Have you ever discovered that your turned pen barrel is not 100% round? You can feel a small ridge on the assembled pen where the pen barrel meets the pen tip, pen cap or centreband and although you know this should not be there, this is not an easy problem to deal with.
Instinctively, you know that something turned on a lathe should be a perfect round (cylindrical). Of course this is true, except when the item you are turning is not accurately centred - this is known as off-centre turning and this technique can be used to produce amazing things on a lathe, but it is frustrating when it happens unintentionally. Like when you are trying to make a perfect pen!
Your mandrel shaft may bend very slightly over time and through use. Since one normally uses a few spacers on the mandrel, if your mandrel is bent, your pen barrel will be off-centre, ultimately producing a very-slightly-oval shaped pen barrel.
This can be solved by returning to basic principles of spindle turning (your pen barrel is basically a spindle) using a specially designed drive centre and live centre pair - the Between Centres Mandrel System.
The Between Centres Mandrel System consists of a drive centre and live centre pair that have been specially designed with a 7mm (actually it is 6.2mm) shaft onto which all your standard pen bushings will fit.
Because it operates using the primary spindle turning principle, using this will enable you to turn your pen blanks into consistent, pure-round, concentric pen barrels. Gone are the days of out-of-round pen barrels as there is no mandrel shaft to flex or bend.
Although this system can be more time consuming than a standard pen mandrel when turning double-barrel pens, it does offer some time-saving when turning single barrels as there is no thumb-screw - when you slide the tailstock forward/back, the barrel is instantly tight/loose.