Varying Wall Thickness

[Brett Bobo]

When starting to turn the wall thickness on the inside of a bowl earlier, I noticed that the wall thickness varied considerably by about an 1/8" around the perimeter of the bowl. To verify this, I marked a 1/2" wall thickness at a random point on the bowl's perimeter and then turned the lathe on to score the line around the perimeter. Again, the wall thickness varied by about 3/32 around the perimeter by measuring from the outer edge of the bowl to the scored line. What is causing this problem?

Thanks,
Brett

[Tom Giacomo]

It could be just drying. This will induce a certain amount of out of round or twist to the turning. The wall thickness may not be changing but the shape may be slightly changing.

[Allen Neighbors]

I would guess that the bowl is warping as you're turning. If the wood is green, you might try spraying the outside to keep it wet while you turn the inside. If it's not green, but still warping, it's just not dry enough. My guess, only, not worth much.

[Don Geiger]

Without seeing it, I'm not sure exactly what your problem is. I speculate the problem is the wall thickness varies from one side of the bowl to the other.

I've had many students that have experienced a problem where, when you look at the edge of a bowl, the inside is not concetric to the outside. When you look at the edge the wall on one side of the bowl is thicker than the other side.

This is caused by not having re-turned the outside once the bowl is flipped around to turn the inside. It's always a good practice that, once the bowl is mounted on a faceplate or a chuck, to re-turn the outside making it concentric to the spindle of the lathe as is now re-mounted. Be sure to re-turn the outside before you venture to the inside, then the inside and and outside will be concentric to each other. This will also help if your wood is warping.

Another little tip: If you are tuning green wood- don't mess around. Get the project done within a reasonable amount of time without taking breaks. Plan your project so you dont' have to take a lunch break etc. until you are done.

Good turning!


Don Geiger

[Brett Bobo]

Don, you're correct, the thickness of the bowl is not consistent around the perimeter so one side is thicker than another. I'm mouting the bowl with a chuck for turning the inside but given that I'm right-handed, I'm not sure I'd be comfortable re-turning the outside from that position and working backwards--from right to left. Are there any alternatives or suggestions to do this with accuracy of the bowl's shape?

The bowl did appear to be slightly out of round when looking at the bowl from the front of the lathe when in the chuck. Should I try flipping it around and try to round it again between centers?

The wood was not very wet and the log itself (12" dia. x18" long) was about 8 months old. The wood is pecan and the strange thing is that I turned a bowl from the other half of this log and didn't have any issues. Do this still sound like a moisture issue? Would trying to get the exact center on each side of the blank (between centers) when mounting the blank on the lathe help remedy this problem?

Thanks for the help!

[Don Geiger]

Since you said the wood is fairly dry, I doubt the bowl moved very much.

I believe the problem is that after the outside was turned between centers then mounted on the chuck the position wasn't exactly the same. I've seen this many times.

I too am right handed but have a technique that allows me to re-turn the outside, once the piece has been re-mounted.

If the piece is side grain (perpendicular to axis of the lathe) you will want to start near the foot of the bowl and work your way toward the rim. I suggest using a deep fluted side-ground bowl gouge if you have one. Because the head stock gets in the way, sometimes it's difficult to have enough room to push the cut- instead try standing to the right and with your gouge on the left, pull the gouge toward you from the foot of the bowl toward the rim. To do this: Use your right hand, in an overhand grip, to hold the shaft of the tool and anchor the meat of your right hand on the tool rest so you have control. The shaft and handle of the tool needs to be level with the floor. Hold the handle of the of the tool with your left hand and anchor the handle against your body. The flute of the gouge needs to be upward at 45 degrees and facing you. At the beginning make the wood near the foot concentric to the way it is now held in the chuck. Once that part is centered then work you way toward the rim in smooth overlapping traverses (always starting on a section of the wood that has been re-centered).

Once you've got the bowl re-centered, you can drop the handle to about 30 to 45 degrees from the floor into the outside shaping cut and again draw the tool from the foot torward the rim and smooth out any imperfections and work on the shape. NOTE: the outside shaping cut is not a bevel riding cut.

Holding the tool the ways I have described will feel awkward at first, but once you mastered these cuts they will be of great benefit to you.

Let me know if I can be of further assistance.


Don Geiger