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Jeffrey Holmes

Well, Michael Molnar showed me his "thing"...

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I know Mike is catching up from his trip and will be posting more information soon, I just wanted to add that I've been doing some research and I believe that a 'handheld' version may be availalble.

OMG I don't believe I said that :)

Oded

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A Digital Hacklinger Gauge

If you have a PC or MAC, you can assemble this digital Hacklinger gauge for about $130. You do not have to be a computer geek, nor a machinist to do this. In addition to being inexpensive, the device eliminates guess work that you have with a mechanical gauge. You will get a digital value and a graph. The data can be copied into another program for conversion into thickness and displayed as you may like.

The device uses a magnetic sensor (Hall Effect Sensor) which is very sensitive. (At Rutgers I made an experiment for students to measure the earth's magnetic field with this sensor.) You need to convert the measured magnetic field to a plate thickness. This is not hard to do. The software and sensor are designed for use by high school students. You will, however, have to develop good data taking skills which is nothing more than being patient and organized.

You will need to make a wood sleeve to hold a small steel set screw between the sensor and the magnet. As you will see this is easy to do.

Here is the list of what you need:

  • PC or MAC computer with a USB port, and a CD drive to read the software.
  • Purchase from Vernier the Logger Lite software and GO! Link interface box ~$61
  • Purchase from Vernier the Magnetic Field Sensor ~$58
  • Get a few small rare earth magnets (~ ¼") from Edmund Scientifics
  • Make a wood sleeve to hold a small set screw on the sensor's face

The Vernier web site is

Logger Lite:

http://www.vernier.com/go/loggerlite.html

Magnetic Sensor:

http://www.vernier.com/probes/probes.html?...e=standard.html

Check the web site for other sensors that may interest you.

Next, you need to make a wood sleeve that can be either round (mine is from a broom handle) or square. Mine is 1.5" tall, 1.25" diameter. These dimensions are not critical. Drill a ½" sleeve hole ~1" deep centered in the wood sleeve, leaving ~ ½" or less on an end. This sleeve hole will fit over the end of the Vernier sensor.

Now drill a set screw hole centered through the rest of the sleeve. Use a drill slightly smaller than the steel set screw you will screw into this end. I used a socket cap ¼" steel set screw so the hole is 7/32" or 15/64". The wood is soft enough for the set screw to make its own thread. The purpose of the set screw is to attract and hold the small rare earth magnet to the inside surface of the plate. It is important that the set screw is centered and butts up against the sensor for best results.

You can make a couple of sleeves with different set screws to see what works best for you. Also try different magnets. Believe me: the magnets are easily lost, so get several of each.

Calibration is straight forward. Get a few pieces of maple/spruce of different thickness and measure the magnetic fields passing through them. The magnet should be able to hold itself against the other side of the wood. If not, get a stronger magnet or larger set screw for the sleeve. This is the hardest part of making the device work.

Once you have the calibration data, try a real plate. You will learn that this can be tricky, but if you are patient you will get nice results.

Tip: Avoid metal desks, tools, power supplies, etc. that can interfere with the magnetic sensor. Move the sensor around where you will take data to find hidden steel bolts and nuts in your bench.

Tip: Always calibrate before every new plate.

Tip for Logger Lite: Select the Experiment tab and choose Data Collection for taking individual data points and not continuous Time Based data.

Below are some self-explanatory photographs to help you.

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Boom, boom, boom!

Not bad, eh??? :)

Michael... Didn't someone give your thing a nickname? The "iHack", wasn't it?

Thanks, Jeffrey. Yes, it was Ryan McLaughlin from Atlanta. Another great guy.

BTW, after I catch up with "things" here, I will post some photos and impressions of both workshops.

Stay tuned.

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Greetings:

Rummaging around the Vernier site I found that this sensor can be used with a 'Palm' handheld.

http://www.vernier.com/palm/

If you wish to take data in the field but don't want to carry a computer around, the 'Palm' device will store the data and can then be docked to a computer and downloaded.

Michael, I'm wondering if one could simply glue a small magnet at the end of the probe? Would it mess up the probe to have a magnet permanently attached?

Best,

Oded

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Several of us were kidding around about re-graduating violins without taking them apart by gluing sandpaper to one rare earth magnet, putting it inside, and moving another magnet over the outside surface. :)

Michael;

Any problem with making the wooden end rounded (convex) to better take measurements where there's a recurve in the arching, like at the edge of the C bouts? Or would this have a tendency to dent or mar the wood?

Any way to attach the magnet to the end of the probe (maybe like Oded was suggesting) to allow the use of a metal sphere (ball bearing) on the inside of the fiddle? I don't understand enough about how the magnetic field is sensed to know if this can possibly be done, but it would be neat if the part inside the fiddle would roll.

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Thanks, Jeffrey. Yes, it was Ryan McLaughlin from Atlanta. Another great guy.

BTW, after I catch up with "things" here, I will post some photos and impressions of both workshops.

Stay tuned.

I can't wait to hear more about the workshops. I wish I dould have been there but family responsibilities keep me really busy lately.

Impressive invention Michael...

That ain't no Boom Boom Boom! :)

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"Several of us were kidding around about re-graduating violins without taking them apart by gluing sandpaper to one rare earth magnet, putting it inside, and moving another magnet over the outside surface."

-------------------------------------------------------

Apparently, it's been done. Well, perhaps not re-graduating per say, but at least attaching sandpaper to magnets to clean up repair work on the inside of guitars. This information was presented in Stewmac's Trade Secrets newsletter a short time ago. I have not tried it, yet :)

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Don't Strads have these same sanding marks inside? :)

magspin07.jpg

Could also make an auto-stirrer so we don't all get the "lung crud" from hovering over varnish while it's cooking.

Michael met quite a few major players in the violin biz during the week. I don't think they were what he expected! :)

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A Digital Hacklinger Gauge

Here is the list of what you need:

Next, you need to make a wood sleeve that can be either round (mine is from a broom handle) or square. Mine is 1.5" tall, 1.25" diameter.

Will any broom handle do?

She's gonna be Mad! :-)

Just a suggestion, maybe it would be wise to lay low for a few days until she forgets who 'hacked' her broom. :)

Thanks!!!!!!!!!!!!!!!!!!!!

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Many years ago, I came up with a design to improve Hacklinger gauge. The method is to measure the magnetic force using strain gauges (commonly used in pocket scales). The output of the strain gauges (arranged in a Wheaston bridge) is converted into digital distance (or thickness). The whole device is no bigger than the dial indicator on your dial caliper. It can move continuously on the surface without the need to pull the magnet apart. I didn't put it into production because I had doubt that I would make much money out of it. Also I am a maker who do not go by thickness; in fact, I don't use a dial caliper. (A pair kitchen salad tongs or fire place tongs is what they use in 18th C.)

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I thought it was quite a nice "thing" (thingy?) and was rather impressed with it.

It can be multi-functional with some attachments and accessories. :)

My gratitude to the preceding workshop for providing me with an inflatable sheep with fetching long eyelashes. Unfortunately, she was stolen by an Australian who is now on his way to Scotland with her. :)

Perhaps a red haired lad?

Joe

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Nah, I'll stick with sheep, thank you! :)

Seems to me that the lad and the sheep got away....but I'll inquire about her good health. As for him...no worries.

Joe

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Hi,

The numbers onthe chart represent 'Gauss' measurements not millimeters. These magnetic strength numbers are then computed into mm. I believe Mike has said that he's found it accurate to a tenth of a milli. About the same as a Hack gage. Which seems accurate enough for most violinmaking applications.

The gaussmeter is quite sensitive- and I believe it has two settings. The web site gives plenty of details.

Oded

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Thanks Oded.

What would the readings be for say 2.8 mm and 2.7 mm spruce top?

I am curious as to how much change we will expect to see in the Gauss numbers with 1/10th of a mm change.

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Would paramagnetic minerals in the ground-varnish, or even in the wood, affect the readings?

If so, this would require considerable calibration-recalibration to achieve accuracy.

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If the error is 0.1 mm throughout the measurement range, that is much superior to the Hacklinger gauge, which is about 0.1 in the 2 to 3 mm range, then increases greatly (to 0.5 mm or more) above 4 mm, as the spring stretches and the numbers on the printed scale get closer together. The $400+ two-magnet version solves this problem somewhat, but is somewhat fussy to use, changing to a different magnet and then reading the second scale. I've used them for years. The variable error depending on thickness always concerned me.

So I bought the Vernier iHack and plan to test it out thoroughly as soon as permanent magnets arrive. So far the tool itself and the software seem very good. The idea is to use blocks of standard thickness to obtain a correction factor, then export the readings and calculate mm thicknesses in Excel. Seems pretty accurate, inexpensive, and simple. Sweet!

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