Fountain Pen Design

Function, Development, Construction and Fabrication

051-7-6 Flex Nib Modifications – 17-19

17. Real-Life Testing

17. A – Increasing the Breather Hole

As a way of applying what we have learned so far, let us look at some of the experiments and results posted at the forum Experiments With Flex on the Fountain Pen Network website.

Photo 8Oval Breather Hole

In one test Bobje widened the breather hole to a 3.5mm oval of an FPR flex steel nib extra fine, no. 6, as shown in Photo 8.  He stated: “After adding the hole much less pressure is required to make the tines separate easily. During writing, the line width varies from about 0.6 mm to 1.2 mm.  Another nib with the same modification gave a line width variation from about 0.4 mm to 1.1mm — about 3X.”

I drew two-dimensional combinations to the photo.  The l1b1 scheme refers to the oval hole.  The original shape (without a hole I assume) is marked as l2b2.  One would expect that after adding the oval hole, the nib would flex more easily.  Calculating an estimation of the tine separation with our well-known formula provided very similar results.

Drawing 11 — Increasing of Breather Hole

In Drawing 11 the original profile is coloured black.  The section taken away by the hole is blue, and the segment lengths are in pink.  Adding the hole shortens the original segment length from b2 to b1.  As a consequence, the initial bending axis distance y2 shortens to y1.

From Drawing 7 in the previous chapter, we discovered the significant impact the cambering of the flat strip had on the resistance to flexing, the profile turned out to be much stiffer.  The opposite happened here: the reduction of the distance y1 and the shortening of the profile segment b1 cause the tines being more flexible.  Assuming that b1 is half of b2 the tines would open four-times wider.

Photo 9 — Intricate Hole – Joseph Gillott#1

Whether the hole merely is oval or as intricate as in Photo 9, for the effective breadth it does not matter.  The pink dotted line shows the location of the very short breadth.  I believe the sanding marks are the residue from removing the burs from stamping the shape.  The marks would influence the surface reaction between ink and metal; however, the marks perpendicular to the slit do not promote the ink flow but rather hold it in place.

Used as a dip-nib an ornamented shape would help to hold more ink to prolong writing between dipping.  In this particular design, Photo 9, unfortunately, the ink would remain in the upper ornament because its shape widens before entering into the slit which interrupts the capillary draft.

17. B – Wing Scallops

Next, let’s look at another modification which is referred to as wing scallopsBobje started this topic, and Bordeaux146 was another major contributor to technical aspects.  The nibs in question are: “FPR no.6 flex” nib as shown in Photo 10 and a standard “FPR no.6 extra-fine” nib in Photo 11.

Photo 10 FPR no.6 flex nib

My comments on the flex nib Photo 10: “The reason for the softening of the tines is the same as above, which was the shortening of the profile segment b (pink dotted line).  The difference is that here the scallops reduce the width of the profile rather than the addition of the oval hole or the increase of an existing hole.  The effective length l is determined by the location of the scallop, the extension of the slit beyond that point is unnecessary.”

Further down I wrote a chapter titled Hole versus Scallops where I look into the difference between the two modifications in more detail.

Photo 11FPR no.6 extra fine nib

Photo 11 shows the alteration to the standard FPR no.6 extra-fine nib.  “In this case, the modification causes the two breadths b1 and b2 to compete with each other, b1 being the original and the combination b1l1 determines the flexibility.  I tend to say that the b2l2 combination hardly contributes toward the flexibility if at all.”

Here is Bobje’s comment: “However, in a standard FPR no.6 extra-fine nib (Photo 11), it improves performance and creates a cushioned writing experience, but doesn’t actually generate a semi-flex nib. There might be a tiny bit more line variation, and the pen is more enjoyable to write with. But it’s nothing like a modified flex nib.”

17. C – Long Slit and Breather Hole

Photo 12 shows another test undertaken by Bobje.  The test object was an FPR no.6 flex nib to which a hole of 5/64″ – 2mm diameter was added at the end of the slit. Bobje’s comment: “The lines varied 3X, from approximately 0.5 mm to 1.6 mm, and the nib rarely railroads.”

Photo 12Long Slit and Breather Hole

He also reported that initially the nibs flooded, which I assume was caused by the combination with a feed whose the air canal ran along the top (underneath the nib).  The problem subsided after matching the nib with a feed where the air canal is located on the underside of the feed.

The location of the effective breadth is at b1 (7mm), which would mean that adding the hole would not change anything. Even if the effective width would be a b2 (15mm) adding a hole of 2mm would reduce b2 from originally 16mm by only 1mm.  We know that a change in width impacts only linearly on tine spreading.  In this case, it would increase by just 6%, a hardly noticeable amount.

Photo 13Flex Nib with Breather Hole

Hypothetically moving b1 to b2 would lengthen the effective length from 14mm by Δl = 3mm but with no effect because b2 is longer than b1. Adding the hole would have no noticeable impact.

Furthermore, (this is about Photo 13) one could consider b3 as the effective breadth; however, the radius of curvature at this location is much smaller, and therefore, the profile is much stiffer than at the b1 position.  This is another reason for suggesting b1 as the effective breadth.

17. D – Radius of Nib Curvature

And since the topic is curvature, let me comment on a stack of questions by Bobje, starting with: “Where should a hole be drilled, how large, and in what shape?”

The most efficient position of the hole is near the end of the slit for reasons discussed in this chapter.  This gives rise to the question: “Now, how long should the slit be?”  Sensibly, it can reach up to the point where the nib protrudes from the section.  However, such a long slit would have no impact on the flexibility of the tines because of the stiffness in that area.  (Refer to Drawing 12, now.)

Drawing 12Curvature and Stiffness

When the slit is combined with a profile of high curvature, like when the nib hugs the feed (see c), then the flexibility is reduced by more than the slitting would cause.  The reason for this is the increase of y1 to y2.  A useful length of the slit is determined by the curvature, or in another way: A shallow curvature is necessary to benefit from the advantage of a long slit.

Drawing 11 Increasing of Breather Hole

Considering the size of the hole and I repeat Drawing 11 here from further up, it demonstrates that enlarging the hole diameter increases flexibility.  The necessary stability of the nib determines the limit of the shortening of length b1 which reduces as b1 shortens.

Photo 9 Intricate Hole

Finally, the shape of the opening can be whatever you like, except, it should not include any sharp corners which would cause high-stress points and subsequent fatigue cracking.  Also, consider the capillary forces; otherwise, the ink does not empty from the opening.  This is important when the opening has the function of a breather hole. I have repeated Photo 9 as a reminder.

17. E – Thinning Nib

“How might a layman thin the material of a nib reliably?”

This is a difficult enterprise, and I would not try it after the forming of the nib.  The precision required to control flexibility predictively is below a tenth of a millimetre.  Remember, the thickness of the material impacts on the area moment of inertia by the power of three.  During nib manufacturing, the accurate thickness is achieved mostly through roll-forming with the added benefit of work hardening.

17. F – Shape and Position of Scallops

“What is the ideal shape and position of the wing scallops?”

Photo 14 Intricate Scallops

The form of the scallops is an artistic, visual matter.  The sample in Photo 14 shows an intricate design; however, it is merely the distance along the dotted line which causes the effective breadth.  The rest is there to impress.

Photo 15 shows the scallops at the very end.  They are too far back to have any function.  My point is, the scallops as well as the previously mentioned openings,

Photo 15Scallop near Grip Section

only work within the vicinity of the slit and in combination with a shallow curvature.

Scallops can have any shape; the only criterion is the reduction of breadth they create.  For their placement to be effective, it must be in an area of shallow curvature and as close as possible near the end of the slit.

Photo 16Long Open Scallops

Since I like simplicity, the nib shown in Photo 16 is my preferred style.  It is a modification of an Ahab flex nib performed Bordeaux146, Fountain Pen Network. This kind of transition would result in a smooth opening of the tines.

17. G – Mating Nibs and Feeds

“What are more reliable methods of mating nib and feed?”

Since I know the painstaking effort invested in designing a writing aggregate and seeing to it being finely tuned, I would not recommend mixing nibs and feeds.  However, I know it is done.  Here are some recommendations:

1.)  Adding a hole to a nib, which sits on a feed with the air canal on its upper side causes such a nib‑feed combination to leak.  The control of airflow into the ink reservoir is the presence or lack of ink in the top end of the slit or a hole if the pair was designed for it.  This membrane of ink is ruptured by the lack of ink caused by the increasing vacuum in the reservoir and allowing air to enter.

Drilling a hole or enlarging one at this position either makes the membrane less stable and more reluctant to form or not to establish at all.  This depends on the size of the opening and the width of the air canal.  A less stable membrane allows more ink to be released per gulp, which may not be absorbed by the feed.  In the worst case when the membrane does not form at all the ink just runs out.

I would only consider adding a hole if the feed has its air canal at the lower side, opposite the nib. It is also against my design principles to combine more than one function with one feature.  Why?  That’s another topic; I will write about it, one day.

Feeds with their air canal at the bottom do not rely on a “breather hole” since their breathing control facility lays further back near the opening of the ink reservoir.  Its shape is intricate, and one finds such designs in feeds manufactured in an injection moulding process.  Since Ebonite feeds are machined, I wonder if they ever come with the air canal at the bottom.

Photo 17Flex Nib in Action

2.)  One can often observe that flex nibs lift off the feed when writing a broader line.  It is at this moment when more ink is required.  This sounds like a contradictory request because, as Photo 17 shows, there is no ink in the slit.

Drawing 13Ink Crossover

Unless the feed has a cross-over design (Drawing 13) and the nib enough capillary force in its slit, the ink flow will be interrupted.  This is demonstrated in Photo 17.  So, how can it work?

Indubitably, the Noodler nib is capable of performing.  How?  Here we go: “When the nib is at rest (no force applied), the nib sits snug on the feed.  The ink cross‑over to the slit is uninterrupted.  As the tines widen the ink meniscus between the feed and the underside of the nib draws back, upwards.  Since the air canal is on top of the feed, it opens more and more as the nib lifts off the feed.

Therefore, more air can get into the reservoir, in consequence, more ink is supplied to the nib‑feed interface.  It is the amount, the weight of ink which pushes the ink into the widening slit it against the reduction of capillarity.  Therefore, the nib can write a succulent line.  At the end of the line, writing pressure reduces, the nib starts hugging the feed, and the air canal closes, and the flow of ink seizes.  All good, aren’t we lucky?  Otherwise, we would need to tell Mr Noodler that the nibs cannot work.”

I remember as a student reading a paper on the mechanical design of insect wings, and it demonstrated that insects could not fly.  Academia has not changed.  The explanation may be somewhat complicated, and if the need exists, I will add a few drawings.

Drawing 14Capillary and Hygroscopy

Back to nibs and wide slits.  You could raise one more question: “Why does the ink not retract in the slit when it widens?  According to the laws of physics, it should, shouldn’t it?”  (If you are not sure on this read about Capillaries on this website.)

If the written line widens gradually while the tip remains in contact with the paper, the superior capillary attraction of the paper, as well as the above mentioned hydrostatic pressure of the ink column acting onto the ink in the slit, prevents the ink from retracting.  See Drawing 14.  Hydrostatic pressure in fountain pens?  It is covered in the article on the principle design of the feed, called Bubbles and Bottles, here, on this website.


The above demonstrates the importance of considering writing with a flex nib as a form of art.  It needs to be learned through practising for hours, days and years.

Drawing 8Radii Variation of a Nib

3.)  The difference between the nib radius and the diameter of the feed may ask for some consideration; however, I would not regard it as critical as long as one observes a few aspects.  For this I show one of my favourite drawings again, Drawing 8.  It is essential that the nib stays in contact with the top perimeter of the feed because that is the location of the feed capillary and the longer the length of contact, the easier the ink can transverse.

In the drawing I show the flattening of the nib as increasing radii; however, they can be parts of ellipses or any other artistic curve.  The criterion is the increasing distance d3 and d2 from the original centre, which varies the flexing characteristics of the tines and assures the contact between nib and feed.

17. H – Underside of the Nib

“Should the underside of a thinned nib be polished, or would a certain direction in the scratches improve ink flow?”

In my experience of nib production, all nibs came with a smooth surface finish as it was caused by roll-forming and subsequent rumbling.  No extra effort was made to alter the surface finish.  Often one sees grain lines on nibs which in my opinion are caused by de-burring after slitting or stamping of holes and ornaments.

Ink acceptance of metal surfaces can be promoted by the cleanliness of the surface rather than by the scratches.  They cause some changes in surface reaction, but they lose effectiveness after 1‑2mm; they rather hold the ink than promote its flow.  I have written about scratches and roughing under the topic: Feeds made of Plastic.  In case of a plastics, a (microscopically) rough surface increases the contact angle and makes the surface more accepting of water (ink) significantly because without this “roughness” plastics repel water (ink).

18. Hole versus Scallops

You may have already wondered about this: “If both modifications soften the flex, what is the difference?”  Ingeneers like to reply: “A drawing says more than a thousand words.”

Drawing 15 — Holes versus Scallops

The two modifications are shown in Drawing 15 showing half of a nib; that’s why F/2 … while F would be the total force applied to separate the tines. The blue and the pink segments indicate where some material has been taken away from the breadth of the tine.  Blue is the opening and red is the scallop.  The forces Fs and Fo act onto the remaining section.  The blue markings relate to the opening near the slit, the pink to the scallops.  The difference shown in the drawing lays in the variation of the force applied to the tine.

Drawing 16 — Forces and Tine Separation

The effective forces Fs and Fo = F/2×cosαs or αo respectively.

Thus, the modification with the opening hole results in a smaller the tine deflection δ, however, since the tine separation is W/2 = δ×sin αs or αo respectively, the two effects (deflection δ and slit widening W) compensate each other.  This means it does not matter, from a dimensional point of view.  (See Drawing 16, which I have copied from a previous chapter… T is equivalent to  Fs and Fo .)

Oh, and there is always a, however: as long as this happens in a position where the curvature of the tine profile is flat or flattish, there is no problem.  Going further back towards a smaller radius the nib with the opening would show some sidewards instability.

19. Flat Tines

Further up, near the start of the paper, I commented on a particular nib, which was shown in Photo 4 and promised to return to it.  It’s late (11:30pm as I am writing), but here the photo is again, and I will be commenting on it.  Earlier on, I also noted that hardly any nibs/tines are flat. By now we have learned that: “The flatter the tines, the easier they bend.”

Photo 4 — Flat Tines

Generally, the two tines are tilted towards each other, not necessarily needing to be flat, but their effective profile is flat and tilted, see Drawing 16.  How can tines separate if they are not?

Photo 18 — Flat Tines Bending

In Photo 18 (same pen as in Photo 4) the abrupt shape change from the round to the flat part of the nib is highlighted by a stark contrasting shadow line.  This significant shape change works like the kink in the material, makes this edge structurally stable.  We had talked about kinks before.

This means that the corners, marked with large pink dots, are in a fixed position and thus define one end of the bending axes, the other is the end of the slit, the location of the hole.  And since the bending axes are angled towards each other, the tines separate when bent upwards.


I also said before that there are other good things about this design.  They have nothing to do with flex‑nibs, but I like to mention them because for me they are the signs of good fountain pen design.

One is the metal ring, which could include several functions.  Firstly, it prevents the section from splitting.  It also appears to me that the narrow edge is used as the counterpart for the inner cap to achieve a reliable and durable seal.

This fountain pen, a Pilot Falcon uses a thread to hold the cap.  In case of a clip‑cap, the metal ring could serve as a definite stop rather than relying on the elasticity of the clip mechanism.


Photo 19 — No Add

Why did I add Photo 19?  It is the same fountain pen and nib as in Photos 4 and 18…  Forgive a designer.  I photoshopped the embossments away!  Does this not look so much better?  The type of nib can still be recognised by the tiny number 1013, located where the nib progresses out from the section.  Now the beautiful line of the nib is clean and uncluttered.

I don’t like advertising, especially not on items I have paid for.  I don’t buy things with logos and prints, and I take labels off garments.  It interferes with my sense of aesthetics, and when writing it disturbs my area of vision and distracts me.  Does this nib in its purity not look so much better?

Perhaps, the reason for this is that I am a designer and ingeneer whose “formative” years of ingeneering were during the sixties, a time of clean and simple design.

Photo 20 — Elegance

I like this style of nibs, and therefore, I add Photo 20. It is a Namiki nib, super fine.  This is a superb example of nib craftsmanship.  Flex writers sing about it in the highest notes; again, I just could not resist photoshop away the embossment, and the sputters of ink from the original photo.


Photo 21 — A Really Flat Nib

Lastly, let me show another sample of a fountain pen with a flat nib but this time with its tines tilted towards each other, very much like the hypothetical model for our early calculations.  It is a Parker 180, shown in Photo 21.  Somewhat unusual is the almost spherical tip.

Some may say: “What has this nib to do with flex-nibs?”  Nothing really, I like it, and it certainly makes a point about flat nibs.

Some suggested that the nib can also be used in its turned‑over position.  This way it should be possible to write two line widths.  During the days when I was drawing pictures I thought it was a brilliant idea; alas, disappointing, it did not work.  Moreover, Parker Germany had not been helpful.  I also realised that almost all other nibs offer a fine line when used upside down.

The metal cover on top of the nib was there for strengthening because without the curved profile the tines would bend easily to plastic deformation.  Much fatigue cracking happened at the end of the slit, and later a tiny cross slit was added to distribute the stress over a larger area.

Also, the feed had a novel idea with the ink being fed through the centre of the feed.  Working in the field of fountain pens at the time, I was keen on the individual new ideas, but for me, the overall performance was not satisfactory.


It seems this paper has come to an end.  I hope it provided some clarity about the flexing of nibs and other a few other odds and ends.  I had expected that during writing some clarity would have occurred about some suitable criteria to establish a technical demarcation between standard nibs,  flex‑nibs and the different characters among the latter.

At least, we have worked out a suitable test method.  The execution and collating of data I would like to leave someone else or a group, who has a considerable collection of fountain pens, sufficient technical interest and a passion for the job.  If needed, I would be available for some guidance.

You have been reading this paper (I can tell), and I can hear you say: “…and what about this, and what about that.”  Let me know about them, and I will add these points to my best knowledge and ability.

Finally, I would like to thank the members of the forum Experiments With Flex at the website Fountain Pen Network.  Without their stimulating, thought-provoking contributions this paper would not have come into existence.  They gave me permission to use the information and display their photos.  My gratitude extends to members of forums for other sites, and I want to acknowledge the information and many photos, freely and anonymously available on the internet.


Amadeus W.

21 December 2017

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