"Jim Wilkins" wrote in message
...
On Jan 16, 12:25 am, "Ed Huntress" wrote:
...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. ...Civil engineers usually know general reinforcement and
prestressing. Now I need to find a few of those who also are amateur
machinists. d8-)
Ed Huntress

Are you thinking of this as a manufacturing process or one that
hobbyists could use to bootstrap a machine shop, like Gingery's cast
aluminum tools?

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.


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.

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
somehow.

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 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
leaks.

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
monolithic structures.

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.

Jim Wilkins

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.

--
Ed Huntress