adam smith wrote:

I want to purchase and learn to use a lathe, because I need to make
new parts for some old 1930's air horns (diaphone fog horns) that are
not otherwise available. One part I need to make is the piston, which
looks like this:

http://www.airraidsirens.com/diaphon..._front_640.jpg
http://www.airraidsirens.com/diaphon..._close_640.jpg

It's 4" in diameter at the wide end, and 2.83" diameter over the rest,
by about 5" long overall. This piece fits concentrically inside a
brass cylinder, and it reciprocates at 180Hz when in use. The original
1918 shop drawings for this device spec the precision of the diameter
of the piston as "4 inches, to gauge limits--machine all over
outside". Newer drawings have the diameter as 4.000, +0.000/-0.002

I know these are made by first casting an aluminum blank (with a core
for the hollow part), but what I do not know is what caliber of lathe
and tooling I will need to do the machine work. I am not a machinist,
but I'm going to learn with the intent of making these parts and other
pieces for various antique sirens and horns. Thus, I'm interested in
buying a lathe to learn on primarily, but I want to be sure to get one
the first time around that,when I'm skilled enough, can turn out these
parts with the specified accuracy.

I'm interested especially in which import lathes are up to the task,
and what size is required. Yes, I realize many of you are now groaning
at my use of the word "import", but please let me explain. My budget
is that of the home enthusiast and does not allow for purchasing new
made-in-the-USA equipment. I've been reading this group for many
years, and many times have I heard "get an old South Bend, Atlas or
Logan". That is undoubtedly great advice for an experienced machinist
setting up a home shop, but I'm just starting out and I do not feel
that I could safely shop for a used lathe and not end up with a
junker. I also want a tool that is ready to use and not a project unto
itself. With these criteria, is an old SB still a possibility?

Thanks,

Adam

Your first photo makes it appear that there may be a joint between the
slotted barrel and the larger portion. Is there?

I'm assuming the part is the moving section of a valve which modulates an
air flow to produce the loud 180 Hz acoustic waveform emitted by the fog
horn

Do you know how those slots were originally made? You could probably tell
from the finish of their walls whether they were cast around fins on the
core or cut with some sort of saw or milling cutter. I'll wager they were
cut though, they look pretty thin for any reasonable core material of
their original era.

Selecting a suitable lathe may well be the easiest part of your project,
and I'll leave it to the experts here to advise you on brands and sizes.
It looks like that part could be turned on most any 6 inch or larger
lathe. Anything but a real junker ought to be able to hold .002" on
diameter on a part that size.

Cutting those slots will require some kind of milling or rotary saw setup,
along with fixturing to hold and position the part in the correct location
for each slot. I suspect they have to be located axially to within maybe
ten thousandths of an inch of their design locations to keep the valving
action optimal. Again, this will probably be a trickier thing to handle
correctly than the lathe work itself.

Some of the things you'll have to learn or figure out for yourself are
what kind of a chuck or other fixturing to use to hold the part and
minimize the chances for slippage or shifting while you're turning it on
the lathe, and what kind of tool bit shape and cutting fluid to use to get
the finish you need. Also, you want to avoid being bit by thermal
expansion, by making sure the part isn't much above room temperature when
you make your final sizing passes.

You didn't mention anything about where you're going to get the castings
to turn these from. If you're thinking DIY on that, you'll probably find
out that there's more skills to learn about the casting portion than what
you'll need to aquire for the simple lathe turning operations.

I suspect those six ribs inside the part were put there there to add
strength to the piece so vibration didn't cause it to crack at those
little solid sections between the ends of the slots. You might be able to
turn the part on a lathe in it's entirety from a piece of modern strong
aluminum bar stock and avoid the whole casting exercise; unless, of
course, your goal is to make exact replicas of the originals.

I'm sure others here will be able to offer helpful suggestions.

Good luck,

Jeff




--
Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"If you can keep smiling when things go wrong, you've thought of someone
to place the blame on."