My Piston pen design

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lukitas

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Dec 27, 2017
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Frankly, I have no idea about kit pens. Never considered making one or even buying one.

What happened was, I got frustrated with the sort of pens I could afford. Grew queasy at the waste implied in cartridges. The converters are too small. Ok, the Twisby vacuum filler is big enough, but has sharp threads and shoulders right in the middle of my grip. I despair of finding a decent vintage wet noodle at an affordable (to me) price, and one wet noodle isn't enough : you need one for every colour, don't you?

The only way was to design a better pen myself. For simplicity's sake, I started with the idea of Noodlers' piston filler : a simple syringe with a hollow piston, a little air-tube in the back of the feed, just long enough to clear the end of the piston when pressed down all the way. It creates enough of a vacuum to almost entirely fill the pen.
Dad taught me to keep my fingers high on the pen : when your fingers are higher, you need less movement to cover a larger surface. It had to be a pen with a smooth section.

I spent a month or two drawing up plans, then bought a lathe and started building. Five parts to build, four to buy. The section and the reservoir are one piece; the piston is hollow, has a foot with a groove for the o-ring and a ridged end to push and pull; the hollow bolt to retain the piston in the reservoir screws into the end of the reservoir; the back cap slips over the piston and screws onto the body, and the top cap has an inside cone that is hardly wider than the cone of the section. When closed, the cap and pen form a thin watertight join at the top of the pen, which is easy to pop open, and closes securely, with no fear of leakage outside the cap.
Ergonomically it works exactly to requirements : it is a pleasure to use. But it is a bear to build.

For one, my tools are limited. I have a mini-lathe from the empire on the other side, with semi permanent analog read-outs, a gear box that has to be unbuilt to change gears, and a tail-stock that is a hundredth of a mm or so too high. And I haven't the wherewithal to scrape it down. I pee with the willie i got.
It gets worse. Acrylic turns ok, but it isn't easy. It starts to melt close to the boiling point of water (see what I did there? No celsius, no farenheit!). A 1/10" drill bit will start to boil the material about 1/2" in, at 175 rpm. I had to develop a ritual : drill 10 mm deep, change to a drill 1/2 mm smaller, drill another 10 mm, rinse and repeat, to size. Finish with a boring bar for a straight hole. Well, straight-ish. The plastic bends under tool pressure. The beginning of a bore is always tighter than the end, the beginning of an outside cut is always wider than near the chuck. Which leaves one with the unthankful task of keeping both ends within tolerance.

The design of the pen leaves but little material for the hollow bolt : I didn't want the pen to be wider than a Montblanc, which is 16mm at its widest, and I wanted a reservoir of at least 10mm diameter, so it could accommodate a hollow piston. All these parts screw together over the same length in the middle of the pen. I decided to give the most wall thickness to the outside, and the least to the hollow bolt : it only has to stop the piston from slipping out, and rarely needs to be manipulated.
I chose triple entry thread. That is to say, I would have tried quadruple entry, but my gearbox isn't big enough to accept the gears for that. As it is, at a speed of 3mm per revolution, I am already over the nominal maximum of the machine. Doesn't matter, I have to turn the threads by hand anyway. You know the procedure : turn one thread to size, advance the tool 1 mm on the compound slide, turn the second, repeat for the third. test for fit and cut a little deeper if required. For the flange on the body that takes both inside and outside threads, I try to position them so that the valleys on one side fall on the ridges of the other, to keep the maximum possible wall thickness.

All of this makes for a pen that costs a lot of time to make, with a result that can never be as polished as it should be. There is no way I could make the bore of the piston perfectly straight and polished : the hole is only 5,5 mm, and the drill bit will weave about the center to leave me with a helicoidal deformation of the bore. However much care I take with the bore of the reservoir, I need to hedge with two sizes of o-ring, and possible adjustments to the o-ring groove. No matter how precisely I try to replicate each cutting procedure, I can make each pen fit with itself, but the pieces are hardly interchangeable.

Here it is, my stupidly good pen that I cannot make good enough to sell :



If you're interested, I'll happily share the plans.
I would dearly love to see my design sold cheap in retail, but I doubt it can be done. Making moulds for the pieces is non-trivial, what with inside and outside shoulders and threads with clearances required - it boggles the mind. CNC machining is feasible, but again, I dont think they could be produced in large enough numbers to be able to sell them at a reasonable price.

If any of you have ideas or suggestions, please go ahead. Anything to minimise the pain of making them and maximise the pleasure of using them.

Cheers.

bonus : here is one fitted with a Zebra G chrome-plated nib. Works beautifully with Noodlers black, Apache Sunset, Saguaro Wine and Nikita Red. Diamine Blue-black will eat the nib.

 
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jalbert

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May 17, 2015
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Good innovating. Syringe fillers are a nice alternative to a traditional differential piston filler, as they're a lot less work, but still hold a similar amount of ink. I made one a while back, but haven't gotten around to trying it again or improving my design.
 

Curly

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Saskatoon SK., Canada.
That's a heck of a first pen. The acrylic you used, is it extruded or cast? Most acrylic rod sold is extruded and is soft and doesn't machine well, even with lubrication. There is cast acrylic rod available, not to be confused with cast PR (polyester resin) that machines better but I don't know where you would find it on your neck of the woods. Comes in colours too.

Clear Cast Acrylic Rod (Unpolished) from Delvie's Plastics

I would be interested in seeing your plans. It would be a some day project though.
 

Woodster Will

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Aug 25, 2017
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The only thing I can suggest is that standard twist drills aren’t the best in plastic. You can also drill undersize and then ream to get a better finish? You could also try drilling first then boring to size. I’m sure there’s a solution.
 

lukitas

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Dec 27, 2017
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Location
Brussels
Thank you all for your kind response!

I think the plastic is extruded. It comes in 1 meter lengths from Kunststofshop.nl. I tried coloured acrylics, but they are too soft : I couldn't keep the reservoir within tolerance, all the pens made out of this were leaky boogers. I tried to solve that by fitting the coloured acrylic as a sleeve around the stiffer transparent acrylic, but I couldn't get the glueing right. UV-hardening acrylic glue doesn't set under the coloured plastic, another 'specialised' glue made everything melt. The best result I got was with cyan-acrylate superglue, which breaks down under the heat of machining the part. But it looks decent :



It should be possible to at least be partially 3D printed. You'll still need a smooth bore for the piston to glide in, and enough resolution to reliably form the threads.

And I do drill undersize, ream and bore where appropriate.

Here is my working drawing :


A few changes were made, but they are mostly cosmetic : the cap stays straight on the outside, and the piston knob is now the same width as the piston, for a bit more grip.

The pen is 16 mm wide at its widest, tapering down to about 10 mm at the tip of the section. The taper is 1 in 18. This makes it easy to set up the compound slide with the help of two micrometers, and leaves just enough material to hold the pieces for turning the tapers.

A ER32 or larger collet chuck is essential. It can be done with a 4Jaw, but it takes hours to center each piece, it is difficult to judge how much force the teeth are exerting on the piece.

You can cut up to about 0,5 mm, on an outside cut with a piece that is supported by a live centre. Most of my pieces must be cut without the help of a centre : the maximum you can go is about 0,25mm. And the minimum is about 0,1mm: at 0,05mm the material tends to bend rather than cut. Cut no more than 0,25, organise the sequence so that your last cut is 0,1 or a tad more.

Let me walk you through the pieces.

The cap is easy. 10mm hole, 47 mm deep. Step 0,5 mm wider, starting from 24 mm deep, with steps 4,5 mm long : there's your 1 in 18. Use the compound to smooth out the steps - If you've cut the steps to the right dimensions, you should have a perfectly dimensioned cone as soon as you have a continuous cut. Polish the insides, with wet micro-finishing cloth wrapped around a 6mm rod.

The piston took some finagling : The first ones I made in one piece : this meant I had to turn the outside first, the foot being nearest the chuck, cut the piece, turn it around and then drill the hole in the remaining material. A nightmare. Too little plastic left to dissipate heat. Now, I bore the piece first, and then glue in a stopper. Inside dimensions : 5,5 by 45 mm, outside 8,4 by 52, with a foot that is 3 mm deep and 9,8mm wide. The groove in the foot is cut to 8mm. Here is one I made earlier : you can see the helicoidal marks left by the drill bit.
I couldn't find a decent reamer that size, and it is too small to polish. Rolled up paper will leave a trumpet shaped hole, making the foot too weak.



The bolt that holds the piston is a fragile little thing : the hole must be at least 8,5 mm, whereas the top of the threads cannot be more than 10,9. At best, the thinnest wall is at 0,8 mm. It is strong enough to do what it needs to do, protected in the middle of the pen, it only needs to stop the piston from slipping out. But it is very easy to break them as you make them.
The threads are 4,7mm long, with a 2mm thread relief cut near the knob. This is it :



I make triple entry threads, 3mm speed, cut to a nominal depth of 0,6mm. I ground a flat on the tip of the tool that I hope to be about 0,125 mm. After cutting to depth, I advance the tool 0,1 mm, for a little extra clearance. These numbers have to be taken with a grain of salt : the material bends back under the cut, and the amount of bend changes with the length of stick-out and wall thickness.

The back cap has been simplified : no more shoulder at the end of the bore. 8,5mm by 49mm. A chamber at the front, 13,1 (+/-0,05) by 8 mm. Threads To 4,7mm. I leave the outside a little oversize, to make a seamless join with the body later.

The last part to be made is the body. First the reservoir, drilled in steps, so that I always have a cold drill bit to drill the next 10 mm. My last cut is 0,25 per side, 0,5 on the diameter. I don't drill to width over the whole length : even a 9 mm drill bit can come dangerously close to the 10 mm nominal, a 9,5 mm drill will probably leave a visible trace. The chamber should be 10,05 (+/-0,05) by 42mm : a long stick-out for a thin boring bar. On the inside, advancing the tool 0,05 mm will cut, but 0,1 is safer. 0,25 is a bit much : the swarf can get stuck under the tool, start rubbing and leave heat marks.
As I noted, the end of my bore is always a bit wider than the opening, and the opening always has a short inverse slope at the entry, about 3mm to the beginning of the 'straight' bit. It's ok, as the threads go in 4,7 mm. Here I have to fit the bolt before finishing work on the threads. It happens that I have to go up to 0,15 mm deeper than the nominal 0,6, which is quite a bit, but I suspect the inside cut meets more resistance than the outside. I cut the flange to a diameter of somewhere between 13,75 and 13,85 before putting in the thread relief and the chamfers, and then cut the threads.
This is done by hand : I put a nut on the drawbar for the chuck, and use a Nr 19 key to turn the chuck; It's only one and a half turns per thread, takes about half an hour to cut a set of three.

To insure the best possible fit, I drill and ream the hole for the nib and feed slightly undersize (6,6mm), and then heat the section in hot water before slipping in the nib and feed. Beware of keeping a piston with an o-ring fitted while you are heating the section : the ring will push into the hot acrylic, and the pen will be leaky. The nibs are Nr 6. I've used nibs from FPR, from JinHao, Brause Rose and Zebra G. The Brause will oxidise. The chrome plated Zebra G does fine with three of the four inks I tested. I havent tried the titanium Zebra G's yet. Wonder if someone can tell me wether they write as well as the steel ones. The feeds are 6,3mm flex cut feeds from FPR. I drill a 1,5 mm hole in the back, and a little chimney to the ink-channel. The tube is 1/32" silicone found in a medical supply store. Grind a little taper at the end, to fit into the too small hole, and to key with a little dab of superglue. Cut it to as close as possible to the bottom of the piston : you get a better vacuum if you can push out more air.

The least fun part is cutting the outside tapers : There is only 10 mm of material gripped in the chuck, and I cannot use a center : the nib is in the way. A most delicate set-up. If you apply too much tool pressure, the part will push sideways. Or just break. The procedure is the same as for the cap : half a mm steps on the diameter, 0,25 per side, steps of 4,5 mm long, monitored with a 50 mm micrometer following the compound on both X and Y axes.

Once the taper of the section is cut, I screw the cap on and turn the straight bit around the seam in one go, to make it as seamless as possible.

Now is a good moment to start polishing. Tapering the back cap and polishing it is the last job. Before testing the new pen.

Phew.
I'm in the process of finishing pens number 12 through 15, only the tapers left to do on two and a half of them. Writing this was almost more work than making the pens.

Cheers,

Lukas
 

ZbR

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May 24, 2016
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Hi Lukas,

congratulations on your design and construction work.

I like very much hollowed piston. I think that, if after initial ink intake, you can reverse the pen upside-down and push the piston to remove the air so you can intake some more ink.
In my pen constructions I also used air tube but I found it actually unnecessary, the pen worked fine without it.
To glue acrylics I use epoxy resin (not these 5 min.).

All the best.
Zb
 

Curly

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Thanks for adding further details. I remember reading somewhere that boiling the extruded rod in water for 20 or 30 minutes improves the machinability. I've never tried it but if you can't get cast acylic it might be worth a try. Probably best to keep it off the bottom of the pot somehow and I wouldn't use the "boss's" pots.
 

lukitas

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Dec 27, 2017
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Brussels
Thanks.

Zb, the tube makes it possible to fill the pen entirely in two push-pulls : the first pull draws ink, the second push pushes more air than water : on the second pull you get more vacuum, if that makes sense.
Epoxy is a good idea. I only need little dabs at a time though. Having to mix up micro-batches of epoxy seems laborious. I'll think about this some more.

Pete, I did try to cook my acrylic. I steamed it in a steamer, keeping the rods off the metal with little rods. The surface went off, with opaque spots, but only micrometers deep. The machining was much the same. I suppose the manufacturer who makes my acrylic does a good job of tempering them. Or maybe most of the stresses are released when I cut off the skin.

One of the four pens I am finishing broke catastrophically. As usual, these things happen when I'm almost done : for one, I'm exited about finishing, and for another, the last cut is the most delicate and dangerous. The back cap hangs out sticks out 46 mm (almost 2 inches!) and is gripped on its thinnest part. Where it broke. Argh.
The two other ones i finished are doing well, one with a Zebra G, the other with an FPR flex nr 6. And I managed to save a too generous stub - I just noticed the nib was sticking out way too far. Now it is much more civilised. Still generous, but only enough to make the pen fly over the paper when writing. So there : a happy ending.

Cheers

Lukas
 

jalbert

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May 17, 2015
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Why don't you stabilize the cap with a live center when you shape it? Leave a little extra material on the end, make a little divot in it with a center drill, then snug up a live center to it. Then you can shape the cap without having to worry about it breaking. You'll have a little nub left on the end of the cap, but that can be cut off and sanded flush with the end of the cap. I do this, and I've never had a problem with the cap breaking.
 

lukitas

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Good idea John. But I'm greedy and avaricious, and I can just squeeze two back caps out of a stick of coloured acrylic, with not much nub left.
In any case, the last breakage was my own fault : it wasn't seated deep enough in the chuck. 3mm deeper, and everything would have been alright.

Thanks!
 

Gregory Hardy

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Well I will be damned.
No pretense.
No hidden agenda.
"Share your plans?" Wow.
Great pen.
Better attitude.
Friend of the forum.
I just sent a private message to this gentleman just for an extra-special "atta boy."
Now I am going to skip supper and go to my shop and see how I might measure up.
Today was sot of a rotten day at work and then some man from Brussels (who I doubt I will ever get to meet in person) made me a hopeful guy again. Two points for him and IAP for creating the venue!
 

lukitas

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Dec 27, 2017
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Brussels
Wow, thanks Greg. What a welcome!

Sharing the plans is no big deal really : none of the ideas in my pen is new. If I recall correctly, the hollow piston syringe was used in the 19th century. The idea of incorporating the converter into the section does not really qualify as a eureka moment, it is an obvious solution towards larger capacity. A long, tapered section has been done before. And the way inside and outside cones work together is self-evident to any machinists : morse cone fits can be strong enough to withstand serious machining forces. I got lucky by obtaining the right wrong fit. The rest was just figuring out how much I could squeeze into a body no wider than a Montblanc 149, while trying to keep at least 1mm of material in the thinnest bits.

I'm sorry, but the working drawing I put in a previous post contains some obsolete dimensions; basically it is correct, and I did add most of the correct numbers in the same post.

As a pen, the design works very well. Sleek to hold, simple to clean. When I get the nib&feed right, it writes like a dream, for pages and pages. As a construction process, it is a nightmare : it breaks several of the rules of safe machining. Long stick-outs are both dangerous and time-consuming, cutting threaded pieces to 0,7mm wall thickness is courting disaster. I'll be surprised to see anyone pick up the challenge of building a pen from my plans. If anyone does, please share your solutions. It takes me more than a week to make one pen. The cost of materials is negligible, compared to that. And it's finish isn't good enough for it to be sold at the price I would need to ask. I'd love to discover ways to simplify and shorten the production process.
I'd be even more surprised to see my plans converted to injection moulds for high-volume production. If they make it cheap enough, I'll buy one. If it is any good, I'll buy a few more, for all the colours, and claim priority. A modest percentage of the proceeds would be acceptable.

Cheers. Keep on turning.
 

magier412

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Feb 6, 2013
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Seattle, WA
That is a great pen! I’m wondering...is the piston totally necessary to achieve your goals? It could be that getting the piston mechanism working IS the goal (LOL), but if not, have you considered a (much) simpler eye dropper set up? In that case, you still get the volume, but getting the ink into the barrel is about using a syringe (which I do with my pens anyway) rather than the more complex (and elegant) piston.

You’ve given me lots to think about here, and I thank you for sharing.

Have a great day!

K
 

lukitas

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Dec 27, 2017
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Brussels
Thank you Kay!

You're spot on : the piston is essential. It's the brand I want to give the pen : 'Le Piston'.

I did consider making an eyedropper first, to test my nib fitting. But after much cogitation, I concluded that it was actually harder to make than the piston version. All my chambers are less than 50mm deep; I can barely stay within tolerance at that depth. With the tools i've got, I couldn't make a 100mm chamber look good : the drill bit would weave a bernini pillar in the plastic, and a boring bar long and thin enough would deflect like crazy. I won't even think of trying to polish a half-blind hole that long. Assuming a tube with a small hole for nib&feed at one end and a larger hole at the other, stoppered at the end with a screw-in knob with an o-ring.

Moreover, an eye-dropper is fine for enthusiasts like you and me, but I wanted to make a pen that would be easy to use for a lay-person. My mom has one. A friend, who is a teacher, uses his to mark student papers. I'm sure he wouldn't want to hassle with dripping syringes and tilting reservoirs. Slapstick in the class-room. The 3,5+ml capacity of my piston already causes some of the problems that can occur with eye-droppers : large bubbles of air heat up in the hand, causing the pen to gush. With the piston, there is an easy fix : push out the air, continue writing.

Forgive my incessant babbling : here are the pens I carry around with me :



The top one seeps through the glue-joint of the stopper in the piston. I'll need to fix that.

Cheers! Keep on turning.
 
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