Please check my thought process here

[Brian Brown]

So am I losing my mind here, or what? :confused: I have read a few threads lately about pen mandrels bowing. One of the theories is that if you overtighten the brass nut on the mandrell, that it will cause the mandrel shaft to bow. Now I'm so engineer, so correct me if I am wrong. It seems to me that if you tighten the nut more, so that you are compressing the wood blank more, that you are in theory pulling on the mandrell shaft, and again, in theory, streaching said shaft. Rather than bowing, this would have the opposite effect of straitening the mandrel. In actual practice, I can't imagine anybody putting enough pressure on the shaft to have any effect. The wood would compress and blow out, or the threads on the brass nut would strip long before the necessary amount of shaft straitening tension was reached. :eek: Not a really important question, but just one of those things that bounces around in my empty head occasionally crossing my thought process.

Now the part about overtightening the tailstock, that would bow the shaft, and cause some jump roping. This is my preferred method for acheiving oval pen shafts! :D:D

[Ken Fitzgerald]

Brian...I agree.

I try not to overtighten the brass thumb nut in case there is any error on the ends of the blank such that the ends of the blank aren't square. If the ends of the blank aren't square and you tightened the brass thumb nut too tight, you could cause the blank to "tilt" slightly on the mandrel.

Really, if the mandrel is true, the only reason to tighten the brass nut is to hold the blanks tight enough for turning. With sharp tools not a lot of torque would be requried.

[Ken Fitzgerald]

Brian,

Overtightening the tailstock quill would have a geater effect IMHO. That's why I use a "Friction" fit....

I lock the tailstock down within quill range. Then I turn the handle and bring the quill out until it just engages the end of the mandrel. Then, with the lathe turning at a slow speed, I rest my fingers on the revolving cone on the live center and apply a little friction. Just a small amount. If my fingers stop the cone from turning, I put just a little more tension on the quill until the revolving cone turns. Then I lock the quill. Again...you just need a small enough amount of tension to cause the mandrel to spin and allow you to turn the wood. With sharp tools, less tension is required.

[Paul Douglass]

I think too tight a tail stock would be the bowing. I like Kens method and I think I will start practicing it. I only turn one half of a pen at a time. Thus a shorter mandril, less likely to bow and wobble.

[Cyril Griesbach]

Over tightening the tail stock will bow the mandrel. No need to ask how I know this :eek:

[David Drickhamer]

Bowing can also occur if you use a dull tool and a heavy pressure to attempt to make it cut.
On some pens I've started turning without a mandrel.

[Curt Fuller]

I think some of the oblong problem with pens is also caused by too much sanding. You don't think of it when sanding but even on something as small as a pen, the sandpaper is removing more material on the flat of the grain than on the end of the grain (if that makes sense). So you should try to make as clean and smooth of a cut with either your skew or spindle gouge as possible and as close to the size of the bushing as possible. Sanding should only be minimal and with light grits to give a fine finish to the wood, but not to finish shaping the wood. Shaping with coarse (320 or coarser) sandpaper will often result in an oblong shape.

But on the original question, I agree Brian. I can't see how tightening the knurled nut on the mandrel is going to tweak anything, unless you've got a pair of vice grips on it or something.

[Michael O'Sullivan]

I would have thought that over tightening the tailstock would be the more likely culprit. Even assuming that the laws of physics would cause the mandrel to bow based on the compression of the nut, I think it is unlikely that my wrist could generate the torque to do so on a 7 mm or so steel shaft.