"Leon Fisk" wrote in message
...
On Fri, 16 Jan 2009 16:38:02 -0500, "Ed Huntress"
wrote:
"Leon Fisk" wrote in message
. ..
On Fri, 16 Jan 2009 00:25:13 -0500, "Ed Huntress"
wrote:
snip
I would have approached this as a long-term research project years ago,
except that I don't want to spend that much of my spare time, and I see
no
money in it.
If a manufacturer could make a lot lighter frame work for
say a lathe, it seems like it would be profitable. The
do-it-your-selfer would buy some decent bags of redi-mix
locally and pour/finish the machine in situ.
First, there's too much labor for it to be commercially viable. That is,
except for the sheet-metal structures with the poured-in concrete. And the
polymer/granite-aggregate machines you see promoted at shows are mostly
viable for special, custom machines.
Second, "pouring" a machine is pretty limited, because it's not that
simple
to get the required tensile strength and resistance to cyclic loading. It
can be done, and it doesn't require a lot of skill. But it does take some
time and you have to know what you're trying to achieve.
Unfortunately, concrete is not cast iron. It requires some engineering for
any kind of structure that needs to handle more than compressive loads.
I don't think this is what you had in mind though 
I wish it were that simple.
Ahh... but what if you purposely built/added strategically
placed threaded rods and such. I'm sure they would have to
be somehow enclosed in a sleeve, a bit protected from the
cement mixture. After the cement has setup/cured, slap on
plates/washers over them and then add tension to provide for
cyclic operations.
That's the "screw-thread" approach to post-tensioning. It has been used for
small projects. My first experiment with PT, over 30 years ago, was done
exactly that way.
Post-tensioning is usually done with plain steel rods encased in plastic
tubes, and a hydraulic jack, with a strain gage to determine when the
elastic limit of the steel has been reached. At that point a clamp is
squeezed onto the rod, and it applies the full elastic potential of the
steel to the concrete.
It's tougher with threaded rod because the rod has to be aligned and kept
very straight, or the threads catch and drag the plastic tube. It works OK
for short spans. Even if you thread the ends of smooth rod, it's more
difficult to determine the elastic limit. But it can be done. They use
torque sensors on the torque wrenches used in building car engines, for just
this purpose.
All of this has to be done after the concrete has cured, of course, and
there is some relaxation of the tension as the concrete shrinks. All of this
is part of the engineering calculations. In something as small as a machine
tool it's fairly trivial. The best bet is to wait a month or so before
tensioning.
My thinking was more towards building a light weight inner
skeleton and let the assembler build the simple outside
forms from whatever they like. You would simply provide
suggested measurements for them to use for the forms.
OK, here's a simplified description of what you're dealing with, using that
approach. First, the steel has to be quite close to the surface of the
concrete to do any good in terms of *ultimate failure*. However, with no
pre-tension, the concrete will be vulnerable to surface cracking from cyclic
loads, or just from tensile loads.
The usual solution to this is to avoid putting the major loads on the
concrete. Those French and Italian machines I mentioned use the concrete
mostly to stabilize the steel elements, preventing them from buckling by
applying mostly compressive loads to the concrete, from the sides of the
steel elements.
Another way to deal with it is to use fine mesh just under the concrete's
surface. You get something like a ferrocement skin that way. It doesn't
completely prevent cracks, but they're very fine and very shallow, and have
no significant effect on the strength or stiffness of the structure. The
wire mess acts as crack-stoppers, so the most you get is a little crazing of
the concrete surface.
Either way, unless you use multiple layers of mesh, the strength of the
structure is really just the strength of the steel that's in it. That's not
necessarily bad, but it takes a lot more steel in the combination to do the
job that way, because you aren't taking advantage of the concrete's
compressive strength, except the light loads that are employed to keep the
steel elements from buckling. And the steel will not stay attached well to
the concrete if the loads are high. The bond will be subject to high sheer
loads.
I'm guessing that most people here know how prestressed concrete works;
post-tensioned works the same way, basically. Ferrocement is more like
fiberglass in polyester or epoxy resin. The extremely short spans of
unsupported concrete -- fractions of an inch -- do not get sufficiently
loaded in tension for them to fail. The steel mesh comes into play as soon
as tension is applied, because of the short spans. Unlike prestressed or
post-tensioned structures, however, there is no pre-stressing on the mesh.
So ferrocement has more compressive strength than tensile strength. It's
strong enough, however, that thin sections of it actually can be bent and
they spring back.
With fuel costs/shipping most likely to start marching
upwards again, seems like you could save a lot in the weight
area with some careful thought/engineering.
This is just off the cuff thoughts... needs a lot of
revision
Well, you're on the right track. Lower shipping costs is what motivated the
French and Italian machining centers. Also, I think that Hardinge made some
machines this way a while back. It's not a bad idea but it results in a
heavy machine, once the concrete is poured, that still has a lot of steel in
it.
What I'm talking about is somewhat different. Stressed-steel and ferrocement
produce a concrete structure, not just one that's stabilized with concrete.
This book is the best on the subject. Take a look at the chapter
descriptions, and you'll see what it's all about
:
http://www.technopress3000.com/FrameB.htm
There are comparable books on prestressed- and post-tensioned structure.
This one, by the same U of M prof as the one above, covers the field:
http://www.technopress3000.com/FrameB.htm
There is a ton of free information on the web about both, but you have to
watch out for the info on ferrocement. It's become the darling of the
greenie-save-the-Third-World types, who want to make it out of spit and
bamboo shoots. g
--
Ed Huntress