Help me understand bent mandrels

[Cliff Holmes]

My son is having an issue with out-of-round pens. Reading on this forum, I'm hearing that a bent mandrel can cause this.

What I'm not comprehending is the geometry of how this happens. First of all, I don't see how the mandrel bends while encased in wood. It seems you'd end up with a round pen with a curved hole in it?

Or, is the mandrel bending the wood as it's being turned, i.e. as you remove more wood the wood *then* begins to deform?

So confused ...

[Chris Stolicky]

A bent mandrel happens when you put too much compression on the mandrel. This happens either by tightening the nut too tight or putting too much pressure on it from the tail stock. The hole in the middle stays round, but the outside becomes more of an oval shape.

The mandrel's diameter is less than the brass tubes, so it actually has room to bend in between the bushings. It does not take a lot of force to bend the little 1/4" rod. It also does not take much bending to be magnified on the outside of the blank.

I have used the adjustable mandrels for while now, turning one barrel at a time, because it is more difficult to bend the mandrel. In engineering speak, "its less of a long slender column in compression."

I hope this helps.

[Thom Sturgill]

Think about a hammer handle, first you turn it round then you move the live center (tail stock end) a little bit off center and turn again. Now you are flattening one side. Move a little to the other side of center and turn and the handle is oval. Similarly, if the mandrel has a very slight arc to it due to too much tailstock pressure (or a physically pre-bent mandrel) then the middle of the mandrel is spinning around the center instead of on the center so one side of the turning is closer to the center than the other. It does not have to be much, only a few thousanths is visible due to the thinness of the wood layer when you are done.
I haven't tried them, but this is why you are seeing special setups to run between centers without the mandrel, of course you have to turn each piece separately.

[Horst Hohoff]

I had the same issue and gave up using a mandrel.
I turn my pens between centers now.
Problem solved.

Horst

[Bob Vavricka]

A bent mandrel happens when you put too much compression on the mandrel. This happens either by tightening the nut too tight or putting too much pressure on it from the tail stock.
I keep hearing that tightening the nut too tight can cause this--are you talking about the nut at the tailstock end? Seems to me that the nut is actually putting tension on the rod and I can't figure out how that would cause it bend the rod.
Bob V.

[Cliff Holmes]

Hmmm, I still don't get it. The ends of the mandrel are still spinning in the same place. What does it matter if the middle is out of this line? The wood is still spinning around the same centerline as it started. I don't see how the path of the mandrel matters at all unless the wood is bending after being rounded.

[Antonio Martinez]

Think of a jump rope being swung by two people. THe ends don't move but the middle does because of the flexing caused by the pressure placed on the mandrel. THe mandrel is flexible and can be easily bent. Now spin that bent rod and you get the out-of-round blank (like watching a jump rope).

[Cliff Holmes]

Ok, there's got to be something I'm just not getting. If the wood's not bending, why does it matter if the mandrel does?

[Mike Wenman]

Echoing on the previous posts, too much pressure applied with the mandrel nut or tail stock would cause the out of round blanks or even a mis-handled mandrel that put an ever-so-slight bend in it along it's length could cause this. My suggestion would be to ditch the mandrel and start turning the pen blanks one at a time betwwn centers. All you need is a 60 deg dead center in the headstock and a 60 degree live center in the tail stock. Insert the bushings into your blank as you normally would, place it on the lathe and start turning. (note: won't be able to do slimlines this way currently) It is amazing how much better the fit and finish can become on your pens. I got converted to this and won't use a mandrel again!

Switch to between centers turning. You'll be glad you did when you compare the differences in 2 turned blanks.

Mike

[Thom Sturgill]

Cliff, try this - point your index fingers at each other and touch them together at an angle. Now spin that connection point - that is the center of the mandrel with the two pieces of brass tubing - one on each finger. The nuckels stay stationary and turn round but the end at the center point get turned off center, when closer to your body you would cut closer to the tubes, when away from you, you would leave more wood on the tubes because the outside would be round. Turning each piece between centers would stop that or if he has an adjustable mandrel he could shorten it and turn only one tube at a time.

The tailstock should apply only enough pressure to keep the end from whipping around.

[Paul Atkins]

I'm with Cliff - off center, not oval. If you could turn an oval pen without changing the centers, I'd like to see it. Might fit your fingers better.

[Jim Myers]

I understand the out of round mandrel causing oval pens but what I am having a question on is the whole nut too tight part.

When you tighten the nut are you not in effect pulling the end of the mandrel away from the headstock through the bushings?

I think my issue was the tailstock was pushing it out of round but I still think the nut is pulling it straight (I am probably wrong)

[Barry Elder]

The analogy of putting 5 pounds of sugar in a 2 pound bag equates to tightening the nut too tight. Something has to move!

[Mike Wenman]

I understand the out of round mandrel causing oval pens but what I am having a question on is the whole nut too tight part.

When you tighten the nut are you not in effect pulling the end of the mandrel away from the headstock through the bushings?

I think my issue was the tailstock was pushing it out of round but I still think the nut is pulling it straight (I am probably wrong)

When you put your blanks on the mandrel, there is still some play or slop between the bushing and the mandrel even though minute. Often a person will over-tighten the nut and that will put force on the blanks which in turn pushes into the mandrel causing it to flex and that will ultimately cause an out of round blank when turned.

As I and another mentioned before, turning 1 blank at a time between a 60 deg live and dead center is the bees knees :D

[Jim Myers]

Mike your reply makes sense thank you.

I am still not there enough to turn between centers as I like to envision how the pen looks from top to bottom while on the mandrel.

I will not tighten it up as much from now on. I might get to turning between centers eventually but I am still in the hobby having fun stage and just gifting the pens as I see all the flaws. The people I give them too do not notice but I know they are there.

[Dennis McGarry]

When I first started, I would tighten the nut tight then the tail stock. Often times I would get a slight oor.

I have since moved to a different method. I tighten the nut just enough to snug, then move the tail stock up and lock down, turn the lath by hand and tighten the tail stock just enough to make the live center move. then tighten the nut a a tad more.

I have found this helps eliminate the flexing of the mandrel and allows me to get a feel for just tight enough.