Here is a video of how the blade functions:

My print settings for this project (Cura):

- 0.3mm layer height

- 0.4mm nozzle size (important for supports)

- Orient pieces as they are in the picture

- 50% fill (you can do more, but I wouldn't do less)

- Support everywhere grid style

-nozzle size *2 = shell thickness

-layer height *2 = bottom/top thickness

I plated all the pieces at once and printed them at once (Update: plate the lever arms and blade on their sides, instead of how it is shown in the Cura picture, to increase strength). It took around 3 hours.

For finishing the parts remove all of the support and all of the 0.8mm supports included in the model (just make it look like the finished picture). Use a file to make the pieces fit into the body. My jerk is a little high so I have to file at the corners, but it also helps with making good fits on the fly. Make sure parts can slide smoothly and that their axle housing are not interfering with each other. File the 1/4" holes until a pin can fit in. Sand down any corners you want.

Hardware:

All of the hardware can be bought from Mcmaster-Carr (listed in bill of materials). The extension spring must fulfill a few criteria or else it will not function. It must have an outer diameter of no more than 0.185 ", it must have a minimum length of no more than 1.5 " , but lower is more ideal, and it must have a maximum length of 2.8" or more. I found that this spring worked very well: http://www.mcmaster.com/#9044k19/=yicj7n

Cut the pins and bolts to size.

Construction:

Build in this order: Insert blade and lever into body, secure with pins and collars, secure spring with bolts and nuts while blade is in open position, secure razor blade. 

The only tricky part here is to squeeze the collars slightly while installing the blade to get a snug, but loose fit.

In the end the blade should not wobble around while it is open and when the lever is pressed the blade should glide out easily (the lever should not take much force to operate). If it is hard to open find out where the parts are interfering and file some more.

Now you are done:

Congratulations! Now go cut open some boxes or something. And post some pictures so I can see it!

If you don't want to/can't make one you can buy one from me! https://www.etsy.com/listing/244290158/spring-assisted-knife (do it)