Calipers for Closed Hollow Forms

[Matt Hutchinson]

As I am finally able to bring my life into a somewhat sane balance I want to dig into a number of hollow forms. I don't have a laser setup, and don't really plan on getting one at this point.

So for those who have experience making closed mouth hollow forms without lasers, which calipers do you prefer? I hope to make some rather large pieces, but most will be 20" and under.

Thanks!

Hutch

P.S. I have seen a few types in catalogs and online, but just wondering which ones prove most helpful/easy to use.

[Wally Dickerman]

Matt, there aren't any really good HF calipers on the market that I'm aware of. I've had to make my own. Pretty hard to describe them well so I'll take some pics later and post then.

The figure 8 calipers work up to a point. You'll get an accurate measurement in the widest part of the vessel but below that you get an inaccurate reading. How much depends on the width of the opening. Dennis Stewart marketed a very good caliper for the area near the bottom of the piece. As far as It's no longer made. I've seen some shop made copies that worked okay. I'll include a pic of mine.

Wally

[John Keeton]

...making closed mouth hollow forms without lasers...I hope to never do that again!!!;)

[Bob Haverstock]

Matt,

I would encouage you to make a mild steel bar that could be inserted into the hollow form. On that bar I would mount a cheap laser pen on a long mount over the probing end of the bar. That will help you sort out the wall thickness. Sam W's. place of bussiness has laser pens for less that $3 each. Offset the laser so that the light falls off the wood at a predeturmined wall thickness.

Bob Haverstock

[Tim Rinehart]

Take a piece of mild steel bar, perhaps coat hanger or a bit larger, and bend into a rough circular/elliptical shape. Bend the ends so that they face each other nice and even.
If the bar is stiff enough that you can open the gap to say, 1/2", and stay like that after inserting into the HF, then you can determine how thick you are by looking at gap on outside of HF. If gap is 1/8"...then your wall is 3/8" and so on.

I've seen people make variations of this mounting to a bar so the gap can be adjusted.

If you do a search on Sorby hollowing tools...you'll see they a swan neck hollower with one installed that let's you know how thick the section is.

[Wally Dickerman]

Matt, there aren't any really good HF calipers on the market that I'm aware of. I've had to make my own. Pretty hard to describe them well so I'll take some pics later and post then.

The figure 8 calipers work up to a point. You'll get an accurate measurement in the widest part of the vessel but below that you get an inaccurate reading. How much depends on the width of the opening. Dennis Stewart marketed a very good caliper for the area near the bottom of the piece. As far as It's no longer made. I've seen some shop made copies that worked okay. I'll include a pic of mine.

Wally

I took a few pics of some of my HF calipers. The first, made by Dennis Stewart but is no longer made. I have seen some that were shop made that worked fine. This tool is the only one that I've found that accurately measures the bottom of a bowl. The opening in the handle shows the thickness.

The other 2 are shop made. They work very well for the area in the lower part of the vessel. Both are showing a measurment of 1/2 inch. The curved leg goes inside the vessel on both. They can be made with other dimensions.

A laser works very well but has limitations when turning thin walled vessels. I turn walls at 1/16th inch when piercing. That thickness doesn't leave much room for error.

Wally

[Baxter Smith]

Thanks for posting the pictures Wally. I purchased a Sorby Caliper advertised for deep hollowing that works well, but I've already gotten into situations where it isn't enough/won't work. Making something like one of these should do it!

[Matt Hutchinson]

Thanks all! I have been doing a lot of tool making and "setup tweaking" lately. It looks like I have some more projects in the near future. :D

Hutch

[Frank Van Atta]

There are lots of ways to measure wall thickness - here's an excellent article on the subject: Measuring Thickness (http://www.woodturners.org/tech_tips/nelson-thickness/nelson-thickness.htm)

Personally, I like the Ellsworth style bent wire calipers.

[Dale Miner]

I don't think there are any calipers out there that will get to the bottom of a 20" tall piece.

The go-to calipers for me have evolved into a small pair of figure 8 for small items, and a large pair of Veritas calipers for larger work. I like the Veritas as the thumbwheel sets the minimum opening, and they can be opened wide to enter and exit the vessel without losing the measurement.

For something 20" deep, bent wires or other shop made tooling would be my suggestion. A laser setup on a captured bar is a great time saver if you decide to go that way. (The thin wall caveat mentioned by Wally applies though)

The bent wire thing works great on Xmas ornaments where the opening is small compared to the size of the caliper leg.

Later,
Dale M