Fountain Pen Design

Function, Development, Construction and Fabrication

4.10 Definitions of Flex

1. All Nibs Flex…

… more precisely: their tines flex.  Precision and accuracy are of paramount importance for an ingeneer, in particular when he talks shop.  Both dictionaries, the Oxford as well as the Cambridge agree on the meaning of “flex” which amongst other meanings is that of flexing a material: “to be capable of warping or bending (under load) and then (after removing the load) reverting to an earlier shape.” What they really talk about is elastic deflection.

In view of preparing the ground for discussion of the behaviour of a nib, I like to add: “When writing with nibs (attached to a holder or fountain pen), even the gentlest force is applied to the pen.  In consequence, the vertical component of that force moves the tines apart, hence, the written line, the trace of ink on paper widens or narrows in response to this force.

After settling my mind on various assumptions on writing angle and possible radius of wrist-movement, this vertical force can be calculated to vary between 20g to 140g or 0.2N to 1.4N, for regular writing and between 300g to 500g or 3N to 5N to cause the tines of a standard nib to separate by about 0.2 – 0.4mm.  The latter force (5N) is uncomfortable to maintain, but it can peak this level when a writer (a teacher crossing a wrong answer) crosses the Ts and dots the Is of a passionate note, for example.

According to ergonomics tables, a sustainable force applied during controlled action maximises around 100g or 1N.  When this action requires sensitive adjustment (as in writing), the recommended load drops to around 50g or 0.5N.  Since we talk about forces and not weight, I use N, Newton from now on; that’s the plan. The following commentary does not spring from my ingeneers mind (and I shouldn’t say it), but my romantic self (50% of me): “Really, whether Newton or grams. it doesn’t matter that much in daily life, a bar of chocolate will always be a bar of chocolate, unless one deals with constructing rocket ships, cyclotrons or high-speed trains.”

2. All Nibs are Preloaded

This may sound like a sidetrack, but you will find it useful later on that you had been introduced to this idea at this early stage because it is often overlooked. Instead, I talk about it now, so I don’t need to interrupt the topic to which it is applied.

The preload is the force, which presses the tines against each other.  This force needs to be overcome before the tines can separate.  Why are nibs preloaded?  Good question!  (Why is always a good question.)  For non‑flex writers, it has the advantage that the line-width remains constant even though the writing pressure varies until it reaches beyond the point where the preload is exceeded. Then the tines start to spread.

Graph 1 — Preload

Compared with standard nibs, flex‑nibs have a much smaller preload and therefore respond more readily to variations of writing pressure. In graph 1 the preloads for stainless steel and gold are shown as the lines with arrowheads parallel to the Force axis.  For more information on how preload works, go to Stresses and Strains.  Just briefly, the gold nib, having a higher preload, lifts its writing force range of operation in the region of the steel nib.  This needs to be considered when a fountain pen is offered with a steel or gold nib and has been conceived for the same target group.

Photo 1 — Setting of Tines

How are nibs preloaded?   It happens during the manufacturing process.  After the slit has been cut it is parallel.  It has some capillarity, but when the tines converge, the capillarity improves significantly.

The tines move together when they are elongated along their outer edge, noted as the “free sides of the leaves (meaning tines)” in photo 1.  This can happen in form of hammering or through applying an embossed pattern on the top surface of the nib where the material is stretched, widened to move the tines inward but also moves the outer edge of the nib downwards, thus reducing the flexibility of the tines.

The embossment also hides any possible blemishes caused by hammering. The latter can also happen on the inner surface which also brings the tines together, flattens the nib radius and makes them more flexible. I have already talked about this in Nib Mechanics.

Photo 2 — Reflection on Tines

Better nibs, and fine, narrow nibs, in particular, are set (tines brought together), manually in my days.  In this process, the curvature of the tines is increased by bending the tines inwards.  The “setter”, one of the stars in the nib department, would grip the tines with a small pair of pliers near the tipped end, and then they would turn the tines inwards, one after the other.  This can be noticed through the change of reflection lines at A and B in photo 2.

This style of setting also causes the tips of the tines to touch each other in an upside-down V shape fashion (looked at frontally), a precondition for pressureless writing.  In case you need a refresher on this topic, have a read on Nib Manufacturing.

3. What’s the Difference?

Since all nibs, or more specifically, their tines are flexible, what is the difference between a regular nib (some call it a nail) and a flex‑nib?  Let’s first tick off what they have in common. All nibs have some proportionality between the applied writing-force and the amount of separation of the tines.

Photo 3 —  Ornament

And inevitably, they require different writing pressures; however:  How much? A flex‑nib can write lines as shown in photo 3.

Being an ingeneer, I am in need of a classification based on technical, measurable criteria originating from reproducible tests, which separate standard nibs from flex nibs.  Then I am happy again to join discussions.

As a start, I laid out the frame for table 1 where I want to set and define three regular writing scenarios (three columns to the left) which apply to both, standard nibs and flex‑nibs (the forces are different). The three scenarios in the columns to the right show the description of nibs under various, increasing writing pressure with the tine separation (spread) being the comparative value.   Presenting them in table 1, I aim to provide sufficient transparency.

During the analysis, I need to use ingeneering terminologies, otherwise, I induce fuzziness, the very thing I want to avoid.  If you struggle, have a browse through the chapter on Stresses and Strains.  Being conversant with the correct terms certainly facilitates our discussions.


Table 1
scenario Everyday writing Signature and drawings Straight line, with emphasis causing a spread of 0.2mm causing a spread of 0.4mm Far above limits of writing
defined by the applied force [g] 30-80 80-200 350-400 550 800 1000 and beyond
movement of tines tines don’t move apart tines may move apart tines may move apart tines move apart tines move apart tines move apart
after removal of the load tines return to the original position tines return to original position tines return to original position tines return to original position tines return to original position tines remain apart, residual deformation
amount of deformation elastic still elastic still elastic elastic + some plastic elastic + more plastic elastic + much more plastic
preload after load removal the same should be the same could be reduced reduced reduced no preload
capillarity while writing as designed as designed can be reduced reduced reduced disabled
ink flow as designed as designed as designed but may be reduced does not proceed to tip does not proceed to tip Far above limits of writing
comment within intended parameters within intended parameters within intended parameters used to increase softness (1) used to increase softness (1) possible break, beyond repair

(1) Experienced writers and nib artisans use this method to adjust the responsiveness, the sensitivity of the nib (the ratio of writing-force versus tine separation).  Since the preload has been reduced, the tines open now more readily.

Table 1 will apply when we define the Useful Line Width in chapter 6 and the Useful Writing Pressure in chapter 7 on page Fountain Pen Flex Nib Classification.  In the next chapter, I have collected some of the commentaries I found on the web.

4. Types of Flex Nibs

Where is the demarcation between flex and nail (a standard nib)?  I have heard verbal expressions of classifications, like: “Hard, Semi-Flex, several stages of Superflex and finally the Easy Full Flex.” However, what are the technical definitions for the various kinds of nibs?

Some people regard a nib to be a flex‑nib when it can spread its tines repeatedly by one millimetre.  Some others want it to separate by four millimetres. But, what about the writing force required to obtain these spreads?

Others suggest, no-flex and the degree of flex are defined by the ratio between the line width when applying little or no pressure and the line width at a comfortable or maximal (non-destructive) pressure.  For example: A nib, subjected to such (imprecise) loads, showing a variation from 0.2mm to 1.2mm would result in a ratio of 6, and hence, noted as 6X nib.  Some flex nibs achieve a ratio of 20X.  Under this method, could we state that a 2X is a standard nib, a 6X is a flex nib and a 20X a soft flex nib?

This suggestion appeals to me because it offers a measurable, repeatable method, except, it was lacking definitions for the standard writing-forces.  Therefore, I suggested this idea in the flex-nib forum. The replies did not offer any suggestions on standard writing-forces or any other technically useful ideas. Disappointingly.

So that we can move on, I would like to offer my philosophical decision: “A flex‑nib must be designed like one, to be one.”  The ingeneer must have had the intention to construct either a regular nib or a flex nib.  Let us respect the ingeneers and let them decide what they want to call it.  There are always some who disagree; it is perfectly reasonable.

Above all: Enjoy!


Amadeus W.

7 December 2017

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