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Bouncing bowls (Read 843 times)
 
Bill Neff
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Bouncing bowls
Feb 26th, 2015 at 2:19pm
 
I've been having an issue with the last few large bowls I've turned.  These are in the 10-14 inch diameter range.  I mount them on a faceplate.  Rough turn them and create the tenon.  I flip them over, using the tailstock for support, and of course they aren't quite round.  It seems like no matter how much I take off they don't smooth out to round.  Even when they are hollowed the gouge bounces and the lathe vibrates if the speed is wrong.

The only thing I can come up with is that I'm using a spindle adapter (Easywoods brand) on my chuck (PennState brand). I'm wondering if this is creating just a little flex that would cause the blank to act like it's out of round (especially if it's a large heavy piece of wood).  I've checked the chuck when on the lathe and it appears to spin without a wobble.

Thanks!
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Richard Pyle
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Re: Bouncing bowls
Reply #1 - Feb 26th, 2015 at 2:37pm
 
Are you sure it's not wood from a rubber tree? Cheesy
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Len Layman
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Re: Bouncing bowls
Reply #2 - Feb 26th, 2015 at 2:55pm
 
Bill, does it turn true if you take away the tail stock for support?
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Tom Coghill
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Re: Bouncing bowls
Reply #3 - Feb 26th, 2015 at 2:57pm
 
So if I understand this correctly, when you invert the bowl, you find it is no longer centered.

This will happen (to some degree) every time a bowl is re-mounted on the lathe.  The larger the bowl, the more you will notice it and the more vibration it will cause.

What I do:  I finish the outside of the turning (no more tool work, just sanding to be done) before I invert it.  I then play with it a bit to center it as best I can before turning the inside.  If it is to be VERY thin, you may still need to re-turn the outside. However, if the bowl is 3/8" or thicker, the very slight difference will not be noticed.

When I am doing THIN turnings I do not use an anti -lock ring, and I try to do as much as possible of my turning using the same set-up and quickly enough that any moisture content changes will not cause changes.

hope this helps,
Tom
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« Last Edit: Feb 26th, 2015 at 2:58pm by Tom Coghill »  
 
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Bill Neff
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Re: Bouncing bowls
Reply #4 - Feb 26th, 2015 at 5:24pm
 
Richard, no rubber tree wood but it's done it with black walnut, hackberry, kentucky coffee tree (which about killed me because not even the scrapers or Easywood tool wanted to cut it and bounced) and ash.

Len, The bowl wobbles with or with out the tool rest. 

Tom,  I tried that on one bowl, going to finish turning and all the sanding.  When the inside was done I could see a difference in wall thickness from one side to the other. 

The problem is that when I put it in the chuck after roughing the outside, I should be able to turn it back to round with a little clean up.  On one bowl I took almost a half inch off the diameter and it still wobbled.  - yes the chuck jaws were tight. Smiley
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Jim Killen
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Re: Bouncing bowls
Reply #5 - Feb 26th, 2015 at 6:28pm
 
I would check the chuck for runout
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Breck Whitworth
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Re: Bouncing bowls
Reply #6 - Feb 26th, 2015 at 7:52pm
 
Bill make sure your live center point matches up to your head stock point when pushed together, if it does then you can be sure it is something else. I turn bowls primarily and with green wood you will get some warping due to moisture loss from heat. Some times a ripple will occur (green or dry) if your gouge is not riding the bevel correctly. Still happens to me by times.
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« Last Edit: Feb 26th, 2015 at 7:52pm by Breck Whitworth »  

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Larry Matchett
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Re: Bouncing bowls
Reply #7 - Feb 26th, 2015 at 8:09pm
 
I learned this from Jimmy Clewes:

If your tool bounces when you are riding the bevel then no matter what you do your surface will still not be round.  You have to start at the edge of the bowl and have smooth surface for the bevel to ride on.  Easier to show you then talk about it.  I usually true up the face and then start my cut.  Oh by the way make sure your tool is firmly planted against your hip.
The other thing is make sure your tenon is not bottoming out in the chuck.  The jaws should be touching all the way around.  You might be making your tenon too long.
Is there another turner around your area.  I find it helps to have someone else observe your problem.  I find it is usually something very simple to solve the problem.
Had a similar problem with our club lathe, thought it was the chuck.  It wasn't it was the bearings and there was a wobble with the spindle.  I replaced the spindle and the bearings and everything was solved.
I know by now your are thinking holy crap all of these things can't be right.  Your are correct probably just one simple thing.
Good luck.
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Harry Robinette
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Re: Bouncing bowls
Reply #8 - Feb 26th, 2015 at 9:18pm
 
I learned a long time ago that this thumping if from moving the gouge faster then the bowl is turning,slow down the gauges travel and let the gouge cut completely around the bowl before you go further.If you cut to fast you'll leave a sorta ridge and the bounce will continue.Start with either a fresh cut at the bottom or the top of the bowl and don't let the bounce start.
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Don Stephan
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Re: Bouncing bowls
Reply #9 - Feb 26th, 2015 at 9:34pm
 
When the bowl is first turned round in the four jaw chuck, turn by hand using the tool rest brought up very close in several places, to see how out of round the outside of the bowl is.  Do the same before taking the bowl off the faceplate.  If round on the faceplate and not on the chuck the problem should like somewhere with the chuck and/or tenon.

I will have very slight out of round sometimes when I re-mount, but can eliminate most of that if I rotate the tenon slightly in the chuck and retighten.  I don't worry about 1/32" of an inch or so, and don't have shaking in the lathe when it is brought up to speed.
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JimQuarles
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Re: Bouncing bowls
Reply #10 - Feb 26th, 2015 at 10:22pm
 
Saw a similar problem when the chuck was not seated fully on the adapter, or the adapter on the spindle.
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Mike Mills
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Re: Bouncing bowls
Reply #11 - Feb 26th, 2015 at 10:24pm
 
I had that once when the chuck screws were loose.  Once you press the bowl to the jaws you can not see that they are loose as everything is pressed back together.  Only a possibility.
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robo_hippy
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Re: Bouncing bowls
Reply #12 - Feb 27th, 2015 at 12:41am
 
There is always some bounce, if from nothing else, going through end grain to side grain, to end grain to side grain, the tools cut differently through each. You say you have a spindle adapter. This will put the bowl out farther from the headstock, so you have a longer lever arm, and it will not be as stable as a shorter one. If it is not running fairly true when you reverse it, that can be a number of things. Like Mike said, the chuck jaw screws can come loose, and that results in a little run out. One shaving between the jaws and the tenon can make it wobble. Some times you are not seating it dead on to begin with, and this is more of a problem when you are turning larger pieces. You can put the bowl on a table, put the chuck on the tenon, tighten it up and then screw it onto the lathe. Some times, just rotating it 45 or 90 degrees can make a difference, even though theoretically it shouldn't make any difference. When you reverse it, if it is 1/16 inch out of perfectly centered, that means + or - 1/32 of an inch, and that is very close. If you have more run out than that, it might be not getting it in the jaws correctly. If it is running fairly true, and you start cutting out the inside, and you get vibrations, this, again could be the spindle adapter. Shorter is better. Tool pressure can also contribute to it as in taking heavy cuts. If you haven't ground back the heel on your gouges, this can contribute. If you are really rubbing the bevel hard, that can do it. Hold the tool (sword in the movie I saw this in) as you would a bird. Too tight and you kill it. Too loose and it flies away. The bevel should rub the wood, but the wood should not know it.

So, it could be a lot of things. I would try to get rid of the spindle adapter first. I do have a video clip up about mounting things on the lathe. In the video section here, or on You Tube if you type in robo hippy.

robo hippy
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Kevin Black
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Re: Bouncing bowls
Reply #13 - Feb 27th, 2015 at 5:26pm
 
Breck Whitworth wrote on Feb 26th, 2015 at 7:52pm:
Bill make sure your live center point matches up to your head stock point when pushed together, if it does then you can be sure it is something else. I turn bowls primarily and with green wood you will get some warping due to moisture loss from heat. Some times a ripple will occur (green or dry) if your gouge is not riding the bevel correctly. Still happens to me by times.


I was also wonder ing if we were talking green or dried wood. Green wood can also be a challeng to hold securely in the chuck.
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