Crackling paint tillering gizmo idea

[Santanasaur]

I’ve  had this idea bubbling around for a tillering gizmo for character bows  but wanted to bounce it off the community because I’m not sure where to start. So I was thinking of locust and how the chrysals can be used as a tillering guide, wondering how to add this quality to any stave.

Basically you would paint the belly with a paint or glue or wax that  would crackle or discolor wherever the bow is overbending. And with some kind of solvent or other process could  adjust the sensitivity of the mixture  for larger or tighter bends.

I’m not sure if this would be practical, but I think it could be helpful for character bows when the ruler-line trick is not giving enough resolution.

Any ideas?

[Weylin]

Interesting. My gut tells me that any spot bending enough to do that would be noticable by feeling a thin spot in the taper or seeing the hinge.

[mmattockx]

There is a product used in experimental stress analysis called brittle lacquer. As its name implies, it is a coating/paint that is brittle and is intended to crack easily at low strain amounts to show where strain (and stress, by extension) is highest on a component as it is loaded. It is normally used on steel and other metals, though, and I don't know how it would work on wood, but it might be worth a look.


Mark

[aaron]

seems impractical because you'd have to reapply after every tillering session.

[Santanasaur]

I wouldn’t use this in normal circumstances where the normal tillering methods are working. The idea is to have something to fall back on once in a while to get a precise map of the stresses on the belly, especially character bows where thickness taper isn’t as easy to feel, or I just can’t tell precisely enough which parts are too stressed. The material would have to be very sensitive to be more usuful than the eye, and even then possibly only usuful for very irregular staves.

Brittle lacquer seems like the the sort of thing I’m looking for, thanks for the suggestion mmattockx, I’m looking into it.

[DC]

I think I can see this working but it would depend on the ease of use. Like if you had to put on multiple coats or if it had extended drying time it might be more trouble than it's worth. But if you're at 45# on a very difficult 40# bow it would be nice to know where the stresses really are.

[willie]

Santanasaur

please report back with what you find. There was a similar need for a product that could locate the neutral plane on a the side of a sample in bending, in a thread a few years back

thanks

[Santanasaur]

That’s a very interesting thought Willie. My thinking is that in order to reveal the neutral plane the material would have to be extremely sensitive, and also give very high resolution information. Simply cracking at the threshold probably won’t reveal a clean neutral plane.

I played around with some slats to get an idea of what might work. Parrafin wax has some promise and carnauba was even better. The best by far was clay slip, which flakes off like thin brittle yew bark after it dries. The thickness of the layer and the sensitivity can be fine tuned by diluting the slip. The problem is that after putting clay on the bow I’d rather not touch it with any bladed tools again.

[DC]

I like the clay idea. If you could get a good read on compression stresses I think putting a new edge on a scraper would be worth it. I think clay would wash off pretty clean anyway.

[RyanY]

Something like this can be done by putting some type of paper/tape on the back of the bow so the paper is standing up perpendicular to the back of the bow. Make even cuts along the paper so when the limb bends there is space between the cuts. Even spaces between the cuts would show even bending. Hard to describe so I hope that makes sense.

[Santanasaur]

 Ryan, assuming I’ve understood you correctly I’ve used that technique before, with perpendicular pieces of tape along the side of the bow. I would say it’s equivalent to the ruler line method in terms of how it works and the resolution of information you get back. These are great tricks but I’m looking for something that indicates exactly on the belly, rather than vaguely hinting at the sides, which areas are bending most—the kind of precise information you get from looking at compression fractures.

Apparently casein paint is very inflexible, so I may give this a try when I make some cheese this week.

[willie]

My thinking is that in order to reveal the neutral plane the material would have to be extremely sensitive

if I recall correctly, Badger found something for that application but considered it too expensive. maybe some other product in the manufactures product line is available for your application.

[bownarra]

Or you could just learn how to tiller...…;)
The wood shows you what it is feeling - just develop the senses needed to spot the signs
This would be a royal pain in the ass.