Take a look at this instruction sheet for a 30 Caliber Bolt Action pen kit :
You will see that to make this pen, you are told that you will need a "7 mm" mandrel and a set
of bushings that match the requirements of the pen kit. You will also need a 3/8" drill bit to drill your blank.
The bushings, of course, have axial holes that allow them to fit onto the mandrel.
At the bottom of the instruction sheet you will see a diagram of the bushing set.
You will see that the bushings have central holes that are 0.248" in diameter.
And these bushings slide onto the "7 mm" mandrel.
But a "7 mm" mandrel, if indeed it has a diameter of 7 mm, would be 0.275" in diameter.
So how can the bushings with holes of 0.248" diameter fit on to a rod (the mandrel) which is 0.275" in diameter ? ? ? ? ?
I did measure the hole diameter and it is indeed 0.248".
But I don't use mandrels any more so I could not check a mandrel diameter, but I suspect it is probably about 0.002" or 0.003" smaller, or about 0.245".
( I do my pen turning by using the TBC method, or Turning-Between-Centers.)
This same scenario plays out in the instruction sheets for many, many pen kits.
So why is the mandrel called a "7 mm mandrel"?
It is more accurately a "6 mm mandrel".
I must be missing something.
Maybe I knew the answer at one time. Please help me remember.
You will see that to make this pen, you are told that you will need a "7 mm" mandrel and a set
of bushings that match the requirements of the pen kit. You will also need a 3/8" drill bit to drill your blank.
The bushings, of course, have axial holes that allow them to fit onto the mandrel.
At the bottom of the instruction sheet you will see a diagram of the bushing set.
You will see that the bushings have central holes that are 0.248" in diameter.
And these bushings slide onto the "7 mm" mandrel.
But a "7 mm" mandrel, if indeed it has a diameter of 7 mm, would be 0.275" in diameter.
So how can the bushings with holes of 0.248" diameter fit on to a rod (the mandrel) which is 0.275" in diameter ? ? ? ? ?
I did measure the hole diameter and it is indeed 0.248".
But I don't use mandrels any more so I could not check a mandrel diameter, but I suspect it is probably about 0.002" or 0.003" smaller, or about 0.245".
( I do my pen turning by using the TBC method, or Turning-Between-Centers.)
This same scenario plays out in the instruction sheets for many, many pen kits.
So why is the mandrel called a "7 mm mandrel"?
It is more accurately a "6 mm mandrel".
I must be missing something.
Maybe I knew the answer at one time. Please help me remember.
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