laboratory.

After printing the first prototype, we encountered several tolerance‑related issues, mainly caused by the thermal shrinkage of PLA and by the fact that our initial CAD file did not yet include an adequate margin to compensate for this material behavior. The star‑shaped pivot with its 20 teeth, as well as the mating holes, turned out to be too tight and did not allow proper assembly.

To avoid interrupting the testing process, we decided to take a very hands‑on approach and modify the prototype directly. Using precision files, a cutter, sandpaper, and a mini drill, we were able to “hack” the part, manually adjusting the fits until we achieved a functional assembly.

This manual tuning allowed us to validate the fundamental geometry of the tool and mount it on the FANUC ER‑4iA robotic arm for the first tests, collecting valuable information for revising the dimensions and preparing the next print.

For the 3D printing of the first prototype, we adopted specific settings to balance mechanical strength and geometric accuracy. We chose a 25% infill with a honeycomb pattern, a structure that provides excellent resistance to the torsional forces generated by the robotic arm without adding unnecessary weight to the tool.

Additionally, due to the geometric complexity of the 20‑tooth star‑shaped pivot, we had to reduce the printing speed to 70% of the printer’s standard parameters. At maximum speed, the printer was unable to properly define the micro‑teeth, resulting in small defects and surface imperfections. By slowing down the process, the extruder was able to deposit the filament with the required precision.

The four‑piece design allows for a modular and intuitive assembly. At this stage, due to printing tolerances, the components required some manual adjustment to validate the overall geometric fit. This first Design Thinking iteration led us to an important engineering revision: we found that a 20‑tooth star‑shaped pivot on a diameter of only 10 mm demands a level of geometric precision that exceeds the capabilities of our 3D printer, as the individual teeth are extremely small and tend to merge together.

For the next version of the project (V2.0), we decided to optimize the design by reducing the number of positions to 12 or 16 teeth. This modification will increase the size of each tooth, significantly improving printability and mechanical engagement while still maintaining excellent modularity for adjusting the pencil angle. The next steps include remodeling the part in Creo with the updated specifications, performing physical tests on the FANUC ER‑4iA robotic arm in the ITS laboratories, and calibrating the Tool Center Point (TCP).