Fountain Pen Repair
Faye has a nice Shaeffer White Dot fountain pen, and brought it to me with a problem. The gold nib is screwed onto the plastic section assembly ("section" is the name for the part of a fountain pen between the nib and the barrel; the bit one holds when writing) with a few fine threads which had stripped.
I had initially planned to bodge the nib back on with glue, or some such, but in dismantling the pen, I created some more problems. The lower part of the section screws into the upper part with one thread, and the nib screws onto it by another. The latter was already stripped, and I stripped the former as I unscrewed it from the rest of the section! The lower part also contains a thin-walled tube section which is a push fit onto the feed. I managed to destroy this as I removed it from the feed. The part is made from some kind of flexible plastic sometimes referred to as "hard rubber'". In my defense, it doesn't seem to be very chemically stable - the ink had eaten away at it from the inside.
In dismantling the pen, the lower section part had progressed from damaged to completely destroyed. Both threads were stripped, and the tube section had broken in two. There was no alternative but to turn a complete replacement part. I chose aluminium as it's nice to machine, and I could anodise it black to blend in with the rest of the pen. I could see no reason to use plastic as Sheaffer had done - the nib is only retained by ~3 very fine threads, which seems pretty dodgy even for the strongest engineering plastics.
Before I could machine a new part, I had to know its precise dimensions. Relying solely on measurements of the mangled threads would be a bit of a gamble, so I first tried googling around for thread data on Sheaffer pens. No luck. I was somewhat hindered by not knowing the model number/name of the pen. The only markings on the pen, other than the signature white dot on the clip, are engraved on the nib. "Sheaffer. Made in USA". Not much help, but at least I knew to expect crazy imperial dimensions and thread standards.
Measuring the section thread pitch was easy enough with a thread gauge, as there were around 6 threads and they were not too horribly mangled. The section thread is 40TPI. The nib thread was much harder to measure, as the plastic was totally munged and there were only three threads to begin with. Instead, I took an impression from the inside of the nib with a bit of blutack on a cocktail stick `and held it against the thread gauge under the microscope. 54TPI for the nib thread.
I was expecting the thread major diameters to be preferred imperial sizes, eg. 1/4", but this did not seem to be the case. Nor did they look like integer or half millimetre sizes. I had to resort to measuring their intact female counterparts with a vernier caliper, and adding on a probable thread depth. This seemed a bit error-prone, but with a bit of trial and error during cutting it worked out ok.
I needed to know the thread profile to calculate the thread depth, and to make an appropriate cutting tool. Looking at the blutack impressions of the female threads in the nib and upper section under the microscope it was clear that they were trapezoidal. The American manufacture prompted me to guess at the ACME form. It's a pretty simple standard - 29 degrees between the thread faces, and the depth is simply equal to half of the pitch. Since the two threads had quite different depths, I needed a custom cutting tool for each one. For each I started with a piece of tool steel, ground it to a 29 degree point, then carefully filed a flat on the tip with a diamond file, repeatedly eyeing it up against the blutack impression under the microscope.This was pretty fiddly - the 54TPI tool was only around 200µm across at the point, and the finished thread was cut only 240µm deep!
Unfortunately I neglected to photograph the microscopy, the blutack, the cutting tools or any of the turning process! Sorry about that.
This was a really interesting repair job. A fountain pen is on my list of up-coming projects, so it was great to strip one down and see how they are designed. Nothing magic in there; pretty sure I can make one. We decided not to anodize the part, as it is a nice complement to the brushed stainless components.
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