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Sawing a bowl in half

R

Roy Hennagir

Joined
Mar 16, 2017
Messages
45
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15
Location
Iowa
Image 3.jpg
Standing dead Elm, cut last month. Dead at least five years if not more.

Bottom to thin and it cracked. Still learning.

Sawing this in half has shown me a lot of room for improvement. What I thought my fingers were telling me was not what cutting the bowl revealed.

After the bottom was compromised I still finished the bowl. I polished it out with the Doctor's Woodshop pens plus finish. I'll keep half as a reminder and make the other into a wall holder thing.
 
You can use your lathe cutting tool as a depth gauge when turning a vessel on the lathe.
You can also easily make an adjustable depth gauge that will measure how far you are
cutting into the recess of the vessel. A set of calipers is also very useful tool for attaining
an even wall thickness on a vessel. There are a number of videos on YouTube that show
how to make your own depth gauge and calipers for wood turning projects. Everyone finds
the bottom of a vessel sooner or later when turning items on a lathe, If this happens again
you could glue the vessel to another layer of wood and turn the additional piece of wood
into a decorative base for the vessel.
 
Maybe just a tad thin on the bottom. 😀

As you saw, that shape can invite problems like cracking. Here is why: movement in wood is much greater across the grain than along the grain. Even a standing dead tree still has lots of moisture in the wood and there will be a lot of movement during the drying process. Even if the wood is bone dry, there are internal stresses that are being thrown out of balanced when wood is removed during the turning process. In this situation the vertical walls and flat bottom is comparable to gluing end grain to cross grain in flat woodworking if you are familiar with that type of joint failure problem. The variation between very thick and very thin walls is also problematic ... if the thick part wants to move, the thin part doesn't have the structural integrity to withstand the pressure.

A design that avoids abrupt transitions in shape will help to more uniformly spread out the internal stresses and greatly reduce problems related to wood movement. Having a uniform wall thickness is also helpful in this respect. A hemispherical shape was mentioned earlier. A variation on that shape, an elliptical bowl is also a good shape to consider especially if making a bowl that is wide and shallow. In the image below, I show an overlay of an elliptical curve over your half bowl to illustrate the difference in cross section between the two. It and other "round" shapes are good practice while developing your tool control.

Image 3a.jpg
 
Bill's diagram shows a form that allows the wood to move.
In my experience a flowing curve is the paramount attribute to prevent cracking.
Even wall thickness helps a lot too. However a bowl with Bill's suggested shape could have a fairly thin bottm and not crack.

I posted a link to demo handouts I use and video taken of a demo I do on working with green wood in the tutorial and tips subforum:
http://www.aawforum.org/community/index.php?threads/working-with-green-wood.11626/

Two slides in the handout show the difference when wood can move and when the shape prevents wood movement. The bowl on the left can move( similar in shape to Bill's diagram)
Notice the bowl on the left has a peak on the rim that forms as the wood shrinks and moves toward the center bottom. The bowl on the right wanted to move in the same way but could not because of the flat bottom so it cracked.


IMG_3951.jpg

IMG_3952.jpg
 
I appreciate the criticism and more importantly the explanations of why.

When I cracked the bottom I decided to finish it out and learn from it. I'm surprised it took finish as well as it did. It was still pretty wet.

I have done a lot of standard flat wood working and a lot of green work for chairs and things of that nature. This bowl turning is a different animal. The wood grain is oriented differently and you have a mixture of grain orientation in each bowl. They start to dry on the lathe and become out of round before I remove them. This happens faster as I turn them thinner. Shape shifting on the lathe presents issues as well that I had not thought of.

It seems so simple when I watch it on TV.

I think I'm going to have a problem with piles of bowls all over the place.
 
Roy Hennagir said:
Sawing this in half has shown me a lot of room for improvement. What I thought my fingers were telling me was not what cutting the bowl revealed.
Click to expand...

Occasionally bandsawing a bowl in half is a good way to see if your fingers are telling you the truth! Remember this, Roy.......With time, your fingers get calibrated! They are a good indicator, but your mechanical depth gauges, and calipers are not going to be replaced for that final measurement! For me, one item that has become a staple in turning bowls is the Tomkins Gage-T.......that thickness gauge might be one of those items that hurt when you initially make the purchase......but, will turn out to be one of the greatest little helpers when doing bowls......! 😀IMG_2024.JPG IMG_1435.JPG
The Tomkins Gage-T is on the left here:
IMG_1440.JPG
ko
 
Odie, my best bud (RIP) used to say, "Buy once, cry once."
There are some tools that, if they disappeared, would be replaced tomorrow if not sooner. I think your Gage-T is one of them.
Last pic, far right caliper, how is the washer used ?
cc
 
Clifton C said:
Odie, my best bud (RIP) used to say, "Buy once, cry once."
There are some tools that, if they disappeared, would be replaced tomorrow if not sooner. I think your Gage-T is one of them.
Last pic, far right caliper, how is the washer used ?
cc
Click to expand...

Howdy Cliff........🙂

Yep.......I had no idea how important that Gage-T would become, when I first decided to buy it. 😉

That little washer attached to the Versa-cal calipers is just a better place to hang it on the wall. Someone taller than me wouldn't need it.....Ha,ha! 😀

ko
 
odie said:
Occasionally bandsawing a bowl in half is a good way to see if your fingers are telling you the truth! Remember this, Roy.......With time, your fingers get calibrated! They are a good indicator, but your mechanical depth gauges, and calipers are not going to be replaced for that final measurement! For me, one item that has become a staple in turning bowls is the Tomkins Gage-T.......that thickness gauge might be one of those items that hurt when you initially make the purchase......but, will turn out to be one of the greatest little helpers when doing bowls......! 😀View attachment 22471 View attachment 22472
The Tomkins Gage-T is on the left here:
View attachment 22476
ko
Click to expand...

Odie,
Funny how you can take a shape for granted your whole life, I've never really considered the sweep or curve of a bowl. It looks so simple to do, and then you try it.

I only need to:
Have a sharp tool
Orient the tool correctly
Not lose the cut
Not run out of tool rest
Make the cut the same depth throughout the sweep
Get it done before it starts to warp
Swing with the tool with my body
Not grip the tool to death
Not cut through the bottom
Make the walls a uniform thickness
Know what cut to use and when
Know when to stop
Know when to sharpen
Etc.

It's all so simple. (When the other guy does it)
 
@Roy Hennagir the trick is to turn enough that you become the " other guy".
Just about everything on your list is second nature to experienced turners.
It will become 2nd nature to you if you stick with it.

There is a lot going on for sure.
Don't overthink it. I have my beginners work on surface.
If I coach them into getting decent surfaces they will eventually figure out decent curves.

Then there are few folks who never get good with curves - they get pretty good with repairing cracks.
 
Last edited:
hockenbery said:
@Roy Hennagir the trick is to turn enough that you become the " other guy".
Just about everything on your list is second nature to experienced turners.
It will become 2nd nature to you if you stick with it.

There is a lot going on for sure.
Don't overthink it. I have my beginners work on surface.
If I coach them intomgetting decent surfaces they will eventually figure our decent curves.

Then there are few folks who never get good with curves - they get pretty good with repairing cracks.
Click to expand...

I'm just turning them all from the firewood pile. I've been doing roughly the same one size wise over and over. I try to learn with each one.

I just got the Lyle Jamieson bowl basic's video. I turned a whole blank into shavings just practicing the push cut he demonstrates in the video. I want to learn why the tool is doing what it's doing. I'm getting ready to turn another into shavings as well. I'm still not able to get the cut to be consistent and uniform. I'm trying to limit my variables as much as possible.
 
odie said:
Occasionally bandsawing a bowl in half is a good way to see if your fingers are telling you the truth! Remember this, Roy.......With time, your fingers get calibrated! They are a good indicator, but your mechanical depth gauges, and calipers are not going to be replaced for that final measurement! For me, one item that has become a staple in turning bowls is the Tomkins Gage-T.......that thickness gauge might be one of those items that hurt when you initially make the purchase......but, will turn out to be one of the greatest little helpers when doing bowls......! 😀View attachment 22471 View attachment 22472
The Tomkins Gage-T is on the left here:
View attachment 22476
ko
Click to expand...
That gauge is probably the best thing I have ever purchased aside from the lathe itself!
 
I see the gage, odie has it with a couple of others. What makes that one better? I understand it's an opinion. Why is it better then the others pictured with it?

Based on some opinions I've read here I own a couple of Thompson tools and I am half way to owning all the robust tool rests for my lathe. I am very, very happy with both of those purchases.
 
Roy The Thompson gauge was designed to work in a lot of shapes and be very fast to use. I use to change calipers to measure my walls depending on the shape of the bowl. With Keith's gauge it's pretty much all I need except for a few really small bowls.
 
Roy,

Every beginner I've assisted except one has made a bowl of that shape for their first bowl. It's a classic!

I'm a little late, but wanted to add that you made a very clean and well shaped recess for your chuck. That's critical to keeping the wood on the lathe and something beginning turners take a while to grasp. If you make them that well shaped, you can make them less deep, which would help with the bottom thickness. You might also consider using a tenon instead of a recess, as probably most turners on this side of the equator do.

You've gotten really good advice here, and DVDs can help, but one of the best ways to make progress quickly is to work with a coach or mentor. If you don't have a friend who is an experienced turner, check out your local turning club, which you can find on the AAW web site. http://www.woodturner.org/?page=Chapters
 
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