1. Welcome new registering member. Your username must be your real First and Last name (for example: John Doe). "Screen names" and "handles" are not allowed and your registration will be deleted if you don't use your real name. Also, do not use all caps nor all lower case.

big bowl trouble

Discussion in 'Getting Started' started by Doug Rush, Dec 8, 2020.

  1. Doug Rush

    Doug Rush

    Joined:
    Mar 6, 2020
    Messages:
    6
    Location (City & State):
    Cambridge, OH
    So I'm new to turning in 2020 and I've been cranking out bowls this last month and feel that I'm starting to get a feel for it (though far from actually good at it). In the beginning all of my bowls were 6x3 from standard bowl blanks that I bought and things went fairly well. Now I've started to rough turn bowls from green wood and my issue is with the bigger ones. I'm using a screw chuck and the tailstock center to mount the log and I rough shape the outside of the bowl, create a dovetailed tenon and then flip it around into vicmarc chuck to start hollowing out the bowl (standard procedure). However when I do that the bowl is ever so slightly "out of round"-- it wobbles some which causes undue vibration when I go to hollow it out. It did not do that when it was in the screw chuck between centers- it spun true before I turned it around. It only happens when my bowls are about 10 inches or more in diameter. So my questions are: does this actually matter? and if so what is the cause of it and how do I fix it?
    thanks!
     
    Emiliano Achaval likes this.
  2. Emiliano Achaval

    Emiliano Achaval Administrator Staff Member

    Joined:
    Dec 14, 2015
    Messages:
    2,328
    Location (City & State):
    Maui, Hawaii
    Home Page:
    A little bit of out of round is ok, but noticeable wobble is not. The cause is your tenon. Since I have too many chucks, I have extra jaws, so I can check the fit of the tenon with a jaw that I keep hanging by the lathe. I have a tool with a 77 degrees point that is the same as the Vicmarc jaws that I use to make a tenon on some woods. I talk about how to make a perfect tenon fit in my demos. I also use my 3/8 detail spindle gouge to cut a nice clean tenon. A good fit is very important. No matter how big the bowl I'm working on, you can't even tell if they have a wobble, that's what you want. Unless you are a lefty and can make a nice push cut from the headstock, a slice cut is how you could make it round again, but it won't be pretty. I would put it in between centers again and do a better tenon.
     
    Bill Boehme likes this.
  3. Tim Connell

    Tim Connell

    Joined:
    Jan 22, 2018
    Messages:
    170
    Location (City & State):
    Cameron, Illinois
    I used to have the same problem when I started, and finally realized that while I had a good tenon, I didn't have a good flat shoulder for the jaws to fit against. I started paying attention to making a good flat for the jaws to rest against and my problem went away.
     
  4. hockenbery

    hockenbery AAW Advisor Staff Member

    Joined:
    Apr 27, 2004
    Messages:
    6,680
    Location (City & State):
    Lakeland, Florida
    Home Page:
    Like above. out of round/off center is ok. Wobble is not.
    Check the tenon mount

    Tenon should not touch the inside bottom of the jaws
    Side Profile should match the jaws ONEWAY dove tail is about 7 degrees vicmarc is 13 or 14. Match yours
    Turn a flat around the tenon to rest on the tops of the jaws.

    the flat on the tops of the jaws keeps the bowl from rocking on the tenon
     
  5. Doug Freeman

    Doug Freeman

    Joined:
    Feb 26, 2019
    Messages:
    332
    Location (City & State):
    Lebanon, Missouri
    I angle the flat around the tenon from the OD to the ID. It can be difficult to get a perfectly flat surface, and end up with top jaw surface contact at the ID, not the best. A slight angle will create contact at the jaw top OD, providing more stability. Slightly bevel the end of the tenon, and the OD of the tenon shoulder to ensure even contact with the jaws. Perfect tenons can still result in a bit of movement once flipped, but if you have enough to cause enough lathe movement to cause a problem with hollowing the bowl, all the above concerning tenon problems are spot on.
     
    Gerald Lawrence likes this.
  6. robo hippy

    robo hippy

    Joined:
    Aug 14, 2007
    Messages:
    3,049
    Location (City & State):
    Eugene, OR
    I figure if I get to less than 1/16 inch of wobble on a 12+ inch diameter bowl, that is pretty good as it translates to +/- 1/32 of an inch. If it is more than that, then there can be several causes. One is loose screws in your chuck jaws, they do work loose.

    Another is not doing a good tool cut when forming your tenon. I use a recess. I have found since I started using a NRS (negative rake scraper) to cut the shoulder an flatten the bottom of the recess, I get more consistent results. For years, I used a specialized dove tail cutting tool, ground to the exact angle of my chuck jaws so all I had to do to get a 'perfect' cut was like the shaft of the tool up with the ways of the lathe and do a plunge cut. The NRS, in flat grain leaves a very clean surface.

    robo hippy
     
  7. Mike Johnson

    Mike Johnson

    Joined:
    Jul 26, 2016
    Messages:
    1,922
    Location (City & State):
    Nebraska
    Many turners starting out make their tenons too long which bottom out against the back of the chuck. Anytime the tenon is bottomed out in the chuck it creates a fulcrum point which will leverage the bowl out of the chuck with just a small tool catch. A shorter tenon allows the bowl blank to come into alignment with the base of the bowl which was rough turned where the edge of the tenon protrudes from the base of the bowl blank. Your chuck jaws need to seat perfectly against this surface to provide proper support for the bowl blank and put it back into the same axis alignment as it was when rough turned. before reverse mounting into the chuck jaws..
     
  8. Doug Rush

    Doug Rush

    Joined:
    Mar 6, 2020
    Messages:
    6
    Location (City & State):
    Cambridge, OH
    Thank you all for your speedy replies! Obviously I need to work on my tenon-making skills. I thought I was doing well but not well enough. Thanks again
     
  9. Mark Jundanian

    Mark Jundanian

    Joined:
    Jun 6, 2018
    Messages:
    755
    Location (City & State):
    La Grange, IL
    @Doug Rush , along with the above, are you using the tail stock to align the piece when you mount it in the chuck?
     
  10. john lucas

    john lucas AAW Forum Expert

    Joined:
    Apr 26, 2004
    Messages:
    7,488
    Location (City & State):
    Cookeville TN USA
    A couple of tricks that work most of the time. First of all shape your tenon such that the wood touches the jaws on the top outside edge and if it's a dovetail should not touch in the bottom of the V. In other words you want the dovetail slightly more acute than the shape of the jaws. Second. Align the jaws so there is an equal amount of side grain and end grain in each jaw. What can easily happen if you have it aligned with side grain in 2 jaws and end grain in the other 2 is the end grain doesn't compress the same as the side grain and will pull it off center. What I do is draw a line parallel to the grain and then perpendicular to the grain. the I place the jaws so they are in between these lines. These 2 things help most but sometimes it's simply a matter of the wood moving. You have removed a lot of wood and relieved stresses in the wood. I have had them move even when very dry.
     
  11. Doug Rush

    Doug Rush

    Joined:
    Mar 6, 2020
    Messages:
    6
    Location (City & State):
    Cambridge, OH
    I use the tail stock when it is on the screw chuck obviously but once I flip the bowl and mount the tenon in the 4 jaw chuck I remove the tail stock so that i can start hollowing out the bowl. Is that not correct?
     
  12. Doug Freeman

    Doug Freeman

    Joined:
    Feb 26, 2019
    Messages:
    332
    Location (City & State):
    Lebanon, Missouri
    Some prefer to leave the tailstock in place as long as possible for improved safety. It’s a choice. I think Mark is saying to use the live center in the bowl top center hole to help align the blank and tenon in the chuck. That is what I do. I also use the TS to apply light pressure, while the jaws are just touching the tenon, to ensure the top of the jaws are well seated against the blank bottom, then tighten the chuck.
     
  13. robo hippy

    robo hippy

    Joined:
    Aug 14, 2007
    Messages:
    3,049
    Location (City & State):
    Eugene, OR
    John brought up a point that I forgot, but think I mention in my video about mounting things on the lathe. This is more important with green wood than it is with dry wood. So, when clamping down, you don't want the end grain clamped in the center of the curve of 2 of the jaws and the side grain clamped in the other 2 jaws. Rotate the piece so that the center of the end grain is on the gap in between the jaws, and the side grain is in between the other 2 jaws. The end grain compresses less than the side grain, so, especially if the wood is green, you may need to retighten once or twice, depending on how fast or slow you are...

    robo hippy
     
  14. Mark Jundanian

    Mark Jundanian

    Joined:
    Jun 6, 2018
    Messages:
    755
    Location (City & State):
    La Grange, IL
    @Doug Rush , what Doug Freeman said. Use a cone center on the tailstock to find the center of the screw hole left from the screw chuck. Then, keeping the cone centered, mount your newly formed, and now perfect, tenon onto the chuck. This helps to maintain alignment. Note if your screw hole is a little chewed up, then this isn't going to work as well.

    If you have two chucks and a spindle adapter for your tail stock, then you can do an upscale varriation of this trick. With your piece still on the lathe via the first chuck, mount the second chuck to the tail stock using the spindle adapter. Now bring the tailstock up to that perfect tenon and tighten up the second chuck. Loosen the first chuck from the piece and swap it for the second chuck with the piece mounted.

    I hope you get the idea. You're maintaining the alignment until you get the second chuck seated and secure.

    Once the piece is reversed, it is possible to start turning the bowl cavity with the tailstock engaged, but the tailstock will need to be removed sooner or later.
     
  15. Lou Jacobs

    Lou Jacobs

    Joined:
    Jul 18, 2018
    Messages:
    244
    Location (City & State):
    Baltimore, MD
    John mentioned wood moving. Just out of curiosity, measure across the bowl perpendicular to the grain and parallel to it (at right angles) and see if the bowl is still round. If you take a long time turning, it is very likely that your bowl is beginning to lose water and shrink differentially.
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice