What size is a "7 mm" mandrel ?

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magpens

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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.
 
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hooked

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When you drill a 7mm hole and insert the tube, the internal of the tube is ~.248. I think that the name refers to the idea that a 7mm tube fits on it rather than the mandrel being 7mm itself. I believe I read this on IAP at one point during a similar discussion.
 

magpens

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When you drill a 7mm hole and insert the tube, the internal diam. of the tube is ~.248. I think that the name refers to the idea that a 7mm tube fits on it rather than the mandrel being 7mm itself. I believe I read this on IAP at one point during a similar discussion.

Thanks ! That's it ! . . . . ( . . bangs head against wall ! )

I'd "forgotten" (although I really hadn't) that the mandrel was initially for slimline ("7 mm") kit tubes which are actually smaller than "7 mm".

Then the same mandrel, through appropriate bushings, has been used repeatedly for the larger pen kits as they have been introduced.
The 30 Caliber Bolt Action pen kit that I referred to in my OP is just one such larger and newer pen kit.

Hopefully my asking the question, together with the above simple answer, will be helpful for some new pen turners as they join our ranks.

I have been talking to a few new turners recently so it may be a good thing that I asked the question explicitly for them.
Of course, reviewing such concepts is never a bad thing for some of the rest of us.
 
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rherrell

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I can confirm a 7mm tube (slimline) has an I.D. of .248", that's the size I make my 7mm pin chucks. 7mm is the size of the drill bit for the hole.
 

Edgar

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Berea's spec for their "A" mandrel (aka "7mm mandrel") is 0.245" to 0.247" diameter. It has to be slightly smaller than the ID of bushings & 7mm tubes.

Some "7mm mandrels" by other manufacturers are not always this precise.
 

ed4copies

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If anyone has a mandrel purchased recently from Rockler, please measure it.

I BELIEVE (have not taken time to buy one to confirm) that their mandrel is a little "fatter" based on recent complaints that
some of our bushings don't fit.

THANKS for any input
 
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