yet another out of round barrel question

[beloff]

Yes, I am fairly new to turning, but I get the basic premise that when you spin something and hold a tool against it, it should be circular in cross section. My pens aren't. At first, I was concerned with alignment and runout on the lathe (rikon mini). I brought the tailstock to the headstock with points in them and was satisfied with the matching. I used a dial indicator to check the headstock morse taper (0.001") and a faceplate (0.003").

I put a jacobs chuck in and the runout of the mandrel was ~0.010"
with one of the "professional" mandrels with the 1/4" collet chuck it was 0.008". With another collet chuck, I'm back to 0.001" at the headstock.

so, I got some bushings for some atlas-style pens and wall st/gatsby types and started turning. I adjusted the tailstock so that it was snug but not tight, and tightened down the mandrel nut to where is was also snug, but not too tight. result-oval tubes.

I noticed that the bushings fit to the mandrel had some slop and they were noticeably not traveling in a circular orbit when turning. is there some accepted fit for a bushing/mandrel connection? Is there any way to fix the bushings that I have to make round pens? is there a manufacturer who is known for producing bearings with a tight fit to a standard mandrel?

by the way, yes, I am aware of the two mandrel sizes. I have purchased all of my kits from penn state and woodcraft who use the standard ~1/4" mandrel.

 

[leehljp]

There are several possible problems to your dilemma. You mentioned that mandrel being snug and the nut being snug, so if we eliminate those two, -
1. Have you checked the mandrel and its runout?
2. Are you using a 60° live center in the tail stock? If not, this could be the problem.
3. You did mentioned a small amount of slop in the bushings - this is also a possible problem.
4. Have you miked the bushings for roundness? It is not uncommon to have OOR (out of round) bushings, some in differing amounts.
5. You mentioned "snug" on the nut and mandrel; while too tight will cause OOR, so will too loose. I think you are aware of that but I just had to mention it.
6. If you purchase a dead drive center (non-live center) you can use the mandrel-less (aka - "no mandrel" or "turning between centers") method, which recent converts have praised in eliminating their OOR. This will not help if the bushings themselves are OOR.

My bet on your problem would be that 1. you are not using a 60° live center, 2. too much slop in the bushings, 3. OOR bushings or 4. a combination of these three.

I JUST RE-READ your post: The bushings should be tight enough that there is no slop. it should be a smooth fit, IMO.

 

[stevers]

You might want to take the guess work out of the mandrel end of the equation. This is simple to do by straightening your mandrel. It's easy to do, just go to this page and near the bottom is a great tutorial on straightening a mandrel. Good luck.
http://www.woodturner-russ.com/Pen12b.html

 

[beloff]

Thanks for the advice guys. from what I can see, one major part of the problem is in the inner hole of my bushings.

my mandrel has 0.001" runout in the center when fully extended and supported by my live center (60 degree).

the runout on some of the bushings is as high as 0.013", there are some in the 0.003" range, but no matched set meeting that tolerance.

so, one option is to get rid of all of my bushings and get the CNC ones for running between centers. another option is to turn my own bushings. does anyone know of a company who makes consistently good bushings?

 

[leehljp]

Being in Japan, it costs too much to be sending things back, so I usually order two or three pair of bushings at the same time and choose the best pair or two pair for use. I do get a bad one once in a while. Also, one thing I learned on the forum a couple of years ago - treat bushings as consumables, they will wear down.

AS to your bushing being OOR to the tune of .013, that is enough to cause your OOR. As it spins faster, it can and will cause more pronounced OOR. I would look to buy at least two more sets of bushings. OR you could pay more and order some from JohnnyCNC and be done with it. But if you do that you would have to go to the mandrel-less method.

For what it is worth, about 2 years ago, for the Sierra - I ordered 2 sets from one vendor and 2 sets from a different vendor. I didn't check them out until I was back in Japan. Out of 8 bushings, I had three that were not acceptable at all. I did not know until the packing was thrown away and I had them in their storage place. Because of this, there was no way to figure out where the bad ones came from. Experience taught me to check them when I receive them!