@MiteyF
Thank you for your comments. . I hope I am neither misinterpreting you nor going too far with this reply. My main reason for specifying the 4-jaw scroll chuck (and mentioning the 4-jaw independent) was to make "newbies" aware of having to make that choice in view of the advantage of using a 4-jaw over a 3-jaw on a mini-lathe for turning pens. . I also hope that I am not being overly academic. . There are several considerations.
I think the concept of accuracy is relative to, and dictated by, the task at hand. . As Dieseldoc said, we pen turners don't ever need the accuracy that a machine shop needs so the rigorous carefulness required in using an independent jaw chuck is both overkill and inconvenient. . If you buy a 4-jaw independent you are forcing yourself, all the time, into a delicate measuring regimen to avoid out-of-round and out-of-balance. . For drilling a pen blank the mini-lathe plus 4-jaw scroll offers very adequate accuracy that far exceeds that achievable with a drill press, plus allows you to accommodate all blank diameters in the appropriate range normally used for pens. . By "very adequate accuracy" I am thinking about 0.5 to 1 thou deviation between the desired pen blank axis and the machine rotational axis (but I could be kidding myself a little bit).
As I see it, the primary use for a 4-jaw independent chuck, in the context of pen turning, would be to deliberately "dial in" (your terminology) some off-axis deviation to achieve, for example, the shaping of the cross-section (through multiple adjustments) to be other than round. I realize that such a chuck can be used for other things, such as gripping material stock which has asymmetrical shape, or converting an oval-shaped cross-section to a circular cross-section, etc., etc. But I think I may be going too far by even suggesting such things here.
The point is .... a 4-jaw independent is just not appropriate for "normal" pen turning .... in my opinion, and in my experience.
EDIT:
I have to admit, sheepishly, that I only now realize that what I should probably have suggested, initially, is a 4-inch diameter 3-jaw chuck for the mini-lathe in order to get the bigger throat diameter that accepts "largish" pen blanks for the drilling operation. . But, in my own defense, one could argue that a 4-inch 4-jaw (scroll chuck) might in some cases be preferable, depending on the particular application. .
That's what I use .... and hence my "bias" !! Enough said .... I guess !! . If I could completely delete this reply to you, I probably would
