Has anyone made this digital thickness gauge?

[fscotte]

This older thread from Maestro indicates how to make a digital magnetic sensor gauge using fairly cheap parts. Scroll down to Mr Molner's description.

http://www.maestronet.com/forum/inde...s-thing/page-2


I had to read it twice to understand it and then go to Youtube to see how the Vernier sensor works, but it seems fairly easy to make. The rare earth magnetic balls can be found here:

http://www.leevalley.com/us/wood/pag...=1,42363,42348


I'm sure smaller balls can be found somewhere. This looks almost too easy. It's fairly similar to the Magic Probe but I don't think I can swing over $200 for something I would rarely use. Plus I like the idea of DIY.

[Marty Jacobson]

It's basically the original version of the MAG-ic probe. Hall effect sensors have been used in industrial automation for decades.
Liam's Snyman's commercialized version (MAG-ic probe) has a lot of refinements that make it easier to use, but whether you build one or buy one, they are great tools.

[fscotte]

Interesting, I assumed the Magic Probe may have operated on a slightly different principle.

Anyway, I just ordered all the parts, found a new Vernier sensor for $40. Total will be about $100 in parts.

Any reason I can't just superglue a small concave steel device to the bottom of the Vernier sensor? Would the superglue affect the ability to sense magnetic fields?

[Marty Jacobson]

Superglue is no problem. Anything metallic that is introduced between the sensor and the reference (magnet) will have to be compensated for in calibration. Because of that, I would go with a delrin or HDPE tip (or any other durable abrasion-resistant plastic), which will not interact with the Hall effect sensor, save for adding distance between the probe and the object.

[fscotte]

Superglue is no problem. Anything metallic that is introduced between the sensor and the reference (magnet) will have to be compensated for in calibration. Because of that, I would go with a delrin or HDPE tip (or any other durable abrasion-resistant plastic), which will not interact with the Hall effect sensor, save for adding distance between the probe and the object.

The original plans from the link above indicate that a steel set screw is used between the probe and the magnet. Is there any reason why I cannot just superglue the steel set screw or some other small object of steel to the sensor? In other words, not worry about making a wood holder for the probe as indicated in the plans by Mr. Molnar? Is Mr Molnar in error by introducing the steel set screw? Without a steel object, how would the magnet be attracted to the probe?

Can you elaborate on what you mean by calibrating the probe? Perhaps I am misunderstanding how the thing works.

Calibration to me, means first taking magnetic force measurements of known thicknesses, taking note of those force measurements, then using those magnetic force measurements as the basis to measure unknown thickness (mandolin plate). I am not aware of any other kind of calibration?

[fscotte]

Well anyhoo, I got the parts and put the thing together. If you follow the link above over at Maestro forums, Mr. Molnar describes the build.



What you see is the 1/2" rare earth ball magnet, 1/2" regular disc magnet, wood sheath to hold the sensor, and the sensor.



The disc magnet is glued to the wood sheath, and the sensor inserted into the wood sheath and glued. Those components have to be stable as the thing is very sensitive.

There is adequate force to hold the ball through very thick plates and keep the thing level against the plate. The disc magnet doesn't seem to bother the finish.

Now accuracy, well you have to calibrate the thing in the free software provided by Vernier. Calibration is simple. Just use a couple known measurements of the same wood you will be using. It's sensitive but accurate.

It's certainly good enough for the quick spot check.

[Grommet]

Good info Thanks Scott.

Scott

[fscotte]

I'll get a video up soon.

[fscotte]

OK just wanted to demonstrate that it works quite well.

[fscotte]

Here's my '98 Gibson A5L built by Sim Daley. The back plate grads had always eluded me until now. Typical Loar style grads with center area being quite thin, and thick near the neck. Took me all of 15 minutes to map out. Hope you can read my writing.

[fscotte]

Made it a little shorter.

[Michael Weaver]

That's pretty cool. We use some expensive ones at work to measure the pipeline thickness of gas mains and the depth of pitting to determine the integrity of the pipe. It uses sonogram technology.

[fscotte]

Calibrating the sensor.

[yaovaja]

Hi. This is very interesting job.

Is that you use 0.5-inch-diameter magnetic ball?
Is that you use 0.5-inch-diameter magnetic cylinder?
Is that you use 0.5-inch long wood cylinder?
What is the depth of your magnetic cylinder?

[fscotte]

Yes to all those.

The cylinder magnet is just a common refrigerator magnet. It's about 1/4" thick by 1/2" wide. Like I said in the video, the exact sizes of magnets are not important. Just as long as they are strong enough to hold well through the thickest plates you would be measuring. You will need at least one rare earth magnet, but you could also use two rare earth magnets.

[yaovaja]

Thank you.
I did hear some problems with a magnetic ball when measuring an electric instrument.
Someone said the ball is gone inside the steel/magnetic part of the instrument.
Can I use the sensor with the an electric instrument?