Help me design a measuring device

[Tony Ennis]

I'm making a black powder powder horn. Experienced makers do all manner of carving in the horn. Now the horn is ruined if it is carved too deep and the wall is pierced. Where most of the carving happens, the horns are curved and narrow making it impossible to use a caliper.

Can anyone think of a tool that could measure the thickness to say, +- .5mm?

It could be a relative thing where you take a measurement before carving then take a measurement during carving and figure out not the thickness but how much material remains proportionately.

[JCHannum]

An adaptation of the Starrett lock joint transfer calipers would serve the purpose. Measurement is taken, the joint locked, the transfer arm released to allow the caliper to be removed from the horn. The arm is then relocked and the measurement is taken.

Scroll down to the bottom of the page;

http://www.starrett.com/download/427_p357_360.pdf

[PeteM]

Low tech approach: make a purpose built caliper with an inside arm meant to follow the curve of the horn. By extending the arms past the pivot point and adding a scale you can make it direct reading.

High tech approach: ultrasonic thickness detector.

[dp]

Are you starting with an actual horn? If so then I'd think an odd leg caliper would come in handy.

[KiddZimaHater]

A version of these might work. Maybe with longer fingers?
Starrett 1017

[Evan]

I have made a powder horn before. Put a light inside it.

[10KPete]

I've done a number of horns and have used nothing but a bent piece of 1/8" gas or TIG steel rod. I bent it into a hairpin shape with each leg long enough to go down to the bottom of the cavity plus a couple of inches. Then I curved the two legs to sorta match the curve of the horn, then bent the tips in so they could touch each other. Yeah, it's long and springy but with careful 'bend and feel' I can easily get within 1/16", which is as close as I need to be. I don't want a horn that's a half millimeter thin!!

If you used some heavier wire you could probably get a stiffer rig and a better feel for less than 1/16".

I've used that same wire caliper for all my horns. Just re-bend as necessary.

Pete

[Your Old Dog]

I think you are over thinking this! This ain't how it was done 150 years ago. The depth you see is more optical illusion than anything else. The eye can resolve small differences in depth and you can use that to advantage. Try doing a practice piece and you might find it isn't the problem you think it is.

The tip end is thicker and if you cut a horn lengthwise you can get a good read on where you are going. If you are going to create an artistic effort with gauges and other measuring tools it might turn out more mechanical looking then you want.

[portlandRon]

Make two arms in an "S" shape. Cut it out so the arms at the top of the "S" overlap when the bottom arms just touch. To get the lower legs to fit inside the horn adjust the shape of the curve. Flip one arm over and join the arms with a pivot point in the center. Then as you open the bottom arms place marks on the overlaping top arms corresponding to the width of the opening.

[Evan]

What YOD said. If you are in doubt as you work shine a bright flashlight inside the horn. It will easily show if you are getting too thin. I'll bet it WAS done that way 150 years ago. Just hold it up to the sun.

It sure does stink it you get it hot when grinding or polishing.

[Frank Ford]

Magnets.

There's a device we use in the shop to measure wood thickness in otherwise inaccessible areas on guitars and other instruments. It's name is Hacklinger.

Expensive, but worth the $$ for professional use:

http://www.stewmac.com/shop/Tools/Special_tools_for:_Violin/Hacklinger_Thickness_Gauge.html?tab=Details

It's basically a magnet hanging on a slinky little spring, attracted to a magnet inside the item being measured. Couldst work up a DIY version for your use. . .

[Tony Ennis]

*struggles desperately but fails to build something Evan hasn't yet built*

10KPete - that's what I did though I suspect yours was better.

I think you are over thinking this!

Story of my life. I over-think everything and generally tie myself up in knots.

I've been on some horn making forums lately. Evan is correct in that a light is the tool of choice. Snake lights are recommended.

If you are going to create an artistic effort with gauges and other measuring tools it might turn out more mechanical looking then you want.

No real danger of mechanical perfection happening no matter what tools I use! But my tools are files, sandpaper, my pocket knife, and electrical tape. I could see making some simple shaping tools from old hacksaw blades and such if I wanted to do better work.

The only place the depth really matters is on the channel cut into the horn around which the hanging strap will be tied. Its got to be deep enough that the strap can't slip off.

The Hacklinger thickness gauge is neato.

[LES A W HARRIS]

Something like this help?

http://www.leevalley.com/US/wood/page.aspx?p=43201&cat=1,330,49237&ap=1

Cheers,

[Evan]

The only place the depth really matters is on the channel cut into the horn around which the hanging strap will be tied. Its got to be deep enough that the strap can't slip off.

There is no need to make a groove for the strap. Cut some leather lacing and soak it in warm water. Use it to lace on a brass ring or a leather loop at each end. Pull it fairly tight when wrapping it and tie it in a square knot (NOT a Granny knot). When it dries it will be glued in place by the keratin from which the horn is composed. Horn and hoofs are used to make glue.

[The Artful Bodger]

Pull it fairly tight when wrapping it and tie it in a square knot****

****Sailors know a 'square' knot as a 'reef' knot.