Re: [RepLab] Waterjets, EDM, ECM, EB F3

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Sam Putman

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Mar 8, 2010, 1:06:48 PM3/8/10
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On Mon, Mar 8, 2010 at 12:27 AM, Erik de Bruijn <erikde...@gmail.com> wrote:
> I'd like to discuss feasibility of an open source water jet cutter. If
> you know of similar attempts in the CNC zone, let me know. I started
> writing about this and then found this RepRap forum discussion about
> it, so I thought we could still have this discussion:
> http://dev.forums.reprap.org/read.php?128,15001,15001
>
> It would seem to me that it would be easier to acquire a few
> specialised parts (I'm thinking of the nozzle) and pumps that have an
> adequate rating than it is to acquire high-power lasers that can cut
> the more bulky types of materials.
>

I've been finding it helpful to think in terms of initial inputs,
continuing inputs, and outputs.

initial inputs:

pump
nozzle
gantry bot

this makes it a small investment on top of the gantry bot, which we're
all fairly convinced can fabricate most of its own structural parts
via a plasma head or other.

continuing inputs (electric power is a given)

water (lots of it)
abrasive particles (not that expensive or hard to come by)
variety of raw materials, specifically metals, stone, and lets throw
glass in there as well.

outputs

finished parts
a slurry containing abrasive, machine oil, and particles from the workpiece.

This last output troubles me quite a bit. I'm not sure there's any
good way to reuse it or break it down safely/cheaply for release into
the environment, and it's going to be the main output by volume of the
machine.

There are other reasons why a waterjet is a hassle, but it has one
great advantage, which is that it can well and truly cut anything,
even stone. That may well be worth developing. Though I think it will
mostly be worth our while to make 'stone' in additive ways, sometimes
nothing but a cut piece of real stone will do, be it aesthetics or
otherwise. Also, cutting stone with a mineral abrasive, using a
biodegradable oil, would produce a swarf that could probably be
rendered inert.

>
> Wire EDM is probably a heck of a lot easier. This rather amusing video
> made me believe it is probably a much better candidate for open
> sourcing: http://www.youtube.com/watch?v=uUN4_-xp1Wc
> I made one myself but I probably need a specific material to get a
> good spark?
>
> Erik
>

Spark is probably a balance of material, current, voltage, and distance.

I've been coming around to Lawrence Kincheloe's point of view that an
ECM machine might turn out to be one of the core technologies here.
Assuming we can work out the chemistry of plating the metal salts out
of the electrolyte solution so it can be reused, it's a non-contact
way to work metals that can give extremely precise and accurate
results and subtracts material fairly quickly.

ECM in combination with EB F3 would let us rapidly build up a metal
workpiece out of wire, and then machine it down to precise dimensions.
EB F3 allows for differential tempering and the building of an alloy
from multiple wires, so the alloy composition can change inside a
single piece. ECM in turn doesn't typically effect the temper of
metals, nor does it care if it's removing metal from a work-hardened
piece.

It seems to me that given a good EB F3 machine, and a comparably good
ECM machine, we should be able to make, for example, blades for a
small steam turbine of seriously high efficiency. As in, with EB F3
and ECM together, we could make a turbine with superior
characteristics for its size to anything that can currently be
manufactured using other methods.

Not to mention monocoque bicycles, airplane and rocket components...
the NASA video on EB F3 covers a lot of this grounds pretty well.

Coming back to the waterjet cutter, I wonder if an ECM head might even
be easier to make, since the liquid is under much less pressure? ECM
has its own dangers, given that it's basically a high-voltage liquid
lye cannon, but in balance, it may prove less demanding on the
machinery.

cheers,
-Sam.

Vitaly Mankevich

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Mar 8, 2010, 3:20:44 PM3/8/10
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> I've been coming around to Lawrence Kincheloe's point of view that an
> ECM machine might turn out to be one of the core technologies here.
> Assuming we can work out the chemistry of plating the metal salts out
> of the electrolyte solution so it can be reused, it's a non-contact
> way to work metals that can give extremely precise and accurate
> results and subtracts material fairly quickly.

I'm not a chemist, but I've looked into both ECM and EDM before and it
seems that EDM doesn't involve as much nasty stuff. IIRC, ECM
machining of stainless steels results in oxides of chromium in the
electrolyte, some of which are quite toxic. From what little search I
did they didn't seem to be trivial to isolate; there were patents
describing machinery to process ECM effluent streams. Also, some
metals like titanium required acid as electrolytes. Again, I'm not a
chemist so I may be over-complicating this.

Vitaly

Sam Putman

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Mar 8, 2010, 3:33:13 PM3/8/10
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I'd love some further reading on the chemistry of ECM, actually. It's
complex but not necessarily excessively so; as a general rule, what
electricity can draw into solution, electricity can plate back into
elemental form.

Any method you choose you have a byproduct: swarf and dulled tools for
milling, toxic electrolyes for ECM, a wire covered with chunky beads
of various metals with EDM. Choosing the most benign consequence of
the Second Law of Thermodynamics is tricky going.

I seem to recall the removal rates for ECM being better than EDM,
which might prove decisive. If it can remove ten times as much metal
in a given unit of time, then the side-effects will just have to be
imported as design constraints.

Most metals would use an alkaline solution, from what I've read (not
enough), but I'd be unsurprised to find that the chemistry of others
would favor an acidic effluent. That part matters less; acids and
bases are to be treated with caution, but aren't particularly scary.

-Sam

Erik de Bruijn

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Mar 8, 2010, 5:09:15 PM3/8/10
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Interesting. It's my ambition to fabricate a bike one day, either by
using stock components (Makerbeam!) combined with 3D printed custom
components (out of composites, is my plan).

EB F3 has the added benefit of being able to add to extisting parts.

I like the idea of combining additive and subtractive. I wonder why
most of that technology is not available commercially yet, as it is
not that complex to combine the two systems. On the other hand I might
have an idea, it might be due to patent reasons. Most additive mfg.
technologies are exploited by single companies that have developed it.
The notion of open innovation hasn't really dawned on them. I hardly
see examples of commercialization of externally originating technology
(as opposed to "Not Invented Here" syndrome), nor do I see many
companies in this industry using external pathways to the market. How
I like open source.... :)

Erik

On Mar 8, 9:33 pm, Sam Putman <atmanis...@gmail.com> wrote:

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