Slick Simple Method of Checking For "Fair Curve"
 

If you turn bowls you typically are shooting for a "fair curve"
a continuous curved line, with no bumps or hollows. Achieving
a fair curve on a turned piece may appear easy - one continuous
final cut. But the eye, and the hand, can percieve when the curved
line is not quite right, though identifying WHERE the problem(s) is/are
is not easy to do when the "not quite right" is small.

Now it's easy enough to see if a "flat" surface is in fact flat. Lay
a known straight edge on it and with a light behind it, look along
its bottom edge. If you see light then the surface is low there
- or the surface is high on either side.

And for circles, a similar method can be used to identify where
something is out of roung.

But a method for checking to see if a curve is "fair" is not so
obvious - until you see a method that does the job.

John Brugo, of the West Bay Area Woodturners, shared this
method at the July meeting of the Silicone Valley Woodturners.

Get ye to your local crafts supply shop or hobby shop and
pick up a hot glue gun glue stick - one of the long ones - 10
to 12" long. Hold one end on the bottom of your bowl and
bend the rest onto the profile of your piece. Sight along the
bottom of the glue stick looking for light coming through.
If you see any light shining through you know where the low
spot is and then know where wood must be removed. The
glue stick can be used to check the inside of the bowl as
well.

Simple, cheap, and effective. Like all great ideas, it's so
very obvious - once you see it done.

Thank you John. Yet another reason for joining a turners
club.

charlie b

Slick Simple Method of Checking For "Fair Curve"
 

The long gluestick is an idea I hadn't considered before...

I have several metal rulers ( thin stainless steel for relatively short
ones, and aluminum for 2, 3 and 4 foot ones ) that I use in a similar
manner. Shorter ones work on inside curves, longer ones work well on
outside curves and tall or large diameter objects.

This works for straight or curved lines. If you lay the ruler flat on
the item, it works for curves (but can't do sharp curves, of course) and
if you put the rule on edge, you can check out straight lines.

It helps a lot if you have a well lit white wall behind the item to see
small dips and such on outside curves, and I've used a flashlight behind
the rule on inside curves.

Harder to see, but still useable is a piece of spring steel wire, or
stainless welding wire (available in 3 foot lengths). For shallow
curves, even mild steel welding rod will work, but once you over-bend
it, it takes on a permanent curve...

Having been a journeyman body & fender/painter back in the early 70's, I
developed a feel for surfaces. Running my hand over something. I will
often feel a dip or projection that is nearly invisible until finish is
applied.... Nearly anyone can develop this ability, it only takes practice.

One trick to see the surface (if you are well into sanding) is to
temporarily saturate the surface with a solvent (such as naptha) and
view it before the solvent evaporates. As solvents go, naptha is
relatively benign when it comes to finishing. An alternative is
alcohol. Both tend to evaporate relatively rapidly without leaving
contaminants behind. (Of course, if you use these, make sure to take
precautions to avoid sources of ignition, and only in a well ventilated
space.)

--Rick

charlieb wrote:
If you turn bowls you typically are shooting for a "fair curve"
a continuous curved line, with no bumps or hollows. Achieving
a fair curve on a turned piece may appear easy - one continuous
final cut. But the eye, and the hand, can percieve when the curved
line is not quite right, though identifying WHERE the problem(s) is/are
is not easy to do when the "not quite right" is small.

Now it's easy enough to see if a "flat" surface is in fact flat. Lay
a known straight edge on it and with a light behind it, look along
its bottom edge. If you see light then the surface is low there
- or the surface is high on either side.

And for circles, a similar method can be used to identify where
something is out of roung.

But a method for checking to see if a curve is "fair" is not so
obvious - until you see a method that does the job.

John Brugo, of the West Bay Area Woodturners, shared this
method at the July meeting of the Silicone Valley Woodturners.

Get ye to your local crafts supply shop or hobby shop and
pick up a hot glue gun glue stick - one of the long ones - 10
to 12" long. Hold one end on the bottom of your bowl and
bend the rest onto the profile of your piece. Sight along the
bottom of the glue stick looking for light coming through.
If you see any light shining through you know where the low
spot is and then know where wood must be removed. The
glue stick can be used to check the inside of the bowl as
well.

Simple, cheap, and effective. Like all great ideas, it's so
very obvious - once you see it done.

Thank you John. Yet another reason for joining a turners
club.

charlie b

Slick Simple Method of Checking For "Fair Curve"
 

On Jul 12, 6:26 pm, charlieb wrote:

Get ye to your local crafts supply shop or hobby shop and pick up a hot glue gun glue stick - one of the long ones - 10 to 12" long. Hold one end on the bottom of your bowl and bend the rest onto the profile of your piece. Sight along the bottom of the glue stick looking for light coming through.

I laughed so hard I almost fell out of my chair. How wonderfully
obvious is that? And yet, I can assure you I would have NEVER thought
of it.

A glue stick. Hah!

Thanks, charlieb. Another reason to read the newsgroup, eh?

Robert

Slick Simple Method of Checking For "Fair Curve"
 

Slick Simple Method of Checking For "Fair Curve"
 

Rick:

Thanks for all your ideas and your observations.
Always nice to have multiple ways of doing something.

charlie b