"Karl Townsend" wrote in message
...
On Fri, 23 Sep 2011 20:35:25 -0400, "Ed Huntress"
wrote:


"Karl Townsend" wrote in message
news

Overall, you probably would learn a lot of useful and interesting things
by
giving it a try. You have the right idea about using spot-welded mesh or
something similar to get a mechancial bond. Concrete doesn't stick well
to
flat steel when it's vibrated a lot.

There's a thread with nearly 3000 posts on CNCzone concerning building
machines with steel weldements and using epoxy resin and rocks to fill
and provide damping. Several machines have been built this way.

Karl

That sounds like a lot of reading, but it might save Bob more time than he
would lose by experimenting with structures that take a while to build.

BTW, those concrete-filled machine tools typically use a polymer-modified
concrete that provides better bonding and more vibration resistance than
Sakrete.

I scanned the thread one night. Lot of discussion on the fill
material. It was called polymer concrete at some points and epoxy
resin at others. Lots of discussion on the proper aggregate and size
distribution of it. The whole idea is to dampen vibration.

Karl

FWIW, there was a lot of money spent in the late '70s and early '80s on
developing "concrete" machine tools, some of which were quite successful but
most of which cost more in materials and labor than a common cast iron
machine tool. It still has some life in it and it continues to hold a lot of
promise, particularly for custom machines. I think it was IMTS '82 or maybe
'84 that a half-dozen or so showed up on display.

They first tried polymer-modified Portland cement; then polyester resin (not
good for the job); and then epoxy with graded granite aggregate.
Epoxy/granite was the trick. It's also cost close to $100/gallon for the
epoxy. Some builders called the latter "polymer concrete."

But just using the "concrete" as a filler for a steel structure is a
different animal and a lot easier to do. Modified Portland cement, something
like the materials they use for concrete building repairs, and common graded
stone aggregate seems to work OK for that, at a pretty cheap price.

The design issues are a little tricky because the concrete cannot be loaded
in tension or in shear at all. It first appears that a small amount of steel
can be used to take up tension loads, but that coupling issue is not a
trivial thing, so it takes some knowledge of machine structural design to
get it right. But it's doable.

--
Ed Huntress