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Longworth --Cole Jaws-- or Jam chucking

Discussion in 'Woodturning Discussion Forum' started by Mark Halleran, Apr 17, 2020.

  1. Mark Halleran

    Mark Halleran

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    Which is your preferred way of finishing the bottom of a bowl and why.

    On my smaller lathe I have used Cole jaws but now that I have a larger lathe I am looking for a way to hold larger work.
     
  2. GRJensen

    GRJensen

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    Vacuum chuck is my first choice ... if that can't be made to work, I get out the Longworth. My Cole Jaws usually get called into action when I am stacking segmented rings.
     
  3. Drew Sumrell

    Drew Sumrell

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    I've tried a number of methods over the years but have settled on leaving a small nub on the bottom with the center hole from the tail stock. Once the bowl is all finished I use a jam chuck to finish my bottom except for the small nub, which at that point is usually about 3/8" to 1/2" in diameter. I cut it off with a Japanese saw and sand that small area or use my micro carver to stipple the very center. The stipple is ringed with a point tool and usually looks pretty neat. Hope this helps.
     
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  4. Mark Jundanian

    Mark Jundanian

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    I agree with GRJensen, vacuum, Longworth or Cole depending on which has the best grasp on the situation.

    Vacuum is probably first choice, but not if I also want to do sanding on the lathe. Between the Longworth and Cole the buttons differ in size and shape, so one may work better for a paticular shape.
     
    Last edited: Apr 18, 2020
  5. hockenbery

    hockenbery AAW Advisor Staff Member

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    Jamb chuck.
    Works with NE bowls, beaded rimmed bowls, delicate rimmed bowls, rims that have been fluted or carved.

    I used Cole jaws for a while.
    I have a vacuum chuck that I use for some bowls but jamb chucking is so fast and easy I use it most of the time.
    If I did lots of cut rimmed bowls where I would have a lot of bottoms to finish I would definitely hook up the vacuum chuck,

    if I want to put a cute texture pattern in the center of the bottom then it’s the vacuum chuck.
    But mostly today I would sandcarved something on the bottom.

    Note: if turners are not skilled enough with a spindle gouge to keep it from running under the nub using the jam chuck, a jamb chuck is the wrong choice for them.
     
    Last edited: Apr 18, 2020
  6. john lucas

    john lucas AAW Forum Expert

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    Last edited: Apr 18, 2020
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  7. Dave Bunge

    Dave Bunge

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    I also prefer the jamb chuck. For what I do, I find it to be faster, more secure, more versatile (works with natural edge bowls and once turned cut rimmed bowls with warped edges) to use than the other options. As they say, YMMV.

    Found this picture that helps make some points about jam chucks.

    upload_2020-4-18_7-34-4.png

    FWIW a few more details on how I use a jamb chuck: I make one for each size of chuck jaws. Make the outer rim of the wood slightly taller than the rest so that's where it contacts the inside of bowl. Attach a piece of router pad to the face with double stick tape. Even draw lines on the wood around the #1 jaw so I can install it in the chuck at the same position each time.

    To remove the nub that's left at the end, use a thin piece of cardboard, like from a cereal box, with a 1/2" hole cut out in the middle. Put the cardboard over the bottom of the bowl to protect it, then use a flush trim Japanese saw to cut off the nub. Use hand chisels to take the nub the rest of the way flush, then 1" sanding pad on drill to get rid of chisel marks. Sanding small area on face grain so no need to use coarse grits. Hand sand with the final grits, round the corner on the hand sandpaper so you can get closer to the inside rim. Put an unused jamb chuck on the bench top and use it to support the bowl when working on the bottom to protect the rim if necessary (for NE bowls with bark edge).

    Whew, after writing out all the steps, it does not sound faster. I have Cole jaws and use them occasionally. But they won't work for NE bowls which is mostly what I make. And I have bad memories of a nice white oak bowl that took a fatal ride (for the bowl, not any living beings) thru the air when I had a catch with it in Cole jaws.

    I have all the parts for making a vacuum chuck, would probably use it if I finished building it.

    Hope this helps.
    Dave
     
    Last edited: Apr 18, 2020
  8. Steve Nix

    Steve Nix

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    I have and use all the reverse chucking methods listed above depending on the piece I’m turning, but find myself using a jam chuck, as Dave has illustrated over 80% of the time.
     
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  9. Roger Wiegand

    Roger Wiegand

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    Whatever works for the piece. A quick and easy trick I use for reverse chucking between centers is to put a silicone potholder over my chuck to cushion the edges and then bring the tailstock up to hold it in place. I hate having the tailstock in the way though, I can almost never make the cuts I'd like to get to on the inside edge of the foot. Since acquiring a vacuum chuck it gets used a lot of the time. Cole jaws are pretty much self-centering for a round piece, so they are quick and easy to use. Vacuum requires a lot of futzing to get good centering, even if you've remembered to leave a center mark (and I somehow seem to routinely turn the center mark away, leaving me with pure trial and error.

    I have no Idea how you would jam chuck a NE bowl-- at least as I was taught, a jam chuck requires you to make a tenon that your piece fits on with a tight enough friction fit to hold the work in place without using the tailstock. (so I would not call the method that Dave so nicely shows above a jam chuck, but perhaps thats just pedantry) I made one day before yesterday for a particular piece I couldn't figure out another approach for, but it's my least favorite as making it is fussy and it requires an additional piece of sometimes large wood. It does allow you to fully finish the bottom without the tailstock in the way.
     
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  10. Mike Johnson

    Mike Johnson

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    If you leave a centering point on your work piece tenon it is quick and easy to reverse the mounting of the piece and jamb chuck the work piece while using the centering point on the tail stack to align the work piece while applying pressure with the quill. They do make extend center points that provide more room when removing the tenon from the work piece.
     
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  11. Don Wattenhofer

    Don Wattenhofer

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    I totally agree all of the other methods have limitations Like natural edge or they just won't hold a vacuum.
    Adding to Drew's explanation: always use a cup center then cut the nub down to a cone shape and you can get down to as small as 1/8" at the finished base then use a carvers gouge to cut off the remainder.
     
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  12. Dave Bunge

    Dave Bunge

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    Roger, I think you're right about the terminology. What I described is probably more accurately called a friction chuck. Vs what you described, and what Stewart Batty and Ashley Harwood teach, where the cut rim bowl fits tightly on a large "tenon" that matches the inside profile of the bowl. That's the true jam chuck. I agree, the true jam chuck seems very tedious to use. I've never tried. it.
     
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  13. Mark Jundanian

    Mark Jundanian

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    What gives jam chucking the reputation of being more secure is the tailstock which is so commonly used with it as to be assumed in the conversation (although there is such a thing as actually jammimg a piece on to a tightly fitted jam chuck w/o a tailstock). However, nothing precludes you from using a tailstock with vacuum or Longworth chucks or with Cole jaws, which makes them just as secure and leaves open the possibility of pulling away the tailstock.

    The answer to OP's question depends (in a primary way) on what it is that a person is turning. If you're making a thin walled natural edge bowl the best solution may be different than if you're turning a heavy calabash or an undercut rimmed bowl. In my case, the pieces I make don't have a solid bottom and certainly not a nub, so nothing that relies on the tailstock is an option (although I suppose I could put a faceplate on a live center). But then sometimes I turn some other form.

    My advice to anyone is that the most valuable tool in your shop is an open mind. After that have a variety of holding methods available so you can decide what is the best solution to the problem you have.
     
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  14. Tom Albrecht

    Tom Albrecht

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    Hmm, I always thought a jam chuck was the type that is a kerf cut it a disc mounted on the headstock and the bowl/ vessel is the "jammed" into the tight fitting recess/kerf.
     
  15. Robert D Evans

    Robert D Evans

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    I can't believe nobody has mentioned using a donut chuck. It's very secure and you can shape, sand and finish the bottom without interference of the tailstock. You just need the donut chuck to fit your piece of work.
     
  16. Gary Beasley

    Gary Beasley

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    One additional piece for vacuum chucking is an adaptor for the live center to mount your chuck onto, makes it very quick and easy to center the bowl on the chuck before turning on the vacuum. Take me about ten to fifteen minutes to set mine up on center.
     
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  17. Arkriver

    Arkriver

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    Based on a couple of club members experience, the Longworth thing is an accident waiting to happen. A jam chuck is easy, cheap, and quick. Allyn
     
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  18. Mark Halleran

    Mark Halleran

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    Thanks for all your responses. I just acquired this lathe it is a PM 3520A. So far I love it and since I basically bought everything the previous owner had I have not gone thru everything. I just found a full vacuum chuck system in one of the crates I bought. It appears that was his reverse turning choice. I just got an adapter to adapt my previous cole jaws to this new spindle size.

    Is there a max speed or some speed I should stay under when on the vacuum chuck.
     
  19. Gary Beasley

    Gary Beasley

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    Depending on the chuck and level of vacuum I would keep the speed down same as you would cole jaws. Because my main vacuum chuck is a flat disk about 20” I avoid going over 750 rpm.
     
  20. Tom Gall

    Tom Gall

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    Dave, you beat me to it regarding the terminology. I've always described it as a "friction" chuck.....thought I was alone on that.
     
  21. Tom Gall

    Tom Gall

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    Mark, once you start using vacuum you will never look back. With a little thought and ingenuity you can hold almost anything by vacuum, but sometimes the extra effort is not worth the time involved.

    Regarding the Cole jaws - see my post back in February in the link below #19 (I hope this works!)
    Tennon and recess size
     

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