Pen offset?

[beep119]

Hello everyone, still learning...
I have turned a few pens, wood and arcrylic, but I find when I do a two blank pen like a slimline I am getting the pen offset in the middle a bit. And it's very hard to fix to make it correct. If you see in the pic the middle part is smooth on one side with the ring and off on the other side. They are of course lined up so it's even on one side. But if I rotate the pen it's off all around.
On the lathe they are right down to the bushings.
I checked the mandrel on a known flat surface and it's straight
Could it be my lathe? I do have a very cheap lathe.
Why could this be happening?

 

[jcm71]

It looks as if the ends of your barrels are not exactly perpendicular to your tubes. Check to make sure they are exactly square before mounting them on the lathe for turning. Also make sure your lathe centers are aligned.

 

[Carl Fisher]

Look at your blank end on after turning. It looks like you have an eccentric turning issue (also commonly called out of round which technically isn't correct). One side of the blank will be thinner around the tube and one side will be thicker.

Something in your setup is out of alignment or bowed. If you're using a mandrel, it could be a bent mandrel or too much pressure. It can also be caused by not getting a good squared end on the blank, tailstock center out of alignment with the headstock center, bushings not true along with a few other possibilities.

Lots of possible problems and you'll just have to work through each piece until you find out what the cause is.

 

[Paul in OKC]

It looks as if the ends of your barrels are not exactly perpendicular to your tubes. Check to make sure they are exactly square before mounting them on the lathe for turning. Also make sure your lathe centers are aligned.

My first thought as well.

 

[CREID]

One thing I used to do was not put both blanks on the mandrel at the same time. It worked for me. I was probably using too much pressure and bending the mandrel. With only one barrel on the mandrel this seems to help.
Just my 2 1/2 cents

Curt

 

[magpens]

Could you be overtightening the nut on the mandrel ?

Are you being careful to trim the ends of the blank square with the axis of the blank ?

Also ... the bushings .... :

I used to have similar problems and decided to do away with bushings. I now turn all pen blanks using the TBC technique (turning between centers). You need a dead center in the headstock, a live center in the tailstock, and a pair of vernier calipers to measure the blank as you turn it. Try doing a search on this website.

If the part of the bushing that slides into the brass tube is a little too small, you can end up with the blank not properly centered on the mandrel. The result is the wall of the turned blank not being the same thickness all the way around.

 

[TurtleTom]

More times than not it's a bent mandrel. There are several, (don't ask me where, Search doesn't work for me) articles on straightening a bent mandrel.

 

[Joey-Nieves]

Few things that affect this; uneven ends, bent or over tightened mandrel, to much drift between head stock and tail stock.

First verify the alignment of your lathe; put a 60 deg center in the head stock and a 60 deg center in the tail they should align perfectly, remember to clean the inside and outside tapper first. some drifts can be corrected. once this is verified check your mandrel for straightness, clean the tapper and insert in the head stock, I have to clean and insert various times in my lathe before getting it straight.
Now that you know that everything is straight, set up you blanks in the mandrel and tighten gently and verify if it's bending in the middle, if so you are over tightening the mandrel. I use a mandrel saver, but these also are subjected to over tightening.
For pens larger than 7mm I turn between centers, also I once I reach the bushings I remove them a clean the ends a little more so that I have the same width of the pen part.
There are a few things to keep in mind;
-pen parts can be bigger or smaller than the bushings, so when possible use the part as reference or a caliper.
-when applying CA keep in mind that it takes up space, so I undercut the blank to compensate, I use a lot of CA in some cases>

 

[beep119]

thank you everyone, gives me things to check.

 

[Sabaharr]

It is my opinion (for what that is worth) that over tightening the knurled nut does not bend a mandrel. If anything it is pulling it out straight. Hunkering down on the mandrel shaft with the tailstock however will make a mandrel bow when spinning but when pressure is released it might just spring back out straight making it difficult to tell that is the problem. Put you mandrel in your headstock naked (the mandrel, not you) and don't bring up the tailstock to meet it. On slow speed turn on the lathe and see if the mandrel wobbles. Yes, bent mandrel, no, something else. Time to start doing all the other tests. I found that my headstock was cocked a little off center on my JET. The mandrel rubbed one side of the mandrel saver and flexed when entering it. This made a very slight off center like you pic showed. Had to do a bit of metal work on the sled to fix it.

If you are wondering how tight to tighten the nut on the mandrel then try this. Take two flat washers and a split washer that will just fit over your mandrel shaft. Put the split one between the flats and slide them all the way down the mandrel toward the headstock end. Then mount everything else as usual. Tighten the mandrel nut until you flatten out the split washer. That's enough. You might find you have been putting a lot more force than necessary on that nut.

 

[Joey-Nieves]

It is my opinion (for what that is worth) that over tightening the knurled nut does not bend a mandrel. If anything it is pulling it out straight. Hunkering down on the mandrel shaft with the tailstock however will make a mandrel bow when spinning but when pressure is released it might just spring back out straight making it difficult to tell that is the problem. Put you mandrel in your headstock naked (the mandrel, not you) and don't bring up the tailstock to meet it. On slow speed turn on the lathe and see if the mandrel wobbles. Yes, bent mandrel, no, something else. Time to start doing all the other tests. I found that my headstock was cocked a little off center on my JET. The mandrel rubbed one side of the mandrel saver and flexed when entering it. This made a very slight off center like you pic showed. Had to do a bit of metal work on the sled to fix it.

If you are wondering how tight to tighten the nut on the mandrel then try this. Take two flat washers and a split washer that will just fit over your mandrel shaft. Put the split one between the flats and slide them all the way down the mandrel toward the head stock end. Then mount everything else as usual. Tighten the mandrel nut until you flatten out the split washer. That's enough. You might find you have been putting a lot more force than necessary on that nut.

Stephen:
There has been much discussion this week about the nut(not me) and the mandrel. I can tell you by experience that to much pressure will make the mandrel bow and even if you use a mandrel saver this will happen.

Point is I like to experiment, so when I can, I actually put most of the stuff I read here in practice.

As I said, I don't' always use a mandrel, but when I do I use (XX) for 7 mm pens, besides the washer, other members have suggested tightening until the blank almost does not spin by hand, small pressure after the nut touches the blank, sneak in to it as you turn etc. what ever works for you, but if you notice that the blank is not round around the bushing you may have to much pressure on your work.

Finally, always verify your lathe to be running true, that your tappers are clean and do not have rust, confirm as Stephen suggested that the mandrel is running true when free and naked in the head stock and that once you work is secure the mandrel is not bowed in the middle. Avoid turning naked because of the splinters!

Stay thirsty my friends!
Joey

 

[rd_ab_penman]

TBC solves all these issues with bent mandrels and out of round blanks.

Les