Critique of shape, please

[Daniel Warren]

Would appreciate your feedback on the curve/shape of the these two. I feel the one beneath the chuck is what Raffan might consider “ponderous”?

Any feedback on improving the catenary curve would be appreciated!

 

[john lucas]

It's getting there. there seems to be a slight flat or concave area about an inch below the rim. Terry Scott of NZ has a good method for finding those. He uses a long hot glue stick. Bend it around the outside of the bowl and it will show any low or high spots that don't make a smooth curve. Another good method is to run your hand from the lip to the foot. It's surprising how many irregularities you can feel. the overall flow is pretty good.

 

[Daniel Warren]

Thanks John. Those are some good methods. I think I've got some glue sticks hanging around. There may be enough material to eliminate the dip, which I see too.

Is the base too large in comparison to the rest of the bowl in first one?

 

[Bill Boehme]

I would call the curve that you have hemispherical where there seems to be a constant radius. Nothing wrong with that, but if you want a catenary curve, try printing one on paper and then cutting it out as a template to check the curvature. Do the inside and then the outside. That will be a challenge, but I think that it is valuable in getting a good inside curve without having an outside curve to guide you. Shoot for a shallower bowl and I think it will look nicer.

 

[hockenbery]

I try to avoid Flats unless they are part of the design.
A simple way to see flats is to roll a straight edge (pencil works) over the curve with some light behind it.
A curve will touch in one spot a flat will touch over a long distance.
Below you see the difference in the base of your two bowls one has a curve one has a flat.

I find the curve more appealing. The flat is the one you don’t like so much and I suggest the difference is the flat.

 

[Dean Center]

I try to avoid Flats unless they are part of the design.
A simple way to see flats is to roll a straight edge (pencil works) over the curve with some light behind it.
A curve will touch in one spot a flat will touch over a long distance.
Below you see the difference in the base of your two bowls one has a curve one has a flat.
View attachment 30618

Very helpful photos, Al. I could tell there was a difference in the curve of the two bowls, but not put a finger on the exact difference.

 

[Daniel Warren]

I see, Thanks Al. Makes sense. Seems I needed to “tuck” the bottom in more or create a better curve, period.

Here’s another attempt on a large piece of pepper wood. This will have some sort of foot. This was rough turned but I do see some flat spots now.

 

[Emiliano Achaval]

I'll be more worry about the tenon you have. Knocking off the sharp edge will help avoid that the tenon will shear off. It doesn't look like you have a good flat surface for the jaws to sit against. Chances of that bowl running true again with that tenon are slim. It is also too small for the size of the bowl. I also like my tenon to be cut smooth, no torn grain... I leave a square lip for the jaws to sit, not the full length of the jaw, about half, so I have like a step before the bowl begins.

 

[john lucas]

Most of my bowls I try to get the curve at the bottom to flow right through the bottom. I miss it more than I should. It looks great on the lathe but when I remove the tenon I can see that I didn't quite get the shape I wanted. The secret to good bowls in my opinion is to constantly ask yourself the question you asked. What could I change to make it better. Personally I prefer a bowl that has a continuously changing curve like the Catenary curve Bill mentioned. A part of a sphere to me is nice but prefer something a little different. I do like a part of a sphere when I'm doing really flat natural edge piece. That curve that runs from lip to lip across the bottom really works in that case.

 

[hockenbery]

Makes sense. Seems I needed to “tuck” the bottom in more or create a better curve, period.

I get students to use a straight edge when turning the outside profile. So they know they have a curve before it goes into the Chuck.

After a while you get an eye for curves and you learn to watch the profile of the bowl develop rather than watching the tool contact with the wood.

One thing that I find really helpful in turning curves is being able to turn with either hand forward.
When body position blocks my view of the curve usually switching hands on the tool lets me see the curve.
Also useful in getting the chips not to hit you.

What I did was force myself to use the other hand when roughing where it didn’t matter so much.
It took a long time to develop confidence in using the off- hand forward but it was time well spent.

 

[Bill Boehme]

If nobody has mentioned it yet, you can "see" better with your fingers than with your eyes. Close your eyes and feel the curve. Your fingers will tell you where there are flat spots that your eyes may not notice until you find them by feel.

 

[Arkriver]

My lathe has an overhead light and a lathe mounted adjustable light that I can aim wherever I want. I few months back I discovered that if I turn the overhead light off and aim the adjustable light at an angle the tool marks, high and low areas really show up. That might help you. Allyn

 

[Doug Rasmussen]

If nobody has mentioned it yet, you can "see" better with your fingers than with your eyes. Close your eyes and feel the curve. Your fingers will tell you where there are flat spots that your eyes may not notice until you find them by feel.

Old timers in the auto body repair business use their hands by running over an area with a soft clothe to determine if a repaired contour is smooth.

 

[Daniel Warren]

I'll be more worry about the tenon you have. Knocking off the sharp edge will help avoid that the tenon will shear off. It doesn't look like you have a good flat surface for the jaws to sit against. Chances of that bowl running true again with that tenon are slim. It is also too small for the size of the bowl. I also like my tenon to be cut smooth, no torn grain... I leave a square lip for the jaws to sit, not the full length of the jaw, about half, so I have like a step before the bowl begins.

I went back and looked at the teknatool specs and yes, my jaws are way too small for these diameter bowls. Although I will say I successfully turned that 17” pepper bowl on 50mm jaws last night.

 

[Doug Rasmussen]

................................ a continuously changing curve like the Catenary curve Bill mentioned. .....................................................................................................

Bill and John, when you guys mention "catenary " are you using it as a generic term to mean a curve with varying radius or a true catenary?

Bill, if a true catenary, how did you mean draw and print one out?

 

[john lucas]

Daniel I have turned 20" bowls with my 2" jaws on my vicmarc before I finally got some larger jaws. Depends a lot on the wood, how you cut, and how thick the bowl is.

Doug. I use the term kind of generic and think of it as a loose chain hanging down. More like this. https://www.mathcurve.com/courbes2d.gb/chainette/chainette.shtml

 

[Bill Boehme]

Bill and John, when you guys mention "catenary " are you using it as a generic term to mean a curve with varying radius or a true catenary?

Bill, if a true catenary, how did you mean draw and print one out?

Well, I guess it helps to have a CAD program to generate a catenary curve (as well as other curves such as ellipse, parabola, circle, Bézier ...). I sometimes print out a curve after I set the constraints then glue the paper to cardboard or poster board and then cut the shape out to use as a template. I usually do this when making a basket illusion that has an elliptical cross section. In order to have enough stiffness to do the beading I have to do the inside first and it really helps to have a template. I usually create a catenary curve for large shallow bowls. Most of the time I can eyeball a good curve.

 

[Emiliano Achaval]

Well, I guess it helps to have a CAD program to generate a catenary curve (as well as other curves such as ellipse, parabola, circle, Bézier ...). I sometimes print out a curve after I set the constraints then glue the paper to cardboard or poster board and then cut the shape out to use as a template. I usually do this when making a basket illusion that has an elliptical cross section. In order to have enough stiffness to do the beading I have to do the inside first and it really helps to have a template. I usually create a catenary curve for large shallow bowls. Most of the time I can eyeball a good curve.

Can you post maybe some PDF's of some lines so we can print them?

 

[Emiliano Achaval]

Without knowing Daniel's skill level, I don't know if I can tell him to keep using the small tenon on large bowls. John, you and I can get away with it, my last catch was decades ago, but a beginner or someone that still has catches having such a small and poorly made tenon is an accident waiting to happen.... A few months ago, I had a Milo NE, all of sudden the bowl flies off the lathe! There was a crack I had not seen and the tenon sheered off. (Is that how you say it?) SO, even with good tenon, stay out of the line of fire.

 

[Charlie Knighton]

I like to remove tendon .....i like the bowl shape.....so if u going to remove it it should be large....

 

[Emiliano Achaval]

I like to remove tendon .....i like the bowl shape.....so if u going to remove it it should be large....

A very sharp scalpel should do the job

 

[Bill Boehme]

I like to remove tendon ......

That sounds really painful.

 

[john lucas]

I think I have a torn tendon in my shoulder.

 

[hockenbery]

I think I have a torn tendon in my shoulder.

It’s probably a dovetail being held by straight jaws

mine feel like that after working a chainsaw too long.

 

[Charlie Knighton]

calabash....no flat surface including bottom.....bowl sits how it wants.....ie. Mahoney

 

[Daniel Warren]

Without knowing Daniel's skill level, I don't know if I can tell him to keep using the small tenon on large bowls. John, you and I can get away with it, my last catch was decades ago, but a beginner or someone that still has catches having such a small and poorly made tenon is an accident waiting to happen.... A few months ago, I had a Milo NE, all of sudden the bowl flies off the lathe! There was a crack I had not seen and the tenon sheered off. (Is that how you say it?) SO, even with good tenon, stay out of the line of fire.

Fair enough. I should likely get some larger jaws but I also have have had success with these jaws and some larger forms, but not without a few catches (scrapers only, not gouges) and some rolling bowls.

@Bill Boehme could you post a few basic catenary curve CAD drawings here, ones that you have used previously?

 

[Mark Jundanian]

Back to the topic of assessing a curve, a tip I got from a Jaques Vesery lecture is to use a uniform background to help see the curve. I use a piece of cardboard flat black on one side and white on the other.

 

[Bill Boehme]

@Bill Boehme could you post a few basic catenary curve CAD drawings here, ones that you have used previously?

I don't think that I have any lying around on my computer, but I think that it would be far more useful if you used one of the many catenary curve generators that can be found online.

Here is one that prints out both numerical values as well as a graph.



Here is another one, but the graph is hard to see because the catenary curve is pale green on a beige background. Otherwise it is pretty good. You probably would be better off plotting the points on graph paper and then drawing the curve through the data. Another approach would be to enter the data into an Excel spreadsheet and then have Excel draw a polynomial curve through the points. As far as I know, none of the curve generators can be scaled to give you inches equals axes values.

For all practical purposes in woodturning there is no appreciable difference between a catenary curve and a parabolic curve when they both have the same sag and span. The dotted line below is a catenary curve while the parabolic curve is shown by the red line.

 

[Emiliano Achaval]

Fair enough. I should likely get some larger jaws but I also have have had success with these jaws and some larger forms, but not without a few catches (scrapers only, not gouges) and some rolling bowls.

@Bill Boehme could you post a few basic catenary curve CAD drawings here, ones that you have used previously?

I think the main thing here is safety, if you feel safe, by all means, go for it. Good looking bowl by the way.

 

[john lucas]

All you really have to do to get close is to take your plank. Lay it horizontal. Then take a piece of chain and hold it up to it. You can put more slack in or take some slack out until the curve fits the bowl. Then kind of sketch this. Most people start a bowl (and yea me too) by just turning and looking. I think your much better off to start with a plan. You think this is the shape it should be. Now that can change of course as you reveal what's inside the wood, but for the most part your better off with a plan. Those of us who have been turning for a while have a sort of visual plan before we put the wood on the lathe. A lot of times because of defects in the wood or details you want to save that plan changes. Still even with a change if you have good curves in mind you can change. One of my favorite bowl from years ago was a huge piece of Osage Orange. It had some cracks around the outer edge so I put those toward the bottom and started cutting them away. Well by the time I had cut them away the bottom was about 3" and the top about 14 with a huge bite take out in between. It became obvious that this bowl had to have a drastically different shape. I formed a long gently curve from rim to foot so it looked like a huge Trumpet bell. It was georgeous. The long soft curve was just the thing a wood with little character needed. My dad loved it and gave it to his best friend. When his friend passed away the family was giving away a lot of his stuff. I was hoping for the bowl but alas they kept it.
When I teach ornament making I stress to the class how difficult it is to make a perfect sphere. And everyone can see if it's not perfect. So your much better off purposely making it a non perfect perfect sphere. So when you make a sketch catenary curves come into play because you purposely don't want it a perfect sphere.