To be honest, some people DO prefer mandrels and do OK. The problem with mandrels, particularly with those new to the lathe - are the number of very minor things that can and do cause issues. Mandrels flex and get bent - so little that it is not noticed by the naked eye, but it causes out of round (or non-concentric) blanks. Mandrels flex just a tiny bit when too much tail stock pressure is applied. Mandrels flex when a chisel of choice is just a tiny bit dull, and the pen turner adds just a bit more pressure.
There are the Mandrel Savers, and some work well. These use mandrels but the mandrel slides - lengthens and shortens according to the need. The shorter the less vibration and flexing.
TBC - Turning Between Centers: It only allows one blank to be turned at once.
Here is a layout - a picture is worth a thousand words:
Custom Bushings on TBC setup
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Mandrel bushings on TBC lathe setup
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Mandrel and custom bushings can both be used on TBC
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PICTURE With Mandrel which I personally rarely use:
Mandrel layout.
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IN the top two, you can see the blank on the lathe with a bushing on each end. On the bottom you can see the blank off the lathe and two sets of bushings. While it is difficult to see without knowing what to look for, one set of bushings is ordinary bushings and the other set of bushings have a cone shaped divot that fits perfectly with the head stock dead drive and tail stock live center. EITHER set of bushings work.
The process is to use the bushings on a blank until it is round and down to size (Use calipers to determine the size, not the bushings). Then take the bushings off for finishing. This works particularly well when finishing oily woods such as ebonies. Taking bushings off of ebony the woods after finishing with CA will often cause the CA to lift up off of the blank. Therefore, finishing is always easier without bushings, unless you use specialty nylon or other non stick bushings to use when finishing.
What is needed for TBC is a 60° live center for the tail stock and a 60° dead or drive center for the head stock and a set of bushings for the pen you are making. That is it.