"Joseph Gwinn" wrote in message
...
In article ,
"Ed Huntress" wrote:
"Joseph Gwinn" wrote in message
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In article ,
"Ed Huntress" wrote:
"Joseph Gwinn" wrote in message
...
In article ,
"Ed Huntress" wrote:
"Jim Wilkins" wrote in message
..
.
On Nov 24, 1:04 pm, "Ed Huntress" wrote:
"Jim Wilkins" wrote in message
...
I just checked my two best sources, Holtzapffel #2 and Oscar
Perrigo's
1916 "Lathe Design", ...
...
As I think about this, I'm remembering what I thought about it at
the
time,
30 years ago. I believed then that the issue was the difficulty,
without
planers, mills, or big surface grinders, of getting the four
planes
of
a
pair of V-ways coordinated for straight and smooth travel. One
way
to
interpret this is that you can adjust the single plane of the
flat
way
a
lot
easier than the pair of planes you have with a second V. So to
say
that
it
was simpler to correct accuracy with the V-and-flat could just
mean
that;
if
the ways are hand-finished, you're correcting accuracy, and
V-and-flat
is
a
lot easier to correct than two V's.
Maybe. g
--
Ed Huntress
If I read Holtzapffel correctly the two sides of inverted vee ways
were at first made separately, joined and aligned afterwards to
fit
the fixed and moving poppit heads (headstock and tailstock to us).
Yeah. In the very beginning of modern lathes, the V-and-flat
combinations
were assembled the same way. The first screw-cutting lathes had
wood
beds
with bolted-on iron ways, IIRC. (I'm doing this from memory; don't
bite
me.
g)
"This slight width of base does not afford sufficient lateral
support
to the heads, which with only moderate force in turning are liable
to
vibration; while exact parallelism of the two angular edged bars
is
also necessary. Improvement in stability was sought by making one
side
of the bearers flat and broad, fig. 72, with a corresponding flat
on
the underside of the lathe heads; retaining one angular side, to
give
the direction or common axis. This arrangement also facilitated
the
construction, as the parallelism of the two bars was no longer
essential,..."
Right. That sounds familiar.
Fig 72 shows one flat and one inverted vee way on a cast iron bed.
The difficulties of the early machine builders that Holtzapffel
recorded aren't that much different from those of a homebrew
machine
tool maker today, except that we can buy ground drill rod and flat
stock and they could hire cheap child labor for tedious hand
fitting.
Right. Maybe you've peeked at my ideas for a ferrocement lathe with
steel
ways. d8-)
(Having finished reading Naaman's _Ferrocement & Laminated
Cementitious
Composites_, I'm less enthusiastic about that construction.)
Significant work has been done on concrete-filled fabricated metal
frames for precision machine tools.
The place to start is MIT professor Alexander H. Slocum
(http://meche.mit.edu:16080/people/index.html?id=80). A good
discussion
and many references may be found in his book "Precision Machine
Design".
Joe Gwinn
Thanks, Joe. I talked to Slocum not too long ago -- maybe a year or
two --
and someone here brought up his book before (maybe you?)
Very likely. I recall posting the reference before. Yes. The thread
was "Epoxy grainite build yer own machine frame" in March 2009.
-- it's on my list
of things to get for my library. I had a copy for a couple of weeks on
an
interlibrary loan, and I read what he had to say about long-term
stability
and so on, which helped a lot.
I bet you can borrow it again.
I can, but I'd like to have it. It ain't cheap, and my list of desired
books
is long.
I've been tempted as well.
I should point out that I was studying and writing about concrete- and
polymer/aggregate-base machines around 30 years ago, and I've visited
manufacturers of them in the US and in France and Italy, mostly around
that
time but as recently as seven years ago. I'm familiar in general with
the
various approaches, the materials, and the design philosophies. But
I've
never claimed expertise; it was more on the level of a good
journalistic
exercise.
The commercial applications are one thing. This wild hair I've been
chasing
has more to do with exploring strategies for building at home, at very
low
cost, and to see what can be done with a material that's intrigued me
for
40
years -- ferrocement. The obvious approach to building a machine like
this
would be either a fairly massive, fiber-filled casting or a
screw-tensioned,
post-tensioned cast structure. The former is my next area of study and
it's
a big one. The latter is something I've abandoned because of problems
with
long-term stability and engineering that's trickier than it looks. But
it
would be a great way to go, if it wasn't for the stability issue.
Slocum runs hot and cold about concrete versus lots of cast iron, but
he
isn't trying for infinite life either.
If it's cheap enough, people will live with something that doesn't last
100 years. Most HSMers won't last that long.
Right. g I don't think that the limitation is the life of the machine,
but
rather of dealing with growth or shrinkage over a period of years. There
are
ways around it. I just haven't thought it through. But, again, we're
talking
about the difference between, say, a South Bend and a Hardinge. You can
build something that will fit into the SB category. But concrete is not
the
stuff to use if you want Hardinge.
Then again, if you want Hardinge, you'd better mortgage the house and buy
Hardinge. g I own a South Bend, and it's all I could want for my hobby
machining.
I've been tempted as well. But I like my house, so Clausing will have
to carry on.
I'm convinced that a concrete machine can deliver ordinary lathe
accuracy
and could be perfectly suitable for the hobbyist. Problems crop up
when
you
go for high-end, toolroom-grade accuracy and long-term stability. The
polymers are better for that but they're *very* expensive, and defeat
the
basic goals I'm starting with.
Anyway, thanks for the tip. I would love to go for this in a big way,
but
it
isn't in the cards right now. I'm engaged in conversation with Dr.
Senft
on
Stirling engine lubrication, and the outcome is going to eat me up for
months to come. d8-)
Is there a good solution? I recall that this was one of the big
issues.
You'll have to read my article, should I succeed in gathering enough info
to
write one. There are solutions that work; different solutions for
different
realms, from low-temperature-differential types to fractional-horsepower
mule motors, up to 100-hp-plus automotive engines. The solutions are all
different. Senft has put me on to a guy who apparently is one of the
world's
experts on automotive Stirlings, and who knows the big-time lubrication
solutions, both for kinematic engines and for free-piston types. I
haven't
talked to him yet. I'm looking forward to doing so.
Please keep us posted. It's even on topic, too.
I will if I get anything useful. There's a big book about the history of the
Philips Stirlings, which I've never seen. I'll have to ask one of the
experts if it contains any good info on lubrication.
The basic story is that free-piston Stirlings are using dynamic gas
bearings -- which is to say, the pistons are designed to center themselves
on a film of the working gas. Low-temperature-differential Stirlings
generally run dry, often with graphite pistons (another issue; it's not as
clear as it seems, because synthetic graphite is not lubricious). Other
bearings in those engines are dry-running ball bearings.
Small stationary engines sometimes are made with oiled bottom ends and, in
the case of beta Stirlings, with Rulon or other syntheic seals to keep the
oil out of the hot end. And sometimes they run dry, too. Here it's important
to avoid lateral loads on the pistons, by using rhombic drives, Scotch
yokes, or other mechanisms that result in straight-line forces applied to
the pistons.
I don't know about the big automotive types. There have been a number of
successful ones, from Ford, Saab, and others, so there must be a way. I'm
looking forward to learning more.
The basic issue, BTW, is that oil that gets into the heat exchangers ruins
their efficiency. And oil that gets into the hot end will carbonize. This is
one of the big engineering problems with Stirlings, as you've apparently
heard.
BTW, if anyone is interested in the approach you mention, which is
various
forms of making a light welded or bolted steel structure and filling
it
with
concrete, it has possibilities for the home builder. But it's a lot
trickier
than you might think. There are bonding issues and problems with the
different coefficients of expansion between steel and various...er,
"fillings," plus retained-stress issues with the steel itself. And
concrete
grows (or shrinks; I forget) for decades after it's cast. It's not
enough
to
matter in building structures but it can be an issue when you're
dealing
with thousandths of an inch.
Yes. Slocum goes over this.
The issue is to replicate the static and dynamic stiffness of cast iron
more cheaply (or with less weight) than cast iron.
A fabricated box-beam frame has plenty of static stiffness, but has far
too little damping, so the dynamic stiffness is scant. Filling the
frame is an attempt to sharply increase the damping and thus dynamic
stiffness, to make the chatter threshold more remote.
Joe Gwinn
Yes. But a torsion box (box-beam) made of concrete has lots of natural
damping. As you say, the filled-base commercial machines have been
attempts
to build in damping with low shipping weight (and lower costs) for the
machines.
It's a very tricky engineering problem, but it doesn't require a lot of
knowledge or heavy math. It just requires a good feel for materials and
structures, combined with a lot of patient thought and analysis.
As I said, Slocum runs hot and cold on this in the book. The savings
were not dramatic, even if he used aluminum weldments for the machine
frame. (Yes, it's a torsion box, bent into a C-form.) I think Slocum
did build such machines, so the tradeoff summary wasn't just a
theoretical musing.
Slocum patented (5,799,924 and 5,743,326) a vibration damper consisting
of a tube with a greased steel rod within, with the space between tube
and greased rod filled with poured-in epoxy resin. The grease acts as a
mold release, so the rod is floating, and later acts as the goop layer
to absorb vibration. I think that this was intended for boring bars, as
an alternative to solid carbide. And one could make the tube from solid
carbide as well.
I suppose one could embed a greased inner box frame inside the box beam
machine frame before filling with resin. From his patent drawings,
Slocum is thinking this way.
Joe Gwinn
If it gets too complicated, it can become a solution in search of a problem.
g When I was writing about machine tools and people were trying to sell me
on the Hexapod, it became one of those. Likewise, an
elliptical-piston-machining lathe I was once involved with.
--
Ed Huntress