To give a brief background -
A friend of mine wants to start supplying ripe coconuts (husk removed) with some stuff etched on the shell.
The etching needs to be only deep enough so as not to puncture the shell. In order to do that, the thickness of the shell needs to be known using a non destructive method, so that the etching tool can be set to cut just short of the complete thickness.
What might be the best way to achieve that? I was thinking of ultrasonic gauge measurement tools, but I am not sure if they work on organic stuff like fruits and nuts. Are there non destructive 2D or 3D modelling tools that can create a profile of the inside of the coconut using lasers or something? Any such specific tool manufacturers that come recommended?
Get a few random coconuts and a pair of calipers. Crack the coconuts and measure the shell thickness of each of them. Assume that these randomly-chosen coconuts are of typical thickness, and choose an etching depth that is considerably less than that average. During production, use mistakes for lunch.
You’re describing pretty deep etching, such that I’m not sure I’d call it etching anymore. Unless your friend is trying to create 3D/relief patterns, the shell shouldn’t need to be machined that deeply in order to create a visible pattern.
I’ve got a little experience of crafting with coconut shell, and I’ve found the thickness to be fairly consistent at about 3mm thick (although the three seams are often thicker, as are parts of the pointed end - you will be able to engrave 1mm deep without much risk of piercing the shell.
I don’t think ultrasonic measurements are likely to work, as the union of nut meat to the inside of the shell is pretty tight - I think you’d end up measuring the thickness of the shell plus the meat.
Are you laser engraving? Because the inconsistency of composition of the shell material is likely to be a bigger factor in penetration than the actual thickness - coconut shell is a bit like a load of strands of fibre fused together - well, that’s actually what it really is - but the point is, not all of those fibres have the same material properties.
For example - if you look here - where I sanded the loose fibres off the outside of the shell, the material underneath is smooth and solid, but made up of light and dark strands (actually very pretty when polished) - a laser is going to penetrate the darker zones more than the lighter ones.
But also make sure you measure multiple points over each shell so that you get an idea of variation of a single shell as well as variation over a population of shells.
Then choose an etching depth that balances the quality of result with an amount of wastage that you are wiling to accept.
Doesn’t ultrasonic thickness measurement use the detection of different density of the materials to give the thickness? If that is so, then shouldn’t it give back a different echo wavelength/frequency at the meat-shell junction?
You guessed correct about the laser engraving. So what you are saying is that laser engraving will not work on the shell because of the different shades of the strands? I don’t know the principles of laser engraving, so I guess I will have to read up on that.
Yeah - I think you’re going to run into troubles - laser engraving works best when the material is uniform - laser cutting is more forgiving of non-uniform materials, because you just dial it high enough to cut right through.
If nothing else, the lighter strands are going to tend to engrave less deeply than the darker ones, just because they reflect rather than absorb light, but I’ve a feeling there could be variations in density of the shell material too.
I think the idea of custom-engraved coconuts could probably still be realised by a cnc rotary tool with the right configuration of axes, and assuming you can scan the shape of the thing first so you only cut to a certain depth below the surface (adjusting for all the imperfections of shape)
The easiest way to predict what a NC laser engraver will do to a coconut shell is to clean up a couple shells like Mmangetout’s cool videos demonstrate, then take 'em to somebody skilled with a laser engraver. In 10 minutes they and you will know whether this will be a piece of cake or an endless headache.
Mature coconuts I recall may have shells as thick as 5mm thick at the base. They’re usually thinnest at the sides. Very young coconuts have thin shells, maybe as thin as 2mm. That’s all from memory.