Mandrel problems

[ChrisD123]

Hey, so I have been making pens for just under a year now and all has gone fairly well however recently i have found that my bushing are not spinning circular to the mandrel and so after finishing, one side is thicker then the other. Which makes for a terrible feeling pen. I have replaced my mandrel and used new bushing and none of these are fixing the problem im just wondering if theres something obvious im missing. i use a metal lathe and chuck a universal mandrel in it with a tailstock and a 60% live centre any help would be great. Thanks!

 

[leehljp]

I don't know if you want to hear this or not, but this is where TBC shines. (turning between centers)

Adjustable mandrels help some in that they reduce flex. But it is just as much technique (the turner) as it is non-precise bushings and other problems.

I ALWAYS ordered 2 complete sets of bushings for a pen type because I learned early on that bushings that come in new are do not necessarily have the hole drilled exactly centered. Use your calipers to measure the hole in the bushing relative to each side. I found that about 25% are off from some to significantly.

As to mandrels, when doing 2 blanks on a mandrel, the mandrel is open to flex and a few other prolems. Flex is more technique than anything else, but still it is a problem that needs to be recognized.

TBC eliminates the flex caused by too much user pressure or too tight of a tail stock. I am assuming that since you are using a metal lathe that you are using a 60° live center. If not, that causes some off center wobble.

 

[randyrls]

Chris; Is the OOR condition at the head stock or tail stock end? That is your first clue. Check and clean the tapers in the head stock and tail stock. Make sure the tapers are squeeky clean. Then check the bushings for the same thing. Bushings don't last forever.

To test for out-of-round problems, mount a bare tube and the bushings on the lathe. Turn the lathe on and lay a gouge or any tool with a round edge across the bushings and tube at both ends. Any vibration means the tube isn't centered on the mandrel, or the bushings are OOR. Use caution when doing this, you don't want to catch the tool on anything.

 

[Richard Gibson]

Also be sure that your blank ends are square to the tube. Had the same problem you did and this was my problem.

 

[Randy Simmons]

You may be over tightening the tailstock as well, if you are using the live center and brass nut. I got myself a mandrel saver, which helps to eliminate pretty much all flex, from then until I went between centers.

 

[frank123]

Since you are using a chuck -I'm assuming a scroll chuck of some sort, not a four jaw independent- the first thing I would do is chuck up a piece of TGP rod -or drill rod lacking that- the same size as the mandrel you are chucking and measure for trueness using a dial indicator at the chuck and at various points along the shaft away from the headstock.

Unless you are using a very expensive and precision chuck on your lathe there is always a potential for the chuck jaws gripping the mandrel shaft off center from the lathe axis, or gripping things off center at some but not other diameters of chucking. Even then there is also a potential for an angular misalignment of the either the chuck or the headstock itself which will magnify itself the closer to the tailstock you measure.

 

[sbell111]

I would first make sure that I wasn't overtightening the tailstock. Then, I would verify that the mandrel is straight. Then, I'd replace the mandrel shaft that I bent by overtightening the tailstock and try to change my future behavior.