Why ?

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magpens

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I have often wondered why the brass tube sizings are as they are in the case of some ballpoint pen kits.
Let's compare a couple of ballpoint kits, a Slimline (which uses a 7mm tube) and a Bolt Action (using a 3/8" tube).

The 7mm tube has an inside diameter of 0.246" and a Schmidt (or Parker ?) ballpoint refill has a diameter of 0.230"

So, the Bolt Action would still accept its usual ballpoint refill if that kit were designed with a 7mm tube.

Now, the 3/8" tube has an actual O.D. of 0.364" and the 7mm tube has an actual O.D. of 0.266", the difference in diameters being 0.098".

That means that if the Bolt Action were designed to use a 7mm tube then the wall thickness of the blank would be 0.049" greater, after drilling for the tube, gluing the tube in, and turning the blank down to the usual bushing size .

And the thicker wall would mean less blank transparency, and hence less need to paint the brass tube and/or the inside of the blank,
thus saving the pen maker both time and money .

Any good reason why kits could not be designed and made with the smaller brass tube in such a case ?

Other kit pen examples could enjoy an even a larger increase in the blank/barrel wall thickness.

Any comments ?

I should add that the 7mm tube size would not be suitable for downsizing ballpoint kit brass tubes in all cases.
My main thrust is to downsize to a suitable size in order to increase the blank wall thickness.
Some ballpoint pen kits utilize a twist mechanism which would require a tube of larger size than 7mm.
Thanks to Drewby108 (whose reply is immediately below) for raising the size requirement of kits that use twist mechanisms.
My questioning the tube sizes of some kits does not apply to fountain pen kits which have to accommodate ink cartridges
but rollerball kits could have their brass tubes reduced just like ballpoints.
Also, obviously, for kits with caps, the sizing considerations for the cap tube depends on what the cap has to fit over.
 
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Drewby108

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I'd agree with you on a large number of kits, especially with differences in standard vs metric tubes only being a few thousandths.

It makes sense to have multiple sizes for kits like cigars and sierras that have internal twist mechanisms. Specifically, the bolt actions move into the tube, or else the top end would have to be longer and therefore heavier. Otherwise, I generally chalk it up to selling more peripherals required for different odds and ends. How many already have a 27/64" drill when starting out?
 
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sorcerertd

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Using the smallest tube possible for the pen kit makes the most sense to me. More "meat" to work with and possibly more durability as the veneer layer left on some tubes kind of scares me just a little sometimes.

less need to paint the brass tube and/or the inside of the blank
I would very much like more of this. Painting and I are not on good terms just yet, but I'm getting better with it.
 

jttheclockman

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I read the question but did not follow it all. With that said I will put forth a few theory's. First the wall thickness for tubes needs to be thin in order to easily press in components. Using small diameter tubes on larger kits is a good idea for casting purposes. But this could lead to heavier pens of the same size. Not sure if that is a reason though. Having common tube diameters and lengths is another good idea but then all the pen stuff needed for the kits can be copied easily so I think these companies just want you to spend more money for their kits.
 

duncsuss

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Smitty made the LeRoi ballpoint kit, a version of the Sierra/Wall St/Mesa, that used 3/8" tubes instead of the more normal 27/64". The difference was only 3/64" but it helped.
 

Curly

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Sometimes it is nothing more than inertia. They started making them that way and want to continue using as much of the same tooling as they can because what they have is already payed for. Making a change to a nib is little more than some programming of the computer running a CNC lathe where making a brass tube that is better suited requires much more costs to make and change the tooling. Besides history shows that whatever they make we will buy with little complaint. Making pens to suit resin casting isn't much of a concern to them since the complaints are minimal and the kits sold. If a bunch of new pens debuted and then never sold because we wouldn't buy them because the blanks ended up thin then change would happen.

Sorry about that last sentence. Two "because"s in one sentence is bad. šŸ˜
 

Paul in OKC

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While the id of the tube may take that refill, that leaves very little room for the wall diameter of the nib end. With a tube of .246 id, and a refill of .230 od that leaves .016, which is .008 wall for the press fit parts. I suppose they could design the nib section with a counterbore and have it press on the od of the tube. Then you would have to cut a tenon, or the material down to the tube for a proper length.
 

sorcerertd

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There are many interesting and valid points here. It would appear that the only escape from all of those limitations is kitless, which no doubt has it's own challenges.
 
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