Expand or compress???

Discussion in 'Newbie' started by Regis Galbach, Jul 28, 2017.

  1. Regis Galbach

    Regis Galbach

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    I've been making a few bowls and always have cut a tenon for my chuck to hold (G3). But, this chuck and most (probably) will expand or contract.

    So, my question is what are the reasons to cut a tenon or a recess? Is it simply style or are there other reasons.

    Thanks,
    Regis
     
  2. Mark Wollschlager

    Mark Wollschlager

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    I find that a recess can be less sturdy for anything other than finishing cuts.
    I use them on platters, but, the amount of material around the recess has to be maintained or it can fail suddenly.
    It can be a pain to remove chuck jaw marks from the inside of the recess. The tenon is made to be reshaped or turned away.
    Just my preferences. I have seen demos from very well known turners who use recess chucking with great success.
     
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  3. Gerald Lawrence

    Gerald Lawrence

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    I have not done a tremendous amount of work with expansion mode but have had pretty good results after a little practice . Do not try on soft woods as may get many more failures from that. Be sure to leave a large enough foot to prevent failure . As mark said sanding out the inside of the expansion could be a slight problem but for me not from the chuck. It is sanding the inside cut that is a challenge. By the way expansion gets better results with dry wood both for finish cuts and to reduce splitting.
     
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  4. Clifton C

    Clifton C

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    I'm in the tenon/bowl recess/platter camp, but, Like Mark, I've seen folks use a recess on a bowl with good results. I'm fairly aggressive while roughing and seem to get better results from a tenon. Why? I'm not sure, but I think the stability comes from the shoulder on the top of the jaws which gives you a bigger (wider?) footprint. With platters, I'm using a bigger jaw set, #3 Strong Hold maybe, with the top outside grove ring thing machined off to make it into a sharper dovetail shape. It just feels like a more positive grip when expanding into a platter recess, and, the stability in a platter is the whole platter the jaws can expand against.
    But as everyone says...YMMV
     
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  5. Mike Johnson

    Mike Johnson

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    I have used a tenon on the 1st turning and then cut a recess for the 2nd turning.
    This process can be simplified by turning a tenon in the interior bottom of the bowl
    and then mounting the bowl to this tenon to cut the recess in the base of the bowl.
    This might sound like extra work but it deletes the process of removing the tenon later on.
     
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  6. hockenbery

    hockenbery AAW Advisor Staff Member

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    I use a tenon for a variety of reasons
    I have never been able to tighten a chuck too much on a tenon. I have split recesses with over tightening. The first chuck I owned only expanded...

    Tennons use less wood on half logs. Easy to show with a drawn circle Cut down to make a 2.5" tenon Cut down to make a 3" recess. Related advance turners can hollow a ball by turning a grove that dovetail jaws grip and you will have the ball shape with a groove.

    Tenon much, much easier to turn when ruffing between centers and the center point can be left in the middle of the tenon for use later.

    In general a recess compromises the design to include a recess.Recesses leave a hole in the bottom. You can of course turn it all away wasting more wood.
    Tennons make it easy to make a round footless bottom or a small foot of an inch or less.

    Twice turned bowls are much, much, much easier to return with a tenon. I usually leave a center mark in the tenon and with balanced grain it is in the center of the dried bowl making it easy to mount the bowl and return the outside and true the tenon.

    Recess doesn't make any sense to me for hollow forms.

    On platters a recess can be a good idea however a tennon leaves a lot more design options for platters.

    All that said I may be using a recess in public for the first time in a month. I have plan to use a couple of 3" forsntner holes with a recess.
     
    Last edited: Jul 28, 2017
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  7. Bill Boehme

    Bill Boehme Administrator Staff Member

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    One other limitation of a recess or mortise is that you are more or less constrained to using the perfect circle diameter in order to have uniform clamping pressure all the way around. Anything larger than that and the contact area quickly goes to zero. This means that you will wind up with four small area high pressure points. A misstep while turning and the piece is at risk of unceremoniously dismounting.
     
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  8. Regis Galbach

    Regis Galbach

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    Now that you mention that, I'm thinking that with a slightly oversize opening the holding pressure would be in the middle of each curve (4 places). Whereas, oversize tenon would be compressed at each corner of each segment (8 points).

    Thanks for everyone's insight,
    Regis
     
  9. Bill Boehme

    Bill Boehme Administrator Staff Member

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    That's true. With dovetail jaws, you get optimum clamping at the perfect diameter whether clamping a tenon or expanding into a recess. But I prefer eight points of contact that are digging in a bit as opposed to four smooth points of contact. I actually prefer the Oneway profiled jaws for general purpose holding where I don't have to worry about getting an exact tenon size. If I am turning something that will be removed from the chuck and then re-mounted, I use a Vicmarc chuck with dovetail jaws and a perfect circle diameter tenon for the most repeatable chucking..
     
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  10. John Turpin

    John Turpin

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    I pretty consistently cut a dovetail mortise for my Nova G3. As a new turner though, I'm never sure how deep of a recess is needed. I've seen some use a mortise that is frightfully shallow, but I tend to fall on the side of caution and go deeper. Maybe unnecessarily deep. I wonder if there is rule for this?
     
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  11. Bill Boehme

    Bill Boehme Administrator Staff Member

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    The only rule is whatever you feel safe doing, but I would feel safer with a short tenon than a shallow mortise.
     
  12. Clifton C

    Clifton C

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    As a turner gets better at turning and skills increase, the stress and strain on a tenon or recess becomes less. When I first started turning, my tenons were beefy and my recesses deep, now, tenons are shorter and recesses shallower. So, your rule is to start off on the safe side, turn comfortably, set yourself up for success. Your tenons should be as long as you can make them without bottoming out in the chuck, others have mentioned the effect of diameter to jaw size and that should also be taken into account. Shoulder should be slightly under cut so it sits tight to the top of the jaws. I feel that tenons should be cut precisely and with care, not quick and dirty cause it's just going to be cut off later. Same with a recess, turn them clean and crisp. With the jaws closed tight, make the opening slightly dovetailed and just barely big enough to fit over the jaws. The top of the jaws should sit on a nice flat surface, the inner corner should be crisp. A lot of good info in this thread, and even more in previous posts, search them out but don't hesitate to ask more questions.
     
  13. robo hippy

    robo hippy

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    Well, as for bowl mounting, when made correctly I have seen no difference in the holding ability between a recess or a tenon. Other than that, I will never use expansion on spindle work. Never understood how some think that one uses more wood than the other. If you turn off the bottom, there is no difference between a 1/8 inch recess, or a 1/8 inch tenon...


    View: https://www.youtube.com/watch?v=5KHkkws9lWA&t=22s


    robo hippy
     
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  14. Bill Boehme

    Bill Boehme Administrator Staff Member

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    I agree, but I've seen lots of bowls and platters where the recess is left on the finished piece. To me it looks half done, but it's not a problem for others.
     
  15. hockenbery

    hockenbery AAW Advisor Staff Member

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    In some designs there is no difference
    In some designs you can save some wood by not hollowing the middle of the recess.

    However recesses often use more wood.

    It is most evident on 1/2 logs and whenever you want a foot smaller than the tenon.
    The opening for the recess needs to be 3/4 to an inch wider than the tenon depending on the chuck.
    So on a round half log you need to give up more wood for a recess and it has to be deeper than the top of the tenon. I would want to go even deeper for a recess than I show in the diagram.
    If you are doing a small foot less than the diameter of the tenon the foot can be turned in the tenon.
    Have to turn away more wood with a recess.
    If you want a round bottom on the bowl a recess wastes more wood.
    A couple of sketches
    IMG_4290.JPG P IMG_4292.JPG
     
    Last edited: Jul 29, 2017
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  16. Regis Galbach

    Regis Galbach

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    Now THAT is a very complete bowl/platter mounting class.
    Thank you,
    Regis
     
  17. robo hippy

    robo hippy

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    Well, I never consider pieces with small bottoms because I am the kind of person who walks into a room and things fall over..... Tenon rule is 1/3 to 1/4 the diameter of the bowl. If I am going for a small bottom and an 'artistic' piece, I turn down to a point and part the bowl off. I seldom do that though, but it works well for madrone if you want to turn really thin. I guess I would probably use a tenon on that one...

    robo hippy
     
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  18. Clifton C

    Clifton C

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    Well, I don't mind small bottoms, but, I'm more partial to shapely, with nice curves and...hmmm, where is the delete button...
     

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