Making a Switchblade Fun & Interesting

I’ve been working on this for roughly the last year, off and on (mostly off), just in the design stage. It took a while to wrap my head around how it works, and to come up with a fairly simple way of doing it that only uses flat parts. Flat parts because I would eventually like to make this into a real knife made from good quality steel, and it’s much easier to machine by hand parts that are of a uniform thickness.

A few days ago I decided to take the next step and actually build it from wood, just to see how well it worked.

I resized the parts up to twice as big and printed it with the dimensions. I figured I’d have a better chance of success by making the prototype bigger, since it would be easier to make the parts from wood:

the plan

I made most of the parts from 1/4″ plywood, carefully cutting them out. These are the main components for the mechanism – the slide that pulls the “U” shaped parts that will launch the blade forward or back:

the slide

partly assembled

I say “launch” because that’s basically how the mechanism works: the “U” shaped parts only move a short distance under spring power, and that is supposed to provide enough force to propel the blade all the way forward or all the way back. This relies on the mass of the blade and momentum.

To lock the blade in the forward (open) position, or the back (closed) position, there are wooden springs that engage with the end of the blade when it’s open, and a notch in the blade when it’s closed:

the gap under the wooden spring
the wooden springs in place

These springs are pushed aside by the slide, which has ramped sections. As the slide moves back, it pushes the spring out of the notch in the blade, releasing the blade. As it slides forward, it pushes the spring aside and the blade is free to come back.

It’s a fairly complex concept and all of the parts need to work precisely for it to function properly, so a prototype is a very good way to actually see the potential problems.
The mechanism needs a good bit of spring tension to propel the blade back and forth, and that was a problem. When I added what I thought was enough, it caused the “U” shaped parts to press down on the slide, which in turn pressed down on the blade. The result was a lot of friction that kept the blade from moving freely. I have since come up with a way to overcome this problem, but there are still a lot of other things to work out. The overall size of the knife – it’s too bulky as it was designed. To make it from steel, I could vastly reduce the size of the parts, but it would still be fairly large, mainly because of the size of the spring needed for it to work.

the mechanism
the knife closed

I’ve been thinking that it could be made smaller if I used two compression springs, rather than one bigger tension spring, and that would be my next step in the design. In the prototype, the blade is made from wood and doesn’t seem to have enough mass to overcome even a small amount of friction, so the real blade would have to be heavier. Possibly drilling out the base of the blade and fill it in with a heavier metal, like lead, would increase the mass enough to carry the blade forward.

the knife with the blade open

If you would like more detail or to show something that you’ve made, I started a forum topic on the switchblade knife design. I’ll be posting details on my progress there.

I made a video talking about the problems with the knife and showing how it works: