I just finished another projec I've been wanting to do for a long time. I have a Norma lead pencil (4-color model) from the 60's that my grandpa gave me when I was a kid. I used it throughout high school, university and I still use it now and then. The problem is the lead size, which is something like 1.5 mm, a long obsolee size that is ever harder to source.
I decided to convert the pen from the obsolete size to the still common 0.9/1.0 mm size.
I won't bother showing all the tools I used but I will describe the process.
This is the Norma pencil in the picture below. You use the 4 noticed slider tabs that circle the pencil to extend one of four colors. I have red, green, blue, and a 2B lead in there. It's a twist pen. You twist the lead holder tip to extend and retract the lead manually. The pencil consists out of quite a few parts and is precision-made. Consider it's about 60 years old and it still works perfectly with regular use. They do not make these things anymore. Under the cap it has an eraser that caps a reservoir that holds about 20 spare leads.
This picture shows one of the internal lead holders with the spring and notched tab attached. The spring is color coded (red, blue, green, black) and visible from the outside so you know what lead you have chosen.
My plan involved inserting a short brass tube that would function as sleeve and reduce the internal diameter of the lead holder tube from 1.15 mm to just a hair under 1 mm. There would have to be sufficient friction for the lead to not fall out.
The read arrow shows where the short brass tube is to be press-fitted once the internal diameter has been sized.
With an 0.9 mm power drill (actual size 0.83 mm) is drilled out the little brass tube insert. I the used a jeweler's/watchmaker's broach to taper the inside of the tube insert. That way the tube has slight restriction that offers the perfect resistance to the lead insert. This is how the lead holding tubes on pencils are made in the factory, although it s done by machines.
Trying to illustrate the idea behind using a broach. A broach is a tool for widening a hole but you can put a taper on the inside of a tube with it. It's more of a sharp-edged cutting tool than a file.
I had to slightly taper one end of the brass insert over a length of 2 mm to be able to press-fit it into the lead holder tip which remained unmodified. I used a handheld micro-power tool, and very fine sandpaper to achieve the proper taper. I then press-fit the sleeve.
A still unmodified original lead holder and the modified with the 0.9 mm press-fit insert. Don't be fooled by the overly large appearance of the unmodified tip. The opening is 1.21 mm but the internal taper restricts the inner diameter to 0.15 mm.
Because of the small size of the parts the process was fiddly. I spent about 2 hours all in all.