Trued up Bowl became out of round as turning progressed????

[Doug Olsen]

Today I had an experience I don't understand. I turned a very dry 10 inch cherry bowl down to 9 inches to get the exterior true. Thereafter as I refined the outside and turned the inside it became very "out of true" as I continued shaping the bowl.

It was turned to a thickness of 3/8 inches but became out of true long before I reached that thickness. I used a six screw face plate and checked to be sure the screws had not loosened.

I had to lower the rpm's from 800 to 500 because vibration set it as I continued turning the bowl. I am at a loss as to why the bowl was true in the beginning and became "untrue" as I progressed.

I'm sure it is "pilot error" but I can't figure out what I did wrong.

Can anyone give me some pointers? Thanks,
Regards,
Doug Olsen

 

[Odie]

Today I had an experience I don't understand. I turned a very dry 10 inch cherry bowl down to 9 inches to get the exterior true. Thereafter as I refined the outside and turned the inside it became very "out of true" as I continued shaping the bowl.

It was turned to a thickness of 3/8 inches but became out of true long before I reached that thickness. I used a six screw face plate and checked to be sure the screws had not loosened.

I had to lower the rpm's from 800 to 500 because vibration set it as I continued turning the bowl. I am at a loss as to why the bowl was true in the beginning and became "untrue" as I progressed.

I'm sure it is "pilot error" but I can't figure out what I did wrong.

Can anyone give me some pointers? Thanks,
Regards,
Doug Olsen

Doug.......

You are correct. "Thinness" may contribute to conditions leading to warping, but it isn't the only thing that matters........

Anytime you remove wood, you are altering the internal stresses in what's left. Most dry and stabilized woods will warp to some degree during final turning......some will not. The amount of warp may, or may not be enough to alter the "game plan" for that bowl.

When I detect a warp, my first thought is to reconsider any plans I may have had for detail work, as these things need a near perfect circular surface for them to look well executed and appealing to the eye.

This sounds like it would have been a good bowl to practice with your newly ordered Oneway Bowl Steady........😀

ooc

 

[john lucas]

Odie's right. Wood often moves when you remove wood from it. It can be unpredictable. As you gain more experience you will turn faster so the bowl often doesn't have time to move before you get it hollowed. However it still happens sometimes. I try to turn the first inch or so to the thickness I want and then never go back if it starts to move. If it stays round I may go back and thin it more but not usually.
I just turned some air dried lumber that had been drying for 20 years. I let it acclimate in my shop for a week, cut the wood and let that sit for another week before the final cutting and gluing. I turned it and 5 days later the door to the piece move again. You would think after this much time it would be stable. It didn't move much but when it's a door the gap is pretty obvious.

 

[Mjonesrdg]

Not pilot error

Doug,

This is a pretty common occurrence, not your fault. Especially if the bowl blank was cut from limb wood. Mother nature arranged the wood fibers so that the top of the limb had good tension fibers, and the bottom of the limb had good compression fibers so as to hold the weight of the limb up. Much of that difference remains even after the wood drys...and when you release the tension, it tries to straighten out.

 

[Bill Boehme]

You probably had a piece of "reaction" wood. Wood from limbs has a lot of built in stresses that are in equilibrium as previously mentioned. However, it does not need to be from a limb to have internal stresses if the trunk had a significant lean. When you remove some of the wood from a piece of reaction wood, it is no longer in equilibrium which will cause it to move until the stresses are back in balance. When you get a piece of wood like this, there is very little chance that you can do anything to prevent it from warping.

 

[MichaelMouse]

As said, wood has its own internal stresses. If you had orchard wood, almost a cinch the piece had been stressed when growing by pruning for picking. Limbs are also full of stress. The cause of the stress should be obvious. Straight grain is predictable, if unexciting. Wood under stress is often gorgeous but challenging. When you interrupt the equilibrium by taking wood away, it becomes a bit dynamic.

Thin is not so much the problem as getting to thin, as you discovered. Wood contracts upon itself, so the more contiguous wood you have, the greater the potential for movement. People who leave broad bottoms on green turnings confront this problem and resulting splits quite often. To a lesser extent, the "10 percenters" are also affected. Leave walls too thick and their 10% tangential contraction can get you in trouble when the grain's not symmetrical.

There's also a possibility that your "dry" piece wasn't so dry inside. Or, perversely, too dry. Those adjustments normally happen slowly, though it's possible to produce them under heat stress from sanding.

 

[KellyDunn]

Everybody is right. Its not your fault and common. Its so wet where I live I pop my bowls back in my kiln before I return them. Even though they went through it once to dry the roughed bowl. If I just grab a bowl from the shelf to finish I pretty much will guess its going to move. Almost all my koa blanks are limb wood. I take down the outside and sand that to finish. Do as has been suggested on the inside and work your way down to finish thickness knowing you may not get the chance to touch a tool to the upper part again. Depending on how out of round lower your sanding speed so your discs do not bounce inside the bowl or you will sand two edges very thin. After you have sanded the outside you should have gotten it damp to raise the grain and before you get onto the inside sand in both forward and reverse to sand down raised grain. On the inside if it goes way wonky chances increase for hidden sanding marks that getting the piece damp will really bring out. If bad go back to whatever grit you think you need to. For me its rare to have to back below 220. But it happens and I do whatever it takes to clean it up.