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Posted by Thomas Womack on June 22, 2008, 6:07 pm
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I'd like to make a Menger sponge
(http://en.wikipedia.org/wiki/Menger_sponge) in metal; that is, I want
to make a number of intersecting through holes of square cross-section
through a cube of metal.
I don't know what the relevant metal would be; it's a decorative item
with a lot of operations done on it, so ease of machining is the major
requirement and attractiveness a secondary one. Free-machining brass
looks good.
Even the base cube of metal seems quite hard to find; I can find
various suppliers for two-inch square aluminium bar stock, and one
ebay auction for an offcut of four-inch square aluminium bar stock,
but I can't find brass square bar stock of more than one inch, which
would be too small.
With four-inch square stock I could presumably produce a cube with
97.2mm sides by face-milling, and it looks as if the big square holes
would not be too hard to mill - what sort of radius is it reasonable
to get on the corners of a 32.4mm x 32.4mm hole?
But I don't have a good idea how to make the third-level holes - 3.6mm
square holes, 97.2mm deep. It seems both a bit small and a bit deep
to mill, and I've no idea how happy milling machines would be with a
hole that repeatedly broke into air - I presume the edges around the
break-throughs would be horribly burry. And I'd need 216 holes to
that spec.
[for second-level holes I could fill the first-level holes with
square-section dowels, mill, then remove the dowels; would it make the
third-level milling easier to fill the first- and second-level holes
with some kind of low-melting alloy that could be melted out
afterwards?]
Could I drill them and then make them square with a broach of some
sort? Custom broaches sound likely to be crazy expensive, so I
presume I'd have to size the project for the size of the nearest
round-to-square broach, and I'm not sure where to look for a
round-to-square broach.
I presume the fourth-level holes, 1.2mm x 1.2mm x 97.2mm, would be
completely impractical by any affordable means - it's 80 times
diameter, which looks barely possible in a good EDM shop, but 648
holes at a good EDM shop would leave me vastly out of pocket.
Plan B is to glue together eight thousand 1cm^3 plastic cubes, which
would involve some trivial jig-making and a lot of very tedious
assembly, and lose a lot of the gee-whizz of fractal geometry machined
in metal, though would make a nice big block in the end. It looks as
if eight thousand little cubes would cost a few hundred dollars; I'm
getting a feeling that small deep holes with reasonably sharp corners
are not to be had for a dollar.
Tom
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Posted by Jim Wilkins on June 22, 2008, 6:29 pm
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wrote:
> I'd like to make a Menger sponge
> (http://en.wikipedia.org/wiki/Menger_sponge) in metal; that is, I want
> to make a number of intersecting through holes of square cross-section
> through a cube of metal.
> ....
> Tom
Round holes, then wire EDM?
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Posted by Jim Wilkins on June 23, 2008, 6:28 am
Please log in for more thread options > wrote:
>
> > I'd like to make a Menger sponge
> > (http://en.wikipedia.org/wiki/Menger_sponge) in metal; that is, I want
> > to make a number of intersecting through holes of square cross-section
> > through a cube of metal.
> > ....
> > Tom
>
> Round holes, then wire EDM?
That was a reality check.
If I had to do this on less than an aerospace budget I'd look into
milling molds of the unit cell and casting them in wax, gluing them
together and making a lost wax casting. Since the outer surfaces are
flat they could be cast a little oversize and fly-cut smooth to remove
the joint lines.
If the metal is soft, like aluminum or pewter, a home-made square
broach of unhardened tool steel should cut at least a few holes before
it dulls and jams, then you could resharpen it by taking a light cut
to the face of each step in a lathe.
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Posted by Ned Simmons on June 22, 2008, 6:31 pm
Please log in for more thread options On 22 Jun 2008 23:07:10 +0100 (BST), Thomas Womack
>I'd like to make a Menger sponge
>(http://en.wikipedia.org/wiki/Menger_sponge) in metal; that is, I want
>to make a number of intersecting through holes of square cross-section
>through a cube of metal.
>
Stereolithography
http://www.bathsheba.com/math/
--
Ned Simmons
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Posted by James Waldby on June 22, 2008, 7:32 pm
Please log in for more thread options On Sun, 22 Jun 2008 23:07:10 +0100, Thomas Womack wrote:
> I'd like to make a Menger sponge
> (http://en.wikipedia.org/wiki/Menger_sponge) in metal; that is, I want
> to make a number of intersecting through holes of square cross-section
> through a cube of metal.
[big snip]
If I were making something like this, I'd start by writing a program to
dissect it into castable layers. Patterns could be made up of short
pieces of square stock in two or three standard lengths, locked up in a
matrix, in the same way that movable-type letters were used to print
newspapers long ago. Most patterns would be used several times per
sponge.
[snip re low-melting alloy]
[snip re round-to-square broach]
If on a limited budget, you might consider buying square files or rasps
and adapting them via a grinder or sander.
[snip re expensive EDM shop for 648 holes]
> Plan B is to glue together eight thousand 1cm^3 plastic cubes, which
> would involve some trivial jig-making and a lot of very tedious
> assembly, and lose a lot of the gee-whizz of fractal geometry machined
> in metal, though would make a nice big block in the end. It looks as if
> eight thousand little cubes would cost a few hundred dollars; I'm
> getting a feeling that small deep holes with reasonably sharp corners
> are not to be had for a dollar.
Or (as before) dissect the sponge into layers that have long surface slots
and short cross-holes and some supporting structure, ie, two or three units
of thickness. Starting with plate stock, either mill the slots and broach
the holes, or punch small holes with a square punch and saw or mill outer
rectangular perimeters as needed. Then epoxy the layers together, or bolt,
or oven braze, or if you did good work, just wring them together.
-jiw
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> (http://en.wikipedia.org/wiki/Menger_sponge) in metal; that is, I want
> to make a number of intersecting through holes of square cross-section
> through a cube of metal.
> ....
> Tom