Watch the quick tip from Stuart Batty on knocking off the edges.
Round in no time:
Quick Tip - Knocking Down the Corners - YouTube
No doubt, there are a infinity of ways and tools that can be used to get a piece of wood from square to round and while the video suggested show a great way and a super sharp and adequate gauge size and shape, this may change slightly when we are dealing with small blanks such as pen blanks and more importantly with blanks that may be a composition of various materials glued or cast together.
The very first aspect of these, is the turning speed, too fast (about 4,000rpm or so) and the blank and the chances are, the blank will disintegrate/vibrate to destruction all by itself, the "G" forces of the centrifugal super speed are enough to break it apart, particularly after being drilled where, material separation may have occurred from heat or drill forces, without you know about it or be able to see it.
Using a "bad" gluing technique will not help much so, and without touching the blank with any tools, putting it through exaggerated rotation speeds, is more than enough to "blow" the blank to bits. A loosely glued tube in a "fractured" blank (wood and resin, particularly), will require very little forces from a cutting tool to "rip" a chunk apart and this is when the use of a flap disc where the 2 surfaces are rotating in opposite directions, offers a much softer/gentler touch to any of the sharp edges of the square blank.
Understand how the flap disc system works, is a lot more important than just grab a 4" grinder and install a flap disc on it and take it to the spinning "piece'" in the lathe so, and like any other "system", there is the right way to go about it and there is the wrong way to go about it, also.
Having options, is all I'm talking about, what suits one person does not suit the next however, I like to know that I'm aware of options and know what they are, I will be in a much better position to find something that suits me...!
However, speed is far too important to be ignored, learn about "G" forces from centrifugal rotation/spinning and you will understand what I'm talking about...!
Cheers
George