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Thread: Bowl out of true when remounting

  1. #1

    Bowl out of true when remounting

    I have a problem.. Or, I'm not sure if it's a problem or not.

    I have some limited experience with bowl turning (using my previous lathe, a few years ago), which makes me wonder if I do have a problem or not. But I want to consult some experts.

    I bought a new (used) lathe a little over a year ago, but haven't been turning bowls until this weekend. Now that I've tried turning a few bowls using almost dry wood (has been drying for five years, but still feels a bit heavy), I do have some doubts. I meant to rough turn them, and let them dry for a while (I have a few such bowls already on the shelf since a couple of years, that I also mean to finish now).

    I first turn the outside, mounted on a screw chuck (I have both a dedicated screw chuck that came with the machine, and a screw insert in my Sorby Patriot chuck). I have tried both screw chucks with the same result. When I have finished with the outside, I make a tenon for my chuck.

    This is where the problem starts. When I remount the bowl on my chuck, no matter how carefully I adjust it, the newly turned outside of the bowl seems out of true. I realize this always happen to some degree, but it just seems like it's too much.

    After I turn the inside, and remove the bowl, I can see that the rim of the bowl is visibly of different thickness depending on where you look. I haven't measured, but it looks like it's several millimeters off. Say 8 mm on one side of the bowl, and 5 mm on the opposite edge. The thickness is visibly different.

    Is this normal? I don't remember this happening in the past. Not to this extent at least.

    If it's not normal, I'd want to find the problem.

    My suspicion is that it has do do with the chuck adaptor (which I had to have specially made for the spindle thread - Whitworth 1 1/4" if I remember correctly).

    I have been checking things with a dial gauge, but can't come to a conclusion. It seems like the outside of the chuck is a bit off, but then, it isn't the outside that is gripping the bowl. I've tried measuring the jaws, but they have a small amount of play when not gripping anything, so I can't come to any conclusion there. Just now, an idea struck me.. Maybe obvious to others, but I should probably measure the jaws when gripping something, or maybe the object they are gripping.

    A friend that used to work at a machine shop helped me make the adapter. He's retired and moved out of town since then, so I probably need to pay someone to make a new adapter. If it's really the adaptor that is the problem... It might as well be the chuck.. And if the chuck is bad, a new adaptor would not help. I would just be throwing my money away.

    I've also been thinking of ordering a new chuck. Maybe a Vicmarc. But, as my spindle thread is unusual today, I can't buy an insert for any chuck. I'd still have to have one made for me.

    Or could it be something else? Maybe the lathe itself? I haven't noticed any problems when turning chisel handles and such things, but that might not mean anything.

    How would you investigate this? What to measure?

    I can also add that I also have a metalworking, 4-jaw scroll chuck that came with the lathe. When checking the outside rim of this chuck with a dial gauge, it is actually only moving 0.01 mm when turning the chuck around. This feels like quality to me. I would like my other chuck to be as good. :-)

    But, still, I'm not sure how much this means. I'm only measuring on the outside rim of the chucks. But it feels like an indication of where the problem is.

  2. #2
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    One thing that might help you is to make sure that your last cut before putting the piece in your chuck is to take a finish cut on your tenon. Lots of folks cut the tenon first then shape the outside of the bowl releasing tensions in the wood and that without recutting the tenon will cause the piece to be a little out of round when chucked on the tenon.

  3. #3
    There could be multiple factors at play, including what Bill mentioned. This situation will happen to some degree most of the time and the proper corrective action, if needed, would be to true up the outside of the bowl after chucking and before hollowing the bowl.

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  4. #4
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    2 thoughts. Bill is correct about the wood moving. Also make your tenon with a shoulder. The wood should sit on this shoulder which helps keep it aligned and also helps keep it from rocking and coming loose in the chuck later. Then tenon should also be cut so the V at the bottom is I guess I'll say out of the way. In other words the jaws should sit against the shoulder and the tenon and never really hit the bottom of the V where the 2 meet. Watched a great Stuart Batty video on this. If the V was not cut perfectly which most of us can't do the jaws can try to grip the bottom of that V and not the 2 aligning reference point of the flat and sides of the tenon. The second thing is to mount your wood in the chuck so that you have equal side grain and end grain on each jaw. I call this 45 degrees to the grain direction. When you just randomly put the tenon in the jaws you may have 2 jaws on side grain which crushes easily and 2 jaws in end grain which doesn't and it can pull the bowl out of alignment.
    That being said some bowls just dont want to go along and you either have to live with it or retrue the outside. If the bowl is thick I don't worry about it. If it's going the be thin or have beads around the lip that will show the irregularity then I re true the bowl.

  5. #5
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    Quote Originally Posted by JohnC Lucas View Post
    2 thoughts. Bill is correct about the wood moving. Also make your tenon with a shoulder. The wood should sit on this shoulder which helps keep it aligned and also helps keep it from rocking and coming loose in the chuck later. Then tenon should also be cut so the V at the bottom is I guess I'll say out of the way. In other words the jaws should sit against the shoulder and the tenon and never really hit the bottom of the V where the 2 meet. Watched a great Stuart Batty video on this. If the V was not cut perfectly which most of us can't do the jaws can try to grip the bottom of that V and not the 2 aligning reference point of the flat and sides of the tenon. The second thing is to mount your wood in the chuck so that you have equal side grain and end grain on each jaw. I call this 45 degrees to the grain direction. When you just randomly put the tenon in the jaws you may have 2 jaws on side grain which crushes easily and 2 jaws in end grain which doesn't and it can pull the bowl out of alignment.
    That being said some bowls just dont want to go along and you either have to live with it or retrue the outside. If the bowl is thick I don't worry about it. If it's going the be thin or have beads around the lip that will show the irregularity then I re true the bowl.
    I'm not a terribly experienced turner but did have the opportunity to attend a class at the local club taught by Stuart Batty. I was going to say what JCL said about the shoulder. Stuart showed us how this is so important but he also showed one other thing. He said he has found the face of the jaws usually are not square. I couldn't believe it when he took a wood turning tool to true one up.

    Mike

  6. #6
    Thanks! I had never thought about the end grain vs side grain issue. I'll think about this next time.

    I'll make sure to check the shoulders and that the V is cut as you describe. I'll check the Stuart Batty video as well!
    (I guess it's the one called "Recess tenons and preparing recesses in bowl blanks").

  7. #7
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    Quote Originally Posted by Staffan Hamala View Post
    I'll check the Stuart Batty video as well!
    (I guess it's the one called "Recess tenons and preparing recesses in bowl blanks").
    If you are looking on Vimeo also watch his "Jaws and Chucks" and "Tenons Part 2". The three as a group cover about everything.
    "I became insane, with long intervals of horrible sanity." - Edgar Allan Poe

  8. #8
    Usually when this happens to me - and I turn a good deal of really green wood - it's because the tenon is not properly formed. I've rarely had wood move so quickly after turning.

    First, what size tenon are you turning compared to what size bowl?
    Second, how deep is your tenon? The tenon shouldn't bottom out in the chuck; instead, the tips of the chuck jaws should touch the bottom of the bowl, and sit flush.
    Third, are you turning a flat on the bowl above the tenon? This will help the jaws sit flush and in the same plane relative to the rim.


    One thing that really helped me is this:

    After you've turned your tenon, with your bowl still mounted on the worm screw on the lathe, attach the chuck to the tenon, lightly (this assumes you have a second chuck...). Then bring the tail stock up into the threads of the chuck. This will center it relative to the headstock. Last, tighten the chuck. Now you can remove the bowl and thread the attached chuck onto the spindle of the headstock. It will be about as centered as it ever will be.

    FWIW, I prefer using a spur center now for starting bowls. That point is easier to line up than the larger hole from the worm screw or face plate when you flip the bowl around.
    Last edited by Prashun Patel; 05-25-2018 at 11:02 AM.

  9. #9
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    I'm still a beginner compared to folks around here, I get a little wobble after turning the blank around, the ones that are really bad are almost always due to the tenon not having a good square shoulder to sit on. The other is not having the blank flat against the face of the jaws- often when I'm trying to mount a big heavy blank with one hand on a chuck already on the lathe. It's better to take the chuck off, turn the bowl upside down and use the weight of the chuck to seat it flat on the bowl as you tighten the jaws.

    But also, the person who taught me how to turn put a really big emphasis on being able to turn right or left handed. Spindle and bowls I had to learn each cut with my hands reversed. I'm by no means proficient but I do feel comfortable swapping hands when the cut calls for it, avoiding awkward body positions. That has been a great help. For me I save my finishing cuts until the bowl is reversed and chucked up and then I finish the outside shape of the bowl before I hollow. Any time you take the blank out of the chuck or off the lathe you risk introducing some out of round to it so I do as much as I can from one chuck position.
    Where did I put that?

  10. #10
    When you reverse the bowl to turn the inside, is it running true before you turn the lathe on? This is some thing I always check before turning the lathe up to speed. If it is running fairly true, then the inside should run true as well. If the outside runs true, and the inside does not turn out true, then that sounds like an alignment problem of some sort. For how far out of round your bowl is, one other possibility is that it is moving as you turn, which should be visible. Having a flat shoulder really helps keep the bowl steady.

    Here is a video clip I did a while back for mounting things on the lathe.

    https://www.youtube.com/watch?v=5KHkkws9lWA

    robo hippy

  11. #11
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    Glad I saw this thread because Stuart Batty's videos are really good. I'd seen him before forgot how well he explains and demonstrates while teaching. Plus, all the videos at the link below are free.

    Here's the link:
    https://vimeo.com/woodturning

    Scroll down that page for the link to load more videos.

    BTW, these are studio made videos and very different than some of the videos shot in turning demos where he is not in control.
    Last edited by Richard Dooling; 05-08-2018 at 3:56 PM.
    Men occasionally stumble over the truth, but most of them pick themselves up and hurry off as if nothing ever happened.
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  12. #12
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    Quote Originally Posted by Richard Dooling View Post
    Glad I saw this thread because Stuart Batty's videos are really good. I'd seen him before forgot how well he explains and demonstrates while teaching. Plus, all the videos at the link below are free.

    Here's the link:
    https://vimeo.com/woodturning

    Scroll down that page for the link to load move videos.

    BTW, these are studio made videos and very different than some of the videos shot in turning demos where he is not in control.
    Great link! Thanks.
    Where did I put that?

  13. #13
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    Quote Originally Posted by Richard Dooling View Post
    Here's the link:
    https://vimeo.com/woodturning
    Scroll down that page for the link to load more videos.
    Here is a similar link I use in alphabetical order.
    Three pages, just go to the next page. Just a little quicker to me than the "load more".
    But I am pretty lazy sometimes.
    https://vimeo.com/woodturning/videos...rmat:thumbnail
    "I became insane, with long intervals of horrible sanity." - Edgar Allan Poe

  14. #14
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    Quote Originally Posted by Michael Mills View Post
    Here is a similar link I use in alphabetical order.
    Three pages, just go to the next page. Just a little quicker to me than the "load more".
    But I am pretty lazy sometimes.
    https://vimeo.com/woodturning/videos...rmat:thumbnail
    Lazy?
    Efficient!

    Good link, thanks.
    Men occasionally stumble over the truth, but most of them pick themselves up and hurry off as if nothing ever happened.
    - Churchill

  15. #15
    I have now watched the Stuart Batty videos, and had a quick check of the tenons I cut the other day. I can see at least one error. The corner of the tenons were not cut as cleanly as they should have been, according to Stuart's demonstration. I never realized that corner was so important. It's obvious now, but it didn't occur to me before.

    I'll try to be more careful when making the next tenon, when I get back to the lathe in the coming weekend.

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