DIY Portable Knife Sharpener With Customizable Angle

Having a tool in good condition is always ideal. A well-sharpened knife not only allows you to work well but also prevents accidents (less force required to cut means more control of the knife) and having tools for this is essential. Accessories for this purpose have many types and angles but they maintain a fixed angle or take up a lot of space, it can be annoying to look for one for a specific knife (it has happened to me, with my leatherman knife) so you can imagine having many accessories for each type of angle needed? why not compact all this in a single tool?

I have designed a simple tool that achieves this end, applying a bit of recycling and design theory it is possible to fulfill the purpose of making the process of sharpening a knife easier in a compact way and that anyone can make their own

Welcome to my instructable :D

Supplies

The following materials will be used for this prototipe:

A very Hard material (Hardened Steel, HSS, Ceramic...)
Mototool with SiliconCarbide/Diamond cutting disk
3D Printer
M4 x 12~14 Phillips Bolt
M4 Nut
Sharp Blade cutter
"Super Glue" Adhesive
a bit of inventiveness :D

Step 1: Material Selection

This selection of material is very important. it is required that the material of the sharpener be harder and more resistant than the material of the knife to be sharpened, for which the type of materials to be used is restricted, an HSS tool steel is a good start, the blades of a scalpel can serve well, but among all those engineering ceramics are the best, but... Where I find that?

I used an old cheap Zirconia Dioxide (ZiO2) knife, "extreme" but brittle sharp, It only lasted 2 months... I discard the plastic handle an only and use an small diamond cutting disk to make a simple groove in both surfaces and break same as a glass cutting

I used two handle pieces and cut in 10mm x 12mm keeping rectangular form (I leave the other cuts as a spare parts), an harder material sandpaper (SiC, black sandpaper) could be used to flatten some surfaces of ceramic plates

Step 2: First Design Steps - Sketch

For the process design I use Autodesk Fusion 360 in a Educational Project. Some people don't know how use this program, but I recommend first take a look at minicourse "Introduction to Modeling and Design for Manufacturing (Link for this Course)"

I start with some initial conditions:

I want to sharpen a fillet knife with 1.5mm thickness in a 15° angle, but I also want to sharpen a hand axe/tomahawk, ~4mm thickness in a 45° angle
Sharpener must be portable and safe to use
Sharpener must have a minimun number of pieces
I use the previous 10x12 ceramic chips

I take the ceramic chip as a base part for my design, In a XY plane using Fusion 360, I draw a sketch and I draw 2 inverted triangles (blue, first image) with 5mm base, 50° and 3mm base,15° with a common center in Y axis (normal knife sharping angles), I draw the 10x12mm rectangle in contact with right side of both triangles and I take the left side of both rectangles to make a reference to obtain the location of center of rotation (red lines, first Image), This metod still requires an extra measure to restrict all measures so I define 6mm of distance to border (Yellow Line, first image). I Try to make a 4-link mechanism to minimize the chip movement but but this implies a greater number of pieces (Second Image)

You can design your own measures! only you can modify the Fusion 360 files to a custom ceramic plate dimention

With the first sketch I define the location of main pivots of mobile parts and extra dimentions, I place a bottom space where the adjusting screw will be located, main pivots are are located 28 mm from the base and 16 mm from the center line, is displayed as drawn in the "Calculations" file (Third Image)

I attached the Fusion360 files so you can interact with the measurements :D

Step 3: 3D Design - Mobile Part

To make the mobile part I take some measures from skecth in step 2 to locate the main pivot (first image), I choose some features to facilitate the sharpener operation, first I have chosen 1 mm of space between the left and lower edges of the piece because they will be in contact with metal parts (knife to sharpen in left side and adjust bolt in lower side, third image), I try to select a "Base angle", so I decided 30° because most of utility knives use this angle, I cut the superior side at an incline of 15° (15° per side, 30° total) to locate this angle faster in reference to fixed part (4th image).

I use a 5mm hole in main pivot to stiffen movement

Mobile Part v8.f3d
Download View in 3D

Step 4: 3D Design - Fixed Part

I take the measurements again from the step 2 and make a main sketch (1st image) I put an extra circular border centered in pivot locations to reinforce the part.

I make a decision about this piece, it does not require screws for its assembly and its design will be totally symmetrical, so I only have to design a single piece and manufacture it twice, same as a mobile part. I extrude a cilinder concentric to pivot location to make a plastic axle, besides being a piece of union

To maintain the location of moving parts I extrude 3 concentric circular sections, because if I choose an unique section, the 3D printing filling will be less rigid than a multiple sections. I make an "extra extrusion" to limit the mobile part up to 15° (min. angle), I extrude 2 extra cilinders lower to main pivots to keep the assembly fixed and I extrude a special middle section to contain the M4 Nut (3rd image).

Finally, I chop the right superior border to indicate some useful angles (15°, 20°, 25° and 30°) in reference to mobile part border (4th image)

Fixed Part v30.f3d
Download View in 3D

Step 5: 3D Design - Assemble

Using the modeled parts I make a general assembly, in the first and second image you can see the operation of angle change, for the minimal position (first image) I can screw the bolt and push both ceramic plates up until they reach the limit (2nd image), When the knife passes through the sharpener, it rests on the screw and on the pivots, maintaining the desired angle.

The result is only using 5 parts, three of them repeated twice. the final size is 43 x 33 x 11 mm

I Attached the Assembly file in Fusion360 and the remain parts

Step 6: 3D Print and Assembly

I print this 2 desings twice in colaboration with a friend, in white PLA plastic (cheaper). Final STL files are attached in this section :D

I use "super glue" to fix the ceramic plate to mobile part, you could use some epoxy adhesive to fix these parts but take more time. An extra suggestion is use a round file to rough up some imperfections in all holes

I glued the M4 nut to one of fixed parts too, to facilitate the assembly proccess

Step 7: Final Assembly and Small Testing

Finally, I fix all parts with some pressure in their fixing axes, As you can see, the final sharpener is small but allows a good manipulation, the linear base could be used to rest in a table to improve the sharpening action.

Step 8: Some Final Considerations

Sharping edges of a knife are dangerous, be careful with your hands!

This final prototipe opens in a 15° ~ 50° apply for vaious angle charts finded. this sharpener could be used in a single bevel knife (using the flat face of knife as a reference) or a doble side bevel knife. A quick skarp in a knife is put a bit higher angle and pass the shatpener 3 or 4 times, to make a minimal "composite edge"

Some interesting theory for knife sharpening could be usefull to read, and you can understand a little more about the importance of the cutting angle in a knife.

I added too an a JustMachete.com resume guide to angle selection (in my consideration)