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Thread: Reliability of mortise on bowl

  1. #1
    Join Date
    Oct 2013
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    Prescott Valley, AZ
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    Reliability of mortise on bowl

    I am turning the inside of the "holey bowl" discussed several weeks ago. To reduce waste, I glued a piece of birch to the bottom and turned a mortise into it while it was on the screw chuck mounted on the spindle. (I don't like to leave a mortise in the bottom of a finished bowl.) I was getting some chatter when I was only an inch into the inside. When I looked at the contact between mortise and the 4" jaws on my Nova2, I noticed that the jaws are contacting the bowl at the midpoint of the arc of each jaw. See photo.

    My question is; Is that contat sufficient to hold the bowl securely?
    I guess that I need to determine the perfect diameter of the mortise for gripping strength.
    Attached Images Attached Images

  2. I would reshape that recess, very lightly, fitting the chuck jaws until I got almost perfect contact. You are only holding at 4 points, and an event like a big catch or too much speed could cause it to fly off.....not a pretty thing to see happen, especially if it hurts you or someone else, and it could ruin that pretty piece. It looks like it might take another 1/8" to bring your contact to a good point, but be careful you do not get that recess too thin, as the strength of the wood may not support your jaw pressure....you have a fine balance to reach with this one! Safety matters more than anything else!
    Remember, in a moments time, everything can change!

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  3. #3
    When I glue a piece on the bottom I always make it a tenon and the chuck grips it all the way around....

  4. #4
    Your recess is too large. For 50mm jaws a recess 2 1/16" should fit perfectly. But, you do not have enough material on the outside of the recess to be safe. A minimum of 1" is best.

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  5. #5
    What Mr. Keeton said. a mortise should be very close to the arc of the chuck jaws for maximum friction over the maximum area. I am constantly guilty of violating this concept and had a nice sycamore platter go frisbee golf on me a week ago. So I turned a round piece just a millimeter or two larger than the closed chuck jaws to use for a template when turning the mortise.

  6. He is using 4” jaws according to his post, but the holding principles are the same. Now that I’ve considered this a bit more, perhaps reverse turn the added glue block completely off, truing up the bottom, using a jam chuck, and put a new one on, turn a new recess that is just a mm or two larger than the jaws are when completely closed.
    Remember, in a moments time, everything can change!

    Vision - not just seeing what is, but seeing what can be!




  7. #7
    1/8 to 3/16 max depth of recess is needed, and like the others have said, that recess is too big. If your jaws measure 2 inches across, then you need 2 1/8 inch max diameter. Also, you want the side angles to match, so if you have 7 degree dovetail on the jaws, then you want 7 degree bevel on the recess walls. Some times you get wobble from not having the jaws seated in the bottom of the recess properly as in dead flat or even one tiny shaving under one of the jaws. Some times just one loose screw in the chuck jaws can do the same thing.

    robo hippy

  8. #8
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    This might be a real soup sandwich. 4" jaws. I test fit the mortise at 13 degrees; it was perfect. I didn't envision a gap between jaws and too-large mortise. I left an island on the top where I can reinstall the screw chuck. I bought the 4" jaws specifically for large bowl. I think I will first reduce the depth of the mortise, as Reed suggests. If that doesn't improve my soup, I will install a new block with smaller diameter recess, as Roger suggests.
    I am glad I asked. Thanks.

  9. #9
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    Better. Because my glue block was 15/16", I turned off the mortise and recut it about an eighth inch diameter larger than the closed jaws, 3/16" deep.
    Attached Images Attached Images
    Last edited by Randy McCreight; 02-02-2019 at 10:03 PM.

  10. Quote Originally Posted by Randy McCreight View Post
    Better. Because my glue block was 15/16", I turned off the mortise and recut it about an eighth inch diameter larger than the closed jaws, 3/16" deep.
    cool deal. Sometimes its better to start fresh!
    Remember, in a moments time, everything can change!

    Vision - not just seeing what is, but seeing what can be!




  11. #11
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    I spun it briefly on its new mortise; I didn't detect any wobble.

  12. #12
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    Jul 2018
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    Knoxville,TN.
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    Face plates and glue blocks are like bread and butter. I use this combination over and over. Just wack the space between work and glue block with a chisel and they will separate, turn the face of block and glue another day. Lyle Jamieson uses this all the time. He uses a ring of superglue I sometimes use wood glue. They both can not stand a swift hammer blow to the side but will hold on like a snapping turtle resisting twisting. God bless, Roger.

  13. #13
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    Thankyou, everyone.

  14. #14
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    Just bit of a different perspective maybe...

    Recess depth. I always go about 3/8" deep. Maybe excessive or maybe not. The last time a bowl was almost orbited my daughter was turning and got a catch.
    One side was pulled from the jaws about 1/4". At 1/4" recess it may have been orbited, at 1/8" it surely would have.
    Also, I have never had anyone remark "that bowl would look a lot better if it were 1/8" taller".
    That said, I do use a shallower recess with platters where there is little wood to start with but the wood is normally very dry.

    Diameter of the recess. I cut mine where I can just get the jaws in when closed fully; my jaws are dovetail. My chucks are Nova so the dovetail is 15. Other brands may be 7 or other angles; you should try to match your jaws as much as possible.
    Nova states there is 2mm of material removed when cutting the jaws so they only have to expand 2mm from fully closed to come to full circle. There is plenty of room inside the recess for the jaws to expand that much. Other brands or straight jaws may vary. Making the recess larger only means less support imo.

    This is a pic I made years ago showing the jaws in the recess.
    Attached Images Attached Images
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  15. #15
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    lufkin tx
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    1,991
    All the forms of holding pieces work if you adjust your techniques to the situation--observe wood strength, wood greenness, weight, hole or tenon dia. (fit), length of piece ect. Leave plenty of wood around a mortise, use strong wood for glueblocks and single screws and observe all the basics. Ps--thick CA will hold anything in my experience and works on wet wood and requires no clamping like most glues

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