Hi Simon
Here's my two cents.. by all means this is just my opinion so take it with a pinch of salt.
Forget how to's & tutorials, make it up as you go along. Focus on your machining skills rather than your pen making skills. In this case, learn about cutting threads. Learn about thread pitch. 0.75mm pitch for instance is what is commonly used on pen kits as its a fine thread.. this means it won't be such a loose fit when screwing it together, but of course, this is just one which has been picked. you can use any thread size you want. just through experiance, you will find some work better than others for different applications.
Going on from there, what size tap should I be using? .. well that depends on how big you want the pen to be in your hand.
a full size gent will use M12 on the lid, M10 on the section.
a jr gent will use an M10 on the lid, and an M8 on the section.
Here's my chain of thought:
I want to make a pen which is 15mm in diameter.
I've decided im going to use an M12 thread on the section for the lid.
This means im going to have to drill the lid out 11mm, so I can tap it M12. this means for the most of the lid, the wall thickness will be 2mm. thats plenty thick enough to be strong. but what if i wanted the pen diameter to be 13mm... well, fine... but then the wall thickness is only 1mm!.. might get away with it? ... don't forget about those M12 threads you cut in the lid... now you only have 0.5mm wall thickness by the threads.. the weakest point which takes the highest amount of stress... you can see why these crack for so many people.
Your focus should always be "but how much meat is left" in the above example, if i wanted to go less than 15mm, id probably just use M10 threads on the lids, simply because it will leave more meat surrounding them - and that is the only reason which dictates what size threads I will use on my pens.
I work with metal as you know, so of course I can get away with alot more than someone can who works with wood or acrylic.
As others have said, plan it out... think it out.... think about it even more while you're making it and don't be afraid to change your mind half way through. Just take a step back, go for a coffee and have a think about it before continuing.
put a piece of material in the chuck of your lathe, and grab a kit pen. now copy it. I like to lay a pen on top of the material, grab a marker pen, and mark its length.. where the threads start... where tapers begin.. go crazy.. colour away. that way you can just eyeball it as you're machining it.. cutting up to your marker pen lines etc. measure the depth of a kit pen, and drill it out to that depth. that way you know the refills and springs will fit etc. they've already done the donkey work for you. Yes, fountain pens require a little more work with the sections. are they complicated? not really.. just a few more steps for you to machine. I'll confess I have more pens than I do sections at the moment lol. I like to be in the right frame of mind to do my sections and il make half a dozen at a time. You will also hate to learn that fountain pen nib carriers use very special threads.. For special threads, you will need a special tap.. and a unique tap will cost a fortune. for my JoWo's, I could only source one from the USA, and by the time that tap cam through customs, it cost me somewhere around the tune of £60. In fact thinking about it.. my first kitless pens, I just used the sections off a gent kit just so i could get to grips with making the pen bodies themselves. once I was comfortable with that, I started on the sections.
Hope I've not rambled on too much
I just want to encourage everyone not to think about making pens.... but to think about how to make a bit of material screw onto another .. and drilling a hole in it. once you get the process down, the rest is child's play only limited by your imagination.
Of course if you're planning on using a metal lathe for this endeavor, just come on over to Blackpool and il teach you in person.
