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.
This week I heard about sharpening knives on mugs and your project is a very nice idea! Do you plan to optimize the use of 3d printing technology? i.e.: there's way more plastic then needed to guide the blade, perhaps some boolean substractions won't hurt :)
This would be a great product for Shapeways, then you can sell direct from them and get a profit too! I'd buy one, the design could use some work tho, I'd be happy to collab if you like :)
Thats a great idea! At first i thought you were cutting a piece of meat!
Very clever, you should go into business with this one. All you need to do is figure out how to make a universal fit for the mug... maybe a tapered neck? Good luck, you're on to something my friend.
I think the design is brilliant. Used properly one can sharpen knives safely. Although, I'm a little disturbed by the way you held the mug in the video. You should never hold it in such a way where the blade is near any part of your body such as your fingers or the back of your hand. In a moment of inattention accidents are liable to happen. You wouldn't want to test the sharpness of the knife on yourself, unless you are into that sort of thing.
anychance you could sell a few of these for people that don't have 3D printers?
I would also be interested ... there are lots of us that do not have a 3D Printer ... yet
I gotta get my RepRap running again so I can print one of these.
Please kickstarter that thing. It seems like a great product.
Very nice! <br> <br>I've known about the coffee cup trick for years, but could never hold the knife at a consistent angle. <br> <br>I'm also just starting to get into 3D printing, and am looking for practical pieces to get started with. Looks like I found a winner here! <br> <br>Good job.

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Bio: I'm passionate about combining engineering and sustainability through DIY technology. I'm a mechanical engineering student at Olin College and currently building things at ... More »
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