Pietron - the Automatic Mod for the Pietro Grinder

We all know the Pietro grinder stands out for its stunning grind quality, thanks to its unique vertical flat burrs. However, let’s be honest: its ergonomics and workflow are... questionable 😂

I started prototyping this project about a year ago. The initial version worked but wasn't refined enough for public release. Recently, I revisited the design to create a robust, reliable version that anyone can replicate.

Why this mod?

1. Budget-Friendly: The total build cost is under $60, using standard, easy-to-find components.
2. Beginner Friendly: You don't need to be an engineer. The assembly requires minimal technical skills.
3. Safety Feature: By repurposing a drill gearbox, we utilize the built-in Torque Clutch. If a stone accidentally gets into the beans, the clutch will slip instead of jamming, protecting your burrs from damage.
4. Non-Destructive: The mod uses existing mounting points. No drilling or glue is required, and it is fully reversible.

❤️ Special Thanks: I want to express my deepest gratitude to my beloved wife. A brilliant person and a first-class engineer, she was the one who took this raw idea and turned it into reality. This project wouldn't exist without her.

What you’ll need:

1. A 3D printer
2. A soldering iron
3. A few parts from AliExpress
4. A pair of steady hands (yours or a friend’s)
5. A bit of patience

Let’s fix the ergonomics and unlock the full potential of the Pietro!

⚠️ Disclaimer & Safety Warning ⚠️

This project involves working with electricity, soldering, and moving mechanical parts. Please proceed with caution.

1. Do it at your own risk: The author is not responsible for any damage to your grinder, personal injury, or fire resulting from improper assembly or usage.
2. Warranty: While the mod is designed to be non-destructive, please be aware that disassembling any part of your grinder may void the manufacturer's warranty.
3. Common Sense: Always disconnect the power supply before making adjustments. Ensure all 3D printed parts are printed with sufficient infill to handle the torque.

Here is the detailed list of components you will need to complete the build.

1. 3D Printer Filament

1. Specs: ~800g (PETG is highly recommended over PLA for flexibility and heat resistance)
2. Where to buy: Any local store
3. Approx. Cost: $11

2. DC Motor

1. Specs: RS-550 DC, 25V, 13 Teeth
2. Link: AliExpress
3. Approx. Cost: $10

3. Gearbox

1. Link 1 (Reference / Out of Stock): AliExpress (Original Link)
2. Link 2 (Available Alternative): AliExpress (New Link)
3. Approx. Cost: ~$7–10
4. Note: I purchased the gearbox from the first link, but it is currently out of stock. I found the second link which, judging by the review photos, appears to be the exact same version.

4. Hex Nuts

1. Specs: 3/8-24 UNF (2 pieces) — these screw onto the gearbox thread
2. Link: AliExpress
3. Approx. Cost: $3

5. Power Supply

1. Specs: 24V 5A
2. Link: AliExpress
3. Approx. Cost: $15

6. Power Socket

1. Specs: DC099 (5.5 x 2.5mm)
2. Link: AliExpress
3. Approx. Cost: $2

7. Potentiometer

1. Specs: 100kΩ with built-in Switch
2. Link: AliExpress
3. Approx. Cost: $2

8. PWM Speed Controller

1. Specs: DC 10-60V 20A (You will need to desolder the original potentiometer and solder the new one with the switch)
2. Link: AliExpress
3. Approx. Cost: $3

9. Threaded Rod

1. Specs: M3 size (4 pieces, minimum 18cm each). It is most convenient to buy a single 1-meter rod and cut it into 4 parts.
2. Where to buy: Local Hardware Store
3. Approx. Cost: $3

10. Chicago Screws (Binding Posts)

1. Specs: M3 30mm (We only use the female "barrel nut" part to screw onto the threaded rod)
2. Link: AliExpress
3. Approx. Cost: $2

11. Wiring

1. Specs: 16 AWG Silicone Wire
2. Where to buy: AliExpress or Local
3. Approx. Cost: $2

Total Estimated Budget: ~$60

The design features simple geometry with little to no supports required, so feel free to use your favorite printer settings. However, here are some baseline recommendations for the best results:

1. Material: PETG is highly recommended (it offers better flexibility and is less brittle than PLA). You can try PLA, but it may crack under stress.
2. Infill: 15-20%
3. Perimeters (Walls): 3 (This ensures the parts are strong enough to handle the torque).

Why .STEP files instead of .STL?

I am providing the files in .STEP format intentionally. Parts sourced from AliExpress can sometimes vary slightly in size. STEP files allow you to easily import the design into CAD software and modify the geometry to fit your specific components if necessary.

Files to Print:

1. 📄 coffe_grinder_back_cover.step
2. 📄 coffe_grinder_cup.step
3. 📄 coffe_grinder_gearbox_holder.step ⚠️ PRINT 2 COPIES
4. 📄 coffe_grinder_gearbox_pin.step
5. 📄 coffe_grinder_holder_front_part_1.step
6. 📄 coffe_grinder_holder_front_part_2.step
7. 📄 coffe_grinder_main_base.step
8. 📄 coffe_grinder_motor_block_holder_part_1.step
9. 📄 coffe_grinder_motor_block_holder_part_2.step
10. 📄 coffe_grinder_motor_holder.step
11. 📄 coffe_grinder_potentiometer_knob.step

Detach the original handle from the grinder body. This is usually a straightforward process, but if you encounter any difficulties or want to be extra careful, I recommend looking up a video tutorial online.

To install the adapter correctly, please follow this exact sequence:

1. Place the metal washer (⌀ 10mm) inside the 3D printed adapter first. (This is crucial to protect the plastic from the spring pressure).
2. Place the original factory spring on top of the washer inside the adapter.
3. Mount the assembled adapter (with the washer and spring inside) onto the grinder shaft.
4. Secure everything by tightening the original factory bolt.

⚠️ Important Disclaimer: Component Variance

This is the most unpredictable variable in the project. Suppliers occasionally update component specifications without notice. Because these 3D printed blocks are designed with maximum precision and tight tolerances, receiving a gearbox with slightly different dimensions may make assembly impossible.

If your components match the design, proceed with the following assembly order:

1. Gearbox Holders: Attach the two gearbox_holder parts to the gearbox.
2. Motor Holder: Attach the motor_holder part to the motor.
3. Part 1 Installation: Install motor_block_holder_part_1. Ensure the cutout for the pin lines up perfectly with the speed selector switch.
4. Locking Pin: Insert the gearbox_pin to lock the speed selector in "Low Speed" mode.
5. Part 2 Installation: Install motor_block_holder_part_2. Finally, squeeze the entire assembly tightly by hand to minimize any gaps.
6. Drive Interface: Screw two nuts onto the rotating gearbox shaft.
7. Mounting to Base: Insert the resulting assembly into the rear opening of the main base and feed the wires down through the bottom.
8. Protective Cover: Secure the protective cover using 4 small bolts or self-tapping screws—whatever you have on hand in your toolbox (⌀ 3mm).

💡 Pro Tip: The "Stone-Proof" Safety Clutch

Since we are repurposing a drill gearbox, we gain a fantastic built-in safety feature: the Adjustable Torque Clutch.

I strongly advise AGAINST setting the gearbox to the "Drill" icon (locked mode). Instead, rotate the adjustment ring to a low setting (around 1-2).

1. Why? This setting provides more than enough torque to grind even light roast beans effortlessly. However, if a rock or foreign object accidentally ends up in your coffee, the clutch will slip (you will hear a clicking sound) instead of forcing the grinder to crush the stone. This simple trick effectively saves your expensive burrs from catastrophic damage.

1. Install the Rods: Screw the 4 threaded rods into the holder_front_part_1 piece (the part that sits near the grind adjustment dial).
2. Grinder installation: Install coffee grinder into the holder_front_part_1.
3. Mount the Holder: Slide the holder_front_part_2 onto the rods. Pay attention to the cut on the bottom part of that piece it should correspond the screw on the grinder. Ensure that both halves wrap securely and tightly around the body of the coffee grinder.
4. Connect to Base & Align Drive: Guide the threaded rods through the holes in the main base. As you slide the coffee grinder into place, pay special attention to the connection point. The hex nuts you attached to the gearbox shaft must slot perfectly into the 3D printed adapter (the part replacing the handle). This mechanical connection is vital for transferring power from the motor to the coffee grinder.
5. Measure & Cut: Check the necessary length of the threaded rods. They need to be short enough to allow the Chicago Screws (Binding Posts) to tighten the assembly completely, but long enough to thread in. Mark the correct length and cut off the excess metal.
6. Final Secure: Tightly screw on the Chicago Screws to pull the whole assembly together, firmly locking the coffee grinder to the base.

⚠️ Disclaimer: I am not a professional electronics engineer. This circuit was designed as a hobbyist project using standard, off-the-shelf components available to everyone.

The Circuit Logic: Before assembling, let's clarify the flow. The Power Socket feeds electricity into the Speed Controller Board. The Potentiometer acts as the command interface (regulating Speed & On/Off status) for the Controller Board. Finally, the Controller Board drives the Motor.

Instructions:

1. Soldering:
2. Power Socket: Solder the wires to the Power Socket terminals.
3. Potentiometer: Desolder the wires from the stock potentiometer (included with the Controller Board) and resolder them to the new potentiometer pins.
4. Mounting Controls: Install the Power Socket and Potentiometer into their slots on the base and secure them tightly with their nuts.
5. Wiring the Controller:
6. Connect the wires from the Power Socket to the Controller Board's Power Input.
7. Connect the wires from the Motor to the Controller Board's Motor Output.
8. Plug the Potentiometer connector into the Controller Board.
9. Final Step: Screw the Controller Board into its designated mount inside the base.

For the best stability, I strongly recommend adding some form of anti-slip feet. You can use generic rubber stick-on feet from a local hardware store, or you can follow my "DIY hack" method to create a custom integrated silicone rim.

The Custom Silicone Rim Method:

1. Prime the Surface: Apply a plastic primer to the bottom of the 3D printed base. This ensures the silicone bonds permanently to the plastic.
2. Apply Silicone: Squeeze a continuous bead of silicone sealant (standard kitchen or automotive grade works fine) along the bottom edge of the base.
3. The "Stamp": Place the coffee grinder down onto a silicone soldering mat. Fresh silicone sealant does not stick to cured silicone mats. The mat will flatten the bottom of the bead, creating a perfectly level surface without bonding to it.
4. The Result: Once cured, lift the coffee grinder. You will have a custom-molded silicone rim that acts as excellent feet, providing superior grip on your countertop.