Measuring the top/back through soundhole.

[Karl Hoyt]

Greetings:

I want to measure the thickness of the tops of intact mandolins. I know others have done it: I'm wondering what your measuring device of choice might be for this. I have a 1" micrometer which is useless other than very close areas around the soundholes. I know stewmac has some sort of thickness gauge..

My hunch is that you have used some sort of shop made tool?

Your responses are appreciated.

Karl

[Joe Mendel]

MAG-ic Probe. magicprobe.net, or a Hacklinger gauge.

[Jonathan Ward]

MAG-ic Probe. magicprobe.net, or a Hacklinger gauge.

I covet a MAG-ic probe. Their "Pro" level software will even draw the instrument and note all the measurement locations. But the gizmo and the software together set you back about $400US with their package deal. That's a bit strong. Maybe someday in the event of a lutherie emergency a business case could be made.

[Marty Jacobson]

I covet a MAG-ic probe. Their "Pro" level software will even draw the instrument and note all the measurement locations..

Not exactly. It lets you click on a photograph and note the last measured value. But taking the photograph, and syncing up the location on the instrument with the measured location on the instrument, is all up to you.

[Jonathan Ward]

Thanks for the clarification. That seemed too good to be true. And it is!

[Karl Hoyt]

I'm with you, Jonathan. That's a little steep for my needs, though it sounds like an excellent tool. I was originally wondering if someone had converted some dividers or deep -reach calipers to do the job. I have an idea coursing through my simple brain. Off to the sketch pad. Thanks to all who replied.

[dave vann]

There's a magnetic thickness caliper on International Violin's website which measures 1-7mm with .01 accuracy for $160.00. On the same page is a Hacklinger gauge that is $365.00. Does anyone have experience with the less expensive caliper? I could get behind $160 much easier than $365.

[j. condino]

You can make your own version of a hacklinger guage for about $8 in scrap parts.....

Having owned both the original Hacklinger and the Majic Probe, I'm not a fan of the MP. What I need on a day to day basis in the shop is a simple, reliable, battery free way to measure plate thicknesses, not a bunch of over hyped digital distractions. It should be a $10 iphone app with a $50 attachment, not a $400 unit. That said, regardless of price or complexity, if you are a digital nerd, get the MP. If you are a crusty old analog geezer, get a Hacklinger. Either one will make a tremendous difference in your knowledge and building process. It is not all about the numbers, but being able to accurately read and understand one of the variables helps- a lot.

[Marty Jacobson]

The real selling point of the Mag-IC probe is that you don't have to separate the magnet every time you take a reading.

[HoGo]

There have been many previous threads about this. Here is one that has some interesting info on home made versions, both electronic and mechanical. I personally converted transparent lipstick into tiny pocket version of magnetic gauge, I believe I posted some pics of that years ago. Not perfect, actually ugly as hell, made of whatever scrap I had within reach at the moment I got the idea, but good enough for simple measurements.
I was thinking about making the electronic version, it's nothing special for electronic guys but never had the time. I think it would be pretty easy programming an app for smart phone these days and just use built in headphone/mic input nd use A/D converter of the phone and simple electronic sensor on cable. (that would be much simpler than using USB or other digital ports which would require your device have its own digital output circuitry)

[HoGo]

Here is my old magnetic "Lipstick gauge" I made some 15-20 years ago. The tip is made of plastic syringe (relatively soft PE, doesn't scratch finish) Body is of transparent lipstick (with strong light you can see the pins moving inside the spiraling notch). You pull the spring by turning it so the scale spirals around the tube.
In the meanwhile I re-calibrated (on the other side) and added dot at each 0.2mm, in lower ranges 0.1mm.

[fscotte]

I built my own magic probe a few years ago. Cost about $100.

[HoGo]

I built my own magic probe a few years ago. Cost about $100.

What sensor did you use? I've seen versions with magnetoresistor or Hall effect sensor...

BTW, I don't remember name but there was another version of magnetic gauge by some guy in OZ if I remember correctly. It was simpler, but cheaper than the Magic probe.

[fscotte]

I bought a device from Vernier and had to dismantle it a bit. Glued in a regular disc magnet at a specific distance from the circuit, and used Vernier software to take measurements.

I bought a ball rare earth magnet. If you look at the probe itself, you’ll see a wood spacer. That’s the distance between the disc magnet and the circuit which is at the end of the plastic housing. It’s not hard to make at all.

Here’s a video I made on how to calibrate and use it.


https://youtu.be/chSTnDjPRgw


Here’s another vid where I describe how the sensor works:


https://youtu.be/HWPr0VUG9GM


And yet another. This is all I have demonstrating it.


https://youtu.be/eJgnJDPRTc8

[fscotte]

Finally, this is the sensor I bought.

https://www.vernier.com/products/sensors/magnetic-field-sensors/mg-bta/

You also need the Go Link to connect it to a computer:

https://www.vernier.com/products/interfaces/go-link/


The software is Vernier Logger Lite and its free to use.

[HoGo]

Thanks Fscotte. I thought you made one from scratch.
I was thinkong about hooking magnetoresistor on audio jack and process the signal via audio input/output of smartphone. WIth some calibration you can sample the signal in real time and convert into numbers on display. With some extra time on hands app could be programmed to take pic of the instrument and save the measurements directly on it....

[Richard500]

Both interesting methods and more versatile than the “metre a danser” a double ended caliper that would be a task in anything with f-holes. Anyway, I guess there are several reasons for measuring an intact instrument, one being figuring out why it’s so good. Do we need to think about also measuring the mechanical properties of the wood itself, like density, elasticity, or any anisotropy in properties? I remember that the violin analysts for some time had theories about the varnish, the age of the wood when harvested, and various speculative things before solid measurements entered the picture, allowing for creative modeling of new build.
Or, for a mandolin, are wood and finish properties smaller than dimensional and architectural ones?
Apologize if I’m being too academic here. Misspent youth.

[Marty Jacobson]

Both interesting methods and more versatile than the “metre a danser” a double ended caliper that would be a task in anything with f-holes. Anyway, I guess there are several reasons for measuring an intact instrument, one being figuring out why it’s so good. Do we need to think about also measuring the mechanical properties of the wood itself, like density, elasticity, or any anisotropy in properties? I remember that the violin analysts for some time had theories about the varnish, the age of the wood when harvested, and various speculative things before solid measurements entered the picture, allowing for creative modeling of new build.
Or, for a mandolin, are wood and finish properties smaller than dimensional and architectural ones?
Apologize if I’m being too academic here. Misspent youth.

I've used all of the usual varnishes on very similar mandolins (lacquer, french polish over shellac, french polish over oil varnish, poly, modern waterborne varnish) and have not been able to see any differences attributable to the type of finish used. The individual wood sample, physical geometry of the instrument, strings, and technique are the main contributing factors for mandolins (in no particular order).

[HoGo]

Anyway, I guess there are several reasons for measuring an intact instrument, one being figuring out why it’s so good. Do we need to think about also measuring the mechanical properties of the wood itself, like density, elasticity, or any anisotropy in properties? I remember that the violin analysts for some time had theories about the varnish, the age of the wood when harvested, and various speculative things before solid measurements entered the picture, allowing for creative modeling of new build.
Or, for a mandolin, are wood and finish properties smaller than dimensional and architectural ones?

From my making I think both wod properties and "architectural ones" are both important though you can only measure some of them on finished instrument (you cannot measure stiffness directly) so we do our best to measure what we can on best examples and do some educated guessing about the rest.
There have been so many "theories" in violin world that upon closer examination won't hold water... Most old violins have not much varnish left, sometimes barely traces... some very well preserved (very rare) examples have sometimes quite thick original layer of varnish. And they play fine... so the finish is hardly huge factor (unless it is really thick or extra hard - I once measured almost 0.7 mm thick nitro lacquer on well known maker mandolin in some areas).
Wood varies but, IMHO, good maker should be able to select piece of wood best suited for tone he wants and be able to adjust the arching and/ or graduations for given piece of wood to get there.
I have not seen bad piece of wood yet (not thinking of knots or defects), I've made mandolins out of spruce that was rejected by violin makers and made tops out of spruce from new growth firewood stash with great results....

[Richard500]

Thanks for your experience, and for not mentioning my wrong name for the maitre a danser, photo of a small one attached. Looking a bit further into the mysteries of bracing, to this non-builder, it seems that mysteries are still there, so it’s an art form. Even holography isn’t providing much that can translate into more than a broad idea about frequency response, let alone the subtle tonal qualities. In my world, it just means that suitable tools are simply not needed, which is just fine.