V2 - Credit Card Utility Knife

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Description

This utility knife handle fits in the form factor of a credit card, transforms from flat card to functional handle & blade, and locks open and closed.

I was inspired by the classic CardSharp, but wanted to make a version that uses utility knife blades and is easily printable. I designed and posted a V1 Card Utility Knife a few weeks ago and got some feedback from the community. Thanks to everyone who printed and/or took the time to comment on V1! Check out V2 and tell me what you think!

V2 Changelog

I have taken the feedback into consideration and designed a V2 with some notable improvements! The main changes are:

• A locking mechanism to keep the blade in place whether it is extended or retracted. 
• A blade guard at the upper tip of the blade when it is retracted.
• An extended overhang that covers the cutting edge better when the blade is retracted.
• Dove-tailed pegs as a second means of constraining the slider to its tracks.
• Minor aesthetic changes to the folding wings
• Changes to thumb grip on slider

A note about the lock

The lock that I designed for this project had to be shoved into an incredibly small amount of space in the thickness of a credit card. Therefore, it keeps the blade in place, but it has the strength you would expect out of something that is 0.6mm of plastic. That is, it should be subjected to heavy loads with the expectation that it will not break. 

 

The lock should keep the blade from being shaken loose and opening or closing accidentally during handling, but SHOULD NOT BE RELIED UPON TO KEEP THE BLADE IN PLACE DURING CUTTING.

What knife is:

• A credit-card-sized folding handle for a utility knife blade
• A safe and easy way to carry a cutting blade in your wallet and have a handle when you need it
• As minimal as possible to allow it to fit in the form factor of a credit card
• For everyday cutting tasks that don't require excessive force

What it isn't:

• Ultra heavy duty
• A prying knife

Blade Specifications:

This handle is designed to carry utility knife blades with the following features:

• Shape: trapezoid with notches for locking
• Length: ~2.4 inches (≈61 mm)
• Height: ~0.75 inches (≈19 mm)
• Thickness: typically ~0.024 in (≈0.6 mm)

Filament:

I printed these in both basic PLA and PETG. PETG definitely holds up better to repeated bending, but PLA also works fine, especially with the print profile that is included (the holes and top/bottom surfaces make a perforated bend).

Assembly:

ASSEMBLE WITH CARE!! Utility knife blades are extremely sharp and should be handled with extreme caution

1. Install the blade into the slider.
   • It should be a secure, but not too tight fit (the blade should stay in position by itself).
2. Install the slider into the handle
   • Place the right edge of the slider into the right side of the track
   • Make sure to align the dove-tail pegs to their installation holes in the slot
   • Slightly bend the card away from you while pressing the left side of the slider into the left track
   • Alternatively: You can slide the slider/blade assembly in from the top. If you opt to do this, just allow the dove-tail pegs to ride along the top of their slot until they can pop through the installation holes.
3. Install the blade guard to the top of the handle
   • Align the pegs on the guard with the holes in the handle and lightly press them in until they stay on their own (not all the way in)
   • Press the pegs down so they are flush with the top of the handle. They should slightly poke out the back of the handle.
   • If the pegs wont go through the holes:
     • I have noticed that sometimes the print will randomly cause one or both of the holes for the blade guard to be under-sized, making it very difficult to install the blade guard.
     • If this happens, you will need to just force the peg through as follows:
       1. Flip the knife over so that the back is facing up and the front of the guard is resting on a sturdy flat surface.
       2. Press down hard on the back of the card near the holes for the guard pegs. This should allow you to get a nice press fit of the pegs into the handle.
       3. If it is hard to press pegs all the way in by hand, try using a tool like a flat-head screw driver to concentrate the pressure closer to the peg/hole.
4. Check for correct installation
   • Slide the blade back and forth. It should move easily, but with some friction.
   • The dove-tail pegs should protrude through the back of the handle, securing the slider in place
   • The slider should get stuck at the top and bottom by the locking mechanism.
   • The lock pegs should slide freely in their slot, similar to the dove-tail pegs.

NOTE: Any use or assembly of this tool with a utility blade is dangerous and should be undertaken at your own risk and using proper protective equipment such as anti-cut gloves, eye protection, and other relevant equipment.