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Filling inaccessible cracks with a liquid filler?

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Forum topic by Silvanus posted 03-30-2020 11:13 PM 842 views 0 times favorited 24 replies Add to Favorites Watch
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Silvanus

9 posts in 66 days


03-30-2020 11:13 PM

Topic tags/keywords: question odd question musical instrument crack filling

I am working on a musical instrument that I designed. It has a very complex internal structure, and I fabricated it by cutting cross sections on a laser cutter, and then gluing the pieces together with wood glue. It is currently in two halves, and I am ready to join them together. However, before doing so, I would like to ensure that any potential cracks between layers are completely sealed so that air cannot bypass the intended path. The surface to seal is charred by the laser cutter, and inaccessible due to the narrow deep gaps in the design.

My thought is to flood the cavity with some sort of liquid such as boiled linseed oil + turpentine, or wood glue thinned by water and then draining out the liquid and spinning the work as it cures. Some sort of thin epoxy with a very long working time would be nice, but don’t know if such a product exists.

I tried googling for information to help me proceed, but this is a rather odd case that didn’t turn up relevant solutions.

Any advice from the experts here on how I should proceed?


24 replies so far

View Aj2's profile

Aj2

2973 posts in 2537 days


#1 posted 03-30-2020 11:30 PM

I’m not sure what your answer is but I do know boiled linseed oil is not a good one. That stuff takes a long time to dry. Months or years adding turpentine will make your drying time longer.
How big of a cavity you working with will definitely steer your choices.

Good Luck

-- Aj

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Foghorn

468 posts in 126 days


#2 posted 03-31-2020 01:17 AM

Fish glue might be an option. Very long open time. It would be easier if we knew what you were building along with some pictures.

-- Darrel

View Phil32's profile

Phil32

1061 posts in 643 days


#3 posted 03-31-2020 01:34 AM

We don’t know if this is a wind, string, or percussion instrument. It might make a difference.

-- Phil Allin - There are mountain climbers and people who talk about climbing mountains. The climbers have "selfies" at the summit!

View CaptainKlutz's profile

CaptainKlutz

3041 posts in 2234 days


#4 posted 03-31-2020 01:36 AM

+1 BLO is bad idea.
Using any hardening oil as sealer will take forever to cure.

Sorry not enough information to suggest actual materials?
What kind of instrument; wind, string, percussion?
Do you care about tone added by sealer?
How well do you want to seal any gaps?
Does it have to be 100% liquid tight with 100psi applied or just room temp air pressure sealed?
Soo many questions unanswered…

There are all kinds of gap and/or pressure vessel sealers available.
The soft ones will dampen vibrations, hard coatings will resonant differently than wood.
For a pressure tight seal could immerse the pieces in pressure pot of resin (cactus juice?), drain/remove excess, and then bake the structure into one solid mass?
etc, etc, etc….
Massive number of options, IMHO,
But then I AM a materials engineer who has worked with adhesives/coatings/polymers all my life. LOL

Cheers!

-- If it wasn't for bad luck, I wouldn't have no luck at all, - Albert King - Born Under a Bad Sign released 1967

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Silvanus

9 posts in 66 days


#5 posted 03-31-2020 04:26 AM

It is a wood wind instrument. A hard finish is definitely preferred to soft for tonal quality. The wood is Baltic Birtch plywood.

Potential cracks would be thin, but definitely need to be airtight under minimal amounts of back pressure that can be created with the lungs pushing air through a very narrow channel with a cross section of about 1/32” by 3/8”. That narrow cross section is external, and in no danger of getting filled by the sealer. The narrowest intended gap on the inside of the cavity is about 3/16” wide.

I will take a picture of the central cross section and upload it.

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Silvanus

9 posts in 66 days


#6 posted 03-31-2020 04:36 AM

This is what the central cross section looks like. It is 4” deep. The outside diameter is about 9.5”.

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Silvanus

9 posts in 66 days


#7 posted 03-31-2020 04:54 AM

I went ahead and unclamped the work so you can get a better idea of the internal surface finish and overall scope.

View CaptainKlutz's profile

CaptainKlutz

3041 posts in 2234 days


#8 posted 03-31-2020 07:04 AM

Wow, PIA to seal with real sealer, and not change the tone?
See why you thought about BLO 1st. :-)

Is there any way to test the chamber sealing?
How about this:
Place a 1/4 thick piece of EDM rubber on top, coated with temporary spray adhesive, with small holes drilled in each chamber location. Use a dispensing syringe with blunt needle, remove the internal stopper, so compressed air flows into chamber, and listen for leaks between chambers.

If there are no leaks, then don’t need to seal it?

Have used dispensing needle with foot switch apparatus in development lab for verifying adhesive sealing on automotive pressure transducers. :-)

Regardless, for a 2 atm human pressure rated seal, my brain thinks might need an immersion process if you have numerous leak issues?

Simple version if all cracks are exposed, and not large gaps:
Dip in it 2lb cut of shellac, wait for bubbles to stop rising up, drain/shake remove bulk liquid, wipe/brush out any remaining, dry a couple days, and sand the glue joint to ensure glue can stick to wood?

If you have larger gaps, then check out cactus juice wood stabilizer?

Just tossing ideas that I might use in your situation. Only instrument I’ve made was a ukulele. Although, I did help make a part that looked like that? Was micro machined inertia sensors, made via etching grooves deep into silicon die about 1/8” square using photo lithography? While lot smaller! LOL

Best Luck!

-- If it wasn't for bad luck, I wouldn't have no luck at all, - Albert King - Born Under a Bad Sign released 1967

View tvrgeek's profile

tvrgeek

950 posts in 2389 days


#9 posted 03-31-2020 11:41 AM

I can only speak from speaker building on air-tight. Yo want it sealed well enough the mass of the air in a crack does not matter, but you do not want it sealed as air pressure differences. A pinhole is good. Nail hole is not. A leak with a longer time constant than the lowest note wil be irrelevant.

View tvrgeek's profile

tvrgeek

950 posts in 2389 days


#10 posted 03-31-2020 12:56 PM

Curious, is this some sort of broad band friction enhanced Helmholtz resonator?

View Silvanus's profile

Silvanus

9 posts in 66 days


#11 posted 03-31-2020 04:07 PM

@CaptainKlutz:

I was actually planning on getting a piece of 12”x12”x1/16” natural rubber gasket material and cutting out the central cross section with the laser cutter and then using it in the center, with some clamps to seal the gasket, in order to allow me to check the tonal quality of the resonator cavity.

Based on a couple of spots I am seeing on the outside, I am concerned about the possibility of internal leaks, so I am trying to plan my next steps if my fears are confirmed.

The cracks should be thin, but there is no way of wiping excess sealer from the surface in the narrow resonator gaps. Hence the requirement that the sealer be a thin liquid. My thought was to spin the chamber as the resonator cures to prevent pooling. Also it wouldn’t require full immersion, as the bottom on one half is completely sealed with the exception of the central 2” hole which I am far less concerned about and is isolated from the rest of the cavity on that side. The other half has a similar 2” hole that a fluid path would lead to, so I would have to seal it off (rubber and clamp style depending on the sealer used). Then the liquid can just be poured into the cavities and left to soak in.

I will take a picture of the exterior flaw that has me concerned so you have an idea of what could have gone wrong in the middle.

Out of curiosity, is cactus juice a hard finish or a soft finish?

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Silvanus

9 posts in 66 days


#12 posted 03-31-2020 04:15 PM

@tvrgeek:

The resonator is a single air column folded back over itself repeatedly. Pinhole leaks between the folds can potentially affect the first fundamental frequency of the resonator, as well as create sideband peaks (which could be interesting). However, this resonator is not used in isolation, and needs to be tuned an octave down from another piece that gets plugged into the resonator.

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tvrgeek

950 posts in 2389 days


#13 posted 03-31-2020 04:34 PM

Wow, hard to imagine the equations or testing you did to account for the friction of such narrow passages and the differences between each ring. It looks like the area of each ring is the same, but friction will be different.
Keep us posted. Sounds like quite a project. I would be quite curious of the Q.

Is each end of a slot radiused? Not sure a pinhole would really hurt it, but only you know for sure.

As this is not being used to mechanically resonate, then flushing it with poly makes sense to me. Increased rigidity probably is a good thing.

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Silvanus

9 posts in 66 days


#14 posted 03-31-2020 04:34 PM

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Silvanus

9 posts in 66 days


#15 posted 03-31-2020 04:53 PM

@tvrgeek:

Yes, the cross sectional area of each ring arc triplet is the same as the central circle. I am not to concerned about friction as the chamber is intended to have a turbulent flow rather than laminar flow anyway, and added back pressure is actually a really good thing.

By poly, i am assuming you mean polyeurathane. Is that correct?

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