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For those who’ve lusted after a hacklinger gauge, but were repelled with equal force by the price, you may want to try building your own.
I’ve always been a bit nervous when scraping recurves in archtops (& a violin), not knowing how much material was left! This gauge allows “one-sided” thickness measurement, using rare earth magnets. When the spring pull overcomes the attraction force (degraded by material thickness) of magnets inside & outside the instrument, the gauge magnet detaches with a click, against a fixed “stopper”. Whatever the indicator reads at that instant is the thickness. The stopper keeps the inner & outer magnets close enough together that the inner doesn’t “fall off” & rattle around inside the (guitar) – it stays firmly in place opposite the gauge. (It will even jump over braces, if they’re not too high).
For component testing, I looped monofilament over my scale, which was placed onto the arm of a radial arm saw. I then tested springs (and magnet combos) to determine the force needed to stretch them 1” (or, for magnets, to pull them apart). The target was a spring (I tried 5) whose 1” force was about the same as a magnet combo force with 1/8” material between the magnets. (That gives the gauge lots of range for thicker & thinner material). I settled on a spring (Traker TE55) with a “1 inch force” of 70g and a combo of two 3/8 magnets inside the gauge & a single ½” magnet inside the guitar.
Anyway…………….here’s how I made it.

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Dave
Milton, ON

Very cool, Dave. Thanks for posting!

Do you have a figure for its accuracy?

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now known around here as Pat Foster
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http://www.patfosterguitars.com

Pat, regarding accuracy, all I can say is remember that it's not a mechanical measurement (like calipers) so three decimal place accuracy is not possible. And using it is an acquired skill......when you're close, pull the plunger VERY slowly, keep the indicator from jumping on release, always keep gauge perpendicular to the surface, etc. But if you've built it & particularly calibrated it carefully, and then use it properly, it works. (eg 10 readings on a 1/8" sample will all be within the .120" & .130" marks)

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Dave
Milton, ON

Dave - One of the most useful things I've seen on the OLF to date! Thank you for taking the time.

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Instagram: @jfrenchluthier
Web: https://www.jfrenchguitars.com/

Dave you are a genius!

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Ken

Very cool Dave. Just a quick question on setting up the scale for measuring...did you just use trial and error to get it right? The difference in force needed to pull the magnets apart when a piece is 3mm thick versus 1mm is exponentially different as opposed to evenly scaled. (I'm not sure I'm making sense here)

Notice on this one how the gradations aren't even.

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Joe Breault
Merrimack, NH
Perpetual novice

The spring force is constant (Hooke's law) so the magnetic force becomes less and less as the distance increases until it lets go. Therefor the scale is not linear.

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Ken

Yes, you do see the graduations becoming farther apart as the material thins. (You can see this a bit in the last pic)
By the way, I found it useful to calibrate on a piece of masking, to make sure of the graduation locations, before transferring the marks with a very sharp file to the tube.

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Dave
Milton, ON