Turning Archive 2005

Concentric circles and a whole lot more *PICs*

Dave Smith, Longview, WA
>These three paragraphs are from an article I have been trying to write about bowl distortion for a follow up on alcohol drying article.

When I first started collecting data on drying bowls using the alcohol soaking method I tried to determine the distortion by measuring the change in longitudinal and radial bowl width. It became apparent after measuring the ovalness of a bowl that it didnít always equate to success in turning a round finished bowl. A better indicator of an adequate roughed out bowl wall thickness is to true the top of the rim to a flat surface. Then using a pencil scribe the largest and smallest circles(concentric circles) possible on the top of the rim. If a roughed out wall thickness of one half inch consistently results in the ability to remount a bowl and scribe concentric circles of 5/16" or larger then that is a viable thickness for the material and geometry used. If a thinner finish wall is desired then perhaps a thinner roughed out wall would suffice.

Pushing the limits of wall thickness of roughed out bowls occasionally leads to the inability to turn a finished bowl. Thinner wall thickness requires greater care in remounting the bowl on the lathe. After mounting a roughed out bowl it is helpful to start truing the rim and determine of the bowl needs adjustment before removing material for the sides. Concentric circles are a good indicator of a good bowl remount.

Turning thin roughed out bowls can also lead to distortion of the finished bowl if only one side of the bowl is turned to finished shape before proceeding with the other side. I have found that shaping one side of the bowl builds up stress that is relieved when the other side is turned to shape. When the high waste is removed on the second side it relieves stress and results in a distorted finished bowl. By removing the high waste on each side incrementally I have effectively eliminated distortion of finished bowls.

Example of Concentric Circles on a white oak bowl.

Bowl profile after removing paper wrapping. This is a crotch piece and you can see the two limbs on the left an right. As you can see I cut close to the pith which results in considerable distortion in the area of small growth rings.

Other side of bowl.

I have no idea where the final bowl data is but since I donít throw anything out it must be around some where. My wife gave the bowl to a friendís daughter as a house warming gift. So I know where to get the measurements. I am fairly certain the final thickness was 5/16".

Below is the data on soaking and drying the bowl. Red Lion is a local motel where my daughter works. I mark all my wood as to origin incase there is a recall on the wood.

Red Lion white oak bowl 1

Rough turned to 3/4" thick x 11" diameter
Alcohol soaking
in 2200 5/7/04
out 1130 5/8/04

5/8 1530 2,11.5
5/9 0845 2,6.4
5/10 0840 2,2.55
5/11 0920 2,0.2
5/12 0830 1,14.45
5/13 0930 1,13.35
5/14 0830 1,12.65
5/15 0930 1,12.2
5/16 1000 1,11.9
5/17 1600 1,11.9
5/18 1730 1,11.5
5/19 1500 1,11.5
5/21 0740 1,11.3
5/24 0810 1,11.05
5/25 1945 1,10.9

Final diameter 11" x 10 5/8".

There are pictures of the finished bowl in the next post.

Dave Smith

With all this manure there has to be a pony somewhere, I just canít find it in Longview, WA.

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