Why progressive/combined springs?
On sporty car suspensions, these springs do the following:
When the car goes over a slight bump in the road, the wheel goes up easily since the spring is light at first, leaving a bit of comfort for the occupants; when the car starts to lean in a corner, the light part of a the spring collapses completely pretty soon, thus becoming solid (i.e. no springyness at all) and the strong part takes over, generating a lot of force to prevent unwanted body roll.
If you think about it more abstractly, that is pretty much what you want in a keyboard as well: Using progressive or combined springs (both use pretty much the same principle, difference being that they require either one or two parts) one could produce a keyboard that is a) light to activate b) rather hard to bottom out, and c) has a good amount of force bringing the key back up, right to the activation point - so it would be good for double-tapping as well.
There are other ways to do this, like using rubber membranes at the end etc, sure, but I'm pretty certain the actual reason why this isn't done by switch manufacturers is because it's more expensive and more complex, meaning more parts to go wrong in each switch. But such petty considerations have never bothered any real Geekhacker, so let's get to it then...
DIYing a combined spring
What you'd need to DIY this: Two springs of differing spring rates, one of them light (think Cherry blue/brown), the other medium strength (think Cherry black); some smallish wire cutting pliers/side cutters, and something that connects the two springs so that they are held together on top of each other and can be inserted into where the springs usually go. I'd suggest soldering or two-component adhesives, unless you happen to have some plastic parts ready that could do the job.
Now, for the perfect spring combination as described above, you'd have to experiment with different lengths of those springs, but before, we should have a look at the three primary points of consideration (less positively inclined people might call them "problems"):
- One phenomenon to keep in mind is that cutting coil springs means to take away some of their "springyness" (let's put it into more technical terms: Some of its soul gets eaten) as obviously, the spring uses these coils to give way to the forces applied. Cut springs take more force to compress.
- Also, just about every switch imaginable uses an amount of pre-tension - when you put together the switch housing, you compress the springs a bit, and therefore the springs won't make too much noise when the key is depressed, as the spring is always held in compression even when you fully let go of the key, and it is therefore not hitting the spring housing when you do that. Thus you do want to keep some pre-tension, unless your goal is to have a keyboard louder than anybody elses.
- And third, your typical coils spring consists of three parts: the springy part in the middle, and a few coils on each end that are sitting on top of each other. These parts are there to provide a good seat for the coil itself and also to generate preload, but they are completely inactive in terms of springyness. This provides a good wayto compensate for the first phenomenon - as we inevitable will have to cut away some of the spring to keep a similar level of pre-tension, we should be cutting away mainly those inactive parts of the spring.
These points must be taken into consideration when stepping into action as your own spring designer. I'd say you should start with the light spring. Cut off as much of the end part as you can, while still having a flat section left to seat it. this is probably not very important, but it could prevent some problems over extended periods of use, such as the sharp edge of the spring working its way into the switch or the keystem. Do that on both sides of the spring. Then replace the length you cut off with a corresponding length out of the middle of "medium" flavored spring, by glueing the two springs together. Try to keep the resulting spring the same length as the one it replaces at first, but experiment with that - you should be able to vary the overall force and also the take-over-point quite a lot by changing the length of the medium strength spring. If you do start to experiment with the length of the weaker spring, just keep in mind that in general the weaker spring collapses completely way before the stronger spring even starts to noticeably compress, so don't make it too short - otherwise you'll just have the benefit of the softer touch for a tiny bit of key-travel.
Happy spring-hacking!
Chris
