Ugh, turning out of round bowls

[Gary Herrmann]

I tried the DNA thing. Its been two weeks, so I'm trying to turn a now oval shaped plum bowl. I've had a few catches which either loosened the dang thing or sent it bouncing off the lathe. After the last one, I decided to call it a day.

I think I was starting to get tentative, which doesn't seem like a good thing. Going to surf a bit, have a beer and when I go back downstairs, it will be to play air hockey with the mancub.

I'm off tomorrow, so if I can get through the honeydos, I'll confront that plum booger again.

[Jim Ketron]

Gary, did you re true up your tenon first before mounting it in the chuck?
Your tenon will distort and warp just like the bowl did.
Take your time and light cuts until you knock down the high spots then you can get down to business.

[Rich Souchek]

Gary,
Jim suggested it above. The mounting tenion also gets oval and needs to be recut.
I put something on the headstock that is close to the bowls curves, then set the bowl inisde to the headstock. Bringing up the tailstock, then trial rotate the piece to get it centered. Then a little more tailstock pressure and light cuts to get tenion round again.
Then flip bowl and mount in chuck.
If bowl doesn't want to stay in chuck after that, and tenion starts getting torn up, I sometimes use thin CA to harden the tenion wood.
Rich S.

[Dominic Greco]

Gary,
Sounds like the grain is tight on one side of the bowl and loose on the other. Several years back Bill Grumbine taught me how to avoid this. Placing your blank across uneven grain will warp even the best intentioned bowl. Even one soaked in DNA.

To explain uneven grain, picture the log with the fresh cut portion facing you. It looks sort of like a bull's eye, right? Only, some of the circles (growth rings) look like they were drawn by a drunken artist. If you look at the distance between the growth rings you'll see some rings are close together, and some are not. Cutting your bowl blank from a portion of the log that has both closely and loosely packed growth rings will get you a warped bowl for sure.

Think of it as uneven forces acting on your wood. To combat this, you must exercise some care when cutting your bowl blank from the log. With wood like cherry, apple or plum, this is real important. Don't straddle two different areas of grain densities. Make it so that your bowl blank will be cut from a portion that either all loose grain, or all tight grain.

Another thing to watch out for is the pith. This is the center of the tree and the area that cracks the most. Most times you cut this right out when you split the log for bowl blanks. But some times your chainsaw cut wanders and you get a blank with more pith in than you intended. It's best to get rid of that area or risk a crack.

I hope this helps.

[Bob Hamilton]

I have this simple sketch to illustrate what Dominic is saying. You would cut the log along the green line rather than the red one.

http://www.bobhamswwing.com/justpics/cut%20line.JPG

Take care
Bob

[Bernie Weishapl]

Holy Cow Bob and Dominic. It is like a light came on. Thank you for your explaination. Gary I had the same problem on some and not on others. If I cut them it seemed like I had problems and the ones I bought I had less problems. Now I understand why when I read this and went out to look at blanks I cut and blanks I bought. Most that I bought were cut on the green line. I hate to admit it most I cut were cut on the red line.

Sorry Gary I didn't mean to jump in like this but it just hit me. I do agree with Jim that light cuts are the order of the day and make sure your tenon is round.

[Dominic Greco]

I have this simple sketch to illustrate what Dominic is saying. You would cut the log along the green line rather than the red one.

http://www.bobhamswwing.com/justpics/cut%20line.JPG

Take care
Bob

Bob,
Thats PERFECT! Just what I was trying to say.

[George Tokarev]

I suppose the other information is useful, but the problem of catches is in the presentation. You want to keep a good firm pivot point and rotate the gouge in and through the shape until you get a place to reference a bevel. Some fat guy roughing the back of a platter in birch with some severe warping and with an off-center heart like the no cut direction at http://s35.photobucket.com/albums/d160/GoodOnesGone/?action=view&current=TrueBottom.flv if you have broadband.

Inside truing cuts also. http://s35.photobucket.com/albums/d160/GoodOnesGone/?action=view&current=Inside.flv

Cutting with the heart off-center will give you a pattern similar to the pictures here. http://www.sawmillcreek.org/showthread.php?t=52195
It started waaaaay out of shape, as you can see.

[Gary Herrmann]

Thanks guys. I had rounded the tenon. I probably wasn't using a light enough touch. The other thing that made a huge difference was making the tenon longer. The jaws had more to grip and I think that helped it not wander off due to my neophyte technique.