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Keyswitch force curve guide 2020

This comprehensive guide teaches you what a force curve is and how to interpret it. We cover measures like gf and cN, see the effect of different springs, tactility and clicks. The ultimate goal of this article is to teach you how to use a force graph to consciously choose your next keyboard switch.

Let's dive right in!

Summary

On the x axis you have travel distance (how deep is the key pressed) expressed in mm. The axis starts at zero and usually stops at total travel distance (at about 3.5 mm) where you are bottoming out. On the y axis the required force to press the key is shown expressed usually in gf (gram-force) or cN (centi-Newton). These units can be considered equivalent in practice (more on this later).

On the grid you can identify points of any force-travel combination. Notable points are tactile, operation, release position, etc.

[[image 1] Example with single point marked.

Quick background

There are a ton of different keyswitch models out there which is a result of the expiring of Cherry MX patents and the renessaince of mechanical keyboards.

Additionally, the advent of hotswap sockets and hotswappable keyboards on the market made it possible to change keyswitches way easier and boosted demand for even more switches.

Understanding the force graph may come in handy to be able to choose from this waste amount of options, to understand the specifications and roughly evaluate a keyboard switch before placing an order.

What's a force curve supposed to do?

A force curve describes the relation of the key travel (x axis) and the required force (y axis) to achieve that movement.

In other words, it shows you how much force you have to exert to depress the key by a certain amount. Beyond some special cases in keyboard switches like pressure point, tactile position, operating point, reset/relese point etc., a force curve contains the information about the force-distance measurement of every travel distance.

Force curve example

Look at the factory force curve graph of a Kailh Box speed gold:

On the x axis you see the travel in mm. Total travel displayed here is 3.5. There's no reason to show higher values because at the total travel (when you are bottoming out) the force skyrockets. It's almost like a brick wall: you could pass this total travel only by breaking your switch.

On the y axis the required force is shown. The measure here is gf (gram-force) or cN (centi-Newton), sometimes simply marked as g or "load".

They are similar and can be considered equal in practice, but let's see how these different units and terms compare anyway.

Measurement units of force curves

You may know that the unit of force is Newton [N]. Since we try to express some kind of force, N is our natural starting point. Because with keyswitches we are in the range of 0-1 Newton, a lower magnitude is more appropriate. With expressing the same in cN (centi-Newton) we can spare the decimal separator. (1 cN of course is equal to 1/100th of a N.)

How can we model this applied force? Eg. by putting small weights on a key. That's how gramms [g] come in the picture. Most of us rarely use force and N in real life and we are more familiar with mass and kg so the possibility to use g instead of N is quite cool. However, gramm is the unit of mass, not force. To turn it into force, we multiply the gramm value by gravity.

The standard value of gravity is 9.80668 m/s2.

Let's say you have a keyswitch with 50gf operation force. Placing a 50g (0.05kg) weight on your key exerts a force of 0.05 x 9.8=0.49N=49cN.

As you can see, there's a 2% difference (50g vs 49cN), but this is usually ignored and gf and cN values can be considered equivalent.

Basic terms: What are some notable values on a force curve?

Manufacturers and vendors publish some numbers regarding their keyswitch products, the most common being:

Total travel. The distance where your keycap hits the housing of the switch. In other words the distance to bottom out. Measured in millimeters. Most MX style switches nowadays have a total travel distance of 3.5-3.6 mm. Kailh Chocs' total travel is 3.0-3.2 mm, while Kailh "X" (notebook?/scissors) switches' have a total travel of 1.8-2.7 mm.

Operating/Activation/Actuation point. The travel distance at which the keypress is registered (circuit of the switch is closed). Kailh speed switches have an actuation of 1.1 mm while 2-2.2 mm is a more general range for other switches.

Operating/Activation/Actuation force. The force required to register a keypress. 40cN is usually considered a light switch whereas 80cN is the heavy end of the spectrum.

Tactile point. Tactile and clicky switches have a bump when pressed. Whith these types of switches overcoming this tactile bump usually requires more force than measured at the actual activation position. Linear switches have no tactile position.

Reset point. As you release the key the circuit is still closed and the keypress is still registered for some time. The reset point is the position where the switch is deactivated and is ready for the next actuation.

Now that we've covered the basic terminology, we are ready to evaluate some force curves.

The problem of simple discrete values

As you will see, these discrete values can be misleading on their own. To get a more clear picture about the behaviour of a switch (before trying it) a force curve is a handy tool.

Force curve of linear switches

Since linear switches don't have a tactile bump, all we see is an increasing force as we compress the spring in the switch.

Force curve of tactile switches

The tactile bump is clearly seen.

Force curve of clicky switches

Again, the tactile bump defines the characteristic shape of this force curve.

What to watch out for when evaluating a switch?

Variability and consistence of switches of the same model

How to use a force curve?

Force curve example

Springs

How do you draw a force curve?

Well, most force curves you see in patents are simple estimations and artworks I guess. But it shouldn't be.

To draw a force curve you have to measure mass/force and travel distance at the same time. Mass is easily measured with a small scale. And a fine gradual increase of distance can be achieved by eg. a 3D printer, an arbor press or anything similar.

HaaTa has a great description of his force curve gauge.

- You can do this with a 3D printer but also with a simple digital scale at home.

Sources