How to center on remount for inside of bowl

[sasjzl]

Hi all,

I am still fighting back but need some specific advice. I have a PSI chuck and just bought a Oneway Termite hollower and am doing better but here is a situation I do not understand. I was going to send a picture but I am sure you have all seen this. I am turning from a slice of a log that has been rounded pretty well on my bandsaw. Turning the outside is not that big a deal. My problem is that after I make my tenon and remount it in my jaw clamps, I have not yet succeeded getting it centered. Two main reasons I know it is not centered. I always break through on one side wall whereas the wall opposite is at least half an inch. Also I can tell by the tool bouncing off the piece. At least this explains to me why I am breaking through one side of the wall when I try to get it thinner than about half an inch. So it seems to me that I am doing one or more of the following things incorrectly.

- Not creating a centerd tenon at the bottom of the bowl for my jaw clamps to hold on to.
Although I am not thinking that this is the problem because I do not see how it would be physically possible to make an uncentered tenon when the piece is sitting on a faceplate alone.
There can only be one axis. How could I create an uncentered tenon turning on one axis?

- I am not mounting, or rather remounting, the bowl on the same axis as when I turned the outside. I really try to create a simple solid tenon with a straight shoulder on it of about half an inch but I must be mounting it off its original axis?

So I think my challenge is to learn how to remount a bowl to turn the inside and somehow make absolutely sure that when I remount it it is true, or at least a heck of a lot more true than it was on my last two bowls.

I am looking forward to taking the segmented turning class at Woodcraft in Raliegh and am hoping alot of my newbie questions will be answered there. The termite cutter does work nicely.

Thanks very much,
Jim Lee

 

[erasmussen]

try bringing the tail stock up with a cone center, to hold the back centered when you remount it

 

[DaveO]

Are you turning green wood? If so you could be having wood movement occurring making your roughed-out "outside" not round. I had a problem with this when I was interrupted while turning a green bowl, when I got back to it the next day it wasn't round. If you are taking a long time turning or creating a lot of heat either by tool use or sanding this could be happening.

Dave

 

[timf67]

I am a novice as well, but I noticed that when I was trying to turn a natural edge bowl that when I remounted it the weight imbalance cased the bowl to "wobble". As a result, I was removing wood unevenly and the tool was bouncing and catching. I think in my case that a better lathe would have helped steady the piece. What kind of lathe are you using? How old is it, and are the bearing worn possibly?

 

[Robert Arrowood]

:icon_scraHave you tried to see if the piece is "true" round when you remount? And are you checking to make sure that the chuck is staying tight? I've had the wood give way after some turning.Good luck.All my stuff is out of round because of the lathe:BangHead:.But thats a whole nother thread.

 

[Splinter]

Hi all,

- I am not mounting, or rather remounting, the bowl on the same axis as when I turned the outside. I really try to create a simple solid tenon with a straight shoulder on it of about half an inch but I must be mounting it off its original axis?

Thanks very much,
Jim Lee

Jim,

Are you saying that your tenon is 1/2" "long"? When you mount the bowl in the chuck does the face of the jaws seat tight up against the bottom of the bowl. If not they should. This provides more support and helps keep the bowl stable while you are turning. Don't let the tenon "bottom out" against the inside back of the jaws.

 

[sasjzl]

"Are you saying that your tenon is 1/2" "long"? When you mount the bowl in the chuck does the face of the jaws seat tight up against the bottom of the bowl. If not they should. This provides more support and helps keep the bowl stable while you are turning. Don't let the tenon "bottom out" against the inside back of the jaws."

The tenon is longer than 1/2". I don't have it right here but it is about an inch or an inch and a half. The jaw clamp chuck is new to me but I am being really careful about putting it in 'evenly' at least by sight. And I am getting it as tight as I dare. Also I made a point of having the end of the clamp against the shoulder of the tenon. The tenon is not bottoming out and by sight it looks even all around and I test the fit periodically and it was good. The tenon itself is not very pretty but it does look round and it has to be centered right?

I am thinking that what I have to do is to be much more careful to check that it is centered as well as possible. I am just real curious as to how it could have gotten so out of whack. At least it answers the question I had about busting through on one side of the wall.

I am doing the turning in one session. It is green, white oak wood. The lathe is a Shopsmith and the bearings are in real good shape. It works tight and accurately when I am doing other things with it.

Thanks,

Jim Lee

 

[sasjzl]

"try bringing the tail stock up with a cone center, to hold the back centered when you remount it".

Earl, Had to think about that for a few but I think I understand what you are suggesting. Bring up the tail stock as if I was going to be turning it holding at both ends but just turn it on to see if it is true and if it is not then adjust it using the tailstock so that I have a fixed point to tighten the chuck clamp on. I am wondering if my problem is that the shoulder of the tenon is not absolutely square to the axis of the bowl. I do not see why it would not be but I am sure that stranger things have happened and I am a rank newbie turner.

Thanks,
Jim Lee

 

[NCTurner]

http://www.gulfcoastwoodturners.org/RESOURCES/Guide to Bowl Making.pdf

 

[Mike Davis]

Jim,

I think you may want an experienced turner to check your set up and your procedure in mounting the bowl both in turning the tenon and in remounting to turn the inside.

There are several things that could cause your problem and could even be a combination of two or more problems.

Several have already been mentioned and I can think of a few more, but I don't want to send you on a wild goose chase. I think it best to have everything checked and verified by an unbiased observer.

That's my advice and I'm sticking to it. :dontknow:

 

[Trent Mason]

Jim,

I think that your main problems are

1. Your tennon is too long. I don't think you need a 1/2" tennon. That is a mistake that I made for a while. It is very difficult to get it centered if it is bottoming out in the chuck. I would go 1/4" or maybe 3/8". Measure the depth of the jaws and just make sure your tennon is less than that.

2. If it is centered and you're going through one wall and the wall thicknesses are off that much, then your piece is not round. It could've been movement, as Dave suggested, but more than likely, it is not round. Here is a picture of a natural edged cypress bowl that I turned a while back. You can clearly see the difference in the walls thickness. HTH. :icon_thum

 

[Gunslinger]

One other thing to check. I had the same problem a few weeks ago, and much to my chagrin, I found I had a screw loose. :embaresse Nay, even missing. While starting to clean up I noticed an odd screw on the floor. Hmmm.. Yep, two screws were missing from the jaws of my chuck and another was about ½ turn loose.
The jaws still tightened properly and the wood was seated properly. When I placed the bowl to the chuck everything was fine until I turned the lathe on. No wonder I could not get it round.
My guess is that the tendon is too long on your bowl but it won’t hurt to check the chuck.
Mike

 

[Gunslinger]

Jim,
If the jaws are loose on the chuck make sure you remove them and clean between the mating services well. Don’t just retighten.
Also, did you check to make sure the head and tail stock line up? Bring the two together with drive centers and lock the tailstock. If they look like they line up, move the tailstock back, adjust the quill all the way forward and recheck with the headstock. I’m not sure but I would assume that a 1/16 error when they touch would be greatly magnified when separated by 6 to 8 inches.
Mike

 

[flyrod444]

The tenon has to be done right to work. It is hard to explain how it is done so you would be best to have someone show you first hand. One has to have the top surface of the jaws flat on the bottom of the bowl for chucks to hold bowls well. I always mark my tenon between the #1 and #2 jaws so that it is centered better when I take a bowl out and put it back in the chuck.
I only rough out the shape of the outside of my bowls and then put my tenon on. I then mount the bowl in the chuck and retrue the outside and do my finial cut. This way your bowl will always be true when you start hollowing out the inside. There really isn't much reason to get the bowl pefect shape before mounting the bowl into the chuck, because one hardly ever gets its to mount in perfectly centered in the chuck. Dove tail type chucks do better than the standard jaws on One Way Chucks on centering the bowl, but I don't like them. The Dove tail has to be perfect or they don't hold well.
Good Luck
Jack

 

[DavidF]

When you say it "has to be on one axis" what you mean is that the tenon will be on the same rotational axis as the face plate you mounted on first. If the face plate isn't in the right place to enable you to turn the tenon in the centre of the wood then you may well go through the side while turning the inside. Just imagine as an extreme case that you mounted the face plate way towards the edge of the top surface. You would end up turning the tenon also on the edge. Are you fully rounding the piece before remounting on the tenon? a lot of turners don't, they just flatten the base enough to turn the tenon and then do all the work from the other end and the tenon doesn't then start in the centre.

 

[Mike Davis]

I completely turn and finish the bottom of my bowls before I turn them around and mount in the chuck. I've never had a problem with them being off center and was not aware that this could be a problem. I rely on my ignorance of these problems to keep me safe and out of harms way. Now that i know it could be a problem i surely will have some out of center bowls. :rotflm:

But, so far when I have checked the wall thickness and roundness they have been within a few thousandths. That is one advantage of turning dry wood.

 

[sasjzl]

Hi David,

You wrote:
"Just imagine as an extreme case that you mounted the face plate way towards the edge of the top surface."

I am not understanding that. My thought is that however I mount the faceplate when turning the outside the piece will either become round or not. If it is fully rounded and I create a tenon that is fully rounded how can the tenon be on a different axis then the rim of the bowl? In the case above where I had the faceplate too far off center I would think that the bowl would never completely round. But even if I had not centered the faceplate on the top of the rough bowl, which I probably never do exactly, after I round the piece it is then centered regardless of where it started on the faceplate. For example if I can round the piece then there is a horizontal axis that should be the same for the bottom or the top.

I think that either I am not getting the jaws to butt against the shoulder of the tenon or the wood is changing while I am working on it from heat or the released tension of the wood. If it is the latter that would suggest to me that I need to minimize the time between remounting and getting the piece finished. Although I do not remember goofing around too much between the those two times on my last two bowl attempts.

I am taking a class in segmented bowl turning at my local Woodcraft store and it is interesting because there is no remounting because we are doing everything from what would be considered the remounted position, bottom to the left, rim to the top, on a normal green bowl made from a solid hunk of wood.

Thanks very much,
Jim Lee



Thanks,
Jim Lee

 

[DavidF]

I am not understanding that. My thought is that however I mount the faceplate when turning the outside the piece will either become round or not. If it is fully rounded and I create a tenon that is fully rounded how can the tenon be on a different axis then the rim of the bowl? In the case above where I had the faceplate too far off center I would think that the bowl would never completely round. But even if I had not centered the faceplate on the top of the rough bowl, which I probably never do exactly, after I round the piece it is then centered regardless of where it started on the faceplate. For example if I can round the piece then there is a horizontal axis that should be the same for the bottom or the top.

That's why I mentioned whether you DO fully round the blank first. You are right of course that if you keep turning until the blank is round then it will be on the same axis, just smaller! If you don't turn it fully round, turn the tenon and then reverse chuck the bowl and start the inside, there is a chance of running through the side.