"Jim Wilkins" wrote in message
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On Nov 24, 8:38 pm, "Ed Huntress" wrote:
"Jim Wilkins" wrote in message

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On Nov 24, 6:42 pm, "Ed Huntress" wrote:





"Jim Wilkins" wrote in message

Interesting, but I think you'd have growth problems at the back end,
unless
you made the spindle float in the inner race of the backside bearing --
or
had the backside bearings run right on the spindle, with clearance.

I'd use a tubular bearing holder (a piece of pipe) cast into a
ferrocement
headstock, with floating rollers at the tail end and a pair of facing
tapered roller bearings on the head end. That's one traditional setup
for
lathes, and it combines Z-axis stability, high load capacity, and
free-floating at the tail end to deal with Z-axis growth of the spindle.

I'd have to look at what kinds of bearings are cheaply available first.
Ed Huntress-
So you have the drive pulley on the end of the spindle instead of
between the bearings?

Yeah, I worked that out in one of my first sketches, and I saw no real
disadvantage. I was thinking about a pretty large (2 inch) hollow spindle.
In my original version, I didn't even have a drive or gears for threading.
It was going to use thread follower attachments, like the ones used on an
old Unimat or some screw machines.

The idea here was to build a little tank of a lathe with only the basics,
which you could build up as you go along. First you make a speed lathe;
learn some freehand turning and spin-forming; then add a tailstock; (do
some
wood-turning); then a cross slide (using it like a gang-turn Wasino), and
eventually, a compound. You could add a power Z-axis feedscrew and my
thought at the time (around 1980) was that CNC was going to become cheap
enough that you might never have geared thread-cutting at all, going
straight to servos for CNC threading.

The reason for the ferrocement over other kinds of reinforced concrete was
twofold. First, it allows free-form shapes, so you could make the
headstock
a hollow monocoque type, like modern lathes, rather than deal with the
lack
of lateral (X-axis) flexibility of old designs with pillow-block or
stantion-type bearing supports. The structure of the bed would be a torque
box, rather than parallel beams. That would give you rigidity throughout,
without adding a lot of mass.

The second reason is that it's isotropic; you don't have to engineer
around
the lines of tensile stress that can be tolerated. It's more like a metal
than reinforced concrete in its mechanical properties. That's easier for
us
amateurs.

It wasn't really a conventional design but it combined separate elements
of
well-tried lathe types. I had some suppositions about the ability of
ferrocement to handle tensile loads which I've since learned, after
reading
Naaman's book, are not exactly true. (It will handle the loads, but it
will
microcrack under tension).

But ferrocement still has some big advantages over other kinds of
reinforcement. It still could be the best choice, perhaps with a bit of
hybridization, using post-tensioned tendons at the bottoms of the lathe
bed,
or maybe some fibers to prevent microcracks. 'Dunno. If I get some time,
I'll have to re-think it now that I have the engineering data to help me
along.

That does allow a higher reduction ratio, and
perhaps a crank handle for threading to a shoulder. Schedule 80 pipe
might have enough wall thickness to make a bearing seat and you could
clamp the bearings with bored-out pipe caps, as long as you can get
the pipe to run true both ways to make the seats parallel.

The idea here was to turn a piece of tubing of some sort to all of the
inside and outside dimensions, including threading for the bearing
retainers, to within a couple of thousandths. One hopes that you have a
friend with a lathe. g Or, if it was a club-type project, these parts
could be made and sold.

Then make a rig that fits tightly into the bore and which has three pads
that rest on the bedways to align the spindle-holding tube close enough
while the structure is curing that you could hone the tube bore to final
dimensions, without a lot of wasted effort.

I would rather be able to adjust the spindle vertically for the Z axis
milling feed, or to turn an oversized pulley or bore a taper in the
spindle end.

OK, there's a lot of room for variation in the basic idea.



I don't see the spindle heating and expanding enough to cause a
problem with the relatively light and flexible bearing support
framework of a homebrew machine.

In good lathe designs, I believe you'll find that the typical setup is to
have a pair of preloaded bearings facing each other at the spindle-nose
end.
In classic designs, these were angular tapered-roller bearings in larger
lathes, and angular-contact ball bearings in smaller ones. All of your
Z-axis location is accomplished with these head-end bearings. Then the
tail
end of the spindle was held in a single- or double-row bearing that
allowed
linear movement -- either straight rollers, or ball bearings that allow
the
spindle to move.

It doesn't take much heat to make the spindle grow substantially, which
will
either overload your bearings or unload the preload on the head-end
bearings. Also, this lathe is no wimp. It would handle much higher loads
than my SB-10L and would be roughly the same size, although a little wider
and probably a little shorter (mine has the 54" bed).

Anyway, it's been a good thought exercise for me from time to time. My
ideas
about it have changed a bit, but, basically, the objective is to design a
lathe that a determined amateur with no experience could build at home.
Some
parts are tricky and will require machining; I've never worked out a
really
good saddle, cross-slide, and compound that can be made easily at home.

As I said, I see it as a progressive design. You could start using it as
soon as you complete the basic steps, learning and gaining some experience
while making some fun and useful things, and then build it up in stages to
a
full-blown thread-cutting engine lathe. With CNC threading, no less. g

I would love to see someone with more time and the ambition for it take
some
of these thoughts and carry it through. I've been diddling with it for so
long that I'm losing heart to do it.

--
Ed Huntress

I used different design criteria, this machine would take on
occasional jobs too large for my lathe and mill. That means it can be
built lighter and cheaper than normal and only have to take finishing
cuts on pieces roughed out with a saw or cutting torch.
(pause to watch Mya dance)
As a bootstrap machine it could make parts to upgrade itself.

The other similar project is a Harig-style cylindrical and tool
grinding attachment for my Toolmaker surface grinder, to make it more
like a Quorn.

This would resemble a small and precise lathe headstock and spindle
that includes sliding motion. The 5C spindle of a spin index might be
a decent start if it can be cleaned up well enough and converted to an
air bearing. The grinder has a Y leadscrew so the Quorn's complexity
can be reduced.

I bought a Morse taper adapter with the outside ground to 1.000 OD for
this project and discovered that it had bulged slightly around the
release key slot.

Those sound like good projects, too. Regarding the big machine, things like
gap-bed lathes with big swings are a natural for the ferrocement
construction method. You can span long distances at very low cost. It's also
a good place for cylindrical bedways. The travels typically are short, and
you have plenty of room to make their diameter as large as you need.

In fact, a big gap-bed lathe with short tool travels would be easier than a
thread-cutting engine lathe by a wide margin. If you have to face big disks,
however, the whole prospect is a lot more difficult.

--
Ed Huntress