Practical Magic: Switching Gears

This month, please welcome Sarah of Technical Services for a Practical Magic guest post!

The Technical Services staff at PPL care very deeply about the tools of our trade.  We prepare and repair your favorite books, DVDs, audiobooks, and devices using markers, pencils, stamps, stamp pads, scissors, X-ACTO knives, paper cutters, label printers, barcode scanners, and no fewer than four tape dispensers.

When one of those tape dispensers appeared on my desk with a note saying “I’m having issues,” I took the situation very, very seriously.  (Not just because his eyebrows made him seem a little cranky.)

tape dispenser having issues

Up in Tech Services, we also like to tinker.  Some of us tinker with bikes, lamps, tools, 3D printers, crafts, or wood – others tinker with images or words.  We all want to figure out how things work!

So apart came the tape dispenser.

This is a “definite length” dispenser, so pressing a lever advances exactly the same amount of tape each time.  The workings are straightforward:  the lever turns a large gear which turns a gear idler.  The gear idler turns two small gears attached to rollers which pull the tape under the cutting blade.  I’ve also started to call it our “Cold War Era” tape dispenser because it is encased in steel and the gears and rollers are all metal, so it weighs more than 5 pounds.   The only plastic components are the lightweight tape holder and the bumper behind the lever.

I admire straightforward tools that use mechanical energy to accomplish simple tasks, but there is a flaw in this design:  a very narrow large gear turns a wider, smaller gear.  Guess what we’d done!  We’d neglected to grease the gears and had worn a groove right through the gear idler. To add insult to injury, I had to break a screw to remove the gears.  No wonder he’d raised his eyebrows at us.

Couldn’t we just order replacement parts?  Unfortunately, no.  3M stopped making this model years ago.

Gutenberg to the rescue!

I knew I wouldn’t be able to find a gear on Thingiverse that matched our original, so I downloaded a gear generator and the free program OpenSCAD to manipulate the design.

gear generator screenshot

Luckily, 3D printing is ideal for quick prototyping.  It took 3 tries, but finally, the gear had the right diameter and the right number, width, and height of teeth.

And I thought THAT was a challenge!  The screw that would hold the new gear was a much bigger puzzle.

I used several Google image searches to figure out the original screw type and how to take its measurements. Once I knew that I needed a 10-32 hex-head shoulder screw I tried very specific searches for 3D printable files, but had no luck.  I even searched Amazon to find out if it was a standard type we could order, but that didn’t pan out either.  Thingiverse contributors have developed screw generator files (like this one), but they weren’t as easy to use as the gear generator, at least for this OpenSCAD novice.

Gutenberg prints files in .stl format, so I used a search technique we often recommend: using Google’s search operators to limit results to a desired file type.  In the search box, I typed “screw filetype:stl” and reviewed the results. (You can use Google’s Advanced Search to do this with more common file types.)

Beyond Thingiverse, there are many websites that provide 3D printable files.  If you’re curious about the next level of 3D printing design, check these out:

3D ContentCentral: “FREE 3D and 2D CAD Models of User-Contributed and Supplier-Certified Parts, Assemblies and more.”

3D Warehouse: millions of models created in SketchUp, contributed by manufacturers and individuals.

GrabCAD: “get access to thousands of free CAD projects. Join our community of 2,540,000 engineers and 1,040,000 CAD files.”

I found a sketchup file in 3D Warehouse for a 10-32 screw, but other parts of the file weren’t right: the screw needed a “shoulder” 1/4” long and 5/16” diameter, and the head needed to be changed to a flat “hex head” type.   Most importantly, any editing we did couldn’t alter the threaded portion.  A 10-32 screw is 3/16” wide and has 32 threads per inch.

Michaela jumped right in to edit the sketchup file in Tinkercad.  After all, a shoulder screw is simply a hexagon, a cylinder, and a threaded cylinder stacked one on top of the other.

We tried a few variations of the design with the threaded cylinder, then had one very informative failure.  When a prototype with supposedly correct dimensions shredded as I screwed it in, I realized that the nature of the PLA filament could make our design much simpler.

Back in Tinkercad, Michaela replaced the threaded cylinder with a smooth one. This time I intended the screw to thread itself, because PLA is soft enough to be carved by the threads in the steel casing.  Success!

We celebrated the return of our tape dispenser with the glee only Tech Services staff can muster.

tape dispenser organ transplant

(Of course, we greased the gears before closing up the wound.  We didn’t want to waste all the work we, and Gutenberg, put into this lifesaving project!)

PPL isn’t the only place where the need for replacement parts inspired someone to learn how to use free web resources and design programs then 3D print the results.  Check out a similar real world problem solved by 3D printing at a high school in Uniontown, Pennsylvania.

Join us for one of our upcoming demos to watch Gute in action:
Friday                    11/20         2:30pm
Wednesday          12/16         2:00pm

Check our Facebook events or our calendar for updates.  And keep an eye out for more information on our 3D design workshop on December 9, 6:30-8pm!


Gutenberg enjoys humming, getting crafty, and causing mischief around the library. Ada enjoys reading up on the latest 3D printing technology and researching tips and tricks to make life easier. Gute and Ada’s people enjoy tinkering, making, and discovering ingenious solutions to everyday problems. If you have questions or suggestions for future Practical Magic posts, please feel free to email us at

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