.iStab

The iStab is a 3D printed iPhone cover which has a folding knife along one edge, producing a blade whenever you might need one. This phone case is "cutting edge", and is designed as an exploration in conceptual 3D printed weapons.
The models I based this idea from are free, and the programs needed edit the 3D files are also free: Netfabb and MeshMixer.

The iStab is a great way to keep a cutting implement on hand at all times. This can be used as a weapon for self-defense for the next time you're in a knife-fight like Michael Jackson in Beat It, or harmlessly used for everyday actions like opening packaging or cutting items to size. Forget highly-contentious 3D printed firearms and flimsy iPhone knuckle dusters, the iStab has you covered for all your cutting needs.

Here's what I used to make my iStab:

materials: 3/16" (5mm) rivets scrap of felt scrap plastic Dollar Store paring knife 3D models: folding knife + iPhone case  (both free)

tools: Meshmixer  (free) Netfabb  (free) rotary tool riveter access to 3D printer

Blades such as the one shown in this project are considered weapons by law enforcement. This project is provided for educational use only and should not be replicated.

Step 1: Find Models

I found a folding knife and iPhone case model for free on Thingaverse:

• folding knife: http://www.thingiverse.com/thing:4725
• iphone 4 case: http://www.thingiverse.com/thing:24734

If you've downloaded .STL files you'll need to convert them to .OBJ files before you can edit them. After you've found the models you want open each model separately in Netfabb and export as an OBJ file: Part > Export Part > As Wavefront OBJ. Next, we'll combine the models in MeshMixer.

Step 2: Combine and Print

Following the directions in this Instructable, I combined the folding knife side plate with the side of the iPhone case.

Once you have your .OBJ files, I followed ShadyLogic's steps on how to edit with MeshMixer and was able to quickly import 2 models. Import one model, then import another making sure to "append" the project to have both models in the same environment.

Using the alt+right-click/alt+left-click mouse button(s) I was able to navigate around the MeshMixer environment and line up my models. I placed the inside edge of the folding knife and lined it up with the inside of the iPhone case. Pay attention to where the knife siding penetrates the plane of the inside of the iPhone case.

Merge the two models by selecting one model then Shift+Click the other model (they should both turn white). Then go to Edits > Combine to merge the models into one.

Now we can export the merged model back to NetFabb so we can make it watertight before sending it to print. In Netfabb go to Extras > Repair then find Automatic Repair on the bottom right, then execute the repair. You now have a watertight, custom 3D model to send to the printers. Save the model as an .STL or .OBJ (depending on what the printing service requires).

After you've got your .STL or .OBJ file you'll need somewhere to print it. A great new product to hit the desktop 3D printer market is the Creality 3D printer, inexpensive and works great!

I included the .STL file you'll need to print your own iStab here ↓

Step 3: Clean Prints

When your 3D prints are finished there may be some additional cleaning required.

There was still support material attached my prints in the nooks and openings. With a damp cloth, a sharp pokey-tool, and a hobby knife, clean all surfaces, grooves, and openings of all your 3D prints.

Step 4: Make Blade and Lock

3D printed material is known for being brittle, and my prints were no exception. The blade and lock were very fragile at teh taped edge, and the blade profile was not to my liking. Wanting to beef up my knife, I decided to make the blade from a real steel kitchen knife, and the blade lock from a thicker piece of plastic. Though both of these modifications make this item less 3D printed than desired, I think they are functional elements that need to made from another type of material to ensure durability.

blade:
Sharped printed 3D plastic isn't very threatening, even after sharpening the blade was brittle and flimsy. I found an inexpensive paring knife at my local Dollar Store, the blade was steel and slightly larger than the profile of the 3D printed blade.

I didn't like the profile of the 3D knife blade. Instead of modifying it in Meshmixer I decided to freehand a portion of the blade profile after tracing the basic outline onto the steel blade. The steel knife handle was disassembled so just the blade and tang were left. Using a rotary tool, the traced knife blade was craved from the steel knife.

I gave the knife a sharp edge with the rotary tool, the final edge can be honed later. A small notch was also carved into the top edge of the knife in both sides, this will allow a fingernail to be used to pry the knife out of the sheath when it's folded closed.

lock:
The taper on the blade lock was tapers so fine that it wouldn't have held the blade at all. A plastic cutting board was used in place of the 3D printed piece. I traced the 3D printed blade lock onto the cutting board, then used a sharp hobby knife to cut the shape.

Step 5: Widen Openings, and Then Rivet

The folding knife handle was held together with 3/16" (5mm) rivets. The openings in both sides of the 3D printed folding knife handle and the steel blade were widened to match this diameter; the blade lock opening was also widened to accept another rivet of the same diameter. 

Rivets are a great way to attach pieces together without leaving a bulky fastener. However, rivets work on compression and can crush your work if your material is soft. 

crushed print - attempt 1 (above):

When using a softer material, it's a good idea to test your rivet on some scrap material to see the outcome. My first attempt at using the riveter was not successful, crushing one of the openings and causing the end to split. In another area the material was binding at an opening and caused warping along the length of the handle, making closing the blade impossible.

proper rivet - attempt 2 (below):
Small washers and a dab of superglue were used in between elements of the print/lock/blade sandwich, then a rivet was pressed onto both ends of the handle to secure both sides of the folding knife. With finesse, you can get the riveter to pop the rivets with a tight bond and without crushing your 3D prints. The type and composition of your prints will play a factor in this (a more rubberized ratio of printing medium will have a greater compression allowance).

If the rivets don't pop and snap off it's better to back off the rivet once it's tight enough and just trim it down with a rotary tool. More on this in the next step.

Step 6: Refine the Action

Before installing your iStab onto your phone you'll want to make any minor tweaks to the blade action. After riveting, my blade was tight, so I lubed it with some oil and stressed the joints some to make the blade action smoother.

Make any final adjustments to the blade and action now. Rivets can be lightly hammered to increase tension, minding not to shatter the brittle casing. Or, a sharp knife can be pried inside one of the rivet joints to loosen the action if it's too tight.

Step 7: Padding Protection

Sometimes the rivets don't pop when they've reached the desired tension. The tails that don't pop can be trimmed with a rotary tool with a grinding disc attachment. The shoulder of the rivets also sit slightly proud and were also trimmed down with a rotary tool to reduce their profile.

After the interior has been smoothed down, scraps of felt were cut into squares and placed over any sharp/metal pieces that might come in contact with the phone body. This will reduce any damage to the phone from the sharp metal when inserted.
Two pads were located on top of the interior head of the rivet and hot glued in place.

Step 8: Add Phone, Commence Stabbing

With the padding in place, the iStab was easily snapped over the iPhone. The felt bumpers added a nice cushion and held the case securely to the phone; more snugly than than other commercially available models, by my observation.

After refining the action, the blade deployed perfectly and can easily be flicked out or folded away. The iStab is perfect for the next time you need a blade for that project you just looked up on your mobile phone, or protecting yourself from an electronic device thief while you're on the phone.

Be safe, and be awesome!