Wall thickness of wood turned bowls can be a challenging aspect to master.
Of all the different components of wood bowl turning, wall thickness seems to be sited the most as a significant hurdle to overcome.
In this article, we will look at why wall thickness might vary, why it matters, how to measure wall thickness, and how to set your bowl up for a successfully turned even wall thickness.
Once or Twice Bowl
Here we are looking at the final wall thickness of a finished bowl. That might be a once-turned green bowl. Or the first turning of a twice-turned bowl. Either way, the concepts are the same.
A once-turned bowl is just like the term says, turned once and done. Making an even wall on a once-turned very green and wet bowl blank is vitally important to prevent cracking due to uneven drying.
The wall thickness of the first turning of a twice-turned bowl is not as critical but does need to be relatively even throughout.
If you’d like to learn more about twice-turning wood bowls, check out this article.
You might be asking why is it so essential to make the wall thickness of a wood turned bowl even?
It doesn’t really matter if the bowl walls are thick, somewhat thin, or very thin. They just need to be as even as possible, but why?
At first, it might seem that it’s purely an aesthetic consideration. Evenly turned walled bowls look and feel better. While that may be true, there is something more going on.
Whether a bowl blank is super green and wet or if it has been sitting around for some time, there is a good chance there is still moisture inside the wood.
When we turn the bowl walls even, the contained moisture has an equal opportunity to escape throughout the whole bowl at about the same rate.
If we leave areas thicker than other on the bowl, the denser regions will hold moisture longer than the thinner areas and cause uneven drying.
Uneven drying is a significant factor in causing wood to stress and crack. We don’t want cracks, so we focus on making evenly thick (or thin) walls on our wooden bowls.
If you have turned more than a couple bowls, you know what an uneven wall thickness looks and feels like.
Our index finger and thumb are incredibly useful calipers and critics of bowl wall thickness.
Early on in wood bowl turning it is common to get excited at the prospect of a finished bowl and rush the last few cutting passes.
It’s easy to make cuts that don’t conform perfectly with the exterior bowl shape and deviate into the side wall, making the wall too thin in areas.
In worse case scenarios, the bowl gouge even makes it through to the other side.
As they say, “making the inside of a bowl wider than the outside is not a desirable effect.“
It is near the end of the bowl turning process when we can quickly start to see the final bowl come to life when we need to slow down and make sure each cut is what we need and nothing more.
The Simple Slip
The biggest culprit to uneven bowl walls is the wandering bowl gouge over-cut.
We’ve all been there. We’re down to the last couple passes, and instead of following the gentle curve of the bowl, our gouge glides a bit too far left and into the wall.
At that point, we’re stuck with an even rim area that fades into a thin area that gets thick again.
If we fuss with it, we will usually make matters worse.
Slow And Steady
Aggressive cuts along the interior bowl wall can be executed after much practice.
Once we are close to the final few cuts, instead of making a few massive cutting passes, it is better to make several smaller cuts.
Making multiple cuts also gives us time to stop the lathe and frequently check the wall thickness.
It’s much easier to shave a little more off than trying to add wood into an area that is too deeply cut.
Make three or four super light passes to gradually get the thickness right where you need versus one large pass that might go a bit too deep.
When you locate an area of the wall that is a bit too thick, be careful to only make a light shaving cut to reduce the surface.
The best way to guarantee the gouge makes a light cut and doesn’t dig in or get too aggressive is to rub the bevel first.
Drop the bowl gouge handle just a bit, so the cutting edge is off the wood and just let the bevel rub gently.
When you are ready to make the pass, slowly raise the gouge handle until the cutting edge engages and proceed with the cut.
Check, Check and Re-Check
It doesn’t cost anything to stop the lathe and check the wall thickness.
Do it often!
Manually Checking Thickness
With the lathe off, use your fingers to pinch the rim and slide down the side wall of your bowl as far as possible.
Check in a few different areas if necessary, and try to ignore any texture created by the wood surface.
Even closing my eyes has helped me detect slightly thicker areas of the bowl wall that need just a bit more attention.
Measuring With Assistance
Our fingers are the best way to measure the rim and top of the bowl. However, we need help with bigger bowls.
Calipers are necessary to visually see the wall thickness of a bowl beyond the reach of our fingers.
Double Ended Calipers
Double-ended calipers work well for visually replicating and indicating the bowl thickness. Here’s a link to the double-ended calipers I use.
Gently pinch one end of the calipers on an area of the bowl side wall. Then the wall thickness is presented as the open space between the two opposite points.
Carefully slide the calipers up or down the bowl wall, and any gap variation indicates thicker or thinner areas on the bowl.
I also use a C-shaped plunger style gauge with a dropped stick to check my larger bowls wall thickness, especially near the bowl base.
The base of the C-shaped portion is positioned on the bowl exterior, and the plunger stick is dropped in on the inside part of the bowl.
The thickness of the bowl wall is indicated by how far the end of the plunger is from the C-shapes gauge.
This plunger style caliper was designed by Mike Jackofsky. Here’s a link to his site for more details.
Two Core Approaches
The conventional way to scoop out the bowl interior is to start in the middle and turn out a small half sphere.
The middle-out approach starts in the bowl middle and progresses with increasingly larger scoops until the last pass becomes the bowl interior.
Middle Out Drawback
The technique of starting in the middle and working outward can work well, especially for bowls with relatively thicker walls.
However, I have a couple issues with this approach.
First of all, it is easy to get carried away, making each scoop bigger and bigger all without paying much attention to the bowl’s exterior.
If the interior hemispherical shape doesn’t match the exterior of the bowl, you may need to do some improvising at the end to adjust the shape of your cutting path.
Also, if you plan to turn a very thin walled bowl, the center mass of the bowl blank is gone by the time you’re ready to make the last delicate pass.
When the walls are thin, and bowl center mass has already been cleared away, the top rim and bowl walls will flex and vibrate making a clean cut impossible. (emphasize)
Inch Worm Approach
I love to turn natural edge winged bowls, and one little secret to getting the top wing edges turned is to do them first.
Yes, this technique is precisely the opposite of the middle first approach. Think of this procedure like an inchworm working its way down the inside of the bowl wall interior.
Even though I use this trick on natural edge bowls, it works for any type of bowl.
Instead of scooping out the center, start right at the top rim and establish the final wall thickness immediately.
With a small groove marking the thickness of the bowl wall, make a few passes to the right to clear away a “ditch.”
Now with a bit cleared away, continue the final wall thickness downward and clear a small amount to the right.
As you continue inching down the inside wall, a tiered pyramid cone will appear in the bowl’s middle.
The bowl’s center mass is holding things together and making it possible to turn the top edge of the bowl rim as thin as you’d like.
As the wall emerges, the center cone can be turned away progressively to make room, until you shape the final inside bowl bottom.
Don’t Back Up
Here is the catch with the inchworm method. Do not back up!
Once you have finished the top rim area, don’t return to that area, and so on down the bowl.
As you progress downward and the center mass is turned away to match the bowl wall, the solid support is lost.
The rim and top sidewalls will flex and flutter when the center core is reduced. So don’t try to back up.
Stacking The Deck In Your Favor
Besides stopping the lathe and measuring the wall thickness frequently, there are some additional things you can do to give yourself an advantage.
The bowl interior corresponds directly with the bowl’s exterior shape.
A more complex bowl shape makes matching the interior wall thickness a challenge, and it increases the chance that the bowl interior might intersect with the bowl’s exterior.
Keep the bowl exterior design simple and fluid, and it will be much easier to make the interior cooperate.
Added Tenon Shoulder
Many times the bowl wall thickness issues seem to be nonexistent until we reach the base or foot of the bowl.
The last little area in the bottom of the bowl is a famous place for a blowout. It’s hard to gauge the thickness in the bowl bottom.
If you’ve ever been turning and the sound of your bowl gouge changed from a cutting sound to a hollow rubbing sound, you probably know what I mean.
A way to combat blowing out the bowl bottom is to give yourself a margin for error.
When you make your tenon, add a nice wide and relatively deep shoulder area under the tenon.
This will give you extra material to work with if needed.
Line of Sight
How we stand and “see” the cutting passes as we work also plays into a successfully turned bowl.
If we bend over and peer into the open end of the bowl, we have no idea or way to gauge the thickness of the bowl walls.
Try positioning yourself standing tall and looking down on the bowl profile. In that position, you can see where the gouge is traveling and even visualize where the tool is down inside the bowl as it cuts.
When turning the interior bowl walls and establishing the final wall thickness, the bowl gouge bevel needs to be parallel to the exterior of the bowl.
Take a moment and check that the bevel is in the right position before and during each pass.
And of course, riding the bevel is critical to achieving a smooth finish. Be sure to check out this article all about riding the bevel.
Wall Thickness Wrap Up
Establishing and maintaining an even wood bowl wall thickness can be a struggle at times.
Don’t sweat the occasional unevenness or the occasional translucent areas. They are all part of the learning process and should be considered a rite of passage.
Take your time and unleash some of the tips from this article on your next bowl.
Let me know if you see a difference in the walls of your bowls. Leave me a comment below.
Thanks and Happy Turning!