I cannot recommend in strong enough terms that you:
1) Drill on the lathe;
2) Buy and use a digital caliper to measure your tube size and hole size and drill size;
3) Regard the drill size recommended in instructions as a guide only;
4) Buy some good quality drill bits ( Norseman ) in a range of sizes you need
If you can afford them, get fractional, metric, and letter-size bits. . It could take a bit of time and experience to figure out what you actually need and can use.
There are two pen kits for which the turned barrels come out notoriously thin at one or both ends - the Executive, and the Sierra (and all its variants).
Take the Executive for example since it is the one you used and it is the worst.
The drill size recommended is 3/8", so the hole size is 0.375" if the hole is PRECISELY drilled.
The bushing size is 0.420". These dimensions are DIAMETERS.
Subtracting: 0.420 - 0.375 = 0.045" . . Divide that last number by 2 to get the acrylic thickness after drilling and after turning the blank down to the bushing size.
The acrylic thickness you get is 0.045"/2 = 0.0225" ... and that is if you have done the drilling and turning PRECISELY to the recommended sizes.
If you set your calipers to 0.0225" and observe the size, you will be amazed at how small it is !! . You can do it, yes, but everything must go perfectly and precisely !!
It is likely that you won't achieve good results consistently !! . So what do you do ?
1) DO NOT use the Executive kit !!
2) If you insist on using it, use a smaller size drill ... but what are your options ?
Check this table of drill sizes: -
Drill Size Conversion Table
You could use a 9.50mm drill = 0.374" ... Hmmm, not much better than 3/8" = 0.375"
How about a U size drill = 0.368" ..... better check the tube size ... it's 0.360" for Exec
So, if you use this U size drill you should end up with a gap of (0.368 - 0.360)/2 = 0.004" on each side of your brass tube .... that's the space the glue will occupy.
Now, that's really not enough room to get a good gluing of the tube into the blank !!
And, because of the nature of acrylic materials, you might not even be able to get the tube into the hole WITHOUT any glue ... drilling causes the acrylic to heat and expand, so when you take the drill out, the hole gets smaller again; maybe less than 0.368"
Another thing is that your acrylic wall thickness is now (0.420 - 0.368)/2 = 0.026".
That's still not very thick, and with most acrylic materials you will be able to see the brass tube and the irregularities in the glue layer through the acrylic. . NOT SO NICE.
Do you see why I said DO NOT USE AN EXECUTIVE KIT ? . Sierra is NOT much better.
Hopefully you will also see why you NEED calipers, and NEED a range of drill bit sizes.
With regard to drill bit size, other than the U-size, there is no other size that is useful.
As we saw, the 9.50mm size is extremely close to the 0.375" size. . Forget it.
There is no possibility of going smaller than U-size = 0.368" because the next smaller size is 23/64" = 0.359" which is smaller than the brass tube size, and therefore no good.
So you are out of luck. . That summarizes why I never use an Executive with acrylic.
But you have already glued your tube in the blank ... and you have a visible gap.
You can perhaps fill the gap with some very thin CA glue as suggested above. . This may not be successful because of the viscosity of the glue that prevents it flowing into a small gap.
Forgetting about the visibility of the brass and the glue through the acrylic, you may want to continue with what you are doing. . That visibility may not be a problem ... perhaps you have used black acrylic. . If you are very careful in pressing in the metal parts, you might be successful in avoiding a crack in the acrylic. . One way to avoid cracks is to "ream out" the inside diameter of the brass tube using some "coarse" sandpaper (180 grit) wrapped around a small dowel. . Another way is sand down the size of the brass pen kit parts that get pressed into the brass tube. . Then glue them in.
Or, you might decide not to proceed, but you want to save the brass tube. . You can turn off the glued acrylic by putting the blank back on your lathe, and very carefully turning the acrylic off down to the brass tube ... as you get close to the brass, the acrylic will likely chip off , perhaps with the help of a sharp knife. . You can then reuse the brass tube.
I sincerely hope that I haven't discouraged you or offended you by my remarks above.
Feel free to question me further if you want to. . Good luck with your turning ... I like following your progress.
BTW, you will find that table of drill sizes extremely useful ... keep it handy. .
http://www.csgnetwork.com/drillsizeconvert.html
