# What actually keeps a converter or cartridge attached to the FP feed / housing?



## FGarbrecht

Having purchased at least 5 different styles of FP converters and cartridges (all 'standard international') exactly _only one_ makes a good friction fit on the nipple end of a Bock type 180 triple.  I assume therefore that it must be standard practice among FP makers that the *section* needs to be designed so that the diameter of the hole that admits the head of the converter must be exactly sized to friction fit that specific converter or cartridge, or risk the converter coming loose in the pen body and making a big mess.  Is this a correct assumption or am I missing some special sauce or other obvious fact of FP design?  I had assumed that the converter and feed/housing attach solely by friction fit of the nipple, but this would appear to be incorrect.
Thanks


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## DrD

Based on what I've with the fountain pens that I've turned, it is friction fit - sometimes good, sometimes, not so good. I have had the converter come off but that was while removing the section from the barrel; no ink was spilled.


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## bmachin

My feeling is that unless you have a good reason (you need a short barrel for instance) to buy something other than a Schmidt converter, why would you? They are the best out there and I have never had one not fit well. Of course there are people here who have built a lot more pens than I and their milage may differ.

Another thing to consider is wear. If you remove and replace one often enough it will loosen up, so my advice is to just get it in place and leave it there. If you need to flush out the nib take it off and use a bulb syringe. Just don't get in the habit of removing and replacing willy-nilly.

Even if it is a little loose, there is probably enough surface tension that you won't end up with a barrel full of ink (fingers crossed) and if you've calculated your hole depth correctly it can't fall off.

Finally, yeah, we all go for a pretty good fit. In my case I go for a slip fit. A friction fit can pull the metal rim off the converter.

Just my opinion which I'm sure is one of many.

Bill


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## magpens

Very interesting topic, and one which has often come to my mind. . Following.


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## monophoto

FGarbrecht said:


> Having purchased at least 5 different styles of FP converters and cartridges (all 'standard international') exactly _only one_ makes a good friction fit on the nipple end of a Bock type 180 triple.



Obviously, I can't contest Frederick's observations because I wasn't there.  But I will say that I own several dozen fountain pens, mostly pens that use international standard cartridges/converters, and I have never had the experience of a cartridge/converter coming loose.  Perhaps Frederick's experience is based on a few converters/cartridges and one style of section with which I have no experience.

Most cartridge/converter fountain pens rely on friction to keep the cartridge/converter attached to the nipple at the back of the section.  And as others have noted, there is a potential for wear to cause the opening of converters to expand after many years of use.   I think - but I can't prove - that this is more likely to happen with cartridges that are designed for one-time use and are therefore disposable.   But in general, as long as the barrel of the pen is in place, there is very little force on the cartridge or converter to counter the friction and cause it to come loose.

But that all being said, I do agree that some converters are better than others, and cheap converters are a source of significant frustration.  While I have not had the experience of having one detach from the nipple on the section, my issue is that the seal around the screw advance on the plunger is really crappy and often leaks.  Conversely, there are some converters that can be disassembled to allow the plunger to be lubricated with silicon grease, thereby improving performance.  You get what you pay for.

Some people reuse cartridges.  The fact that they were designed to be disposable suggests that they might be made of material that is more subject to stretching due to wear, which could be a problem if they are reused.  I have a J. Herbin roller-ball pen that uses liquid ink in international standard cartridges that I refill using a syringe (the body of the pen is too short to use a converter), and it is just as tight as the day I received it four or five years ago.


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## Penultimate

I have had some fit up issue using non-Schmidt converters. The fit around the nipple can be tight or too loose for my comfort level to leave in a pen. I recommend supporting the converter above the nipple. It does have to be an interference fit but I believe it helps support the converter. 

Good luck. 


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## darrin1200

It is a friction fit to the section, not the feed tube. I’d have to double check, but I drill my section through with a 7mm bit. It gives me a friction fit to the converter just behind the metal ring on the K5 converter. I prefer the K5 because it can be disassembled, as mentioned above.


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## FGarbrecht

darrin1200 said:


> It is a friction fit to the section, not the feed tube. I’d have to double check, but I drill my section through with a 7mm bit. It gives me a friction fit to the converter just behind the metal ring on the K5 converter. I prefer the K5 because it can be disassembled, as mentioned above.


Thanks to all who have replied but this is the answer I was looking for.  Virtually none of the 'standard international' convertors I have grab onto the end of the Bock feed tube.  The Schmidt K5 does, but I've been making some shortie pens where it doesn't fit.  The gold-tone squeeze converters (like from RL Greenwald and Beaufort Ink) don't really grab onto the feed tube but they seem to work for my shortie design because I have the section drilled out just the right amount (accidentally) to provide a firm grip on the outside of the convertor.


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## budnder

For a custom fountain pen, note that Schmidt actually makes a threaded converter (the K6):



			Schmidt K6 Threaded Piston Converter
		


I think I'm going to give these a try, as it seems like a good idea.

I think maybe some Delta fountain pens use these?


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## bmachin

That is not a standard metric thread. Richard Greenwald also carries the K6 as well as the m7.5x.75 tap (rather spendy) here:









						Tap, Metric M7.5 x .75 Tapered - Richard L Greenwald LLC
					

Tap, Metric M7.5 x .75, Tapered The thread fits our item #10101 and 10140 mechanisms.  Drill a 6.8 mm pilot hole for proper threading.  This is a tapered tap to be used for a through hole.  For a plug tap – carefully grind to leave 3 – 4 starter threads; for a bottom tap – […]




					richardlgreenwald.com
				




Bill


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## budnder

bmachin said:


> That is not a standard metric thread. Richard Greenwald also carries the K6 as well as the m7.5x.75 tap (rather spendy) here:
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> Tap, Metric M7.5 x .75 Tapered - Richard L Greenwald LLC
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> Tap, Metric M7.5 x .75, Tapered The thread fits our item #10101 and 10140 mechanisms.  Drill a 6.8 mm pilot hole for proper threading.  This is a tapered tap to be used for a through hole.  For a plug tap – carefully grind to leave 3 – 4 starter threads; for a bottom tap – […]
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> richardlgreenwald.com
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> Bill



Ah... the economy of 3D printed threads...


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## bmachin

budnder said:


> Ah... the economy of 3D printed threads...


I sort of expected that. Just thought somebody else might like to know.


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## magpens

Roy and Bill, 
Thanks for providing the info about the threaded K6 converter. . Interesting and useful.


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## MikeinSC

As you've noticed, not all standard international cartridge converters are made to the same standards. In general, any Schmidt made (or rebranded Schmidt) K5 converter will be consistent. 
The K5 is friction fit to the nipple that comes off the back of the nib housing. 

I would caution against making a section that is too tight against the converter. It could lead to a cracked section. It isn't necessary for a converter or cartridge to stay securely fastened to the nib. A 7mm or "J" (which is barely nominally larger than 7mm) would work. I personally use a good quality J bit.

Also consider that a customer is going to appreciate simplicity and part availability in the long run. K5's are common, a K6 is not.

A pen that they can easily take apart for cleaning. And parts that can be found easily should they need to be replaced. While K5's do come apart, there really is little to no need to disassemble one with any frequency.

Make it easy on yourself and the customer in your pen design should warranty work ever need to be done.


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## More4dan

bmachin said:


> That is not a standard metric thread. Richard Greenwald also carries the K6 as well as the m7.5x.75 tap (rather spendy) here:
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> Tap, Metric M7.5 x .75 Tapered - Richard L Greenwald LLC
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> Tap, Metric M7.5 x .75, Tapered The thread fits our item #10101 and 10140 mechanisms.  Drill a 6.8 mm pilot hole for proper threading.  This is a tapered tap to be used for a through hole.  For a plug tap – carefully grind to leave 3 – 4 starter threads; for a bottom tap – […]
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> richardlgreenwald.com
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> Bill



M7.5mm x 0.75 taps are available on Amazon for $9 and $5.50 shipping. EBay has it for $7.95 and free shipping but will have to wait 3-4 weeks to get from China. I went the EBay route. Good quality and works with the K6 converter. Gives a good snug fit you won’t have to worry about coming loose. 

Danny




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## budnder

I was thinking it looked like a K5 converter could still be used in a section designed for a K6... when my K6's arrive I'll investigate that.


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## More4dan

budnder said:


> I was thinking it looked like a K5 converter could still be used in a section designed for a K6... when my K6's arrive I'll investigate that.



You are correct, a K5 will fit a section made for a K6. The K6 has threads on the clear barrel but below is the same. 

Danny


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## More4dan

I’ve finally gotten around to making a section threaded for the K6 converter. Wow! I should have done this for every Kitless FP I’ve made. No more leaking pens, at least from a loose converter. The thread is a M7.5 x 0.75 and the tap is available on EBay. 


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## FGarbrecht

More4dan said:


> I’ve finally gotten around to making a section threaded for the K6 converter. Wow! I should have done this for every Kitless FP I’ve made. No more leaking pens, at least from a loose converter. The thread is a M7.5 x 0.75 and the tap is available on EBay.
> 
> 
> Sent from my iPhone using Penturners.org mobile app


Any chance you've tried these with Bock feeds?  The only converter I've found that mates well with Bock #5 feeds is the Beaufort ink universal piston converter (I'm tired of buying converters that don't fit .  It also looks (from the picture of the K6) that the end of the section would need to be redesigned a little bit longer to accommodate the threading.  Are you using a 6.8 mm drill for the tap hole?


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## More4dan

I’ve just tried on JOWO and Schmidt so far. I’ve used a K5 on a Bock #5 several years ago. I seem to remember that the converter would come loose from time to time and it took some effort to push it in. The threaded converter would have helped. 

I’ve stoped using the Bock #5 for 2 reasons. The section is a bit too long for my taste. But more importantly about half of the last batch I received required a lot of tuning to get to write correctly. For one I could never get it to feed ink consistently. Once tuned the Bock is a great nib, but took hours of time for some.  The JOWOs have been just about perfect right out of the box.  

For the drill, a 17/64 gives a 77% thread or an H gives a 76%. 

Here is the K6 pushed into a Bock #5 feed, fits fine.


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## darrin1200

My concern using the K6 would be the fragility of the section thread. As it is, the thread portion of the section is thin. If it gets an additional thread on the inside, the wall thickness will be even thinner. If the converter gets ink on the threads, it could become difficult to remove. 
I have never had a converter slip out of my section, I have used K1, K2 and K5.


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## More4dan

darrin1200 said:


> My concern using the K6 would be the fragility of the section thread. As it is, the thread portion of the section is thin. If it gets an additional thread on the inside, the wall thickness will be even thinner. If the converter gets ink on the threads, it could become difficult to remove.
> I have never had a converter slip out of my section, I have used K1, K2 and K5.



Very good point. I’m using a M9 x 0.75 section thread. I measured my M9 x 0.75 die and it cuts to 0.317”. I measured my M7.5 x 0.75 tap and it will cut to 0.294”. This leaves 0.023” or a 0.011” wall minimum where the threads line up and 0.042” maximum if they don’t. 

11 -12 thousandths wall thickness is getting pretty thin for acrylic but would be fine for metal. However, it’s still stiff when I squeeze it. I’ve broken an acrylic section trying to remove a stuck non-threaded converter once. I’m thinking the threaded one will always go in and out straight reducing the chances of side loading the wall. Time and testing will tell. Once the converter is screwed into the section, the whole assembly feels much more solid than the ones I’ve made for the K5 with a slip fit design. 

My 9mm die is adjustable and is currently at the minimum ID, maximum cut. I plan to open it up to max ID and it should get my min wall thickness to 14 thousandths. 

I could also use a M9 x 0.5 section thread and get the min wall thickness to 19 thousandths. 

My current test section is the worst case with 7.5mm tap and maximum depth threads from my 9mm die. I’ll let folks know how it holds up. 

Get a piece of scrap material and do some testing. 

I also like the fact I don’t have to get the dimension for the hole for the converter drilled “just right” where I get enough tension to hold the converter snug. Too loose and I scrap the section and start over. 

Danny


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## More4dan




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## More4dan

Here is a great resource for checking the dimensions for threads and seeing how they stack up for your kitless designs. 







						Metric screw thread: M Profile calculator
					






					theoreticalmachinist.com
				




Danny


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## darrin1200

I'm not trying to be argumentative Danny. I have been struggling with the idea of going to a K6 for a while. I like the idea of a secured ink source, but these are the arguments I come up with for adding a second thread on the interior of the section.

I also want to add, I generally use #6 jowo nibs in a section using a M10x.75 section thread.

Another thing to consider, is the strength of the section when a normal cartridge is pushed in. That's when you may see some side loading.
Also, unlike the K6,  the non threaded converters are readily available almost everywhere. 

Another risk of side loading on an overly thin wall, is if someone uses your pen as an eyedropper. Just about any pen can, if you add a little silicone grease to the section thread. With the converter installed, it acts as a support to the thread section. With out it, a small drop could snap the section right off.

True, if you go with metal these arguments would be nullified. But, personally, I don't wish to add metal to my sections.


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## More4dan

Darrin you raise some really good points. I may have to make some examples from different materials and do some destructive testing.


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## More4dan

Darrin, when using a JOWO #6, the thread for the feed is just about the same as the K6 thread, 7.4mm vs. 7.5mm. You would have the same minimum wall thickness (within 0.002”) in both cases. I guess it depends on wether your section design has the ID and OD threads aligned, stacked on top of each other. 

Danny


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## darrin1200

More4dan said:


> Darrin, when using a JOWO #6, the thread for the feed is just about the same as the K6 thread, 7.4mm vs. 7.5mm. You would have the same minimum wall thickness (within 0.002”) in both cases. I guess it depends on wether your section design has the ID and OD threads aligned, stacked on top of each other.
> 
> Danny
> 
> Sent from my iPad using Penturners.org mobile app



In my design, the inside threads for the nib housing don’t go as far as the section tenon, for this reason.
I think I am going to have to order a couple of K6 converters, to play around with.


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## More4dan

darrin1200 said:


> In my design, the inside threads for the nib housing don’t go as far as the section tenon, for this reason.
> I think I am going to have to order a couple of K6 converters, to play around with.



Got it, makes good sense. When you make pens for sale, you can’t be too careful. Reputation is very important. Thanks for the feedback. 

Danny


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## budnder

FYI - a pic from what I've been 3D printing for a K6/Jowo #6... I find that the K5 for this, which implies I need to back off the threads a bit that I have specced for the K6. But... not sure I have much left to give in that direction. I recall measuring and thinking that it made sense you could have K6 threading and a K5 would be a friction fit in there, but I'm kinda wondering if that is really practical. 





FYI - the little "feet" on the section are only there for support when I print (e.g. where I anchor the print) - I clip those off post printing. Also, keep in mind that the printing isn't 100% accurate. In general, I think there's about 2-3% swelling, so that soaks up some of the tolerance.


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## More4dan

So, I have a #5 nib (JOWO) section that I made for a K6 converter. The section threads are M9 x 0.75. I just tried fitting a K5 and a K2 converter. Both fit nice and snug, with a good firm connection. I drilled with an “H” (0.266”) drill bit prior to threading for the K6 converter (M7.7 x 0.75)


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## More4dan

More4dan said:


> So, I have a #5 nib (JOWO) section that I made for a K6 converter. The section threads are M9 x 0.75. I just tried fitting a K5 and a K2 converter. Both fit nice and snug, with a good firm connection. I drilled with an “H” (0.266”) drill bit prior to threading for the K6 converter (M7.7 x 0.75)
> 
> 
> Sent from my iPhone using Penturners.org mobile app



Typo M7.5 x 0.75. 


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## darrin1200

More4dan said:


> So, I have a #5 nib (JOWO) section that I made for a K6 converter. The section threads are M9 x 0.75. I just tried fitting a K5 and a K2 converter. Both fit nice and snug, with a good firm connection. I drilled with an “H” (0.266”) drill bit prior to threading for the K6 converter (M7.7 x 0.75)
> 
> 
> Sent from my iPhone using Penturners.org mobile app


That is a close fit. If my numbers are right, that leaves about .015” wall thickness between the threads. Have you tested its durability.
One of my biggest problems with the thin walls, was actually cutting the thread. The tenon had a tendency to break when I was cutting the second thread.


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## More4dan

I’ve just tried the K6 threading, so time will tell. I’ve been making the section with a 9mm thread and #5 nib for years. I’ve had one break when I had a pen in my back pocket and sat down on concrete. It was the cap thread that broke, not the section. I did have a section break once during fabrication when threading the outside of the section last. I now thread the 9mm OD threads before drilling the section. 

I’ll make up some sections to test this weekend from some drop pieces of acrylic and ebonite. I still working on a repeatable way to test. 


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## darrin1200

More4dan said:


> I’ve just tried the K6 threading, so time will tell. I’ve been making the section with a 9mm thread and #5 nib for years. I’ve had one break when I had a pen in my back pocket and sat down on concrete. It was the cap thread that broke, not the section. I did have a section break once during fabrication when threading the outside of the section last. I now thread the 9mm OD threads before drilling the section.
> 
> I’ll make up some sections to test this weekend from some drop pieces of acrylic and ebonite. I still working on a repeatable way to test.
> 
> 
> Sent from my iPad using Penturners.org mobile app


Looking forward to reading your results.


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## More4dan

darrin1200 said:


> That is a close fit. If my numbers are right, that leaves about .015” wall thickness between the threads. Have you tested its durability.
> One of my biggest problems with the thin walls, was actually cutting the thread. The tenon had a tendency to break when I was cutting the second thread.



Just had one break while threading for the K6 into a 9mm threaded section. Built another and reversed the order. Threaded the ID first and the 9mm OD last without an issue. I’ll build a few more to see if I can get consistent results with the sequence and dimensions. 

Danny


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