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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

Reply to
charlieb
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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:

Reply to
Rick Frazier

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

Reply to
nailshooter41

snip

Hey, I don't come up with this stuff - I just pass them along. John Brugo got it from someone at the Portland WWA thing. Haveno idea who came up with the idea originally.

And yes, there is much to be learned - and shared - in forums like this.

charlie b

Reply to
charlieb

Rick:

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

charlie b

Reply to
charlieb

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