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Hive13 Project
Save The Pencil Sharpener
Original (broken) gear and 3-D printed replacement gear
Status: Complete
Project Owner: Kerensa Juniper
Project Location: At Home
Where to contribute: N/A
Start Date: 06/20/2018
End Date: 06/27/2018

Materials Used

  • SketchUp 2017
  • Customizable Gear model from Thingiverse
  • MakerBot Ultimaker2 3-D Printer
  • Utility Knife
  • Filing tool

Project Needs

  • Pencils to sharpen.

Project Information


This is my electric pencil sharpener, a Boston Model 18. This USA-made device, though it is about 20-30 years old, still makes a nice, sharp, straight point, unlike some manual sharpeners (think of the old-school manual types one would use

One day, I needed to sharpen a pencil. To my dismay, I could hear the motor making noises, but pencils would not sharpen.

Saved from the Recycle Pile

The pencil sharpener wasn't working, and I had to consider that maybe it was time to let it go. I had some other materials, including defunct motors, that I was gathering to sell for scrap. I was going to dissect the pencil sharpener for the cord and the motor.

Identifying the Real Problem

When I removed the cover of the pencil sharpener, I found the real reason it wouldn't sharpen my pencils: There was broken gear that prevented the sharpening mechanism from turning.

A friend identified the gear material to likely be urethane. The material is highly brittle and fragile. I saw this as an opportunity to re-create the gear with 3-D modeling and then 3-D print a replacement.

3-D Modeling

3-D modeling for this project was created with SketchUp 2017, along with some assistance through Thingiverse, where I found a customizable gear model. But even after modeling the gear through Thingiverse, it still needed cleaning up and re-configuring in SketchUp.

To get measurements, I used digital calipers. For the areas that were hard to measure or reach with calipers, I clicked out a length of mechanical pencil lead, then used the calipers to measure that lead.


3-D Printing

It took me about 5 attempts to create the 3-D model. Each time, though, I learned from my mistakes and were able to start over faster. When I had a model that I liked, I exported the file to .STL format and imported that into Cura, which prepares the 3-D model for printing.

The first 3-D printout was a rough draft. I wanted to make sure the gear teeth would match up with the existing (broken) gear. The teeth matched well. But my biggest challenge was to make the spindle hole so it would fit on the pencil sharpener's spindle.

After tweaking the spindle hole, I was confident enough to make a higher-quality 3-D printout. As this part is not generally visible, I just went with what filament was already loaded in the 3-D printer.

Fine-Tuning the Printout

After 3-D printing, there still needed to be some refining that had to be done manually. Cura added support structures to parts of the gear; otherwise, the printout would sag in places that needed to be kept straight. That support structure needed to be removed. Next, The spindle hole needed some minor shaving-down which I did with both a filing tool, and a little whittling with a utility knife. Furthermore, a few extra filaments between gear teeth needed to be filed down.

Testing the Printed Gear

Once I was able to fit the gear onto the spindle, it was time to test it out. First, I ran an "unplugged" test by manually turning the gear. So far, so good.

Sharpener Gear Test.gif

Then it was time to do a "plugged-in" test. I was thrilled to see and hear the motor and gears turn in the pencil sharpener once again. I was a bit baffled, though, because while the sharpening mechanism turned, I couldn't get my pencils sharp. I then realized that I had put the motor's magnet back on, backwards; thus, the motor was turning the wrong direction! After I took everything back apart, reversed the magnet, and re-assembled everything, the sharpening mechanism turned in the correct direction, and it sharpened my pencils perfectly, as before!