Dual-Position Pencil Holder for Fanuc Robotic Arm (CAD to 3D Print

In this project, I will show you how I designed and manufactured a custom dual-position pencil holder (end-effector) for a Fanuc robotic arm.

The main goal was to create a versatile tool for trajectory testing and robotic drawing. This tool allows the pencil to be inserted in two different configurations:

1. Vertically: Ideal for precise path calibrations and geometric drawing.
2. Diagonally: Perfect for simulating the natural tilt of a human hand during writing.

The entire workflow goes from the initial paper sketches to professional CAD modeling in Creo Parametric, ending with 3D printing the final functional part.

Software:

1. Creo Parametric (for CAD modeling)
2. 3D Slicing Software (e.g., Cura / PrusaSlicer)

Hardware & Materials:

1. Fanuc Robotic Arm (with standard flange)
2. 3D Printer
3. PLA or PETG Filament
4. Standard Pencil or Marker
5. M3/M4 screws (if used to lock the pencil)

Every good engineering project starts away from the computer. Before opening the CAD software, we grabbed paper, a pen, and a caliber to collect all the fundamental dimensions required to perfectly interface our tool with both the robot and the drawing instrument

Baseline Measurements:

1. Fanuc Robot Flange: We measured the robotic arm's end flange, which has a side of 48 mm and a height of 6 mm. These dimensions are crucial for designing the tool's mounting base.
2. The Pencil: A standard pencil has a diameter of 6 mm and a total length of 162 mm.

Design Logic:

The goal was to create a compact yet sturdy vertical rod capable of holding the pencil in two different modes. We designed a main body with an overall height of approximately 120 mm (excluding the pencil). With the pencil inserted, the total tool height reaches around 210 mm.

For the placement holes, we made specific geometric choices:

1. Vertical Hole: It has a depth of 70 mm to ensure the pencil stays perfectly perpendicular to the workspace (90°) and doesn't wobble during robot movements.
2. Angled Hole: Designed at a 45-degree angle to simulate the natural tilt of human handwriting.

Moving from paper sketches to a 3D model is where the project truly takes shape. For this phase, we used Creo Parametric, an excellent parametric CAD software that ensures maximum geometric precision.

The modeling process was divided into four main stages:

1. The Mounting Base

To ensure the tool would be perfectly stable, we started by designing the base that directly connects to the Fanuc robot flange. We created a simple square extrusion of 48 mm per side and 6 mm thick, adding the mounting holes with the exact diameters previously measured using the caliper.

2. The Vertical Rod

Once the base was complete, we started a new sketch exactly at its center. From there, we performed a simple vertical extrusion to create a rod about 11 cm (110 mm) high. Combined with the 6 mm base, the total structure height perfectly hits our ~120 mm target.

3. Extruding the Vertical Hole

For the straight pencil sleeve, we used an extrude cut (material removal) on the top surface of the rod. The hole features a 5.9 mm diameter and goes straight down to a depth of 70 mm, providing excellent support to eliminate any pencil wobble during robotic movements.

4. Creating the Diagonal Hole (The Geometric Trick)

Making an angled hole requires an extra step, as it cannot be done from standard flat surfaces. To solve this, we created an angled Datum Plane offset at exactly 45 degrees. Using this new plane as our sketch plane, we performed the extrude cut to create the second slot, allowing the robot to draw with a natural human-like hand tilt.

Once the 3D model was completed in Creo Parametric, it was time to prepare it for the physical world. We exported the project as an .STL file and imported it into the QidiStudio slicing software to configure the printing parameters.

To ensure the tool could withstand the mechanical stress and rapid movements of the Fanuc robotic arm, we carefully adjusted the settings for maximum durability:

1. Infill Density: We selected a 35% infill. This density is optimal for functional parts, making the tool solid, rigid, and impact-resistant without wasting unnecessary filament.
2. Orientation & Supports: The tool was oriented on the build plate to optimize structural strength and ensure that both pencil sleeves (especially the 45-degree angled one) printed cleanly without warping.

After fine-tuning all the necessary parameters (layer height, speed, and temperatures), the slicer calculated a print time of around 2 hours. Finally, we sliced the model and generated the G-code file, ready to be sent to the 3D printer.

After transferring the G-code file to the 3D printer, we started the print job. Two hours later, our custom tool transitioned from a digital CAD model into a physical object. We then moved to the hands-on testing phase and ran into a classic additive manufacturing challenge: the pencil did not fit into the holes.

While the dimensions were mathematically perfect in the CAD software, we did not account for 3D printing tolerances. As hot plastic filament is extruded, it tends to expand slightly inward when forming small cylindrical holes due to material expansion and slicing paths. Consequently, our intentional 5.9 mm tight fit shrank even further in reality, making it impossible to insert the 6 mm pencil.

If we had more time for a second version of the project, the ideal fix would be updating the model in Creo Parametric to scale the hole diameter up to 6.1 mm or 6.2 mm, leaving enough clearance to offset the plastic's natural expansion. Another quick workshop fix would be manually clearing out the existing printed piece using a 6 mm drill bit or a hand reamer to remove the excess plastic.

Even though the tool requires a tolerance calibration before being fully operational on the Fanuc robot, the project was highly educational. It perfectly demonstrates the full workflow and taught us a vital lesson about physical tolerances when transitioning from digital designs to 3D printed parts!