From: "Ed Huntress" <mike234@bellatlantic.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: designing a machine for hobby use.
Date: Tue, 20 Feb 2001 05:49:30 GMT

>>there is no better material on the market for machine tools simply because
of its natural lubricity and dampening qualities.  A machine tool made from
steel instead of iron would, in all probability, not come close to doing the
same job, all conditions being equal.    If steel were a better choice for
machines, it would have been used through the years.<<

Gray cast iron is still the preferred machine tool material for the reasons
you cite. There are welded steel machine tools, mostly larger European ones,
but they're built that way only because it's impractical to make castings
for a 30-foot-high bridge mill. The limitations are that it's hard to make
them rigid (because they're not as massive), they transmit too much
vibration because of the inherent property of the material, and, in the case
of weldments, it's extremely difficult to avoid distortion in welding, at
least at the levels of accuracy machine tools require. They're prone to
residual stresses that can give you grief for years, as the stresses get
relieved over time.

There are two other materials that have even better vibration-damping
properties than cast iron. One is epoxy/crushed-granite mixes and similar
polymer/stone mixes, used by Hardinge ("Harcrete") and others. The other is
ordinary Portland-cement concrete, with more or less addition of polymer
modifiers. This is used either to stiffen and dampen machine tools that are
actually built lightly of steel, or to provide the structure itself. There
have been a few experiments in the latter case with post-tensioned concrete.
The Italians and the French built a number of machines this way in the '70s
and the '80s. It's less popular now.

Concrete works well because most of the loads on a machine tool are
compression loads or tension loads that are simple to deal with, through
reinforcement of some kind. Reinforced concrete has about the same density
and elastic modulus as aluminum. The main problem with concrete, once the
tension issues are solved, is that it can take years to settle down, much
like steel. It keeps on curing and changing chemically for a long time. We
usually discount the changes that occur after initial drying, but you can't
ignore them when you're dealing with precision machine-tool accuracies.

Still, I think that concrete is the hobbyist's machine-tool material of the
future. A kit-type machine could be made that way, with ground flat ways
that could be studded on the bottom and set into epoxy in the concrete bed.
The saddle and cross slide are the parts that would require the most thought
and cleverness. Flat ways, like those of an Atlas, are the easiest to align,
and they work very well.

I made a simple concrete lathe bed as an experiment around 15 years ago and
went so far as to cast the base, using a plywood form. It had two steel pads
cast into the bed at each end to take stanchions for a pair of round ways. I
filed the pads flat and scraped them, using a precision machinists' level to
get all four on a plane, and it was quite easy. I didn't work out the
spindle but that wasn't what I was interested in at the time.

But it was a thought experiment, not a complete project. I know of another
HSM who designed a really massive mill that way. He got hung up on his head
design, in which he cast an iron tube into the head and then tried to figure
out, after the fact, how to bore it so it was perpendicular to the bed. He
never did figure that one out. I think it eventually became the rubble under
his patio. <g>

Next time you're at a big machine tool show, look up the company that makes
epoxy/crushed-granite machine bases for custom machine builders. They're
things of beauty.

Ed Huntress

From: "Ed Huntress" <mike234@bellatlantic.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: designing a machine for hobby use.
Date: Wed, 21 Feb 2001 03:07:13 GMT

Well, you took the concrete lathe a few steps farther than I did. One way
you could seal those bearings is with epoxy. But there are other ways to do
it. My design was based on the idea that I'd use a commercial cartridge-type
spindle, with a variable-speed Fuji DC motor and direct drive. I wanted high
speed for small parts and extreme smoothness for fine finish. The spindle
would cost more than my South Bend, but that was what the project idea was
about.

Anyway, I see that $400 for 3 cu. ft. figure again, and I have to remark
that you're looking at some higher-class material than you need -- and more
than most builders use. I have some stuff that I patch cracks with that says
"polymer concrete" on the bag. I think I paid $6 for it. There is polymer
concrete and there is polymer concrete. You don't need the
high-tensile-strength stuff. It's just nice to have a little polymer in
there to minimize crazing and microcracks on the surface. It probably
doesn't matter squat what you use, if you're going to reinforce with steel
wherever there's a tensile load. And I would.

Keep those experiments going. You're on a productive track.

Ed Huntress

From: "Ed Huntress" <mike234@bellatlantic.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: designing a machine for hobby use.
Date: Thu, 22 Feb 2001 04:52:56 GMT

>>How did you build the bed and what were the results?<<

I made a plywood box for a mold, brushed it with motor oil, mixed up a
standard sidewalk-type concrete mix (as dry as I could handle it) and packed
it into the mold. I had made four steel pads out of box-section mild steel
and placed them where I planned the ends of the two round-stock bedways. A
2x3 went into the top of the casting (the bottom of the mold) to make a
trough for chips and oil. I also laid some pieces of rebar at the bottom,
where the machine base would be stressed in tension, and left the bottom of
the base flat, planning to stand it on some kind of corner pads.

It worked very well. I never built a complete lathe from it, but I did jump
on the bed a lot, with the ends braced up to put maximum tensile load on the
bottom of the bed. It didn't crack.

>>Was it a viable way of building the bed for hobbyists?<<

Oh, yes, without a doubt. It needs some good thinking about how to make
better ways (I have some thoughts on that, and some experience to relay
sometime), how to handle the head and spindle, and what to do about a saddle
and cross slide. But it's all doable, economical, and quite practical.
Someone just has to experiment with it to make it all work right.

>>Were there special design considerations?<<

Concrete is easy to design with in most considerations. It won't handle
tensile loads on its own, so it pays to know something about statics and
about the different ways to pre- or post-stress concrete. But my experiment
shows that you don't really need pre-stressing for a small lathe bed (about
30") if you just lay in enough rebar.

My feeling is that it would be best to try some simple, specialty machine
first, before one tackles an engine lathe. For example, a crankshaft grinder
would be easier, since it wouldn't need a spindle-drive motor as such. You
could make it with a pair of dead centers and an outboard belt drive that
worked directly on the workpiece. And a lathe-type grinding attachment can
be rigged up without the complication of a full lathe carriage.

Ed Huntress