Subject: Re: Turning a parabola

From: [email-masked] (MORRISON ROBERT)
Date: Thu, 21 Oct 1993 14:23:40 GMT

Gary Rosenkranz explains that rather than turning a parabola on a
lathe, he'd manufacture it by stamping.  I fully agree this would
be the best way to *manufacture* a reflector, but in my case I was
asking from the viewpoint of a home machinist who might want to
make one or two, and doesn't have ready access to a stamping machine,
nor any way to make the stamping dies easily.  For a small quantity,
stamping would probably be prohibitive.  But it's instructive to
view alternatives, and to balance mass-production methods against
"prototyping" methods.

Just to take it a bit farther, stamping may not work well if the
parabola is very deep.  Take, as an example, a flashlight reflector.
I would think that stamping such a deep shape from flat stock would
be difficult without "pleats" forming from the extreme deformation
near the outside lip.  Probably for that reason, as well as expense
issues, such reflectors are usually molded plastic.  Just another
alternative.  For shallow curves such as solar cigarette lighters,
stamping seems to be fine.

Dave Parsons described a method which I didn't quite follow, and
I'm hoping he might find time to elaborate.

Thanks for all the suggestions!

Jim Kirkpatrick
[email-masked]  (still posting from a "borrowed" system!)

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From: [email-masked] (Michael Collins)
Date: Thu, 21 Oct 1993 18:05:55 GMT

Jim Kirkpatrick wrote:
|> ...stamping may not work well if the
|> parabola is very deep.  Take, as an example, a flashlight reflector.
|> I would think that stamping such a deep shape from flat stock would
|> be difficult without "pleats" forming from the extreme deformation
|> near the outside lip

  You might think that, but you'd be wrong :-).  Most metals can
be stamped ("drawn" is the correct term for this particular type of
operation) to shapes far deeper and more elaborate than a simple
flashlight reflector.  Look at a common aluminum beverage can --
no seams, no "pleats".  In the forming die set, the material is held
under tension around the periphery of the punch and (usually) die,
and is actually stretched uniformly to shape.  Getting this to happen
consistently is no simple task, but there's a lot of science (and plenty
of art) in the business of volume sheet metal fabrication.

					-- MC --
--
[email-masked]

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From: [email-masked] (JOHN FARMER)
Date: 22 Oct 1993 13:22:20 -0400

In article <[email-masked]> [email-masked] (MORRISON ROBERT) writes:

	...[Some stuff deleted]...

>issues, such reflectors are usually molded plastic.  Just another
				     ^^^^^^^^^^^^^^
>[email-masked]  (still posting from a "borrowed" system!)

Sorry to disappoint you Jim, But the use of plastic for flashlight
reflecteors is a fairly new thing.  I have (somewhere...) one or more
old lights that have metal reflectors.  Granted, they are _old_ but
the changeover to plastic was not (I don't think) due to problems
forming the shape.  I remember the first time I got/purchased a
new non-cheap flashligh and it had a plastic reflector!  
Disappointed I was!  But it appears to be a sign of the times....

John T.

Just say that I'm old enough to remember lots of neat things...
		And young enough to not remember the bad things!

-- 
John T. Farmer, Jr.					[email-masked]
Full-time Daddy, Part-time Grad Student,
Full-time wage slave, Wood butcher when I can.
My son's hero's are Capt. Jean Luc Picard, Norm Abrams, and Mr. Rogers!