I recently made a 3D printed device that attaches to a standard ceramic mug and lets you use it to sharpen knives. It's a fairly well known idea that it is possible to use the ring of unglazed ceramic on the bottom of a coffee mug to sharpen a knife. Usually this is done by turning the mug up-side down and rubbing your knife along the surface while maintaining a 20 degree angle between the two. There's a nice article about it at BackwoodsHome.com.

I have been exploring the uses of 3D printing that have a net-positive impact on the environment and this is one good example. If you break the handle on one of the mugs in your house, you would probably normally just throw the rest away. If you instead turned it into a knife sharpener, you are extending the useful life of the mug. Additionally you are extending the lifetimes of your knives meaning less knives will need to be produced. Printing the plastic part does have an environmental impact, but if it is used successfully, it will result in more environmental good than bad.

If you're interested in the overlap between sustainability and 3D printing, please check out my blog entry on the Autodesk Sustainability Workshop Blog.

Step 1: Design It!

I designed the knife sharpener, first using pencil and paper, then in CAD using Autodesk's free 123D. This design is made so that it can be used horizontally or vertically. When it is used horizontally, the mug lays partially on its side, but the stand holds it 20 degrees from horizontal. This means that the knife stays vertical while you sharpen it. When it is used vertically, the mug sits up side down on its rim and the knife is angled at 20 degrees. Both work well so it's a matter of user preference. I used calipers to measure lots of mugs and found that most common mugs have a diameter of 3.2" plus or minus about 0.05". I sized my design to fit the larger of this range and I used a small amount of electrical tape to take up the slack for slightly smaller mugs.

Step 2: Print It Out!

I printed mine first on a Makerbot Replicator out of PLA. PLA is nice since it is food safe and biodegradable. The first few print attempts failed so I was able to crush up the half-printed versions and compost them in the municipal compost.

Step 3: Test It

After breaking off all the support material, I tried it out on one of my knives. It held onto the mug snugly, but had a few problems. The slots for the knife were too skinny so the knife couldn't slide back and forth smoothly. Also, in the horizontal position the base was a bit too small to keep everything stable. I decided to fix those parts in the design and also move to a higher resolution printer.

Step 4: Version 2

I modified the model with some updates and printed using the Objet Connex 500, a much higher resolution printer. The results were great! Try printing your own and let me know how it goes! Feel free to modify, improve and add on to it. It's also on Thingiverse!