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Chiseling away at rock is something of an ancient pastime.
The general process has remained the same since sphinxes
were stylish additions to the built environment. You take a
sharpened bit of metal, put it against some rock then bash
the bit of metal with a heavy thing. If you're lucky, the
tip
will cause a localized fracture and a bit of rock will fall
off.
If you get good at it, you may find that you can drive a
chisel into the rock and split large sections off. This is
often desirable as it's much less work. To enhance this
effect I propose a chisel with an electrically actuated
expanding/contracting tip. The idea is to narrow the tip,
drive in the chisel and then expand the tip. A functional
equivalent to how you get to the front at a busy bar.
To do this, the section just behind the very tip will be
composed of two halves with piezoelectric crystals
mounted between them. By applying a voltage to the
crystals you can get them to change shape. This isn't a
dramatic shift. Typically, it's around a 0.1% change in
length. So a 10mm piezoelectric crystal stack might expand
only 10 microns. This isn't much, but they generate huge
forces. 30N/mm2 is normal, and this would give 30,000N or
3000kg for a 100x10mm strip in the middle of the chisel.
Now, how to control it. The cool thing about the
piezoelectric actuators is that they're very fast. 30
microseconds or so for the shape change. Now the speed of
sound in steel is about 4000m/s, that's not just noise, but
the speed of the pressure wave from the hammer blow
travelling down the chisel to the tip. That means we have
over 50 microseconds to play with between hitting the
thing and the force arriving at the tip-rock interface. If we
can get the signal to the piezoelectric actuator in 20
microseconds we can narrow the chisel just before the
pressure wave arrives.
So, the chisel is grounded the hammer is charged. Hammer
hits the end of the chisel which is faced with a conductor
connected to the piezoelectric actuator. The electrical
signal and the force are delivered at the same time, but
the electrical signal is way faster. The piezoelectric
actuator narrows the chisel just before the pressure wave
arrives and drives the chisel into the rock. When the
hammer lifts off, the actuator changes to wide formation
expanding the hole by a few microns. Since rock is very
bad in tension and elasticity, you should be splitting off big
chunks in no time.
Piezoelectric actuators
https://www.piceram...cement-modes/#c2518
[bs0u0155, Nov 30 2017]
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Annotation:
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//A functional equivalent to how you get to the front at a
busy bar.// |
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Just what kind of a bar are we talkin' about here? |
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Cold Chisels usually get ground and resharpened a few dozen
times or more. How strong is the metal you're working with
such that the piezo device doesn't become a part of the
environment? |
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Why not use shaped charges ? |
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No, they just do holes. Pretty much any size, but in the end, just
holes. |
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Hmm. So, the aim is to widen the chisel just after it has been driven into the rock? |
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I sort of get this, but chisels are normally wedge-shaped which surely accomplishes the same thing? |
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//Why not use shaped charges ?// |
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Well, quite a lot of technology would be required to get
explosive penetration of a rock face. In fact if an explosion
were imminent, I think I would choose rock as one of the
preferred things between me and it. I view drilling and
explosives as very much complementary technologies. A
drilled hole is dynamite's best friend when it comes to
making a mess of a lot of rock. |
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// quite a lot of technology would be required to get explosive
penetration of a rock face // |
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There speaks the voice of one with little or no direct personal
experience of the use of high explosives. |
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This is not a criticism, merely a reasonable observation of a
deficiency in your education and life experience, and one which,
given the slightest opportunity, you would no doubt rectify with
indecent haste. |
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I'm with [bs0] on this one. Miners, who blow up rocks
professionally, always drill them first. I'm not saying HE is
ineffectual (thinking of the Japanese bombardment of Corregidor
and the RAF's grand slam bombs for U-boat pens) but, in
peacetime conditions, when one's drill rig is moderately safe from
RPGs, etc., it's for amateurs. Actually, in both the examples I
quoted, the HE was backed by a fair amount of kinetic energy
anyway. I'm not sure how much joy you'd get from a demolition
charge that you'd just blu-tacked to a flat cliff face, or placed on
the ground. But I daresay [8th] can enlighten me. |
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Piezoelectric crystals that are placed as facings on chisel-
sides, being crystals, are likely to break when the chisel is
hammered. Brittle, you see.... |
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// I'm not sure how much joy you'd get from a demolition charge
that you'd just blu-tacked to a flat cliff face, or placed on the
ground. // |
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An immeasurable amount. Of course, it won't achieve very much
other than making a tremendous amount of noise and mess and
throwing stuff around, breaking glass,knocking things over, and
generally upsetting people, but if you know a better definition of
"joy" we'd be interested to know . |
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Well, maybe not exactly joy ... more a sort of sociopathic
ecstasy,blended with malicious glee and and atavistic fascination
with destruction. But it's a very positive thing. |
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// But I daresay [8th] can enlighten me. // |
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Of course. Let's see ... microcrystalline pyrophoric magnesium
powder ... thermite ... white phosphorus ... RFNA ... chlorine
trifluoride ... er ... matches, matches, matches ! |
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OK, ready. Where do you want to be enlightened first ? Any
colour preferences ? We can add metal salts to change the
colour .... |
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//An immeasurable amount [of joy]. Of course, it won't achieve
very much [...]// |
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... which nicely sums up "amateur", quid* est demoliendum. |
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*I refer of course to insurance premia. |
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//Piezoelectric crystals that are placed as facings on
chisel- sides, being crystals, are likely to break when the
chisel is hammered. Brittle, you see....// |
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Steel remains the tip and sides. The piezos will form a
core between two divided halves. The halves can be
keyed together for longitudinal stability. The piezos are
so tough you could weld a 1mm plate over them and
they'd just make the plate bulge out. There's a few ways
of doing it. I should underline that the crystals
themselves take none or very little of the longitudinal
load. |
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// which nicely sums up "amateur", // |
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Not necessarily; it depends what you're trying to achieve. |
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If your objective is to randomly damage stuff and upset and frighten a lot of people, and you do that, how is it "amateur" ? |
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Because it's not a profession. More of a confession. |
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This is a scale thing. Want to get through rock, explosive. Want to carve a David , the chisel. Thing is, where you want a chisel, do you want extra uncontrolled force. Jack hammers already do extra force chiseling for the in-between need. |
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