Mounting cored bowl blanks

[Ben Richardson]

I have been given several bowl blanks that were cored from a larger piece. They don't have a clean center point on the bottom and I am not sure what the best way to mount them on the lathe. Should I just use a closed chuck on the drive side and pick the best spot for the tail stock? They slip a lot if I use this method. Any thoughts?
TIA!
Ben

[Marty Tippin]

I'd suggest reverse mounting and centering it the best you can by hand, then cut either a recess or a tenon to match your chuck. Smooth sailing from there.

[Thom Sturgill]

+1 on Marty's suggestion.

[Faust M. Ruggiero]

Here is how I do it. It works for me and I do a lot of cores. Mount the bowl in the vacuum chuck with inside facing tailstock. Use a morse taper extension and hold the bowl with the live center. True a small portion of the inside for the vacuum chuck to fit against. Just true enough for the chuck. Use a gouge rolled on it's side flute facing the center of the bowl. Then take the bowl off the vac chuck and turn it around. You now have a really true surface for the vacuum chuck. Finish the outside, including sanding. Turn it around, rempunt on the vac chuck and finish the inside. I usually drill a hole near the bottom indicating the desired thickness because you can't get a caliper inside the vac chuck.
Why do I do it that way? A bowl distorts while drying causing it to become oblong. If you face the inside toward the vac chuck or even your four jaw chuck and tighten the tail stock too hard you risk splitting the bowl. You can put the convex side into the vac chuck and tighten the tailstock as much as you want and the bowl will not break because the forces are holding it together. Besides, hollowing a bowl on the vacuum is a treat because the chuck is supporting the sides and you get very little chatter, if any. Sounds like a long way to go. Try it, you might be surprised.
faust

[Col Smith]

I would mount a round of MDF etc on the lathe & with a pencil mark circles every 5 or 10mm
Place the bowl in the centre using the pencil lines to centralise.
Bring up the tailstock & cut tenon or recess. If the bowl has a nub remove this to make a flat area which makes mounting easier.

HTH
Col

[Larry Matchett]

If you core from the largest to the smallest you will always have a tenon to work with. When you core the largest core be sure you have a recess on the face before you core the largest. After you core chuck it up and either turn a tenon or a recess. Do the same thing for each core. If you are doing very thin cores from the smallest to the largest then reverse chucking or vacuum chucking is your only option. I have found that reverse chucking works best. I have to admit I am not nearly as proficient as Mike Mahoney.

[Bob Hamilton]

I use a chuck or disk as a friction drive when I bring back my cores, depending on the size of the core. As Faust said, if the drive is large enough in diameter that it is seating against the bowl walls fairly high in the bowl then the tailstock pressure could cause the bowl to split. I aim for my contact on the interior of the bowl to be just a little up from the bottom when I am using a disk. I use the tailstock to pinch the bowl against the friction drive and turn a tenon, and usually bring the entire exterior of the bowl fairly close to its final shape before turning at around and mounting it in a chuck to work the inside. I don't have any problem finding center on the core. The stub on the bottom where it finally got small enough to twist itself off is usually a pretty good guide, or the tool marks left by the coring cutter are reasonably concentric circles that let me eyeball a center close enough to get the bowl round again within the thickness of the walls. I use the 10% rule when coring so the walls on my cores are fairly thick and if I am off by 1/8" or even 1/4" on center it really doesn't make a lot of difference.

Good Luck!
Bob

[charlie knighton]

+1 Larry, Mahoney is a treat

[Robert Edington]

Or you can put the bottom of the core against a disk sander or belt sander and sand a flat on the bottom. Figure out where the best center of the core is, and glue a block to the core. Then use a face plate, or have a tenon already on the glue block and use a chuck.
RP

[Bob Hamilton]

Hi, Ben:
I have just uploaded a video showing how I go about it: https://www.youtube.com/watch?v=N-7EUemtumo

Take care
Bob