just thinking out loud here...

Re: just thinking out loud here...

I believe, as a backer plate, Cole jaws would be too thin and bend under the pressure of the clamping bolts.

hmmm... interesting point

I wouldn't be concerned about the metal plates themselves, but your point makes me think about the force transmitted to the chuck slides.  I would not overtighten it, so hopefully this might not be a problem, but definitely something to think about.  

Cole?

Slaw's brother

Another point

Here is a pic (mock up) showing my donut chuck with PVC pipe used as a stand-off for a warped rim bowl. Some grooves have since been cut for the PVC. � With the tie rods at the extreme circumference and a small footprint in the center (PVC pipe) I can understand there would be concentrated stress at the center. If this was set up on Cole jaws, there would be a lot of stress where the jaws attached to the slides. But, if instead of the stand-off, a flat rim bowl closer to the diameter of the Cole jaws was clamped flat on the jaws, and the tie rods were closer to the rim diameter, it seems to me, the stress would move out from the center, toward the rim, i.e., less stress on the slides. How much less? Not sure. Turnable? Maybe. Safe? We know what Nova would say... � I'm not going to say if I would try, but after I finished, I'd make a dedicated donut chuck. I have a vacuum chuck I use all the time, but I still find uses for the donut... It may be old school, but it's a versatile piece of kit.�

Arrrggghhhh!

Where are the emoticons when I need them?  

I'll have to think more about that...

From an engineering mechanics POV, I'm not sure I would agree that the vertical forces would change based on the height of the object.  There's no question, though, that side forces will increase with the length of the torque arm, and I suppose those may have a vertical component.  I have to admit I've never made one because the thought of all that stuff flying around makes me nervous.  But yours with the PVC jam chuck enclosed apparently works for you, so that's a plus.  

Re: Cole?

Now that's funny; I don't care what anybody says.

Vertical forces, fulcrums and Carolina Coleslaw...

The only Coleslaw is Carolina Coleslaw... � Ok... movin on...

I don't think vertical forces would change based on the height either, until they did.... � I think if you had a small diameter bowl clamped with the tie rods far from the center you would have more stress on the jaw to slide attachment � then if you had a larger diameter bowl with the tie rids closer to the rim of the bowl. My thought being, less stress on slide attachment moving the fulcrum farther away from the centre. I admit to pushing the limits sometimes but mostly the donut chuck is used for cleaning up the little left over nubbin, not heavy duty turning, so, not a lot of stress. As far as whirling hardware, low speed, stay out of line of fire and appendages on the safe side of the tool rest... (Surprised face emoji...)

Alternative

Yo,  I could have mentioned Nat King's dad

But I didn't

yep, totally agree with all you said.

I don't know if I'll do it or not, but if I do, I'll post it.  It would only be for small things and very light cuts.

Re: just thinking out loud here...

Gary & others - here is another option for Cole jaws or Straka chucks.  Below is my quote from another forum on a similar topic on Nov. 2, 2020. [edited]

"Re: Cole / Jumbo Jaws. I never trusted them and they are too fiddly when changing sizes. I like Flat Jaws with added softwood plates which are easily adaptable for different sizes and probably a lot more secure. Only problem with Flat Jaws is size limitations due to the small size of the metal plates that attach to the chuck....you can't go too large with the wood plates for safety. When I had the opportunity to purchase a set of Cole Jaws (Oneway Jumbo Jaws in my case) at an estate sale (cheap) I did so...and then added some PVC foam board (instead of MDF or soft wood) to the jaws. Now I have a 18"Ø chuck (20" swing on my lathe).....way more than I need! Open the jaws slightly and simply turn a groove with a parting tool a little larger than your bowl rim (even dovetailed). Expand or contract the jaws to grip the rim. Perfect centering and almost 360º of gripping on the rim ... not just on 8 points. Can be used for many sizes, many times and easily swapped out when totally used up."

  In the photos below the carved bowl has a rounded rim (made about 2004? - and     never finished) so I made the recess about 1/4" deep or a little less. Slightly dovetailed to hold on the undercut interior in expansion mode.

The last photo are my old Oneway Flat Jaws - same principle just smaller. BTW- MDF is perfect for this use...and cheaper.

"

Thanks, Tom,

Good reminder.  I've been meaning to make some wooden jaws.  You have encouraged me.  Tnx.