Twenty-five years ago I got very interested in concrete and polymer-matrix
machine tools, and studied the hell out of them for a while, but never did a
darned thing about it. I'm wondering if there is enough interest among
hobbyists to try to get a discussion group or something like that going.
There are a lot of routes that might be taken -- post-tensioned,
ferrocement, polymer-modified, fiber-reinforced, not to mention
polymer-matrix machines -- and there is room for a lot of experimentation on
such things as aligning spindles, embedding bedways, and so on. A lot of the
testing and experimenting can be done on the cheap. All it takes is time. I
don't have the time to do it all myself but I'd like to see something come
Is anyone interested? And for you guys who are more web-savvy than I am,
suggestions for a way to upload and display photos and drawings?
I'm just looking for a show of hands to see how many might be interested.
Sounds interesting Ed. A quick Google search turned up this:
Ha! That's interesting. The first reference on that page is to an article
written by one of my old co-editors at _American Machinist_, Joe
I've heard of that book and it's one I should get for my reference shelf --
as soon as I win the lottery <g>. Thanks, Pete.
There's been a HUGE thread on this very suject on CNCzone.com. Several
thousand posts and a few actual machines built. I've only got a few
minutes till the Marathon Lady leaves the dock. I'll search for the
Thanks, Karl. It took some time to go through all of those, which is why I'm
slow getting back to you.
There are a few ideas in there but, frankly, those guys are mostly not up to
speed with this material. The machine designer who complained about cracks
in the polymer concrete reinforced it incorrectly. Most of the others are
talking about making sheet metal structures and trying to beef them up with
concrete. That's really not what I'm talking about. That's been done for
decades (particularly by the Italians and the French), and the structural
design issues they've had to face are much more complex than those posters
are talking about. That kind of structure, while it sounds simple, is
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. No time, no money...I don't do those things anymore. <g> I'd
get involved now just for the hobbyist satisfaction of it IF there were at
least a dozen other people who were serious about it and who would
contribute. But I realize after reading those threads you linked to that
even guys who are knowledgeable machinists may not understand stressed
concrete and concrete composites enough to do serious things without a lot
of study. Some boatbuilders know ferrocement, and some architects know FC
plus post-tensioning. Civil engineers usually know general reinforcement and
prestressing. Now I need to find a few of those who also are amateur
So, I'll put it back in the can for now. Maybe when I retire -- if I ever
retire. Thanks again for your effort; it kept me from reinventing a wheel.
Yea, I know what you mean. its a bit like mining gold. You got to
shovel a lot of ore to find that nugget. My hobby time has morfed from
cutting metal to rebuilding old CNC iron to like new with a brand new
control. Among all the idiots, there are a couple gems.
For example, I learned that my old wire EDM is the best possible
candidate for upgrade to a new control and I found a guy that knows
this machine like the back of his hand.
Say, I think the project after this is to build a four foot by ten
foot Plasma, torch, router combination machine. I'm thinking weldament
for the machine frame. Does this technology have any benefit for
vibration dampening on this application?
Yes, concrete is a good vibration damper and you could make an effective and
inexpensive router frame out of it. But it would take some real
understanding of the material to avoid potential problems. And a good
structure is not going to be light.
For something like that I'd look at prestressed concrete sections that are
available commercially. If you wanted a perimeter frame, you could use small
beams, which tie together with special mortar at the corners. Or, if you
wanted a slab, you can buy hollow floor slabs. These things are heavily
reinforced and prestressed so that they'll handle a lot of tension loads.
But after it was built, I don't think you could move it without wrecking it.
That's one of the limitations. Small, squat machine tools are one thing.
Larger structures are another.
Some of the first machine tool applications were oil-country-type lathes
built by the Soviets back in the '40s, so big machines are entirely
possible. In fact, that's a major attraction of concrete for machines. But
you pretty much have to build them in place. Unlike cast iron or steel, they
have to be built for the specific loads they're going to be subjected to.
Put a forklift under a long span that's stressed for heavy loads from the
top, and you can crack it.
Small machines, like a lathe chucker or a squat bridge-type mill, are an
entirely different story. You can build them with ferrocement and they'll be
capable of handling stresses in all directions. And they can be much lighter
than girder-type structures. The material itself, loaded with a dense matrix
of welded steel mesh, has an overall density similar to that of aluminum.
A big frame like your router idea is something for one's second or third
project, IMO. It's a challenge unless you've been designing bridges all of
Are you thinking of this as a manufacturing process or one that
hobbyists could use to bootstrap a machine shop, like Gingery's cast
I'll build things with whatever works, castings or welded steel or
logs chainsawed flat or anchors drilled into a big flat rock. Assuming
a hobbyist without FEA or the knowledge to apply it, I think metal
bearings and support plates jigged in place and connected by the cast
material might make sense. I would join the pieces with threaded rods
so it could be disassembled for repair or improvement, and the casting
wouldn't have to take much tension. Otherwise no matter how strong it
is, it has to bond to the metal and they have very different thermal
expansions. If the casting came out badly the metal could be reused. I
have a vacuum oven to remove trapped bubbles but I don't suppose too
many others do. I bought it cheap at an auction and spent most of a
day chipping the old spilled resin out of the chamber and fixing
Nonmetallic castings seem to be used hesitantly in industry. My
Powermate generator has one alternator bearing cast into the plastic
end housing. DeWalt chose magnesium for the gear case on their high-
end cordless drills, reportedly because they couldn't make a plastic
one strong enough. I think that was in Design News. The Drill Doctor
chuck is an example of a casting that isn't quite as satisfactory as a
metal one. Polymer pistols have metal inserts cast in, I don't have
one to decribe how, and the Segway which I've dis and reassembled
many times has a very solid metal structure under the plastic.
Not the first, and not exactly the latter. I'm thinking more about building
some simple machines that might be useful to someone who already has a
couple of machine tools. Starting from scratch is interesting, and someone
might want to build on these ideas to do it, but I think it's too many
unknowns to deal with at once, for a beginning.
The machines I have in mind are things like a between-centers grinder and
lapping machine; a speed lathe (that could be built up into an engine
lathe); a sliding-head drillpress; a simple surface grinder; a benchtop
mill; and a manual-feed gap-bed lathe. The latter few are a lot more
complicated but they still can be simpler than a screw-cutting engine lathe.
That's the first thought of most people who have knowledge of machines and
machine elements. I wouldn't want to foreclose options, but my own approach
is to start with the structure and to figure out what means are practical to
mount the guideways or rails, and the dynamic elements.
Using a metal structure to establish relationships among the elements is an
attractive idea -- like a Glock pistol. As I mentioned, a few European
builders have taken that approach. But the concrete is more than glue and
mass. If you use it right, it's the structure itself.
Concrete has some properties that make it a challenge for machine
structures. There is the obvious one that it has virtually no tensile
strength. You deal with that by reinforcement, either by means of tensioned
steel rods or fine fabric composites, such as ferrocement. When you get into
machine tools, there are potential problems with loading it to avoid
tension/compression cycling, and there is the problem, particularly for
large machines, that it shrinks (slightly) for up to three years after
casting. It's like dealing with a wood structure in some ways; you have to
be mindful of expansions and contractions, preferred loading directions, and
so on. You have to design around those issues or compensate for them
The upside is that it's so cheap and so versatile. And it's a great damping
material that's also relatively stable, compared to most alternatives with
comparable costs and versatility. The challenges can be solved with good
engineering, mostly of the clever-hobbyist variety. That's what I find
interesting about the whole idea. This project is more about making
something with creative ideas than it is about building a machine shop --
although I would want to stick to useful machines for projects. I don't see
it as a lab exercise, either.
I don't know about the effect of trapped bubbles in reinforced concrete.
Considering the astonishing number of research papers on advanced concrete
that are out there (from Europe and developing countries, as well as from
the US), I'm sure someone has investigated it.
As for the thermal expansions, yes, and that's something that limits the
approach of making a light structure out of steel and bonding it all
together with concrete. Those differential expansions are not a problem in
finely reinforced composites such as ferrocement. And they're well-known and
characterized for pre-stressed and post-tensioned concrete.
When you use a material such as ferrocement, you can minimize the number of
separate elements that have to be bolted or otherwise fastened together.
Most of those machines I listed above can be built as more or less
There are lots of ways to engineer a structure, and there are ways that are
appropriate for volume manufacturing that can be done better in some other
way by the one-at-a-time hobbyist. Monolithic ferrocement structures, or
post-tensioned structures, are one-at-a-time deals. They can be better than
anything made in production, like many other things we make in our shops.
The idea won't let me go; I've been thinking about it for roughly 30 years,
during which time I've acquired a pretty good sense of concrete structures.
Unfortunately, the time to do it has never magically appeared, and the
incentive is purely one of personal satisfaction.
On Fri, 16 Jan 2009 00:25:13 -0500, "Ed Huntress"
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.
I don't think this is what you had in mind though :)
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.
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
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.
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
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
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
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
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
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.
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
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:
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:
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>
Yup - that's post-tensioning, an extremely effective (and adjustable)
method for reinforcing concrete by placing it in compression. Works
great right up until some idiot decides to cut a hole in the floor and
chops though a tensioning cable (Third hand at best, but I gather it
In mulling this a bit more, one thought for dealing with differential
expansion on long embedded metal parts (ie, ways that you are "beefing
up" with concrete) is to arrange so that the part is only locked in at
one point, and is physically captured by shape (a dovetail, say) but
free to slide (even hit with some mold release wax before casting)
One thing mentioned rather early in the referenced thread was effective
damping by casting epoxy granite (I think) inside innertubes or rubber
bags inside hollow section steel. While it would not add much to
strength, the simplicity of fabricating a frame from large square
section steel and then damping it by stuffing the box sections with
material might be a worthy low-end route. IIRC the rubber-encased was
better than direct cast, probably due both to damping from the rubber,
and not having the voids that will develop due to differential expansion
of direct-cast material in a steel tube - the rubber would allow for
slippage with expansion/contraction.
Then there are the granite tools which slide granite over granite on air
bearings...but those seem rather far out for DIY. What I read (I don't
claim to have read every post) of the threads at CNCzone seemed to have
a lot of churn and not much experimenting, at least not in a functional
way. The guy who's still fussing over epoxy formulations 2 years on and
has evidently never even made a test sample with aggregate is over-fussy
or something like that. The one guy participating in the thread
(Walter?) who seemed to be willing to toss some stuff in a form and see
what happened evidently gave up and moved on. AFAICT the guys in Germany
that were doing this and inspired the 2-year, 3000+ post thread have
been happily making actual stuff with the process while CNCzone has
pondered its perfection at length...but I could have missed something.
Heck, I thought I'd get to see some pictures of fish. <g> I walked across a
nearby pond today, looking for bluegills frozen in the ice. I had my ice
axe, but no luck.
I stopped in there and searched on "concrete"; 'found some comments, but
Anyway, it all depends on whether enough people really want to get into it.
There is plenty of information around to get one dabbling with the idea, but
I'm wondering if people want to dig a little deeper.
200 years ago lathe ways were made by mortaring iron strips into
granite blocks. The closest I've come was welding a framework from
steel channel and angle and shimming the pillow block bearings into
alignment. As a long-time prototype machine builder I prefer a design
and construction method that is easy to modify. Plastic castings don't
have much tensile strength, especially where material has to be added
and the reinforcing fibers don't cross the joint. Rapid prototyping
resins were the worst, I've needed to cut away a large bonding surface
and sculpt several batches of epoxy to build up a boss that would hold
a new bearing.
My Picasa photo links have remained on one line since I started
prefixing them with >. I don't know how you could post drawings except
as graphic images, there isn't any common drawing package other than
MS Paint. I draw machine parts with a Mentor Graphics circuit board
layout program that theoretically writes and reads DXF, but I've been
unable to open one from another CAD program with it.
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